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0001 /*
0002  *  linux/mm/vmscan.c
0003  *
0004  *  Copyright (C) 1991, 1992, 1993, 1994  Linus Torvalds
0005  *
0006  *  Swap reorganised 29.12.95, Stephen Tweedie.
0007  *  kswapd added: 7.1.96  sct
0008  *  Removed kswapd_ctl limits, and swap out as many pages as needed
0009  *  to bring the system back to freepages.high: 2.4.97, Rik van Riel.
0010  *  Zone aware kswapd started 02/00, Kanoj Sarcar (kanoj@sgi.com).
0011  *  Multiqueue VM started 5.8.00, Rik van Riel.
0012  */
0013 
0014 #define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
0015 
0016 #include <linux/mm.h>
0017 #include <linux/module.h>
0018 #include <linux/gfp.h>
0019 #include <linux/kernel_stat.h>
0020 #include <linux/swap.h>
0021 #include <linux/pagemap.h>
0022 #include <linux/init.h>
0023 #include <linux/highmem.h>
0024 #include <linux/vmpressure.h>
0025 #include <linux/vmstat.h>
0026 #include <linux/file.h>
0027 #include <linux/writeback.h>
0028 #include <linux/blkdev.h>
0029 #include <linux/buffer_head.h>  /* for try_to_release_page(),
0030                     buffer_heads_over_limit */
0031 #include <linux/mm_inline.h>
0032 #include <linux/backing-dev.h>
0033 #include <linux/rmap.h>
0034 #include <linux/topology.h>
0035 #include <linux/cpu.h>
0036 #include <linux/cpuset.h>
0037 #include <linux/compaction.h>
0038 #include <linux/notifier.h>
0039 #include <linux/rwsem.h>
0040 #include <linux/delay.h>
0041 #include <linux/kthread.h>
0042 #include <linux/freezer.h>
0043 #include <linux/memcontrol.h>
0044 #include <linux/delayacct.h>
0045 #include <linux/sysctl.h>
0046 #include <linux/oom.h>
0047 #include <linux/prefetch.h>
0048 #include <linux/printk.h>
0049 #include <linux/dax.h>
0050 
0051 #include <asm/tlbflush.h>
0052 #include <asm/div64.h>
0053 
0054 #include <linux/swapops.h>
0055 #include <linux/balloon_compaction.h>
0056 
0057 #include "internal.h"
0058 
0059 #define CREATE_TRACE_POINTS
0060 #include <trace/events/vmscan.h>
0061 
0062 struct scan_control {
0063     /* How many pages shrink_list() should reclaim */
0064     unsigned long nr_to_reclaim;
0065 
0066     /* This context's GFP mask */
0067     gfp_t gfp_mask;
0068 
0069     /* Allocation order */
0070     int order;
0071 
0072     /*
0073      * Nodemask of nodes allowed by the caller. If NULL, all nodes
0074      * are scanned.
0075      */
0076     nodemask_t  *nodemask;
0077 
0078     /*
0079      * The memory cgroup that hit its limit and as a result is the
0080      * primary target of this reclaim invocation.
0081      */
0082     struct mem_cgroup *target_mem_cgroup;
0083 
0084     /* Scan (total_size >> priority) pages at once */
0085     int priority;
0086 
0087     /* The highest zone to isolate pages for reclaim from */
0088     enum zone_type reclaim_idx;
0089 
0090     unsigned int may_writepage:1;
0091 
0092     /* Can mapped pages be reclaimed? */
0093     unsigned int may_unmap:1;
0094 
0095     /* Can pages be swapped as part of reclaim? */
0096     unsigned int may_swap:1;
0097 
0098     /* Can cgroups be reclaimed below their normal consumption range? */
0099     unsigned int may_thrash:1;
0100 
0101     unsigned int hibernation_mode:1;
0102 
0103     /* One of the zones is ready for compaction */
0104     unsigned int compaction_ready:1;
0105 
0106     /* Incremented by the number of inactive pages that were scanned */
0107     unsigned long nr_scanned;
0108 
0109     /* Number of pages freed so far during a call to shrink_zones() */
0110     unsigned long nr_reclaimed;
0111 };
0112 
0113 #ifdef ARCH_HAS_PREFETCH
0114 #define prefetch_prev_lru_page(_page, _base, _field)            \
0115     do {                                \
0116         if ((_page)->lru.prev != _base) {           \
0117             struct page *prev;              \
0118                                     \
0119             prev = lru_to_page(&(_page->lru));      \
0120             prefetch(&prev->_field);            \
0121         }                           \
0122     } while (0)
0123 #else
0124 #define prefetch_prev_lru_page(_page, _base, _field) do { } while (0)
0125 #endif
0126 
0127 #ifdef ARCH_HAS_PREFETCHW
0128 #define prefetchw_prev_lru_page(_page, _base, _field)           \
0129     do {                                \
0130         if ((_page)->lru.prev != _base) {           \
0131             struct page *prev;              \
0132                                     \
0133             prev = lru_to_page(&(_page->lru));      \
0134             prefetchw(&prev->_field);           \
0135         }                           \
0136     } while (0)
0137 #else
0138 #define prefetchw_prev_lru_page(_page, _base, _field) do { } while (0)
0139 #endif
0140 
0141 /*
0142  * From 0 .. 100.  Higher means more swappy.
0143  */
0144 int vm_swappiness = 60;
0145 /*
0146  * The total number of pages which are beyond the high watermark within all
0147  * zones.
0148  */
0149 unsigned long vm_total_pages;
0150 
0151 static LIST_HEAD(shrinker_list);
0152 static DECLARE_RWSEM(shrinker_rwsem);
0153 
0154 #ifdef CONFIG_MEMCG
0155 static bool global_reclaim(struct scan_control *sc)
0156 {
0157     return !sc->target_mem_cgroup;
0158 }
0159 
0160 /**
0161  * sane_reclaim - is the usual dirty throttling mechanism operational?
0162  * @sc: scan_control in question
0163  *
0164  * The normal page dirty throttling mechanism in balance_dirty_pages() is
0165  * completely broken with the legacy memcg and direct stalling in
0166  * shrink_page_list() is used for throttling instead, which lacks all the
0167  * niceties such as fairness, adaptive pausing, bandwidth proportional
0168  * allocation and configurability.
0169  *
0170  * This function tests whether the vmscan currently in progress can assume
0171  * that the normal dirty throttling mechanism is operational.
0172  */
0173 static bool sane_reclaim(struct scan_control *sc)
0174 {
0175     struct mem_cgroup *memcg = sc->target_mem_cgroup;
0176 
0177     if (!memcg)
0178         return true;
0179 #ifdef CONFIG_CGROUP_WRITEBACK
0180     if (cgroup_subsys_on_dfl(memory_cgrp_subsys))
0181         return true;
0182 #endif
0183     return false;
0184 }
0185 #else
0186 static bool global_reclaim(struct scan_control *sc)
0187 {
0188     return true;
0189 }
0190 
0191 static bool sane_reclaim(struct scan_control *sc)
0192 {
0193     return true;
0194 }
0195 #endif
0196 
0197 /*
0198  * This misses isolated pages which are not accounted for to save counters.
0199  * As the data only determines if reclaim or compaction continues, it is
0200  * not expected that isolated pages will be a dominating factor.
0201  */
0202 unsigned long zone_reclaimable_pages(struct zone *zone)
0203 {
0204     unsigned long nr;
0205 
0206     nr = zone_page_state_snapshot(zone, NR_ZONE_INACTIVE_FILE) +
0207         zone_page_state_snapshot(zone, NR_ZONE_ACTIVE_FILE);
0208     if (get_nr_swap_pages() > 0)
0209         nr += zone_page_state_snapshot(zone, NR_ZONE_INACTIVE_ANON) +
0210             zone_page_state_snapshot(zone, NR_ZONE_ACTIVE_ANON);
0211 
0212     return nr;
0213 }
0214 
0215 unsigned long pgdat_reclaimable_pages(struct pglist_data *pgdat)
0216 {
0217     unsigned long nr;
0218 
0219     nr = node_page_state_snapshot(pgdat, NR_ACTIVE_FILE) +
0220          node_page_state_snapshot(pgdat, NR_INACTIVE_FILE) +
0221          node_page_state_snapshot(pgdat, NR_ISOLATED_FILE);
0222 
0223     if (get_nr_swap_pages() > 0)
0224         nr += node_page_state_snapshot(pgdat, NR_ACTIVE_ANON) +
0225               node_page_state_snapshot(pgdat, NR_INACTIVE_ANON) +
0226               node_page_state_snapshot(pgdat, NR_ISOLATED_ANON);
0227 
0228     return nr;
0229 }
0230 
0231 bool pgdat_reclaimable(struct pglist_data *pgdat)
0232 {
0233     return node_page_state_snapshot(pgdat, NR_PAGES_SCANNED) <
0234         pgdat_reclaimable_pages(pgdat) * 6;
0235 }
0236 
0237 unsigned long lruvec_lru_size(struct lruvec *lruvec, enum lru_list lru)
0238 {
0239     if (!mem_cgroup_disabled())
0240         return mem_cgroup_get_lru_size(lruvec, lru);
0241 
0242     return node_page_state(lruvec_pgdat(lruvec), NR_LRU_BASE + lru);
0243 }
0244 
0245 unsigned long lruvec_zone_lru_size(struct lruvec *lruvec, enum lru_list lru,
0246                    int zone_idx)
0247 {
0248     if (!mem_cgroup_disabled())
0249         return mem_cgroup_get_zone_lru_size(lruvec, lru, zone_idx);
0250 
0251     return zone_page_state(&lruvec_pgdat(lruvec)->node_zones[zone_idx],
0252                    NR_ZONE_LRU_BASE + lru);
0253 }
0254 
0255 /*
0256  * Add a shrinker callback to be called from the vm.
0257  */
0258 int register_shrinker(struct shrinker *shrinker)
0259 {
0260     size_t size = sizeof(*shrinker->nr_deferred);
0261 
0262     if (shrinker->flags & SHRINKER_NUMA_AWARE)
0263         size *= nr_node_ids;
0264 
0265     shrinker->nr_deferred = kzalloc(size, GFP_KERNEL);
0266     if (!shrinker->nr_deferred)
0267         return -ENOMEM;
0268 
0269     down_write(&shrinker_rwsem);
0270     list_add_tail(&shrinker->list, &shrinker_list);
0271     up_write(&shrinker_rwsem);
0272     return 0;
0273 }
0274 EXPORT_SYMBOL(register_shrinker);
0275 
0276 /*
0277  * Remove one
0278  */
0279 void unregister_shrinker(struct shrinker *shrinker)
0280 {
0281     down_write(&shrinker_rwsem);
0282     list_del(&shrinker->list);
0283     up_write(&shrinker_rwsem);
0284     kfree(shrinker->nr_deferred);
0285 }
0286 EXPORT_SYMBOL(unregister_shrinker);
0287 
0288 #define SHRINK_BATCH 128
0289 
0290 static unsigned long do_shrink_slab(struct shrink_control *shrinkctl,
0291                     struct shrinker *shrinker,
0292                     unsigned long nr_scanned,
0293                     unsigned long nr_eligible)
0294 {
0295     unsigned long freed = 0;
0296     unsigned long long delta;
0297     long total_scan;
0298     long freeable;
0299     long nr;
0300     long new_nr;
0301     int nid = shrinkctl->nid;
0302     long batch_size = shrinker->batch ? shrinker->batch
0303                       : SHRINK_BATCH;
0304     long scanned = 0, next_deferred;
0305 
0306     freeable = shrinker->count_objects(shrinker, shrinkctl);
0307     if (freeable == 0)
0308         return 0;
0309 
0310     /*
0311      * copy the current shrinker scan count into a local variable
0312      * and zero it so that other concurrent shrinker invocations
0313      * don't also do this scanning work.
0314      */
0315     nr = atomic_long_xchg(&shrinker->nr_deferred[nid], 0);
0316 
0317     total_scan = nr;
0318     delta = (4 * nr_scanned) / shrinker->seeks;
0319     delta *= freeable;
0320     do_div(delta, nr_eligible + 1);
0321     total_scan += delta;
0322     if (total_scan < 0) {
0323         pr_err("shrink_slab: %pF negative objects to delete nr=%ld\n",
0324                shrinker->scan_objects, total_scan);
0325         total_scan = freeable;
0326         next_deferred = nr;
0327     } else
0328         next_deferred = total_scan;
0329 
0330     /*
0331      * We need to avoid excessive windup on filesystem shrinkers
0332      * due to large numbers of GFP_NOFS allocations causing the
0333      * shrinkers to return -1 all the time. This results in a large
0334      * nr being built up so when a shrink that can do some work
0335      * comes along it empties the entire cache due to nr >>>
0336      * freeable. This is bad for sustaining a working set in
0337      * memory.
0338      *
0339      * Hence only allow the shrinker to scan the entire cache when
0340      * a large delta change is calculated directly.
0341      */
0342     if (delta < freeable / 4)
0343         total_scan = min(total_scan, freeable / 2);
0344 
0345     /*
0346      * Avoid risking looping forever due to too large nr value:
0347      * never try to free more than twice the estimate number of
0348      * freeable entries.
0349      */
0350     if (total_scan > freeable * 2)
0351         total_scan = freeable * 2;
0352 
0353     trace_mm_shrink_slab_start(shrinker, shrinkctl, nr,
0354                    nr_scanned, nr_eligible,
0355                    freeable, delta, total_scan);
0356 
0357     /*
0358      * Normally, we should not scan less than batch_size objects in one
0359      * pass to avoid too frequent shrinker calls, but if the slab has less
0360      * than batch_size objects in total and we are really tight on memory,
0361      * we will try to reclaim all available objects, otherwise we can end
0362      * up failing allocations although there are plenty of reclaimable
0363      * objects spread over several slabs with usage less than the
0364      * batch_size.
0365      *
0366      * We detect the "tight on memory" situations by looking at the total
0367      * number of objects we want to scan (total_scan). If it is greater
0368      * than the total number of objects on slab (freeable), we must be
0369      * scanning at high prio and therefore should try to reclaim as much as
0370      * possible.
0371      */
0372     while (total_scan >= batch_size ||
0373            total_scan >= freeable) {
0374         unsigned long ret;
0375         unsigned long nr_to_scan = min(batch_size, total_scan);
0376 
0377         shrinkctl->nr_to_scan = nr_to_scan;
0378         ret = shrinker->scan_objects(shrinker, shrinkctl);
0379         if (ret == SHRINK_STOP)
0380             break;
0381         freed += ret;
0382 
0383         count_vm_events(SLABS_SCANNED, nr_to_scan);
0384         total_scan -= nr_to_scan;
0385         scanned += nr_to_scan;
0386 
0387         cond_resched();
0388     }
0389 
0390     if (next_deferred >= scanned)
0391         next_deferred -= scanned;
0392     else
0393         next_deferred = 0;
0394     /*
0395      * move the unused scan count back into the shrinker in a
0396      * manner that handles concurrent updates. If we exhausted the
0397      * scan, there is no need to do an update.
0398      */
0399     if (next_deferred > 0)
0400         new_nr = atomic_long_add_return(next_deferred,
0401                         &shrinker->nr_deferred[nid]);
0402     else
0403         new_nr = atomic_long_read(&shrinker->nr_deferred[nid]);
0404 
0405     trace_mm_shrink_slab_end(shrinker, nid, freed, nr, new_nr, total_scan);
0406     return freed;
0407 }
0408 
0409 /**
0410  * shrink_slab - shrink slab caches
0411  * @gfp_mask: allocation context
0412  * @nid: node whose slab caches to target
0413  * @memcg: memory cgroup whose slab caches to target
0414  * @nr_scanned: pressure numerator
0415  * @nr_eligible: pressure denominator
0416  *
0417  * Call the shrink functions to age shrinkable caches.
0418  *
0419  * @nid is passed along to shrinkers with SHRINKER_NUMA_AWARE set,
0420  * unaware shrinkers will receive a node id of 0 instead.
0421  *
0422  * @memcg specifies the memory cgroup to target. If it is not NULL,
0423  * only shrinkers with SHRINKER_MEMCG_AWARE set will be called to scan
0424  * objects from the memory cgroup specified. Otherwise, only unaware
0425  * shrinkers are called.
0426  *
0427  * @nr_scanned and @nr_eligible form a ratio that indicate how much of
0428  * the available objects should be scanned.  Page reclaim for example
0429  * passes the number of pages scanned and the number of pages on the
0430  * LRU lists that it considered on @nid, plus a bias in @nr_scanned
0431  * when it encountered mapped pages.  The ratio is further biased by
0432  * the ->seeks setting of the shrink function, which indicates the
0433  * cost to recreate an object relative to that of an LRU page.
0434  *
0435  * Returns the number of reclaimed slab objects.
0436  */
0437 static unsigned long shrink_slab(gfp_t gfp_mask, int nid,
0438                  struct mem_cgroup *memcg,
0439                  unsigned long nr_scanned,
0440                  unsigned long nr_eligible)
0441 {
0442     struct shrinker *shrinker;
0443     unsigned long freed = 0;
0444 
0445     if (memcg && (!memcg_kmem_enabled() || !mem_cgroup_online(memcg)))
0446         return 0;
0447 
0448     if (nr_scanned == 0)
0449         nr_scanned = SWAP_CLUSTER_MAX;
0450 
0451     if (!down_read_trylock(&shrinker_rwsem)) {
0452         /*
0453          * If we would return 0, our callers would understand that we
0454          * have nothing else to shrink and give up trying. By returning
0455          * 1 we keep it going and assume we'll be able to shrink next
0456          * time.
0457          */
0458         freed = 1;
0459         goto out;
0460     }
0461 
0462     list_for_each_entry(shrinker, &shrinker_list, list) {
0463         struct shrink_control sc = {
0464             .gfp_mask = gfp_mask,
0465             .nid = nid,
0466             .memcg = memcg,
0467         };
0468 
0469         /*
0470          * If kernel memory accounting is disabled, we ignore
0471          * SHRINKER_MEMCG_AWARE flag and call all shrinkers
0472          * passing NULL for memcg.
0473          */
0474         if (memcg_kmem_enabled() &&
0475             !!memcg != !!(shrinker->flags & SHRINKER_MEMCG_AWARE))
0476             continue;
0477 
0478         if (!(shrinker->flags & SHRINKER_NUMA_AWARE))
0479             sc.nid = 0;
0480 
0481         freed += do_shrink_slab(&sc, shrinker, nr_scanned, nr_eligible);
0482     }
0483 
0484     up_read(&shrinker_rwsem);
0485 out:
0486     cond_resched();
0487     return freed;
0488 }
0489 
0490 void drop_slab_node(int nid)
0491 {
0492     unsigned long freed;
0493 
0494     do {
0495         struct mem_cgroup *memcg = NULL;
0496 
0497         freed = 0;
0498         do {
0499             freed += shrink_slab(GFP_KERNEL, nid, memcg,
0500                          1000, 1000);
0501         } while ((memcg = mem_cgroup_iter(NULL, memcg, NULL)) != NULL);
0502     } while (freed > 10);
0503 }
0504 
0505 void drop_slab(void)
0506 {
0507     int nid;
0508 
0509     for_each_online_node(nid)
0510         drop_slab_node(nid);
0511 }
0512 
0513 static inline int is_page_cache_freeable(struct page *page)
0514 {
0515     /*
0516      * A freeable page cache page is referenced only by the caller
0517      * that isolated the page, the page cache radix tree and
0518      * optional buffer heads at page->private.
0519      */
0520     return page_count(page) - page_has_private(page) == 2;
0521 }
0522 
0523 static int may_write_to_inode(struct inode *inode, struct scan_control *sc)
0524 {
0525     if (current->flags & PF_SWAPWRITE)
0526         return 1;
0527     if (!inode_write_congested(inode))
0528         return 1;
0529     if (inode_to_bdi(inode) == current->backing_dev_info)
0530         return 1;
0531     return 0;
0532 }
0533 
0534 /*
0535  * We detected a synchronous write error writing a page out.  Probably
0536  * -ENOSPC.  We need to propagate that into the address_space for a subsequent
0537  * fsync(), msync() or close().
0538  *
0539  * The tricky part is that after writepage we cannot touch the mapping: nothing
0540  * prevents it from being freed up.  But we have a ref on the page and once
0541  * that page is locked, the mapping is pinned.
0542  *
0543  * We're allowed to run sleeping lock_page() here because we know the caller has
0544  * __GFP_FS.
0545  */
0546 static void handle_write_error(struct address_space *mapping,
0547                 struct page *page, int error)
0548 {
0549     lock_page(page);
0550     if (page_mapping(page) == mapping)
0551         mapping_set_error(mapping, error);
0552     unlock_page(page);
0553 }
0554 
0555 /* possible outcome of pageout() */
0556 typedef enum {
0557     /* failed to write page out, page is locked */
0558     PAGE_KEEP,
0559     /* move page to the active list, page is locked */
0560     PAGE_ACTIVATE,
0561     /* page has been sent to the disk successfully, page is unlocked */
0562     PAGE_SUCCESS,
0563     /* page is clean and locked */
0564     PAGE_CLEAN,
0565 } pageout_t;
0566 
0567 /*
0568  * pageout is called by shrink_page_list() for each dirty page.
0569  * Calls ->writepage().
0570  */
0571 static pageout_t pageout(struct page *page, struct address_space *mapping,
0572              struct scan_control *sc)
0573 {
0574     /*
0575      * If the page is dirty, only perform writeback if that write
0576      * will be non-blocking.  To prevent this allocation from being
0577      * stalled by pagecache activity.  But note that there may be
0578      * stalls if we need to run get_block().  We could test
0579      * PagePrivate for that.
0580      *
0581      * If this process is currently in __generic_file_write_iter() against
0582      * this page's queue, we can perform writeback even if that
0583      * will block.
0584      *
0585      * If the page is swapcache, write it back even if that would
0586      * block, for some throttling. This happens by accident, because
0587      * swap_backing_dev_info is bust: it doesn't reflect the
0588      * congestion state of the swapdevs.  Easy to fix, if needed.
0589      */
0590     if (!is_page_cache_freeable(page))
0591         return PAGE_KEEP;
0592     if (!mapping) {
0593         /*
0594          * Some data journaling orphaned pages can have
0595          * page->mapping == NULL while being dirty with clean buffers.
0596          */
0597         if (page_has_private(page)) {
0598             if (try_to_free_buffers(page)) {
0599                 ClearPageDirty(page);
0600                 pr_info("%s: orphaned page\n", __func__);
0601                 return PAGE_CLEAN;
0602             }
0603         }
0604         return PAGE_KEEP;
0605     }
0606     if (mapping->a_ops->writepage == NULL)
0607         return PAGE_ACTIVATE;
0608     if (!may_write_to_inode(mapping->host, sc))
0609         return PAGE_KEEP;
0610 
0611     if (clear_page_dirty_for_io(page)) {
0612         int res;
0613         struct writeback_control wbc = {
0614             .sync_mode = WB_SYNC_NONE,
0615             .nr_to_write = SWAP_CLUSTER_MAX,
0616             .range_start = 0,
0617             .range_end = LLONG_MAX,
0618             .for_reclaim = 1,
0619         };
0620 
0621         SetPageReclaim(page);
0622         res = mapping->a_ops->writepage(page, &wbc);
0623         if (res < 0)
0624             handle_write_error(mapping, page, res);
0625         if (res == AOP_WRITEPAGE_ACTIVATE) {
0626             ClearPageReclaim(page);
0627             return PAGE_ACTIVATE;
0628         }
0629 
0630         if (!PageWriteback(page)) {
0631             /* synchronous write or broken a_ops? */
0632             ClearPageReclaim(page);
0633         }
0634         trace_mm_vmscan_writepage(page);
0635         inc_node_page_state(page, NR_VMSCAN_WRITE);
0636         return PAGE_SUCCESS;
0637     }
0638 
0639     return PAGE_CLEAN;
0640 }
0641 
0642 /*
0643  * Same as remove_mapping, but if the page is removed from the mapping, it
0644  * gets returned with a refcount of 0.
0645  */
0646 static int __remove_mapping(struct address_space *mapping, struct page *page,
0647                 bool reclaimed)
0648 {
0649     unsigned long flags;
0650 
0651     BUG_ON(!PageLocked(page));
0652     BUG_ON(mapping != page_mapping(page));
0653 
0654     spin_lock_irqsave(&mapping->tree_lock, flags);
0655     /*
0656      * The non racy check for a busy page.
0657      *
0658      * Must be careful with the order of the tests. When someone has
0659      * a ref to the page, it may be possible that they dirty it then
0660      * drop the reference. So if PageDirty is tested before page_count
0661      * here, then the following race may occur:
0662      *
0663      * get_user_pages(&page);
0664      * [user mapping goes away]
0665      * write_to(page);
0666      *              !PageDirty(page)    [good]
0667      * SetPageDirty(page);
0668      * put_page(page);
0669      *              !page_count(page)   [good, discard it]
0670      *
0671      * [oops, our write_to data is lost]
0672      *
0673      * Reversing the order of the tests ensures such a situation cannot
0674      * escape unnoticed. The smp_rmb is needed to ensure the page->flags
0675      * load is not satisfied before that of page->_refcount.
0676      *
0677      * Note that if SetPageDirty is always performed via set_page_dirty,
0678      * and thus under tree_lock, then this ordering is not required.
0679      */
0680     if (!page_ref_freeze(page, 2))
0681         goto cannot_free;
0682     /* note: atomic_cmpxchg in page_freeze_refs provides the smp_rmb */
0683     if (unlikely(PageDirty(page))) {
0684         page_ref_unfreeze(page, 2);
0685         goto cannot_free;
0686     }
0687 
0688     if (PageSwapCache(page)) {
0689         swp_entry_t swap = { .val = page_private(page) };
0690         mem_cgroup_swapout(page, swap);
0691         __delete_from_swap_cache(page);
0692         spin_unlock_irqrestore(&mapping->tree_lock, flags);
0693         swapcache_free(swap);
0694     } else {
0695         void (*freepage)(struct page *);
0696         void *shadow = NULL;
0697 
0698         freepage = mapping->a_ops->freepage;
0699         /*
0700          * Remember a shadow entry for reclaimed file cache in
0701          * order to detect refaults, thus thrashing, later on.
0702          *
0703          * But don't store shadows in an address space that is
0704          * already exiting.  This is not just an optizimation,
0705          * inode reclaim needs to empty out the radix tree or
0706          * the nodes are lost.  Don't plant shadows behind its
0707          * back.
0708          *
0709          * We also don't store shadows for DAX mappings because the
0710          * only page cache pages found in these are zero pages
0711          * covering holes, and because we don't want to mix DAX
0712          * exceptional entries and shadow exceptional entries in the
0713          * same page_tree.
0714          */
0715         if (reclaimed && page_is_file_cache(page) &&
0716             !mapping_exiting(mapping) && !dax_mapping(mapping))
0717             shadow = workingset_eviction(mapping, page);
0718         __delete_from_page_cache(page, shadow);
0719         spin_unlock_irqrestore(&mapping->tree_lock, flags);
0720 
0721         if (freepage != NULL)
0722             freepage(page);
0723     }
0724 
0725     return 1;
0726 
0727 cannot_free:
0728     spin_unlock_irqrestore(&mapping->tree_lock, flags);
0729     return 0;
0730 }
0731 
0732 /*
0733  * Attempt to detach a locked page from its ->mapping.  If it is dirty or if
0734  * someone else has a ref on the page, abort and return 0.  If it was
0735  * successfully detached, return 1.  Assumes the caller has a single ref on
0736  * this page.
0737  */
0738 int remove_mapping(struct address_space *mapping, struct page *page)
0739 {
0740     if (__remove_mapping(mapping, page, false)) {
0741         /*
0742          * Unfreezing the refcount with 1 rather than 2 effectively
0743          * drops the pagecache ref for us without requiring another
0744          * atomic operation.
0745          */
0746         page_ref_unfreeze(page, 1);
0747         return 1;
0748     }
0749     return 0;
0750 }
0751 
0752 /**
0753  * putback_lru_page - put previously isolated page onto appropriate LRU list
0754  * @page: page to be put back to appropriate lru list
0755  *
0756  * Add previously isolated @page to appropriate LRU list.
0757  * Page may still be unevictable for other reasons.
0758  *
0759  * lru_lock must not be held, interrupts must be enabled.
0760  */
0761 void putback_lru_page(struct page *page)
0762 {
0763     bool is_unevictable;
0764     int was_unevictable = PageUnevictable(page);
0765 
0766     VM_BUG_ON_PAGE(PageLRU(page), page);
0767 
0768 redo:
0769     ClearPageUnevictable(page);
0770 
0771     if (page_evictable(page)) {
0772         /*
0773          * For evictable pages, we can use the cache.
0774          * In event of a race, worst case is we end up with an
0775          * unevictable page on [in]active list.
0776          * We know how to handle that.
0777          */
0778         is_unevictable = false;
0779         lru_cache_add(page);
0780     } else {
0781         /*
0782          * Put unevictable pages directly on zone's unevictable
0783          * list.
0784          */
0785         is_unevictable = true;
0786         add_page_to_unevictable_list(page);
0787         /*
0788          * When racing with an mlock or AS_UNEVICTABLE clearing
0789          * (page is unlocked) make sure that if the other thread
0790          * does not observe our setting of PG_lru and fails
0791          * isolation/check_move_unevictable_pages,
0792          * we see PG_mlocked/AS_UNEVICTABLE cleared below and move
0793          * the page back to the evictable list.
0794          *
0795          * The other side is TestClearPageMlocked() or shmem_lock().
0796          */
0797         smp_mb();
0798     }
0799 
0800     /*
0801      * page's status can change while we move it among lru. If an evictable
0802      * page is on unevictable list, it never be freed. To avoid that,
0803      * check after we added it to the list, again.
0804      */
0805     if (is_unevictable && page_evictable(page)) {
0806         if (!isolate_lru_page(page)) {
0807             put_page(page);
0808             goto redo;
0809         }
0810         /* This means someone else dropped this page from LRU
0811          * So, it will be freed or putback to LRU again. There is
0812          * nothing to do here.
0813          */
0814     }
0815 
0816     if (was_unevictable && !is_unevictable)
0817         count_vm_event(UNEVICTABLE_PGRESCUED);
0818     else if (!was_unevictable && is_unevictable)
0819         count_vm_event(UNEVICTABLE_PGCULLED);
0820 
0821     put_page(page);     /* drop ref from isolate */
0822 }
0823 
0824 enum page_references {
0825     PAGEREF_RECLAIM,
0826     PAGEREF_RECLAIM_CLEAN,
0827     PAGEREF_KEEP,
0828     PAGEREF_ACTIVATE,
0829 };
0830 
0831 static enum page_references page_check_references(struct page *page,
0832                           struct scan_control *sc)
0833 {
0834     int referenced_ptes, referenced_page;
0835     unsigned long vm_flags;
0836 
0837     referenced_ptes = page_referenced(page, 1, sc->target_mem_cgroup,
0838                       &vm_flags);
0839     referenced_page = TestClearPageReferenced(page);
0840 
0841     /*
0842      * Mlock lost the isolation race with us.  Let try_to_unmap()
0843      * move the page to the unevictable list.
0844      */
0845     if (vm_flags & VM_LOCKED)
0846         return PAGEREF_RECLAIM;
0847 
0848     if (referenced_ptes) {
0849         if (PageSwapBacked(page))
0850             return PAGEREF_ACTIVATE;
0851         /*
0852          * All mapped pages start out with page table
0853          * references from the instantiating fault, so we need
0854          * to look twice if a mapped file page is used more
0855          * than once.
0856          *
0857          * Mark it and spare it for another trip around the
0858          * inactive list.  Another page table reference will
0859          * lead to its activation.
0860          *
0861          * Note: the mark is set for activated pages as well
0862          * so that recently deactivated but used pages are
0863          * quickly recovered.
0864          */
0865         SetPageReferenced(page);
0866 
0867         if (referenced_page || referenced_ptes > 1)
0868             return PAGEREF_ACTIVATE;
0869 
0870         /*
0871          * Activate file-backed executable pages after first usage.
0872          */
0873         if (vm_flags & VM_EXEC)
0874             return PAGEREF_ACTIVATE;
0875 
0876         return PAGEREF_KEEP;
0877     }
0878 
0879     /* Reclaim if clean, defer dirty pages to writeback */
0880     if (referenced_page && !PageSwapBacked(page))
0881         return PAGEREF_RECLAIM_CLEAN;
0882 
0883     return PAGEREF_RECLAIM;
0884 }
0885 
0886 /* Check if a page is dirty or under writeback */
0887 static void page_check_dirty_writeback(struct page *page,
0888                        bool *dirty, bool *writeback)
0889 {
0890     struct address_space *mapping;
0891 
0892     /*
0893      * Anonymous pages are not handled by flushers and must be written
0894      * from reclaim context. Do not stall reclaim based on them
0895      */
0896     if (!page_is_file_cache(page)) {
0897         *dirty = false;
0898         *writeback = false;
0899         return;
0900     }
0901 
0902     /* By default assume that the page flags are accurate */
0903     *dirty = PageDirty(page);
0904     *writeback = PageWriteback(page);
0905 
0906     /* Verify dirty/writeback state if the filesystem supports it */
0907     if (!page_has_private(page))
0908         return;
0909 
0910     mapping = page_mapping(page);
0911     if (mapping && mapping->a_ops->is_dirty_writeback)
0912         mapping->a_ops->is_dirty_writeback(page, dirty, writeback);
0913 }
0914 
0915 /*
0916  * shrink_page_list() returns the number of reclaimed pages
0917  */
0918 static unsigned long shrink_page_list(struct list_head *page_list,
0919                       struct pglist_data *pgdat,
0920                       struct scan_control *sc,
0921                       enum ttu_flags ttu_flags,
0922                       unsigned long *ret_nr_dirty,
0923                       unsigned long *ret_nr_unqueued_dirty,
0924                       unsigned long *ret_nr_congested,
0925                       unsigned long *ret_nr_writeback,
0926                       unsigned long *ret_nr_immediate,
0927                       bool force_reclaim)
0928 {
0929     LIST_HEAD(ret_pages);
0930     LIST_HEAD(free_pages);
0931     int pgactivate = 0;
0932     unsigned long nr_unqueued_dirty = 0;
0933     unsigned long nr_dirty = 0;
0934     unsigned long nr_congested = 0;
0935     unsigned long nr_reclaimed = 0;
0936     unsigned long nr_writeback = 0;
0937     unsigned long nr_immediate = 0;
0938 
0939     cond_resched();
0940 
0941     while (!list_empty(page_list)) {
0942         struct address_space *mapping;
0943         struct page *page;
0944         int may_enter_fs;
0945         enum page_references references = PAGEREF_RECLAIM_CLEAN;
0946         bool dirty, writeback;
0947         bool lazyfree = false;
0948         int ret = SWAP_SUCCESS;
0949 
0950         cond_resched();
0951 
0952         page = lru_to_page(page_list);
0953         list_del(&page->lru);
0954 
0955         if (!trylock_page(page))
0956             goto keep;
0957 
0958         VM_BUG_ON_PAGE(PageActive(page), page);
0959 
0960         sc->nr_scanned++;
0961 
0962         if (unlikely(!page_evictable(page)))
0963             goto cull_mlocked;
0964 
0965         if (!sc->may_unmap && page_mapped(page))
0966             goto keep_locked;
0967 
0968         /* Double the slab pressure for mapped and swapcache pages */
0969         if (page_mapped(page) || PageSwapCache(page))
0970             sc->nr_scanned++;
0971 
0972         may_enter_fs = (sc->gfp_mask & __GFP_FS) ||
0973             (PageSwapCache(page) && (sc->gfp_mask & __GFP_IO));
0974 
0975         /*
0976          * The number of dirty pages determines if a zone is marked
0977          * reclaim_congested which affects wait_iff_congested. kswapd
0978          * will stall and start writing pages if the tail of the LRU
0979          * is all dirty unqueued pages.
0980          */
0981         page_check_dirty_writeback(page, &dirty, &writeback);
0982         if (dirty || writeback)
0983             nr_dirty++;
0984 
0985         if (dirty && !writeback)
0986             nr_unqueued_dirty++;
0987 
0988         /*
0989          * Treat this page as congested if the underlying BDI is or if
0990          * pages are cycling through the LRU so quickly that the
0991          * pages marked for immediate reclaim are making it to the
0992          * end of the LRU a second time.
0993          */
0994         mapping = page_mapping(page);
0995         if (((dirty || writeback) && mapping &&
0996              inode_write_congested(mapping->host)) ||
0997             (writeback && PageReclaim(page)))
0998             nr_congested++;
0999 
1000         /*
1001          * If a page at the tail of the LRU is under writeback, there
1002          * are three cases to consider.
1003          *
1004          * 1) If reclaim is encountering an excessive number of pages
1005          *    under writeback and this page is both under writeback and
1006          *    PageReclaim then it indicates that pages are being queued
1007          *    for IO but are being recycled through the LRU before the
1008          *    IO can complete. Waiting on the page itself risks an
1009          *    indefinite stall if it is impossible to writeback the
1010          *    page due to IO error or disconnected storage so instead
1011          *    note that the LRU is being scanned too quickly and the
1012          *    caller can stall after page list has been processed.
1013          *
1014          * 2) Global or new memcg reclaim encounters a page that is
1015          *    not marked for immediate reclaim, or the caller does not
1016          *    have __GFP_FS (or __GFP_IO if it's simply going to swap,
1017          *    not to fs). In this case mark the page for immediate
1018          *    reclaim and continue scanning.
1019          *
1020          *    Require may_enter_fs because we would wait on fs, which
1021          *    may not have submitted IO yet. And the loop driver might
1022          *    enter reclaim, and deadlock if it waits on a page for
1023          *    which it is needed to do the write (loop masks off
1024          *    __GFP_IO|__GFP_FS for this reason); but more thought
1025          *    would probably show more reasons.
1026          *
1027          * 3) Legacy memcg encounters a page that is already marked
1028          *    PageReclaim. memcg does not have any dirty pages
1029          *    throttling so we could easily OOM just because too many
1030          *    pages are in writeback and there is nothing else to
1031          *    reclaim. Wait for the writeback to complete.
1032          */
1033         if (PageWriteback(page)) {
1034             /* Case 1 above */
1035             if (current_is_kswapd() &&
1036                 PageReclaim(page) &&
1037                 test_bit(PGDAT_WRITEBACK, &pgdat->flags)) {
1038                 nr_immediate++;
1039                 goto keep_locked;
1040 
1041             /* Case 2 above */
1042             } else if (sane_reclaim(sc) ||
1043                 !PageReclaim(page) || !may_enter_fs) {
1044                 /*
1045                  * This is slightly racy - end_page_writeback()
1046                  * might have just cleared PageReclaim, then
1047                  * setting PageReclaim here end up interpreted
1048                  * as PageReadahead - but that does not matter
1049                  * enough to care.  What we do want is for this
1050                  * page to have PageReclaim set next time memcg
1051                  * reclaim reaches the tests above, so it will
1052                  * then wait_on_page_writeback() to avoid OOM;
1053                  * and it's also appropriate in global reclaim.
1054                  */
1055                 SetPageReclaim(page);
1056                 nr_writeback++;
1057                 goto keep_locked;
1058 
1059             /* Case 3 above */
1060             } else {
1061                 unlock_page(page);
1062                 wait_on_page_writeback(page);
1063                 /* then go back and try same page again */
1064                 list_add_tail(&page->lru, page_list);
1065                 continue;
1066             }
1067         }
1068 
1069         if (!force_reclaim)
1070             references = page_check_references(page, sc);
1071 
1072         switch (references) {
1073         case PAGEREF_ACTIVATE:
1074             goto activate_locked;
1075         case PAGEREF_KEEP:
1076             goto keep_locked;
1077         case PAGEREF_RECLAIM:
1078         case PAGEREF_RECLAIM_CLEAN:
1079             ; /* try to reclaim the page below */
1080         }
1081 
1082         /*
1083          * Anonymous process memory has backing store?
1084          * Try to allocate it some swap space here.
1085          */
1086         if (PageAnon(page) && !PageSwapCache(page)) {
1087             if (!(sc->gfp_mask & __GFP_IO))
1088                 goto keep_locked;
1089             if (!add_to_swap(page, page_list))
1090                 goto activate_locked;
1091             lazyfree = true;
1092             may_enter_fs = 1;
1093 
1094             /* Adding to swap updated mapping */
1095             mapping = page_mapping(page);
1096         } else if (unlikely(PageTransHuge(page))) {
1097             /* Split file THP */
1098             if (split_huge_page_to_list(page, page_list))
1099                 goto keep_locked;
1100         }
1101 
1102         VM_BUG_ON_PAGE(PageTransHuge(page), page);
1103 
1104         /*
1105          * The page is mapped into the page tables of one or more
1106          * processes. Try to unmap it here.
1107          */
1108         if (page_mapped(page) && mapping) {
1109             switch (ret = try_to_unmap(page, lazyfree ?
1110                 (ttu_flags | TTU_BATCH_FLUSH | TTU_LZFREE) :
1111                 (ttu_flags | TTU_BATCH_FLUSH))) {
1112             case SWAP_FAIL:
1113                 goto activate_locked;
1114             case SWAP_AGAIN:
1115                 goto keep_locked;
1116             case SWAP_MLOCK:
1117                 goto cull_mlocked;
1118             case SWAP_LZFREE:
1119                 goto lazyfree;
1120             case SWAP_SUCCESS:
1121                 ; /* try to free the page below */
1122             }
1123         }
1124 
1125         if (PageDirty(page)) {
1126             /*
1127              * Only kswapd can writeback filesystem pages to
1128              * avoid risk of stack overflow but only writeback
1129              * if many dirty pages have been encountered.
1130              */
1131             if (page_is_file_cache(page) &&
1132                     (!current_is_kswapd() ||
1133                      !test_bit(PGDAT_DIRTY, &pgdat->flags))) {
1134                 /*
1135                  * Immediately reclaim when written back.
1136                  * Similar in principal to deactivate_page()
1137                  * except we already have the page isolated
1138                  * and know it's dirty
1139                  */
1140                 inc_node_page_state(page, NR_VMSCAN_IMMEDIATE);
1141                 SetPageReclaim(page);
1142 
1143                 goto keep_locked;
1144             }
1145 
1146             if (references == PAGEREF_RECLAIM_CLEAN)
1147                 goto keep_locked;
1148             if (!may_enter_fs)
1149                 goto keep_locked;
1150             if (!sc->may_writepage)
1151                 goto keep_locked;
1152 
1153             /*
1154              * Page is dirty. Flush the TLB if a writable entry
1155              * potentially exists to avoid CPU writes after IO
1156              * starts and then write it out here.
1157              */
1158             try_to_unmap_flush_dirty();
1159             switch (pageout(page, mapping, sc)) {
1160             case PAGE_KEEP:
1161                 goto keep_locked;
1162             case PAGE_ACTIVATE:
1163                 goto activate_locked;
1164             case PAGE_SUCCESS:
1165                 if (PageWriteback(page))
1166                     goto keep;
1167                 if (PageDirty(page))
1168                     goto keep;
1169 
1170                 /*
1171                  * A synchronous write - probably a ramdisk.  Go
1172                  * ahead and try to reclaim the page.
1173                  */
1174                 if (!trylock_page(page))
1175                     goto keep;
1176                 if (PageDirty(page) || PageWriteback(page))
1177                     goto keep_locked;
1178                 mapping = page_mapping(page);
1179             case PAGE_CLEAN:
1180                 ; /* try to free the page below */
1181             }
1182         }
1183 
1184         /*
1185          * If the page has buffers, try to free the buffer mappings
1186          * associated with this page. If we succeed we try to free
1187          * the page as well.
1188          *
1189          * We do this even if the page is PageDirty().
1190          * try_to_release_page() does not perform I/O, but it is
1191          * possible for a page to have PageDirty set, but it is actually
1192          * clean (all its buffers are clean).  This happens if the
1193          * buffers were written out directly, with submit_bh(). ext3
1194          * will do this, as well as the blockdev mapping.
1195          * try_to_release_page() will discover that cleanness and will
1196          * drop the buffers and mark the page clean - it can be freed.
1197          *
1198          * Rarely, pages can have buffers and no ->mapping.  These are
1199          * the pages which were not successfully invalidated in
1200          * truncate_complete_page().  We try to drop those buffers here
1201          * and if that worked, and the page is no longer mapped into
1202          * process address space (page_count == 1) it can be freed.
1203          * Otherwise, leave the page on the LRU so it is swappable.
1204          */
1205         if (page_has_private(page)) {
1206             if (!try_to_release_page(page, sc->gfp_mask))
1207                 goto activate_locked;
1208             if (!mapping && page_count(page) == 1) {
1209                 unlock_page(page);
1210                 if (put_page_testzero(page))
1211                     goto free_it;
1212                 else {
1213                     /*
1214                      * rare race with speculative reference.
1215                      * the speculative reference will free
1216                      * this page shortly, so we may
1217                      * increment nr_reclaimed here (and
1218                      * leave it off the LRU).
1219                      */
1220                     nr_reclaimed++;
1221                     continue;
1222                 }
1223             }
1224         }
1225 
1226 lazyfree:
1227         if (!mapping || !__remove_mapping(mapping, page, true))
1228             goto keep_locked;
1229 
1230         /*
1231          * At this point, we have no other references and there is
1232          * no way to pick any more up (removed from LRU, removed
1233          * from pagecache). Can use non-atomic bitops now (and
1234          * we obviously don't have to worry about waking up a process
1235          * waiting on the page lock, because there are no references.
1236          */
1237         __ClearPageLocked(page);
1238 free_it:
1239         if (ret == SWAP_LZFREE)
1240             count_vm_event(PGLAZYFREED);
1241 
1242         nr_reclaimed++;
1243 
1244         /*
1245          * Is there need to periodically free_page_list? It would
1246          * appear not as the counts should be low
1247          */
1248         list_add(&page->lru, &free_pages);
1249         continue;
1250 
1251 cull_mlocked:
1252         if (PageSwapCache(page))
1253             try_to_free_swap(page);
1254         unlock_page(page);
1255         list_add(&page->lru, &ret_pages);
1256         continue;
1257 
1258 activate_locked:
1259         /* Not a candidate for swapping, so reclaim swap space. */
1260         if (PageSwapCache(page) && mem_cgroup_swap_full(page))
1261             try_to_free_swap(page);
1262         VM_BUG_ON_PAGE(PageActive(page), page);
1263         SetPageActive(page);
1264         pgactivate++;
1265 keep_locked:
1266         unlock_page(page);
1267 keep:
1268         list_add(&page->lru, &ret_pages);
1269         VM_BUG_ON_PAGE(PageLRU(page) || PageUnevictable(page), page);
1270     }
1271 
1272     mem_cgroup_uncharge_list(&free_pages);
1273     try_to_unmap_flush();
1274     free_hot_cold_page_list(&free_pages, true);
1275 
1276     list_splice(&ret_pages, page_list);
1277     count_vm_events(PGACTIVATE, pgactivate);
1278 
1279     *ret_nr_dirty += nr_dirty;
1280     *ret_nr_congested += nr_congested;
1281     *ret_nr_unqueued_dirty += nr_unqueued_dirty;
1282     *ret_nr_writeback += nr_writeback;
1283     *ret_nr_immediate += nr_immediate;
1284     return nr_reclaimed;
1285 }
1286 
1287 unsigned long reclaim_clean_pages_from_list(struct zone *zone,
1288                         struct list_head *page_list)
1289 {
1290     struct scan_control sc = {
1291         .gfp_mask = GFP_KERNEL,
1292         .priority = DEF_PRIORITY,
1293         .may_unmap = 1,
1294     };
1295     unsigned long ret, dummy1, dummy2, dummy3, dummy4, dummy5;
1296     struct page *page, *next;
1297     LIST_HEAD(clean_pages);
1298 
1299     list_for_each_entry_safe(page, next, page_list, lru) {
1300         if (page_is_file_cache(page) && !PageDirty(page) &&
1301             !__PageMovable(page)) {
1302             ClearPageActive(page);
1303             list_move(&page->lru, &clean_pages);
1304         }
1305     }
1306 
1307     ret = shrink_page_list(&clean_pages, zone->zone_pgdat, &sc,
1308             TTU_UNMAP|TTU_IGNORE_ACCESS,
1309             &dummy1, &dummy2, &dummy3, &dummy4, &dummy5, true);
1310     list_splice(&clean_pages, page_list);
1311     mod_node_page_state(zone->zone_pgdat, NR_ISOLATED_FILE, -ret);
1312     return ret;
1313 }
1314 
1315 /*
1316  * Attempt to remove the specified page from its LRU.  Only take this page
1317  * if it is of the appropriate PageActive status.  Pages which are being
1318  * freed elsewhere are also ignored.
1319  *
1320  * page:    page to consider
1321  * mode:    one of the LRU isolation modes defined above
1322  *
1323  * returns 0 on success, -ve errno on failure.
1324  */
1325 int __isolate_lru_page(struct page *page, isolate_mode_t mode)
1326 {
1327     int ret = -EINVAL;
1328 
1329     /* Only take pages on the LRU. */
1330     if (!PageLRU(page))
1331         return ret;
1332 
1333     /* Compaction should not handle unevictable pages but CMA can do so */
1334     if (PageUnevictable(page) && !(mode & ISOLATE_UNEVICTABLE))
1335         return ret;
1336 
1337     ret = -EBUSY;
1338 
1339     /*
1340      * To minimise LRU disruption, the caller can indicate that it only
1341      * wants to isolate pages it will be able to operate on without
1342      * blocking - clean pages for the most part.
1343      *
1344      * ISOLATE_CLEAN means that only clean pages should be isolated. This
1345      * is used by reclaim when it is cannot write to backing storage
1346      *
1347      * ISOLATE_ASYNC_MIGRATE is used to indicate that it only wants to pages
1348      * that it is possible to migrate without blocking
1349      */
1350     if (mode & (ISOLATE_CLEAN|ISOLATE_ASYNC_MIGRATE)) {
1351         /* All the caller can do on PageWriteback is block */
1352         if (PageWriteback(page))
1353             return ret;
1354 
1355         if (PageDirty(page)) {
1356             struct address_space *mapping;
1357 
1358             /* ISOLATE_CLEAN means only clean pages */
1359             if (mode & ISOLATE_CLEAN)
1360                 return ret;
1361 
1362             /*
1363              * Only pages without mappings or that have a
1364              * ->migratepage callback are possible to migrate
1365              * without blocking
1366              */
1367             mapping = page_mapping(page);
1368             if (mapping && !mapping->a_ops->migratepage)
1369                 return ret;
1370         }
1371     }
1372 
1373     if ((mode & ISOLATE_UNMAPPED) && page_mapped(page))
1374         return ret;
1375 
1376     if (likely(get_page_unless_zero(page))) {
1377         /*
1378          * Be careful not to clear PageLRU until after we're
1379          * sure the page is not being freed elsewhere -- the
1380          * page release code relies on it.
1381          */
1382         ClearPageLRU(page);
1383         ret = 0;
1384     }
1385 
1386     return ret;
1387 }
1388 
1389 
1390 /*
1391  * Update LRU sizes after isolating pages. The LRU size updates must
1392  * be complete before mem_cgroup_update_lru_size due to a santity check.
1393  */
1394 static __always_inline void update_lru_sizes(struct lruvec *lruvec,
1395             enum lru_list lru, unsigned long *nr_zone_taken)
1396 {
1397     int zid;
1398 
1399     for (zid = 0; zid < MAX_NR_ZONES; zid++) {
1400         if (!nr_zone_taken[zid])
1401             continue;
1402 
1403         __update_lru_size(lruvec, lru, zid, -nr_zone_taken[zid]);
1404 #ifdef CONFIG_MEMCG
1405         mem_cgroup_update_lru_size(lruvec, lru, zid, -nr_zone_taken[zid]);
1406 #endif
1407     }
1408 
1409 }
1410 
1411 /*
1412  * zone_lru_lock is heavily contended.  Some of the functions that
1413  * shrink the lists perform better by taking out a batch of pages
1414  * and working on them outside the LRU lock.
1415  *
1416  * For pagecache intensive workloads, this function is the hottest
1417  * spot in the kernel (apart from copy_*_user functions).
1418  *
1419  * Appropriate locks must be held before calling this function.
1420  *
1421  * @nr_to_scan: The number of pages to look through on the list.
1422  * @lruvec: The LRU vector to pull pages from.
1423  * @dst:    The temp list to put pages on to.
1424  * @nr_scanned: The number of pages that were scanned.
1425  * @sc:     The scan_control struct for this reclaim session
1426  * @mode:   One of the LRU isolation modes
1427  * @lru:    LRU list id for isolating
1428  *
1429  * returns how many pages were moved onto *@dst.
1430  */
1431 static unsigned long isolate_lru_pages(unsigned long nr_to_scan,
1432         struct lruvec *lruvec, struct list_head *dst,
1433         unsigned long *nr_scanned, struct scan_control *sc,
1434         isolate_mode_t mode, enum lru_list lru)
1435 {
1436     struct list_head *src = &lruvec->lists[lru];
1437     unsigned long nr_taken = 0;
1438     unsigned long nr_zone_taken[MAX_NR_ZONES] = { 0 };
1439     unsigned long nr_skipped[MAX_NR_ZONES] = { 0, };
1440     unsigned long scan, nr_pages;
1441     LIST_HEAD(pages_skipped);
1442 
1443     for (scan = 0; scan < nr_to_scan && nr_taken < nr_to_scan &&
1444                     !list_empty(src);) {
1445         struct page *page;
1446 
1447         page = lru_to_page(src);
1448         prefetchw_prev_lru_page(page, src, flags);
1449 
1450         VM_BUG_ON_PAGE(!PageLRU(page), page);
1451 
1452         if (page_zonenum(page) > sc->reclaim_idx) {
1453             list_move(&page->lru, &pages_skipped);
1454             nr_skipped[page_zonenum(page)]++;
1455             continue;
1456         }
1457 
1458         /*
1459          * Account for scanned and skipped separetly to avoid the pgdat
1460          * being prematurely marked unreclaimable by pgdat_reclaimable.
1461          */
1462         scan++;
1463 
1464         switch (__isolate_lru_page(page, mode)) {
1465         case 0:
1466             nr_pages = hpage_nr_pages(page);
1467             nr_taken += nr_pages;
1468             nr_zone_taken[page_zonenum(page)] += nr_pages;
1469             list_move(&page->lru, dst);
1470             break;
1471 
1472         case -EBUSY:
1473             /* else it is being freed elsewhere */
1474             list_move(&page->lru, src);
1475             continue;
1476 
1477         default:
1478             BUG();
1479         }
1480     }
1481 
1482     /*
1483      * Splice any skipped pages to the start of the LRU list. Note that
1484      * this disrupts the LRU order when reclaiming for lower zones but
1485      * we cannot splice to the tail. If we did then the SWAP_CLUSTER_MAX
1486      * scanning would soon rescan the same pages to skip and put the
1487      * system at risk of premature OOM.
1488      */
1489     if (!list_empty(&pages_skipped)) {
1490         int zid;
1491         unsigned long total_skipped = 0;
1492 
1493         for (zid = 0; zid < MAX_NR_ZONES; zid++) {
1494             if (!nr_skipped[zid])
1495                 continue;
1496 
1497             __count_zid_vm_events(PGSCAN_SKIP, zid, nr_skipped[zid]);
1498             total_skipped += nr_skipped[zid];
1499         }
1500 
1501         /*
1502          * Account skipped pages as a partial scan as the pgdat may be
1503          * close to unreclaimable. If the LRU list is empty, account
1504          * skipped pages as a full scan.
1505          */
1506         scan += list_empty(src) ? total_skipped : total_skipped >> 2;
1507 
1508         list_splice(&pages_skipped, src);
1509     }
1510     *nr_scanned = scan;
1511     trace_mm_vmscan_lru_isolate(sc->reclaim_idx, sc->order, nr_to_scan, scan,
1512                     nr_taken, mode, is_file_lru(lru));
1513     update_lru_sizes(lruvec, lru, nr_zone_taken);
1514     return nr_taken;
1515 }
1516 
1517 /**
1518  * isolate_lru_page - tries to isolate a page from its LRU list
1519  * @page: page to isolate from its LRU list
1520  *
1521  * Isolates a @page from an LRU list, clears PageLRU and adjusts the
1522  * vmstat statistic corresponding to whatever LRU list the page was on.
1523  *
1524  * Returns 0 if the page was removed from an LRU list.
1525  * Returns -EBUSY if the page was not on an LRU list.
1526  *
1527  * The returned page will have PageLRU() cleared.  If it was found on
1528  * the active list, it will have PageActive set.  If it was found on
1529  * the unevictable list, it will have the PageUnevictable bit set. That flag
1530  * may need to be cleared by the caller before letting the page go.
1531  *
1532  * The vmstat statistic corresponding to the list on which the page was
1533  * found will be decremented.
1534  *
1535  * Restrictions:
1536  * (1) Must be called with an elevated refcount on the page. This is a
1537  *     fundamentnal difference from isolate_lru_pages (which is called
1538  *     without a stable reference).
1539  * (2) the lru_lock must not be held.
1540  * (3) interrupts must be enabled.
1541  */
1542 int isolate_lru_page(struct page *page)
1543 {
1544     int ret = -EBUSY;
1545 
1546     VM_BUG_ON_PAGE(!page_count(page), page);
1547     WARN_RATELIMIT(PageTail(page), "trying to isolate tail page");
1548 
1549     if (PageLRU(page)) {
1550         struct zone *zone = page_zone(page);
1551         struct lruvec *lruvec;
1552 
1553         spin_lock_irq(zone_lru_lock(zone));
1554         lruvec = mem_cgroup_page_lruvec(page, zone->zone_pgdat);
1555         if (PageLRU(page)) {
1556             int lru = page_lru(page);
1557             get_page(page);
1558             ClearPageLRU(page);
1559             del_page_from_lru_list(page, lruvec, lru);
1560             ret = 0;
1561         }
1562         spin_unlock_irq(zone_lru_lock(zone));
1563     }
1564     return ret;
1565 }
1566 
1567 /*
1568  * A direct reclaimer may isolate SWAP_CLUSTER_MAX pages from the LRU list and
1569  * then get resheduled. When there are massive number of tasks doing page
1570  * allocation, such sleeping direct reclaimers may keep piling up on each CPU,
1571  * the LRU list will go small and be scanned faster than necessary, leading to
1572  * unnecessary swapping, thrashing and OOM.
1573  */
1574 static int too_many_isolated(struct pglist_data *pgdat, int file,
1575         struct scan_control *sc)
1576 {
1577     unsigned long inactive, isolated;
1578 
1579     if (current_is_kswapd())
1580         return 0;
1581 
1582     if (!sane_reclaim(sc))
1583         return 0;
1584 
1585     if (file) {
1586         inactive = node_page_state(pgdat, NR_INACTIVE_FILE);
1587         isolated = node_page_state(pgdat, NR_ISOLATED_FILE);
1588     } else {
1589         inactive = node_page_state(pgdat, NR_INACTIVE_ANON);
1590         isolated = node_page_state(pgdat, NR_ISOLATED_ANON);
1591     }
1592 
1593     /*
1594      * GFP_NOIO/GFP_NOFS callers are allowed to isolate more pages, so they
1595      * won't get blocked by normal direct-reclaimers, forming a circular
1596      * deadlock.
1597      */
1598     if ((sc->gfp_mask & (__GFP_IO | __GFP_FS)) == (__GFP_IO | __GFP_FS))
1599         inactive >>= 3;
1600 
1601     return isolated > inactive;
1602 }
1603 
1604 static noinline_for_stack void
1605 putback_inactive_pages(struct lruvec *lruvec, struct list_head *page_list)
1606 {
1607     struct zone_reclaim_stat *reclaim_stat = &lruvec->reclaim_stat;
1608     struct pglist_data *pgdat = lruvec_pgdat(lruvec);
1609     LIST_HEAD(pages_to_free);
1610 
1611     /*
1612      * Put back any unfreeable pages.
1613      */
1614     while (!list_empty(page_list)) {
1615         struct page *page = lru_to_page(page_list);
1616         int lru;
1617 
1618         VM_BUG_ON_PAGE(PageLRU(page), page);
1619         list_del(&page->lru);
1620         if (unlikely(!page_evictable(page))) {
1621             spin_unlock_irq(&pgdat->lru_lock);
1622             putback_lru_page(page);
1623             spin_lock_irq(&pgdat->lru_lock);
1624             continue;
1625         }
1626 
1627         lruvec = mem_cgroup_page_lruvec(page, pgdat);
1628 
1629         SetPageLRU(page);
1630         lru = page_lru(page);
1631         add_page_to_lru_list(page, lruvec, lru);
1632 
1633         if (is_active_lru(lru)) {
1634             int file = is_file_lru(lru);
1635             int numpages = hpage_nr_pages(page);
1636             reclaim_stat->recent_rotated[file] += numpages;
1637         }
1638         if (put_page_testzero(page)) {
1639             __ClearPageLRU(page);
1640             __ClearPageActive(page);
1641             del_page_from_lru_list(page, lruvec, lru);
1642 
1643             if (unlikely(PageCompound(page))) {
1644                 spin_unlock_irq(&pgdat->lru_lock);
1645                 mem_cgroup_uncharge(page);
1646                 (*get_compound_page_dtor(page))(page);
1647                 spin_lock_irq(&pgdat->lru_lock);
1648             } else
1649                 list_add(&page->lru, &pages_to_free);
1650         }
1651     }
1652 
1653     /*
1654      * To save our caller's stack, now use input list for pages to free.
1655      */
1656     list_splice(&pages_to_free, page_list);
1657 }
1658 
1659 /*
1660  * If a kernel thread (such as nfsd for loop-back mounts) services
1661  * a backing device by writing to the page cache it sets PF_LESS_THROTTLE.
1662  * In that case we should only throttle if the backing device it is
1663  * writing to is congested.  In other cases it is safe to throttle.
1664  */
1665 static int current_may_throttle(void)
1666 {
1667     return !(current->flags & PF_LESS_THROTTLE) ||
1668         current->backing_dev_info == NULL ||
1669         bdi_write_congested(current->backing_dev_info);
1670 }
1671 
1672 static bool inactive_reclaimable_pages(struct lruvec *lruvec,
1673                 struct scan_control *sc, enum lru_list lru)
1674 {
1675     int zid;
1676     struct zone *zone;
1677     int file = is_file_lru(lru);
1678     struct pglist_data *pgdat = lruvec_pgdat(lruvec);
1679 
1680     if (!global_reclaim(sc))
1681         return true;
1682 
1683     for (zid = sc->reclaim_idx; zid >= 0; zid--) {
1684         zone = &pgdat->node_zones[zid];
1685         if (!managed_zone(zone))
1686             continue;
1687 
1688         if (zone_page_state_snapshot(zone, NR_ZONE_LRU_BASE +
1689                 LRU_FILE * file) >= SWAP_CLUSTER_MAX)
1690             return true;
1691     }
1692 
1693     return false;
1694 }
1695 
1696 /*
1697  * shrink_inactive_list() is a helper for shrink_node().  It returns the number
1698  * of reclaimed pages
1699  */
1700 static noinline_for_stack unsigned long
1701 shrink_inactive_list(unsigned long nr_to_scan, struct lruvec *lruvec,
1702              struct scan_control *sc, enum lru_list lru)
1703 {
1704     LIST_HEAD(page_list);
1705     unsigned long nr_scanned;
1706     unsigned long nr_reclaimed = 0;
1707     unsigned long nr_taken;
1708     unsigned long nr_dirty = 0;
1709     unsigned long nr_congested = 0;
1710     unsigned long nr_unqueued_dirty = 0;
1711     unsigned long nr_writeback = 0;
1712     unsigned long nr_immediate = 0;
1713     isolate_mode_t isolate_mode = 0;
1714     int file = is_file_lru(lru);
1715     struct pglist_data *pgdat = lruvec_pgdat(lruvec);
1716     struct zone_reclaim_stat *reclaim_stat = &lruvec->reclaim_stat;
1717 
1718     if (!inactive_reclaimable_pages(lruvec, sc, lru))
1719         return 0;
1720 
1721     while (unlikely(too_many_isolated(pgdat, file, sc))) {
1722         congestion_wait(BLK_RW_ASYNC, HZ/10);
1723 
1724         /* We are about to die and free our memory. Return now. */
1725         if (fatal_signal_pending(current))
1726             return SWAP_CLUSTER_MAX;
1727     }
1728 
1729     lru_add_drain();
1730 
1731     if (!sc->may_unmap)
1732         isolate_mode |= ISOLATE_UNMAPPED;
1733     if (!sc->may_writepage)
1734         isolate_mode |= ISOLATE_CLEAN;
1735 
1736     spin_lock_irq(&pgdat->lru_lock);
1737 
1738     nr_taken = isolate_lru_pages(nr_to_scan, lruvec, &page_list,
1739                      &nr_scanned, sc, isolate_mode, lru);
1740 
1741     __mod_node_page_state(pgdat, NR_ISOLATED_ANON + file, nr_taken);
1742     reclaim_stat->recent_scanned[file] += nr_taken;
1743 
1744     if (global_reclaim(sc)) {
1745         __mod_node_page_state(pgdat, NR_PAGES_SCANNED, nr_scanned);
1746         if (current_is_kswapd())
1747             __count_vm_events(PGSCAN_KSWAPD, nr_scanned);
1748         else
1749             __count_vm_events(PGSCAN_DIRECT, nr_scanned);
1750     }
1751     spin_unlock_irq(&pgdat->lru_lock);
1752 
1753     if (nr_taken == 0)
1754         return 0;
1755 
1756     nr_reclaimed = shrink_page_list(&page_list, pgdat, sc, TTU_UNMAP,
1757                 &nr_dirty, &nr_unqueued_dirty, &nr_congested,
1758                 &nr_writeback, &nr_immediate,
1759                 false);
1760 
1761     spin_lock_irq(&pgdat->lru_lock);
1762 
1763     if (global_reclaim(sc)) {
1764         if (current_is_kswapd())
1765             __count_vm_events(PGSTEAL_KSWAPD, nr_reclaimed);
1766         else
1767             __count_vm_events(PGSTEAL_DIRECT, nr_reclaimed);
1768     }
1769 
1770     putback_inactive_pages(lruvec, &page_list);
1771 
1772     __mod_node_page_state(pgdat, NR_ISOLATED_ANON + file, -nr_taken);
1773 
1774     spin_unlock_irq(&pgdat->lru_lock);
1775 
1776     mem_cgroup_uncharge_list(&page_list);
1777     free_hot_cold_page_list(&page_list, true);
1778 
1779     /*
1780      * If reclaim is isolating dirty pages under writeback, it implies
1781      * that the long-lived page allocation rate is exceeding the page
1782      * laundering rate. Either the global limits are not being effective
1783      * at throttling processes due to the page distribution throughout
1784      * zones or there is heavy usage of a slow backing device. The
1785      * only option is to throttle from reclaim context which is not ideal
1786      * as there is no guarantee the dirtying process is throttled in the
1787      * same way balance_dirty_pages() manages.
1788      *
1789      * Once a zone is flagged ZONE_WRITEBACK, kswapd will count the number
1790      * of pages under pages flagged for immediate reclaim and stall if any
1791      * are encountered in the nr_immediate check below.
1792      */
1793     if (nr_writeback && nr_writeback == nr_taken)
1794         set_bit(PGDAT_WRITEBACK, &pgdat->flags);
1795 
1796     /*
1797      * Legacy memcg will stall in page writeback so avoid forcibly
1798      * stalling here.
1799      */
1800     if (sane_reclaim(sc)) {
1801         /*
1802          * Tag a zone as congested if all the dirty pages scanned were
1803          * backed by a congested BDI and wait_iff_congested will stall.
1804          */
1805         if (nr_dirty && nr_dirty == nr_congested)
1806             set_bit(PGDAT_CONGESTED, &pgdat->flags);
1807 
1808         /*
1809          * If dirty pages are scanned that are not queued for IO, it
1810          * implies that flushers are not keeping up. In this case, flag
1811          * the pgdat PGDAT_DIRTY and kswapd will start writing pages from
1812          * reclaim context.
1813          */
1814         if (nr_unqueued_dirty == nr_taken)
1815             set_bit(PGDAT_DIRTY, &pgdat->flags);
1816 
1817         /*
1818          * If kswapd scans pages marked marked for immediate
1819          * reclaim and under writeback (nr_immediate), it implies
1820          * that pages are cycling through the LRU faster than
1821          * they are written so also forcibly stall.
1822          */
1823         if (nr_immediate && current_may_throttle())
1824             congestion_wait(BLK_RW_ASYNC, HZ/10);
1825     }
1826 
1827     /*
1828      * Stall direct reclaim for IO completions if underlying BDIs or zone
1829      * is congested. Allow kswapd to continue until it starts encountering
1830      * unqueued dirty pages or cycling through the LRU too quickly.
1831      */
1832     if (!sc->hibernation_mode && !current_is_kswapd() &&
1833         current_may_throttle())
1834         wait_iff_congested(pgdat, BLK_RW_ASYNC, HZ/10);
1835 
1836     trace_mm_vmscan_lru_shrink_inactive(pgdat->node_id,
1837             nr_scanned, nr_reclaimed,
1838             sc->priority, file);
1839     return nr_reclaimed;
1840 }
1841 
1842 /*
1843  * This moves pages from the active list to the inactive list.
1844  *
1845  * We move them the other way if the page is referenced by one or more
1846  * processes, from rmap.
1847  *
1848  * If the pages are mostly unmapped, the processing is fast and it is
1849  * appropriate to hold zone_lru_lock across the whole operation.  But if
1850  * the pages are mapped, the processing is slow (page_referenced()) so we
1851  * should drop zone_lru_lock around each page.  It's impossible to balance
1852  * this, so instead we remove the pages from the LRU while processing them.
1853  * It is safe to rely on PG_active against the non-LRU pages in here because
1854  * nobody will play with that bit on a non-LRU page.
1855  *
1856  * The downside is that we have to touch page->_refcount against each page.
1857  * But we had to alter page->flags anyway.
1858  */
1859 
1860 static void move_active_pages_to_lru(struct lruvec *lruvec,
1861                      struct list_head *list,
1862                      struct list_head *pages_to_free,
1863                      enum lru_list lru)
1864 {
1865     struct pglist_data *pgdat = lruvec_pgdat(lruvec);
1866     unsigned long pgmoved = 0;
1867     struct page *page;
1868     int nr_pages;
1869 
1870     while (!list_empty(list)) {
1871         page = lru_to_page(list);
1872         lruvec = mem_cgroup_page_lruvec(page, pgdat);
1873 
1874         VM_BUG_ON_PAGE(PageLRU(page), page);
1875         SetPageLRU(page);
1876 
1877         nr_pages = hpage_nr_pages(page);
1878         update_lru_size(lruvec, lru, page_zonenum(page), nr_pages);
1879         list_move(&page->lru, &lruvec->lists[lru]);
1880         pgmoved += nr_pages;
1881 
1882         if (put_page_testzero(page)) {
1883             __ClearPageLRU(page);
1884             __ClearPageActive(page);
1885             del_page_from_lru_list(page, lruvec, lru);
1886 
1887             if (unlikely(PageCompound(page))) {
1888                 spin_unlock_irq(&pgdat->lru_lock);
1889                 mem_cgroup_uncharge(page);
1890                 (*get_compound_page_dtor(page))(page);
1891                 spin_lock_irq(&pgdat->lru_lock);
1892             } else
1893                 list_add(&page->lru, pages_to_free);
1894         }
1895     }
1896 
1897     if (!is_active_lru(lru))
1898         __count_vm_events(PGDEACTIVATE, pgmoved);
1899 }
1900 
1901 static void shrink_active_list(unsigned long nr_to_scan,
1902                    struct lruvec *lruvec,
1903                    struct scan_control *sc,
1904                    enum lru_list lru)
1905 {
1906     unsigned long nr_taken;
1907     unsigned long nr_scanned;
1908     unsigned long vm_flags;
1909     LIST_HEAD(l_hold);  /* The pages which were snipped off */
1910     LIST_HEAD(l_active);
1911     LIST_HEAD(l_inactive);
1912     struct page *page;
1913     struct zone_reclaim_stat *reclaim_stat = &lruvec->reclaim_stat;
1914     unsigned long nr_rotated = 0;
1915     isolate_mode_t isolate_mode = 0;
1916     int file = is_file_lru(lru);
1917     struct pglist_data *pgdat = lruvec_pgdat(lruvec);
1918 
1919     lru_add_drain();
1920 
1921     if (!sc->may_unmap)
1922         isolate_mode |= ISOLATE_UNMAPPED;
1923     if (!sc->may_writepage)
1924         isolate_mode |= ISOLATE_CLEAN;
1925 
1926     spin_lock_irq(&pgdat->lru_lock);
1927 
1928     nr_taken = isolate_lru_pages(nr_to_scan, lruvec, &l_hold,
1929                      &nr_scanned, sc, isolate_mode, lru);
1930 
1931     __mod_node_page_state(pgdat, NR_ISOLATED_ANON + file, nr_taken);
1932     reclaim_stat->recent_scanned[file] += nr_taken;
1933 
1934     if (global_reclaim(sc))
1935         __mod_node_page_state(pgdat, NR_PAGES_SCANNED, nr_scanned);
1936     __count_vm_events(PGREFILL, nr_scanned);
1937 
1938     spin_unlock_irq(&pgdat->lru_lock);
1939 
1940     while (!list_empty(&l_hold)) {
1941         cond_resched();
1942         page = lru_to_page(&l_hold);
1943         list_del(&page->lru);
1944 
1945         if (unlikely(!page_evictable(page))) {
1946             putback_lru_page(page);
1947             continue;
1948         }
1949 
1950         if (unlikely(buffer_heads_over_limit)) {
1951             if (page_has_private(page) && trylock_page(page)) {
1952                 if (page_has_private(page))
1953                     try_to_release_page(page, 0);
1954                 unlock_page(page);
1955             }
1956         }
1957 
1958         if (page_referenced(page, 0, sc->target_mem_cgroup,
1959                     &vm_flags)) {
1960             nr_rotated += hpage_nr_pages(page);
1961             /*
1962              * Identify referenced, file-backed active pages and
1963              * give them one more trip around the active list. So
1964              * that executable code get better chances to stay in
1965              * memory under moderate memory pressure.  Anon pages
1966              * are not likely to be evicted by use-once streaming
1967              * IO, plus JVM can create lots of anon VM_EXEC pages,
1968              * so we ignore them here.
1969              */
1970             if ((vm_flags & VM_EXEC) && page_is_file_cache(page)) {
1971                 list_add(&page->lru, &l_active);
1972                 continue;
1973             }
1974         }
1975 
1976         ClearPageActive(page);  /* we are de-activating */
1977         list_add(&page->lru, &l_inactive);
1978     }
1979 
1980     /*
1981      * Move pages back to the lru list.
1982      */
1983     spin_lock_irq(&pgdat->lru_lock);
1984     /*
1985      * Count referenced pages from currently used mappings as rotated,
1986      * even though only some of them are actually re-activated.  This
1987      * helps balance scan pressure between file and anonymous pages in
1988      * get_scan_count.
1989      */
1990     reclaim_stat->recent_rotated[file] += nr_rotated;
1991 
1992     move_active_pages_to_lru(lruvec, &l_active, &l_hold, lru);
1993     move_active_pages_to_lru(lruvec, &l_inactive, &l_hold, lru - LRU_ACTIVE);
1994     __mod_node_page_state(pgdat, NR_ISOLATED_ANON + file, -nr_taken);
1995     spin_unlock_irq(&pgdat->lru_lock);
1996 
1997     mem_cgroup_uncharge_list(&l_hold);
1998     free_hot_cold_page_list(&l_hold, true);
1999 }
2000 
2001 /*
2002  * The inactive anon list should be small enough that the VM never has
2003  * to do too much work.
2004  *
2005  * The inactive file list should be small enough to leave most memory
2006  * to the established workingset on the scan-resistant active list,
2007  * but large enough to avoid thrashing the aggregate readahead window.
2008  *
2009  * Both inactive lists should also be large enough that each inactive
2010  * page has a chance to be referenced again before it is reclaimed.
2011  *
2012  * The inactive_ratio is the target ratio of ACTIVE to INACTIVE pages
2013  * on this LRU, maintained by the pageout code. A zone->inactive_ratio
2014  * of 3 means 3:1 or 25% of the pages are kept on the inactive list.
2015  *
2016  * total     target    max
2017  * memory    ratio     inactive
2018  * -------------------------------------
2019  *   10MB       1         5MB
2020  *  100MB       1        50MB
2021  *    1GB       3       250MB
2022  *   10GB      10       0.9GB
2023  *  100GB      31         3GB
2024  *    1TB     101        10GB
2025  *   10TB     320        32GB
2026  */
2027 static bool inactive_list_is_low(struct lruvec *lruvec, bool file,
2028                         struct scan_control *sc)
2029 {
2030     unsigned long inactive_ratio;
2031     unsigned long inactive;
2032     unsigned long active;
2033     unsigned long gb;
2034     struct pglist_data *pgdat = lruvec_pgdat(lruvec);
2035     int zid;
2036 
2037     /*
2038      * If we don't have swap space, anonymous page deactivation
2039      * is pointless.
2040      */
2041     if (!file && !total_swap_pages)
2042         return false;
2043 
2044     inactive = lruvec_lru_size(lruvec, file * LRU_FILE);
2045     active = lruvec_lru_size(lruvec, file * LRU_FILE + LRU_ACTIVE);
2046 
2047     /*
2048      * For zone-constrained allocations, it is necessary to check if
2049      * deactivations are required for lowmem to be reclaimed. This
2050      * calculates the inactive/active pages available in eligible zones.
2051      */
2052     for (zid = sc->reclaim_idx + 1; zid < MAX_NR_ZONES; zid++) {
2053         struct zone *zone = &pgdat->node_zones[zid];
2054         unsigned long inactive_zone, active_zone;
2055 
2056         if (!managed_zone(zone))
2057             continue;
2058 
2059         inactive_zone = lruvec_zone_lru_size(lruvec, file * LRU_FILE, zid);
2060         active_zone = lruvec_zone_lru_size(lruvec, (file * LRU_FILE) + LRU_ACTIVE, zid);
2061 
2062         inactive -= min(inactive, inactive_zone);
2063         active -= min(active, active_zone);
2064     }
2065 
2066     gb = (inactive + active) >> (30 - PAGE_SHIFT);
2067     if (gb)
2068         inactive_ratio = int_sqrt(10 * gb);
2069     else
2070         inactive_ratio = 1;
2071 
2072     return inactive * inactive_ratio < active;
2073 }
2074 
2075 static unsigned long shrink_list(enum lru_list lru, unsigned long nr_to_scan,
2076                  struct lruvec *lruvec, struct scan_control *sc)
2077 {
2078     if (is_active_lru(lru)) {
2079         if (inactive_list_is_low(lruvec, is_file_lru(lru), sc))
2080             shrink_active_list(nr_to_scan, lruvec, sc, lru);
2081         return 0;
2082     }
2083 
2084     return shrink_inactive_list(nr_to_scan, lruvec, sc, lru);
2085 }
2086 
2087 enum scan_balance {
2088     SCAN_EQUAL,
2089     SCAN_FRACT,
2090     SCAN_ANON,
2091     SCAN_FILE,
2092 };
2093 
2094 /*
2095  * Determine how aggressively the anon and file LRU lists should be
2096  * scanned.  The relative value of each set of LRU lists is determined
2097  * by looking at the fraction of the pages scanned we did rotate back
2098  * onto the active list instead of evict.
2099  *
2100  * nr[0] = anon inactive pages to scan; nr[1] = anon active pages to scan
2101  * nr[2] = file inactive pages to scan; nr[3] = file active pages to scan
2102  */
2103 static void get_scan_count(struct lruvec *lruvec, struct mem_cgroup *memcg,
2104                struct scan_control *sc, unsigned long *nr,
2105                unsigned long *lru_pages)
2106 {
2107     int swappiness = mem_cgroup_swappiness(memcg);
2108     struct zone_reclaim_stat *reclaim_stat = &lruvec->reclaim_stat;
2109     u64 fraction[2];
2110     u64 denominator = 0;    /* gcc */
2111     struct pglist_data *pgdat = lruvec_pgdat(lruvec);
2112     unsigned long anon_prio, file_prio;
2113     enum scan_balance scan_balance;
2114     unsigned long anon, file;
2115     bool force_scan = false;
2116     unsigned long ap, fp;
2117     enum lru_list lru;
2118     bool some_scanned;
2119     int pass;
2120 
2121     /*
2122      * If the zone or memcg is small, nr[l] can be 0.  This
2123      * results in no scanning on this priority and a potential
2124      * priority drop.  Global direct reclaim can go to the next
2125      * zone and tends to have no problems. Global kswapd is for
2126      * zone balancing and it needs to scan a minimum amount. When
2127      * reclaiming for a memcg, a priority drop can cause high
2128      * latencies, so it's better to scan a minimum amount there as
2129      * well.
2130      */
2131     if (current_is_kswapd()) {
2132         if (!pgdat_reclaimable(pgdat))
2133             force_scan = true;
2134         if (!mem_cgroup_online(memcg))
2135             force_scan = true;
2136     }
2137     if (!global_reclaim(sc))
2138         force_scan = true;
2139 
2140     /* If we have no swap space, do not bother scanning anon pages. */
2141     if (!sc->may_swap || mem_cgroup_get_nr_swap_pages(memcg) <= 0) {
2142         scan_balance = SCAN_FILE;
2143         goto out;
2144     }
2145 
2146     /*
2147      * Global reclaim will swap to prevent OOM even with no
2148      * swappiness, but memcg users want to use this knob to
2149      * disable swapping for individual groups completely when
2150      * using the memory controller's swap limit feature would be
2151      * too expensive.
2152      */
2153     if (!global_reclaim(sc) && !swappiness) {
2154         scan_balance = SCAN_FILE;
2155         goto out;
2156     }
2157 
2158     /*
2159      * Do not apply any pressure balancing cleverness when the
2160      * system is close to OOM, scan both anon and file equally
2161      * (unless the swappiness setting disagrees with swapping).
2162      */
2163     if (!sc->priority && swappiness) {
2164         scan_balance = SCAN_EQUAL;
2165         goto out;
2166     }
2167 
2168     /*
2169      * Prevent the reclaimer from falling into the cache trap: as
2170      * cache pages start out inactive, every cache fault will tip
2171      * the scan balance towards the file LRU.  And as the file LRU
2172      * shrinks, so does the window for rotation from references.
2173      * This means we have a runaway feedback loop where a tiny
2174      * thrashing file LRU becomes infinitely more attractive than
2175      * anon pages.  Try to detect this based on file LRU size.
2176      */
2177     if (global_reclaim(sc)) {
2178         unsigned long pgdatfile;
2179         unsigned long pgdatfree;
2180         int z;
2181         unsigned long total_high_wmark = 0;
2182 
2183         pgdatfree = sum_zone_node_page_state(pgdat->node_id, NR_FREE_PAGES);
2184         pgdatfile = node_page_state(pgdat, NR_ACTIVE_FILE) +
2185                node_page_state(pgdat, NR_INACTIVE_FILE);
2186 
2187         for (z = 0; z < MAX_NR_ZONES; z++) {
2188             struct zone *zone = &pgdat->node_zones[z];
2189             if (!managed_zone(zone))
2190                 continue;
2191 
2192             total_high_wmark += high_wmark_pages(zone);
2193         }
2194 
2195         if (unlikely(pgdatfile + pgdatfree <= total_high_wmark)) {
2196             scan_balance = SCAN_ANON;
2197             goto out;
2198         }
2199     }
2200 
2201     /*
2202      * If there is enough inactive page cache, i.e. if the size of the
2203      * inactive list is greater than that of the active list *and* the
2204      * inactive list actually has some pages to scan on this priority, we
2205      * do not reclaim anything from the anonymous working set right now.
2206      * Without the second condition we could end up never scanning an
2207      * lruvec even if it has plenty of old anonymous pages unless the
2208      * system is under heavy pressure.
2209      */
2210     if (!inactive_list_is_low(lruvec, true, sc) &&
2211         lruvec_lru_size(lruvec, LRU_INACTIVE_FILE) >> sc->priority) {
2212         scan_balance = SCAN_FILE;
2213         goto out;
2214     }
2215 
2216     scan_balance = SCAN_FRACT;
2217 
2218     /*
2219      * With swappiness at 100, anonymous and file have the same priority.
2220      * This scanning priority is essentially the inverse of IO cost.
2221      */
2222     anon_prio = swappiness;
2223     file_prio = 200 - anon_prio;
2224 
2225     /*
2226      * OK, so we have swap space and a fair amount of page cache
2227      * pages.  We use the recently rotated / recently scanned
2228      * ratios to determine how valuable each cache is.
2229      *
2230      * Because workloads change over time (and to avoid overflow)
2231      * we keep these statistics as a floating average, which ends
2232      * up weighing recent references more than old ones.
2233      *
2234      * anon in [0], file in [1]
2235      */
2236 
2237     anon  = lruvec_lru_size(lruvec, LRU_ACTIVE_ANON) +
2238         lruvec_lru_size(lruvec, LRU_INACTIVE_ANON);
2239     file  = lruvec_lru_size(lruvec, LRU_ACTIVE_FILE) +
2240         lruvec_lru_size(lruvec, LRU_INACTIVE_FILE);
2241 
2242     spin_lock_irq(&pgdat->lru_lock);
2243     if (unlikely(reclaim_stat->recent_scanned[0] > anon / 4)) {
2244         reclaim_stat->recent_scanned[0] /= 2;
2245         reclaim_stat->recent_rotated[0] /= 2;
2246     }
2247 
2248     if (unlikely(reclaim_stat->recent_scanned[1] > file / 4)) {
2249         reclaim_stat->recent_scanned[1] /= 2;
2250         reclaim_stat->recent_rotated[1] /= 2;
2251     }
2252 
2253     /*
2254      * The amount of pressure on anon vs file pages is inversely
2255      * proportional to the fraction of recently scanned pages on
2256      * each list that were recently referenced and in active use.
2257      */
2258     ap = anon_prio * (reclaim_stat->recent_scanned[0] + 1);
2259     ap /= reclaim_stat->recent_rotated[0] + 1;
2260 
2261     fp = file_prio * (reclaim_stat->recent_scanned[1] + 1);
2262     fp /= reclaim_stat->recent_rotated[1] + 1;
2263     spin_unlock_irq(&pgdat->lru_lock);
2264 
2265     fraction[0] = ap;
2266     fraction[1] = fp;
2267     denominator = ap + fp + 1;
2268 out:
2269     some_scanned = false;
2270     /* Only use force_scan on second pass. */
2271     for (pass = 0; !some_scanned && pass < 2; pass++) {
2272         *lru_pages = 0;
2273         for_each_evictable_lru(lru) {
2274             int file = is_file_lru(lru);
2275             unsigned long size;
2276             unsigned long scan;
2277 
2278             size = lruvec_lru_size(lruvec, lru);
2279             scan = size >> sc->priority;
2280 
2281             if (!scan && pass && force_scan)
2282                 scan = min(size, SWAP_CLUSTER_MAX);
2283 
2284             switch (scan_balance) {
2285             case SCAN_EQUAL:
2286                 /* Scan lists relative to size */
2287                 break;
2288             case SCAN_FRACT:
2289                 /*
2290                  * Scan types proportional to swappiness and
2291                  * their relative recent reclaim efficiency.
2292                  */
2293                 scan = div64_u64(scan * fraction[file],
2294                             denominator);
2295                 break;
2296             case SCAN_FILE:
2297             case SCAN_ANON:
2298                 /* Scan one type exclusively */
2299                 if ((scan_balance == SCAN_FILE) != file) {
2300                     size = 0;
2301                     scan = 0;
2302                 }
2303                 break;
2304             default:
2305                 /* Look ma, no brain */
2306                 BUG();
2307             }
2308 
2309             *lru_pages += size;
2310             nr[lru] = scan;
2311 
2312             /*
2313              * Skip the second pass and don't force_scan,
2314              * if we found something to scan.
2315              */
2316             some_scanned |= !!scan;
2317         }
2318     }
2319 }
2320 
2321 /*
2322  * This is a basic per-node page freer.  Used by both kswapd and direct reclaim.
2323  */
2324 static void shrink_node_memcg(struct pglist_data *pgdat, struct mem_cgroup *memcg,
2325                   struct scan_control *sc, unsigned long *lru_pages)
2326 {
2327     struct lruvec *lruvec = mem_cgroup_lruvec(pgdat, memcg);
2328     unsigned long nr[NR_LRU_LISTS];
2329     unsigned long targets[NR_LRU_LISTS];
2330     unsigned long nr_to_scan;
2331     enum lru_list lru;
2332     unsigned long nr_reclaimed = 0;
2333     unsigned long nr_to_reclaim = sc->nr_to_reclaim;
2334     struct blk_plug plug;
2335     bool scan_adjusted;
2336 
2337     get_scan_count(lruvec, memcg, sc, nr, lru_pages);
2338 
2339     /* Record the original scan target for proportional adjustments later */
2340     memcpy(targets, nr, sizeof(nr));
2341 
2342     /*
2343      * Global reclaiming within direct reclaim at DEF_PRIORITY is a normal
2344      * event that can occur when there is little memory pressure e.g.
2345      * multiple streaming readers/writers. Hence, we do not abort scanning
2346      * when the requested number of pages are reclaimed when scanning at
2347      * DEF_PRIORITY on the assumption that the fact we are direct
2348      * reclaiming implies that kswapd is not keeping up and it is best to
2349      * do a batch of work at once. For memcg reclaim one check is made to
2350      * abort proportional reclaim if either the file or anon lru has already
2351      * dropped to zero at the first pass.
2352      */
2353     scan_adjusted = (global_reclaim(sc) && !current_is_kswapd() &&
2354              sc->priority == DEF_PRIORITY);
2355 
2356     blk_start_plug(&plug);
2357     while (nr[LRU_INACTIVE_ANON] || nr[LRU_ACTIVE_FILE] ||
2358                     nr[LRU_INACTIVE_FILE]) {
2359         unsigned long nr_anon, nr_file, percentage;
2360         unsigned long nr_scanned;
2361 
2362         for_each_evictable_lru(lru) {
2363             if (nr[lru]) {
2364                 nr_to_scan = min(nr[lru], SWAP_CLUSTER_MAX);
2365                 nr[lru] -= nr_to_scan;
2366 
2367                 nr_reclaimed += shrink_list(lru, nr_to_scan,
2368                                 lruvec, sc);
2369             }
2370         }
2371 
2372         cond_resched();
2373 
2374         if (nr_reclaimed < nr_to_reclaim || scan_adjusted)
2375             continue;
2376 
2377         /*
2378          * For kswapd and memcg, reclaim at least the number of pages
2379          * requested. Ensure that the anon and file LRUs are scanned
2380          * proportionally what was requested by get_scan_count(). We
2381          * stop reclaiming one LRU and reduce the amount scanning
2382          * proportional to the original scan target.
2383          */
2384         nr_file = nr[LRU_INACTIVE_FILE] + nr[LRU_ACTIVE_FILE];
2385         nr_anon = nr[LRU_INACTIVE_ANON] + nr[LRU_ACTIVE_ANON];
2386 
2387         /*
2388          * It's just vindictive to attack the larger once the smaller
2389          * has gone to zero.  And given the way we stop scanning the
2390          * smaller below, this makes sure that we only make one nudge
2391          * towards proportionality once we've got nr_to_reclaim.
2392          */
2393         if (!nr_file || !nr_anon)
2394             break;
2395 
2396         if (nr_file > nr_anon) {
2397             unsigned long scan_target = targets[LRU_INACTIVE_ANON] +
2398                         targets[LRU_ACTIVE_ANON] + 1;
2399             lru = LRU_BASE;
2400             percentage = nr_anon * 100 / scan_target;
2401         } else {
2402             unsigned long scan_target = targets[LRU_INACTIVE_FILE] +
2403                         targets[LRU_ACTIVE_FILE] + 1;
2404             lru = LRU_FILE;
2405             percentage = nr_file * 100 / scan_target;
2406         }
2407 
2408         /* Stop scanning the smaller of the LRU */
2409         nr[lru] = 0;
2410         nr[lru + LRU_ACTIVE] = 0;
2411 
2412         /*
2413          * Recalculate the other LRU scan count based on its original
2414          * scan target and the percentage scanning already complete
2415          */
2416         lru = (lru == LRU_FILE) ? LRU_BASE : LRU_FILE;
2417         nr_scanned = targets[lru] - nr[lru];
2418         nr[lru] = targets[lru] * (100 - percentage) / 100;
2419         nr[lru] -= min(nr[lru], nr_scanned);
2420 
2421         lru += LRU_ACTIVE;
2422         nr_scanned = targets[lru] - nr[lru];
2423         nr[lru] = targets[lru] * (100 - percentage) / 100;
2424         nr[lru] -= min(nr[lru], nr_scanned);
2425 
2426         scan_adjusted = true;
2427     }
2428     blk_finish_plug(&plug);
2429     sc->nr_reclaimed += nr_reclaimed;
2430 
2431     /*
2432      * Even if we did not try to evict anon pages at all, we want to
2433      * rebalance the anon lru active/inactive ratio.
2434      */
2435     if (inactive_list_is_low(lruvec, false, sc))
2436         shrink_active_list(SWAP_CLUSTER_MAX, lruvec,
2437                    sc, LRU_ACTIVE_ANON);
2438 }
2439 
2440 /* Use reclaim/compaction for costly allocs or under memory pressure */
2441 static bool in_reclaim_compaction(struct scan_control *sc)
2442 {
2443     if (IS_ENABLED(CONFIG_COMPACTION) && sc->order &&
2444             (sc->order > PAGE_ALLOC_COSTLY_ORDER ||
2445              sc->priority < DEF_PRIORITY - 2))
2446         return true;
2447 
2448     return false;
2449 }
2450 
2451 /*
2452  * Reclaim/compaction is used for high-order allocation requests. It reclaims
2453  * order-0 pages before compacting the zone. should_continue_reclaim() returns
2454  * true if more pages should be reclaimed such that when the page allocator
2455  * calls try_to_compact_zone() that it will have enough free pages to succeed.
2456  * It will give up earlier than that if there is difficulty reclaiming pages.
2457  */
2458 static inline bool should_continue_reclaim(struct pglist_data *pgdat,
2459                     unsigned long nr_reclaimed,
2460                     unsigned long nr_scanned,
2461                     struct scan_control *sc)
2462 {
2463     unsigned long pages_for_compaction;
2464     unsigned long inactive_lru_pages;
2465     int z;
2466 
2467     /* If not in reclaim/compaction mode, stop */
2468     if (!in_reclaim_compaction(sc))
2469         return false;
2470 
2471     /* Consider stopping depending on scan and reclaim activity */
2472     if (sc->gfp_mask & __GFP_REPEAT) {
2473         /*
2474          * For __GFP_REPEAT allocations, stop reclaiming if the
2475          * full LRU list has been scanned and we are still failing
2476          * to reclaim pages. This full LRU scan is potentially
2477          * expensive but a __GFP_REPEAT caller really wants to succeed
2478          */
2479         if (!nr_reclaimed && !nr_scanned)
2480             return false;
2481     } else {
2482         /*
2483          * For non-__GFP_REPEAT allocations which can presumably
2484          * fail without consequence, stop if we failed to reclaim
2485          * any pages from the last SWAP_CLUSTER_MAX number of
2486          * pages that were scanned. This will return to the
2487          * caller faster at the risk reclaim/compaction and
2488          * the resulting allocation attempt fails
2489          */
2490         if (!nr_reclaimed)
2491             return false;
2492     }
2493 
2494     /*
2495      * If we have not reclaimed enough pages for compaction and the
2496      * inactive lists are large enough, continue reclaiming
2497      */
2498     pages_for_compaction = compact_gap(sc->order);
2499     inactive_lru_pages = node_page_state(pgdat, NR_INACTIVE_FILE);
2500     if (get_nr_swap_pages() > 0)
2501         inactive_lru_pages += node_page_state(pgdat, NR_INACTIVE_ANON);
2502     if (sc->nr_reclaimed < pages_for_compaction &&
2503             inactive_lru_pages > pages_for_compaction)
2504         return true;
2505 
2506     /* If compaction would go ahead or the allocation would succeed, stop */
2507     for (z = 0; z <= sc->reclaim_idx; z++) {
2508         struct zone *zone = &pgdat->node_zones[z];
2509         if (!managed_zone(zone))
2510             continue;
2511 
2512         switch (compaction_suitable(zone, sc->order, 0, sc->reclaim_idx)) {
2513         case COMPACT_SUCCESS:
2514         case COMPACT_CONTINUE:
2515             return false;
2516         default:
2517             /* check next zone */
2518             ;
2519         }
2520     }
2521     return true;
2522 }
2523 
2524 static bool shrink_node(pg_data_t *pgdat, struct scan_control *sc)
2525 {
2526     struct reclaim_state *reclaim_state = current->reclaim_state;
2527     unsigned long nr_reclaimed, nr_scanned;
2528     bool reclaimable = false;
2529 
2530     do {
2531         struct mem_cgroup *root = sc->target_mem_cgroup;
2532         struct mem_cgroup_reclaim_cookie reclaim = {
2533             .pgdat = pgdat,
2534             .priority = sc->priority,
2535         };
2536         unsigned long node_lru_pages = 0;
2537         struct mem_cgroup *memcg;
2538 
2539         nr_reclaimed = sc->nr_reclaimed;
2540         nr_scanned = sc->nr_scanned;
2541 
2542         memcg = mem_cgroup_iter(root, NULL, &reclaim);
2543         do {
2544             unsigned long lru_pages;
2545             unsigned long reclaimed;
2546             unsigned long scanned;
2547 
2548             if (mem_cgroup_low(root, memcg)) {
2549                 if (!sc->may_thrash)
2550                     continue;
2551                 mem_cgroup_events(memcg, MEMCG_LOW, 1);
2552             }
2553 
2554             reclaimed = sc->nr_reclaimed;
2555             scanned = sc->nr_scanned;
2556 
2557             shrink_node_memcg(pgdat, memcg, sc, &lru_pages);
2558             node_lru_pages += lru_pages;
2559 
2560             if (memcg)
2561                 shrink_slab(sc->gfp_mask, pgdat->node_id,
2562                         memcg, sc->nr_scanned - scanned,
2563                         lru_pages);
2564 
2565             /* Record the group's reclaim efficiency */
2566             vmpressure(sc->gfp_mask, memcg, false,
2567                    sc->nr_scanned - scanned,
2568                    sc->nr_reclaimed - reclaimed);
2569 
2570             /*
2571              * Direct reclaim and kswapd have to scan all memory
2572              * cgroups to fulfill the overall scan target for the
2573              * node.
2574              *
2575              * Limit reclaim, on the other hand, only cares about
2576              * nr_to_reclaim pages to be reclaimed and it will
2577              * retry with decreasing priority if one round over the
2578              * whole hierarchy is not sufficient.
2579              */
2580             if (!global_reclaim(sc) &&
2581                     sc->nr_reclaimed >= sc->nr_to_reclaim) {
2582                 mem_cgroup_iter_break(root, memcg);
2583                 break;
2584             }
2585         } while ((memcg = mem_cgroup_iter(root, memcg, &reclaim)));
2586 
2587         /*
2588          * Shrink the slab caches in the same proportion that
2589          * the eligible LRU pages were scanned.
2590          */
2591         if (global_reclaim(sc))
2592             shrink_slab(sc->gfp_mask, pgdat->node_id, NULL,
2593                     sc->nr_scanned - nr_scanned,
2594                     node_lru_pages);
2595 
2596         if (reclaim_state) {
2597             sc->nr_reclaimed += reclaim_state->reclaimed_slab;
2598             reclaim_state->reclaimed_slab = 0;
2599         }
2600 
2601         /* Record the subtree's reclaim efficiency */
2602         vmpressure(sc->gfp_mask, sc->target_mem_cgroup, true,
2603                sc->nr_scanned - nr_scanned,
2604                sc->nr_reclaimed - nr_reclaimed);
2605 
2606         if (sc->nr_reclaimed - nr_reclaimed)
2607             reclaimable = true;
2608 
2609     } while (should_continue_reclaim(pgdat, sc->nr_reclaimed - nr_reclaimed,
2610                      sc->nr_scanned - nr_scanned, sc));
2611 
2612     return reclaimable;
2613 }
2614 
2615 /*
2616  * Returns true if compaction should go ahead for a costly-order request, or
2617  * the allocation would already succeed without compaction. Return false if we
2618  * should reclaim first.
2619  */
2620 static inline bool compaction_ready(struct zone *zone, struct scan_control *sc)
2621 {
2622     unsigned long watermark;
2623     enum compact_result suitable;
2624 
2625     suitable = compaction_suitable(zone, sc->order, 0, sc->reclaim_idx);
2626     if (suitable == COMPACT_SUCCESS)
2627         /* Allocation should succeed already. Don't reclaim. */
2628         return true;
2629     if (suitable == COMPACT_SKIPPED)
2630         /* Compaction cannot yet proceed. Do reclaim. */
2631         return false;
2632 
2633     /*
2634      * Compaction is already possible, but it takes time to run and there
2635      * are potentially other callers using the pages just freed. So proceed
2636      * with reclaim to make a buffer of free pages available to give
2637      * compaction a reasonable chance of completing and allocating the page.
2638      * Note that we won't actually reclaim the whole buffer in one attempt
2639      * as the target watermark in should_continue_reclaim() is lower. But if
2640      * we are already above the high+gap watermark, don't reclaim at all.
2641      */
2642     watermark = high_wmark_pages(zone) + compact_gap(sc->order);
2643 
2644     return zone_watermark_ok_safe(zone, 0, watermark, sc->reclaim_idx);
2645 }
2646 
2647 /*
2648  * This is the direct reclaim path, for page-allocating processes.  We only
2649  * try to reclaim pages from zones which will satisfy the caller's allocation
2650  * request.
2651  *
2652  * If a zone is deemed to be full of pinned pages then just give it a light
2653  * scan then give up on it.
2654  */
2655 static void shrink_zones(struct zonelist *zonelist, struct scan_control *sc)
2656 {
2657     struct zoneref *z;
2658     struct zone *zone;
2659     unsigned long nr_soft_reclaimed;
2660     unsigned long nr_soft_scanned;
2661     gfp_t orig_mask;
2662     pg_data_t *last_pgdat = NULL;
2663 
2664     /*
2665      * If the number of buffer_heads in the machine exceeds the maximum
2666      * allowed level, force direct reclaim to scan the highmem zone as
2667      * highmem pages could be pinning lowmem pages storing buffer_heads
2668      */
2669     orig_mask = sc->gfp_mask;
2670     if (buffer_heads_over_limit) {
2671         sc->gfp_mask |= __GFP_HIGHMEM;
2672         sc->reclaim_idx = gfp_zone(sc->gfp_mask);
2673     }
2674 
2675     for_each_zone_zonelist_nodemask(zone, z, zonelist,
2676                     sc->reclaim_idx, sc->nodemask) {
2677         /*
2678          * Take care memory controller reclaiming has small influence
2679          * to global LRU.
2680          */
2681         if (global_reclaim(sc)) {
2682             if (!cpuset_zone_allowed(zone,
2683                          GFP_KERNEL | __GFP_HARDWALL))
2684                 continue;
2685 
2686             if (sc->priority != DEF_PRIORITY &&
2687                 !pgdat_reclaimable(zone->zone_pgdat))
2688                 continue;   /* Let kswapd poll it */
2689 
2690             /*
2691              * If we already have plenty of memory free for
2692              * compaction in this zone, don't free any more.
2693              * Even though compaction is invoked for any
2694              * non-zero order, only frequent costly order
2695              * reclamation is disruptive enough to become a
2696              * noticeable problem, like transparent huge
2697              * page allocations.
2698              */
2699             if (IS_ENABLED(CONFIG_COMPACTION) &&
2700                 sc->order > PAGE_ALLOC_COSTLY_ORDER &&
2701                 compaction_ready(zone, sc)) {
2702                 sc->compaction_ready = true;
2703                 continue;
2704             }
2705 
2706             /*
2707              * Shrink each node in the zonelist once. If the
2708              * zonelist is ordered by zone (not the default) then a
2709              * node may be shrunk multiple times but in that case
2710              * the user prefers lower zones being preserved.
2711              */
2712             if (zone->zone_pgdat == last_pgdat)
2713                 continue;
2714 
2715             /*
2716              * This steals pages from memory cgroups over softlimit
2717              * and returns the number of reclaimed pages and
2718              * scanned pages. This works for global memory pressure
2719              * and balancing, not for a memcg's limit.
2720              */
2721             nr_soft_scanned = 0;
2722             nr_soft_reclaimed = mem_cgroup_soft_limit_reclaim(zone->zone_pgdat,
2723                         sc->order, sc->gfp_mask,
2724                         &nr_soft_scanned);
2725             sc->nr_reclaimed += nr_soft_reclaimed;
2726             sc->nr_scanned += nr_soft_scanned;
2727             /* need some check for avoid more shrink_zone() */
2728         }
2729 
2730         /* See comment about same check for global reclaim above */
2731         if (zone->zone_pgdat == last_pgdat)
2732             continue;
2733         last_pgdat = zone->zone_pgdat;
2734         shrink_node(zone->zone_pgdat, sc);
2735     }
2736 
2737     /*
2738      * Restore to original mask to avoid the impact on the caller if we
2739      * promoted it to __GFP_HIGHMEM.
2740      */
2741     sc->gfp_mask = orig_mask;
2742 }
2743 
2744 /*
2745  * This is the main entry point to direct page reclaim.
2746  *
2747  * If a full scan of the inactive list fails to free enough memory then we
2748  * are "out of memory" and something needs to be killed.
2749  *
2750  * If the caller is !__GFP_FS then the probability of a failure is reasonably
2751  * high - the zone may be full of dirty or under-writeback pages, which this
2752  * caller can't do much about.  We kick the writeback threads and take explicit
2753  * naps in the hope that some of these pages can be written.  But if the
2754  * allocating task holds filesystem locks which prevent writeout this might not
2755  * work, and the allocation attempt will fail.
2756  *
2757  * returns: 0, if no pages reclaimed
2758  *      else, the number of pages reclaimed
2759  */
2760 static unsigned long do_try_to_free_pages(struct zonelist *zonelist,
2761                       struct scan_control *sc)
2762 {
2763     int initial_priority = sc->priority;
2764     unsigned long total_scanned = 0;
2765     unsigned long writeback_threshold;
2766 retry:
2767     delayacct_freepages_start();
2768 
2769     if (global_reclaim(sc))
2770         __count_zid_vm_events(ALLOCSTALL, sc->reclaim_idx, 1);
2771 
2772     do {
2773         vmpressure_prio(sc->gfp_mask, sc->target_mem_cgroup,
2774                 sc->priority);
2775         sc->nr_scanned = 0;
2776         shrink_zones(zonelist, sc);
2777 
2778         total_scanned += sc->nr_scanned;
2779         if (sc->nr_reclaimed >= sc->nr_to_reclaim)
2780             break;
2781 
2782         if (sc->compaction_ready)
2783             break;
2784 
2785         /*
2786          * If we're getting trouble reclaiming, start doing
2787          * writepage even in laptop mode.
2788          */
2789         if (sc->priority < DEF_PRIORITY - 2)
2790             sc->may_writepage = 1;
2791 
2792         /*
2793          * Try to write back as many pages as we just scanned.  This
2794          * tends to cause slow streaming writers to write data to the
2795          * disk smoothly, at the dirtying rate, which is nice.   But
2796          * that's undesirable in laptop mode, where we *want* lumpy
2797          * writeout.  So in laptop mode, write out the whole world.
2798          */
2799         writeback_threshold = sc->nr_to_reclaim + sc->nr_to_reclaim / 2;
2800         if (total_scanned > writeback_threshold) {
2801             wakeup_flusher_threads(laptop_mode ? 0 : total_scanned,
2802                         WB_REASON_TRY_TO_FREE_PAGES);
2803             sc->may_writepage = 1;
2804         }
2805     } while (--sc->priority >= 0);
2806 
2807     delayacct_freepages_end();
2808 
2809     if (sc->nr_reclaimed)
2810         return sc->nr_reclaimed;
2811 
2812     /* Aborted reclaim to try compaction? don't OOM, then */
2813     if (sc->compaction_ready)
2814         return 1;
2815 
2816     /* Untapped cgroup reserves?  Don't OOM, retry. */
2817     if (!sc->may_thrash) {
2818         sc->priority = initial_priority;
2819         sc->may_thrash = 1;
2820         goto retry;
2821     }
2822 
2823     return 0;
2824 }
2825 
2826 static bool pfmemalloc_watermark_ok(pg_data_t *pgdat)
2827 {
2828     struct zone *zone;
2829     unsigned long pfmemalloc_reserve = 0;
2830     unsigned long free_pages = 0;
2831     int i;
2832     bool wmark_ok;
2833 
2834     for (i = 0; i <= ZONE_NORMAL; i++) {
2835         zone = &pgdat->node_zones[i];
2836         if (!managed_zone(zone) ||
2837             pgdat_reclaimable_pages(pgdat) == 0)
2838             continue;
2839 
2840         pfmemalloc_reserve += min_wmark_pages(zone);
2841         free_pages += zone_page_state(zone, NR_FREE_PAGES);
2842     }
2843 
2844     /* If there are no reserves (unexpected config) then do not throttle */
2845     if (!pfmemalloc_reserve)
2846         return true;
2847 
2848     wmark_ok = free_pages > pfmemalloc_reserve / 2;
2849 
2850     /* kswapd must be awake if processes are being throttled */
2851     if (!wmark_ok && waitqueue_active(&pgdat->kswapd_wait)) {
2852         pgdat->kswapd_classzone_idx = min(pgdat->kswapd_classzone_idx,
2853                         (enum zone_type)ZONE_NORMAL);
2854         wake_up_interruptible(&pgdat->kswapd_wait);
2855     }
2856 
2857     return wmark_ok;
2858 }
2859 
2860 /*
2861  * Throttle direct reclaimers if backing storage is backed by the network
2862  * and the PFMEMALLOC reserve for the preferred node is getting dangerously
2863  * depleted. kswapd will continue to make progress and wake the processes
2864  * when the low watermark is reached.
2865  *
2866  * Returns true if a fatal signal was delivered during throttling. If this
2867  * happens, the page allocator should not consider triggering the OOM killer.
2868  */
2869 static bool throttle_direct_reclaim(gfp_t gfp_mask, struct zonelist *zonelist,
2870                     nodemask_t *nodemask)
2871 {
2872     struct zoneref *z;
2873     struct zone *zone;
2874     pg_data_t *pgdat = NULL;
2875 
2876     /*
2877      * Kernel threads should not be throttled as they may be indirectly
2878      * responsible for cleaning pages necessary for reclaim to make forward
2879      * progress. kjournald for example may enter direct reclaim while
2880      * committing a transaction where throttling it could forcing other
2881      * processes to block on log_wait_commit().
2882      */
2883     if (current->flags & PF_KTHREAD)
2884         goto out;
2885 
2886     /*
2887      * If a fatal signal is pending, this process should not throttle.
2888      * It should return quickly so it can exit and free its memory
2889      */
2890     if (fatal_signal_pending(current))
2891         goto out;
2892 
2893     /*
2894      * Check if the pfmemalloc reserves are ok by finding the first node
2895      * with a usable ZONE_NORMAL or lower zone. The expectation is that
2896      * GFP_KERNEL will be required for allocating network buffers when
2897      * swapping over the network so ZONE_HIGHMEM is unusable.
2898      *
2899      * Throttling is based on the first usable node and throttled processes
2900      * wait on a queue until kswapd makes progress and wakes them. There
2901      * is an affinity then between processes waking up and where reclaim
2902      * progress has been made assuming the process wakes on the same node.
2903      * More importantly, processes running on remote nodes will not compete
2904      * for remote pfmemalloc reserves and processes on different nodes
2905      * should make reasonable progress.
2906      */
2907     for_each_zone_zonelist_nodemask(zone, z, zonelist,
2908                     gfp_zone(gfp_mask), nodemask) {
2909         if (zone_idx(zone) > ZONE_NORMAL)
2910             continue;
2911 
2912         /* Throttle based on the first usable node */
2913         pgdat = zone->zone_pgdat;
2914         if (pfmemalloc_watermark_ok(pgdat))
2915             goto out;
2916         break;
2917     }
2918 
2919     /* If no zone was usable by the allocation flags then do not throttle */
2920     if (!pgdat)
2921         goto out;
2922 
2923     /* Account for the throttling */
2924     count_vm_event(PGSCAN_DIRECT_THROTTLE);
2925 
2926     /*
2927      * If the caller cannot enter the filesystem, it's possible that it
2928      * is due to the caller holding an FS lock or performing a journal
2929      * transaction in the case of a filesystem like ext[3|4]. In this case,
2930      * it is not safe to block on pfmemalloc_wait as kswapd could be
2931      * blocked waiting on the same lock. Instead, throttle for up to a
2932      * second before continuing.
2933      */
2934     if (!(gfp_mask & __GFP_FS)) {
2935         wait_event_interruptible_timeout(pgdat->pfmemalloc_wait,
2936             pfmemalloc_watermark_ok(pgdat), HZ);
2937 
2938         goto check_pending;
2939     }
2940 
2941     /* Throttle until kswapd wakes the process */
2942     wait_event_killable(zone->zone_pgdat->pfmemalloc_wait,
2943         pfmemalloc_watermark_ok(pgdat));
2944 
2945 check_pending:
2946     if (fatal_signal_pending(current))
2947         return true;
2948 
2949 out:
2950     return false;
2951 }
2952 
2953 unsigned long try_to_free_pages(struct zonelist *zonelist, int order,
2954                 gfp_t gfp_mask, nodemask_t *nodemask)
2955 {
2956     unsigned long nr_reclaimed;
2957     struct scan_control sc = {
2958         .nr_to_reclaim = SWAP_CLUSTER_MAX,
2959         .gfp_mask = (gfp_mask = memalloc_noio_flags(gfp_mask)),
2960         .reclaim_idx = gfp_zone(gfp_mask),
2961         .order = order,
2962         .nodemask = nodemask,
2963         .priority = DEF_PRIORITY,
2964         .may_writepage = !laptop_mode,
2965         .may_unmap = 1,
2966         .may_swap = 1,
2967     };
2968 
2969     /*
2970      * Do not enter reclaim if fatal signal was delivered while throttled.
2971      * 1 is returned so that the page allocator does not OOM kill at this
2972      * point.
2973      */
2974     if (throttle_direct_reclaim(gfp_mask, zonelist, nodemask))
2975         return 1;
2976 
2977     trace_mm_vmscan_direct_reclaim_begin(order,
2978                 sc.may_writepage,
2979                 gfp_mask,
2980                 sc.reclaim_idx);
2981 
2982     nr_reclaimed = do_try_to_free_pages(zonelist, &sc);
2983 
2984     trace_mm_vmscan_direct_reclaim_end(nr_reclaimed);
2985 
2986     return nr_reclaimed;
2987 }
2988 
2989 #ifdef CONFIG_MEMCG
2990 
2991 unsigned long mem_cgroup_shrink_node(struct mem_cgroup *memcg,
2992                         gfp_t gfp_mask, bool noswap,
2993                         pg_data_t *pgdat,
2994                         unsigned long *nr_scanned)
2995 {
2996     struct scan_control sc = {
2997         .nr_to_reclaim = SWAP_CLUSTER_MAX,
2998         .target_mem_cgroup = memcg,
2999         .may_writepage = !laptop_mode,
3000         .may_unmap = 1,
3001         .reclaim_idx = MAX_NR_ZONES - 1,
3002         .may_swap = !noswap,
3003     };
3004     unsigned long lru_pages;
3005 
3006     sc.gfp_mask = (gfp_mask & GFP_RECLAIM_MASK) |
3007             (GFP_HIGHUSER_MOVABLE & ~GFP_RECLAIM_MASK);
3008 
3009     trace_mm_vmscan_memcg_softlimit_reclaim_begin(sc.order,
3010                               sc.may_writepage,
3011                               sc.gfp_mask,
3012                               sc.reclaim_idx);
3013 
3014     /*
3015      * NOTE: Although we can get the priority field, using it
3016      * here is not a good idea, since it limits the pages we can scan.
3017      * if we don't reclaim here, the shrink_node from balance_pgdat
3018      * will pick up pages from other mem cgroup's as well. We hack
3019      * the priority and make it zero.
3020      */
3021     shrink_node_memcg(pgdat, memcg, &sc, &lru_pages);
3022 
3023     trace_mm_vmscan_memcg_softlimit_reclaim_end(sc.nr_reclaimed);
3024 
3025     *nr_scanned = sc.nr_scanned;
3026     return sc.nr_reclaimed;
3027 }
3028 
3029 unsigned long try_to_free_mem_cgroup_pages(struct mem_cgroup *memcg,
3030                        unsigned long nr_pages,
3031                        gfp_t gfp_mask,
3032                        bool may_swap)
3033 {
3034     struct zonelist *zonelist;
3035     unsigned long nr_reclaimed;
3036     int nid;
3037     struct scan_control sc = {
3038         .nr_to_reclaim = max(nr_pages, SWAP_CLUSTER_MAX),
3039         .gfp_mask = (gfp_mask & GFP_RECLAIM_MASK) |
3040                 (GFP_HIGHUSER_MOVABLE & ~GFP_RECLAIM_MASK),
3041         .reclaim_idx = MAX_NR_ZONES - 1,
3042         .target_mem_cgroup = memcg,
3043         .priority = DEF_PRIORITY,
3044         .may_writepage = !laptop_mode,
3045         .may_unmap = 1,
3046         .may_swap = may_swap,
3047     };
3048 
3049     /*
3050      * Unlike direct reclaim via alloc_pages(), memcg's reclaim doesn't
3051      * take care of from where we get pages. So the node where we start the
3052      * scan does not need to be the current node.
3053      */
3054     nid = mem_cgroup_select_victim_node(memcg);
3055 
3056     zonelist = &NODE_DATA(nid)->node_zonelists[ZONELIST_FALLBACK];
3057 
3058     trace_mm_vmscan_memcg_reclaim_begin(0,
3059                         sc.may_writepage,
3060                         sc.gfp_mask,
3061                         sc.reclaim_idx);
3062 
3063     current->flags |= PF_MEMALLOC;
3064     nr_reclaimed = do_try_to_free_pages(zonelist, &sc);
3065     current->flags &= ~PF_MEMALLOC;
3066 
3067     trace_mm_vmscan_memcg_reclaim_end(nr_reclaimed);
3068 
3069     return nr_reclaimed;
3070 }
3071 #endif
3072 
3073 static void age_active_anon(struct pglist_data *pgdat,
3074                 struct scan_control *sc)
3075 {
3076     struct mem_cgroup *memcg;
3077 
3078     if (!total_swap_pages)
3079         return;
3080 
3081     memcg = mem_cgroup_iter(NULL, NULL, NULL);
3082     do {
3083         struct lruvec *lruvec = mem_cgroup_lruvec(pgdat, memcg);
3084 
3085         if (inactive_list_is_low(lruvec, false, sc))
3086             shrink_active_list(SWAP_CLUSTER_MAX, lruvec,
3087                        sc, LRU_ACTIVE_ANON);
3088 
3089         memcg = mem_cgroup_iter(NULL, memcg, NULL);
3090     } while (memcg);
3091 }
3092 
3093 static bool zone_balanced(struct zone *zone, int order, int classzone_idx)
3094 {
3095     unsigned long mark = high_wmark_pages(zone);
3096 
3097     if (!zone_watermark_ok_safe(zone, order, mark, classzone_idx))
3098         return false;
3099 
3100     /*
3101      * If any eligible zone is balanced then the node is not considered
3102      * to be congested or dirty
3103      */
3104     clear_bit(PGDAT_CONGESTED, &zone->zone_pgdat->flags);
3105     clear_bit(PGDAT_DIRTY, &zone->zone_pgdat->flags);
3106 
3107     return true;
3108 }
3109 
3110 /*
3111  * Prepare kswapd for sleeping. This verifies that there are no processes
3112  * waiting in throttle_direct_reclaim() and that watermarks have been met.
3113  *
3114  * Returns true if kswapd is ready to sleep
3115  */
3116 static bool prepare_kswapd_sleep(pg_data_t *pgdat, int order, int classzone_idx)
3117 {
3118     int i;
3119 
3120     /*
3121      * The throttled processes are normally woken up in balance_pgdat() as
3122      * soon as pfmemalloc_watermark_ok() is true. But there is a potential
3123      * race between when kswapd checks the watermarks and a process gets
3124      * throttled. There is also a potential race if processes get
3125      * throttled, kswapd wakes, a large process exits thereby balancing the
3126      * zones, which causes kswapd to exit balance_pgdat() before reaching
3127      * the wake up checks. If kswapd is going to sleep, no process should
3128      * be sleeping on pfmemalloc_wait, so wake them now if necessary. If
3129      * the wake up is premature, processes will wake kswapd and get
3130      * throttled again. The difference from wake ups in balance_pgdat() is
3131      * that here we are under prepare_to_wait().
3132      */
3133     if (waitqueue_active(&pgdat->pfmemalloc_wait))
3134         wake_up_all(&pgdat->pfmemalloc_wait);
3135 
3136     for (i = 0; i <= classzone_idx; i++) {
3137         struct zone *zone = pgdat->node_zones + i;
3138 
3139         if (!managed_zone(zone))
3140             continue;
3141 
3142         if (!zone_balanced(zone, order, classzone_idx))
3143             return false;
3144     }
3145 
3146     return true;
3147 }
3148 
3149 /*
3150  * kswapd shrinks a node of pages that are at or below the highest usable
3151  * zone that is currently unbalanced.
3152  *
3153  * Returns true if kswapd scanned at least the requested number of pages to
3154  * reclaim or if the lack of progress was due to pages under writeback.
3155  * This is used to determine if the scanning priority needs to be raised.
3156  */
3157 static bool kswapd_shrink_node(pg_data_t *pgdat,
3158                    struct scan_control *sc)
3159 {
3160     struct zone *zone;
3161     int z;
3162 
3163     /* Reclaim a number of pages proportional to the number of zones */
3164     sc->nr_to_reclaim = 0;
3165     for (z = 0; z <= sc->reclaim_idx; z++) {
3166         zone = pgdat->node_zones + z;
3167         if (!managed_zone(zone))
3168             continue;
3169 
3170         sc->nr_to_reclaim += max(high_wmark_pages(zone), SWAP_CLUSTER_MAX);
3171     }
3172 
3173     /*
3174      * Historically care was taken to put equal pressure on all zones but
3175      * now pressure is applied based on node LRU order.
3176      */
3177     shrink_node(pgdat, sc);
3178 
3179     /*
3180      * Fragmentation may mean that the system cannot be rebalanced for
3181      * high-order allocations. If twice the allocation size has been
3182      * reclaimed then recheck watermarks only at order-0 to prevent
3183      * excessive reclaim. Assume that a process requested a high-order
3184      * can direct reclaim/compact.
3185      */
3186     if (sc->order && sc->nr_reclaimed >= compact_gap(sc->order))
3187         sc->order = 0;
3188 
3189     return sc->nr_scanned >= sc->nr_to_reclaim;
3190 }
3191 
3192 /*
3193  * For kswapd, balance_pgdat() will reclaim pages across a node from zones
3194  * that are eligible for use by the caller until at least one zone is
3195  * balanced.
3196  *
3197  * Returns the order kswapd finished reclaiming at.
3198  *
3199  * kswapd scans the zones in the highmem->normal->dma direction.  It skips
3200  * zones which have free_pages > high_wmark_pages(zone), but once a zone is
3201  * found to have free_pages <= high_wmark_pages(zone), any page is that zone
3202  * or lower is eligible for reclaim until at least one usable zone is
3203  * balanced.
3204  */
3205 static int balance_pgdat(pg_data_t *pgdat, int order, int classzone_idx)
3206 {
3207     int i;
3208     unsigned long nr_soft_reclaimed;
3209     unsigned long nr_soft_scanned;
3210     struct zone *zone;
3211     struct scan_control sc = {
3212         .gfp_mask = GFP_KERNEL,
3213         .order = order,
3214         .priority = DEF_PRIORITY,
3215         .may_writepage = !laptop_mode,
3216         .may_unmap = 1,
3217         .may_swap = 1,
3218     };
3219     count_vm_event(PAGEOUTRUN);
3220 
3221     do {
3222         bool raise_priority = true;
3223 
3224         sc.nr_reclaimed = 0;
3225         sc.reclaim_idx = classzone_idx;
3226 
3227         /*
3228          * If the number of buffer_heads exceeds the maximum allowed
3229          * then consider reclaiming from all zones. This has a dual
3230          * purpose -- on 64-bit systems it is expected that
3231          * buffer_heads are stripped during active rotation. On 32-bit
3232          * systems, highmem pages can pin lowmem memory and shrinking
3233          * buffers can relieve lowmem pressure. Reclaim may still not
3234          * go ahead if all eligible zones for the original allocation
3235          * request are balanced to avoid excessive reclaim from kswapd.
3236          */
3237         if (buffer_heads_over_limit) {
3238             for (i = MAX_NR_ZONES - 1; i >= 0; i--) {
3239                 zone = pgdat->node_zones + i;
3240                 if (!managed_zone(zone))
3241                     continue;
3242 
3243                 sc.reclaim_idx = i;
3244                 break;
3245             }
3246         }
3247 
3248         /*
3249          * Only reclaim if there are no eligible zones. Check from
3250          * high to low zone as allocations prefer higher zones.
3251          * Scanning from low to high zone would allow congestion to be
3252          * cleared during a very small window when a small low
3253          * zone was balanced even under extreme pressure when the
3254          * overall node may be congested. Note that sc.reclaim_idx
3255          * is not used as buffer_heads_over_limit may have adjusted
3256          * it.
3257          */
3258         for (i = classzone_idx; i >= 0; i--) {
3259             zone = pgdat->node_zones + i;
3260             if (!managed_zone(zone))
3261                 continue;
3262 
3263             if (zone_balanced(zone, sc.order, classzone_idx))
3264                 goto out;
3265         }
3266 
3267         /*
3268          * Do some background aging of the anon list, to give
3269          * pages a chance to be referenced before reclaiming. All
3270          * pages are rotated regardless of classzone as this is
3271          * about consistent aging.
3272          */
3273         age_active_anon(pgdat, &sc);
3274 
3275         /*
3276          * If we're getting trouble reclaiming, start doing writepage
3277          * even in laptop mode.
3278          */
3279         if (sc.priority < DEF_PRIORITY - 2 || !pgdat_reclaimable(pgdat))
3280             sc.may_writepage = 1;
3281 
3282         /* Call soft limit reclaim before calling shrink_node. */
3283         sc.nr_scanned = 0;
3284         nr_soft_scanned = 0;
3285         nr_soft_reclaimed = mem_cgroup_soft_limit_reclaim(pgdat, sc.order,
3286                         sc.gfp_mask, &nr_soft_scanned);
3287         sc.nr_reclaimed += nr_soft_reclaimed;
3288 
3289         /*
3290          * There should be no need to raise the scanning priority if
3291          * enough pages are already being scanned that that high
3292          * watermark would be met at 100% efficiency.
3293          */
3294         if (kswapd_shrink_node(pgdat, &sc))
3295             raise_priority = false;
3296 
3297         /*
3298          * If the low watermark is met there is no need for processes
3299          * to be throttled on pfmemalloc_wait as they should not be
3300          * able to safely make forward progress. Wake them
3301          */
3302         if (waitqueue_active(&pgdat->pfmemalloc_wait) &&
3303                 pfmemalloc_watermark_ok(pgdat))
3304             wake_up_all(&pgdat->pfmemalloc_wait);
3305 
3306         /* Check if kswapd should be suspending */
3307         if (try_to_freeze() || kthread_should_stop())
3308             break;
3309 
3310         /*
3311          * Raise priority if scanning rate is too low or there was no
3312          * progress in reclaiming pages
3313          */
3314         if (raise_priority || !sc.nr_reclaimed)
3315             sc.priority--;
3316     } while (sc.priority >= 1);
3317 
3318 out:
3319     /*
3320      * Return the order kswapd stopped reclaiming at as
3321      * prepare_kswapd_sleep() takes it into account. If another caller
3322      * entered the allocator slow path while kswapd was awake, order will
3323      * remain at the higher level.
3324      */
3325     return sc.order;
3326 }
3327 
3328 static void kswapd_try_to_sleep(pg_data_t *pgdat, int alloc_order, int reclaim_order,
3329                 unsigned int classzone_idx)
3330 {
3331     long remaining = 0;
3332     DEFINE_WAIT(wait);
3333 
3334     if (freezing(current) || kthread_should_stop())
3335         return;
3336 
3337     prepare_to_wait(&pgdat->kswapd_wait, &wait, TASK_INTERRUPTIBLE);
3338 
3339     /* Try to sleep for a short interval */
3340     if (prepare_kswapd_sleep(pgdat, reclaim_order, classzone_idx)) {
3341         /*
3342          * Compaction records what page blocks it recently failed to
3343          * isolate pages from and skips them in the future scanning.
3344          * When kswapd is going to sleep, it is reasonable to assume
3345          * that pages and compaction may succeed so reset the cache.
3346          */
3347         reset_isolation_suitable(pgdat);
3348 
3349         /*
3350          * We have freed the memory, now we should compact it to make
3351          * allocation of the requested order possible.
3352          */
3353         wakeup_kcompactd(pgdat, alloc_order, classzone_idx);
3354 
3355         remaining = schedule_timeout(HZ/10);
3356 
3357         /*
3358          * If woken prematurely then reset kswapd_classzone_idx and
3359          * order. The values will either be from a wakeup request or
3360          * the previous request that slept prematurely.
3361          */
3362         if (remaining) {
3363             pgdat->kswapd_classzone_idx = max(pgdat->kswapd_classzone_idx, classzone_idx);
3364             pgdat->kswapd_order = max(pgdat->kswapd_order, reclaim_order);
3365         }
3366 
3367         finish_wait(&pgdat->kswapd_wait, &wait);
3368         prepare_to_wait(&pgdat->kswapd_wait, &wait, TASK_INTERRUPTIBLE);
3369     }
3370 
3371     /*
3372      * After a short sleep, check if it was a premature sleep. If not, then
3373      * go fully to sleep until explicitly woken up.
3374      */
3375     if (!remaining &&
3376         prepare_kswapd_sleep(pgdat, reclaim_order, classzone_idx)) {
3377         trace_mm_vmscan_kswapd_sleep(pgdat->node_id);
3378 
3379         /*
3380          * vmstat counters are not perfectly accurate and the estimated
3381          * value for counters such as NR_FREE_PAGES can deviate from the
3382          * true value by nr_online_cpus * threshold. To avoid the zone
3383          * watermarks being breached while under pressure, we reduce the
3384          * per-cpu vmstat threshold while kswapd is awake and restore
3385          * them before going back to sleep.
3386          */
3387         set_pgdat_percpu_threshold(pgdat, calculate_normal_threshold);
3388 
3389         if (!kthread_should_stop())
3390             schedule();
3391 
3392         set_pgdat_percpu_threshold(pgdat, calculate_pressure_threshold);
3393     } else {
3394         if (remaining)
3395             count_vm_event(KSWAPD_LOW_WMARK_HIT_QUICKLY);
3396         else
3397             count_vm_event(KSWAPD_HIGH_WMARK_HIT_QUICKLY);
3398     }
3399     finish_wait(&pgdat->kswapd_wait, &wait);
3400 }
3401 
3402 /*
3403  * The background pageout daemon, started as a kernel thread
3404  * from the init process.
3405  *
3406  * This basically trickles out pages so that we have _some_
3407  * free memory available even if there is no other activity
3408  * that frees anything up. This is needed for things like routing
3409  * etc, where we otherwise might have all activity going on in
3410  * asynchronous contexts that cannot page things out.
3411  *
3412  * If there are applications that are active memory-allocators
3413  * (most normal use), this basically shouldn't matter.
3414  */
3415 static int kswapd(void *p)
3416 {
3417     unsigned int alloc_order, reclaim_order, classzone_idx;
3418     pg_data_t *pgdat = (pg_data_t*)p;
3419     struct task_struct *tsk = current;
3420 
3421     struct reclaim_state reclaim_state = {
3422         .reclaimed_slab = 0,
3423     };
3424     const struct cpumask *cpumask = cpumask_of_node(pgdat->node_id);
3425 
3426     lockdep_set_current_reclaim_state(GFP_KERNEL);
3427 
3428     if (!cpumask_empty(cpumask))
3429         set_cpus_allowed_ptr(tsk, cpumask);
3430     current->reclaim_state = &reclaim_state;
3431 
3432     /*
3433      * Tell the memory management that we're a "memory allocator",
3434      * and that if we need more memory we should get access to it
3435      * regardless (see "__alloc_pages()"). "kswapd" should
3436      * never get caught in the normal page freeing logic.
3437      *
3438      * (Kswapd normally doesn't need memory anyway, but sometimes
3439      * you need a small amount of memory in order to be able to
3440      * page out something else, and this flag essentially protects
3441      * us from recursively trying to free more memory as we're
3442      * trying to free the first piece of memory in the first place).
3443      */
3444     tsk->flags |= PF_MEMALLOC | PF_SWAPWRITE | PF_KSWAPD;
3445     set_freezable();
3446 
3447     pgdat->kswapd_order = alloc_order = reclaim_order = 0;
3448     pgdat->kswapd_classzone_idx = classzone_idx = 0;
3449     for ( ; ; ) {
3450         bool ret;
3451 
3452 kswapd_try_sleep:
3453         kswapd_try_to_sleep(pgdat, alloc_order, reclaim_order,
3454                     classzone_idx);
3455 
3456         /* Read the new order and classzone_idx */
3457         alloc_order = reclaim_order = pgdat->kswapd_order;
3458         classzone_idx = pgdat->kswapd_classzone_idx;
3459         pgdat->kswapd_order = 0;
3460         pgdat->kswapd_classzone_idx = 0;
3461 
3462         ret = try_to_freeze();
3463         if (kthread_should_stop())
3464             break;
3465 
3466         /*
3467          * We can speed up thawing tasks if we don't call balance_pgdat
3468          * after returning from the refrigerator
3469          */
3470         if (ret)
3471             continue;
3472 
3473         /*
3474          * Reclaim begins at the requested order but if a high-order
3475          * reclaim fails then kswapd falls back to reclaiming for
3476          * order-0. If that happens, kswapd will consider sleeping
3477          * for the order it finished reclaiming at (reclaim_order)
3478          * but kcompactd is woken to compact for the original
3479          * request (alloc_order).
3480          */
3481         trace_mm_vmscan_kswapd_wake(pgdat->node_id, classzone_idx,
3482                         alloc_order);
3483         reclaim_order = balance_pgdat(pgdat, alloc_order, classzone_idx);
3484         if (reclaim_order < alloc_order)
3485             goto kswapd_try_sleep;
3486 
3487         alloc_order = reclaim_order = pgdat->kswapd_order;
3488         classzone_idx = pgdat->kswapd_classzone_idx;
3489     }
3490 
3491     tsk->flags &= ~(PF_MEMALLOC | PF_SWAPWRITE | PF_KSWAPD);
3492     current->reclaim_state = NULL;
3493     lockdep_clear_current_reclaim_state();
3494 
3495     return 0;
3496 }
3497 
3498 /*
3499  * A zone is low on free memory, so wake its kswapd task to service it.
3500  */
3501 void wakeup_kswapd(struct zone *zone, int order, enum zone_type classzone_idx)
3502 {
3503     pg_data_t *pgdat;
3504     int z;
3505 
3506     if (!managed_zone(zone))
3507         return;
3508 
3509     if (!cpuset_zone_allowed(zone, GFP_KERNEL | __GFP_HARDWALL))
3510         return;
3511     pgdat = zone->zone_pgdat;
3512     pgdat->kswapd_classzone_idx = max(pgdat->kswapd_classzone_idx, classzone_idx);
3513     pgdat->kswapd_order = max(pgdat->kswapd_order, order);
3514     if (!waitqueue_active(&pgdat->kswapd_wait))
3515         return;
3516 
3517     /* Only wake kswapd if all zones are unbalanced */
3518     for (z = 0; z <= classzone_idx; z++) {
3519         zone = pgdat->node_zones + z;
3520         if (!managed_zone(zone))
3521             continue;
3522 
3523         if (zone_balanced(zone, order, classzone_idx))
3524             return;
3525     }
3526 
3527     trace_mm_vmscan_wakeup_kswapd(pgdat->node_id, zone_idx(zone), order);
3528     wake_up_interruptible(&pgdat->kswapd_wait);
3529 }
3530 
3531 #ifdef CONFIG_HIBERNATION
3532 /*
3533  * Try to free `nr_to_reclaim' of memory, system-wide, and return the number of
3534  * freed pages.
3535  *
3536  * Rather than trying to age LRUs the aim is to preserve the overall
3537  * LRU order by reclaiming preferentially
3538  * inactive > active > active referenced > active mapped
3539  */
3540 unsigned long shrink_all_memory(unsigned long nr_to_reclaim)
3541 {
3542     struct reclaim_state reclaim_state;
3543     struct scan_control sc = {
3544         .nr_to_reclaim = nr_to_reclaim,
3545         .gfp_mask = GFP_HIGHUSER_MOVABLE,
3546         .reclaim_idx = MAX_NR_ZONES - 1,
3547         .priority = DEF_PRIORITY,
3548         .may_writepage = 1,
3549         .may_unmap = 1,
3550         .may_swap = 1,
3551         .hibernation_mode = 1,
3552     };
3553     struct zonelist *zonelist = node_zonelist(numa_node_id(), sc.gfp_mask);
3554     struct task_struct *p = current;
3555     unsigned long nr_reclaimed;
3556 
3557     p->flags |= PF_MEMALLOC;
3558     lockdep_set_current_reclaim_state(sc.gfp_mask);
3559     reclaim_state.reclaimed_slab = 0;
3560     p->reclaim_state = &reclaim_state;
3561 
3562     nr_reclaimed = do_try_to_free_pages(zonelist, &sc);
3563 
3564     p->reclaim_state = NULL;
3565     lockdep_clear_current_reclaim_state();
3566     p->flags &= ~PF_MEMALLOC;
3567 
3568     return nr_reclaimed;
3569 }
3570 #endif /* CONFIG_HIBERNATION */
3571 
3572 /* It's optimal to keep kswapds on the same CPUs as their memory, but
3573    not required for correctness.  So if the last cpu in a node goes
3574    away, we get changed to run anywhere: as the first one comes back,
3575    restore their cpu bindings. */
3576 static int kswapd_cpu_online(unsigned int cpu)
3577 {
3578     int nid;
3579 
3580     for_each_node_state(nid, N_MEMORY) {
3581         pg_data_t *pgdat = NODE_DATA(nid);
3582         const struct cpumask *mask;
3583 
3584         mask = cpumask_of_node(pgdat->node_id);
3585 
3586         if (cpumask_any_and(cpu_online_mask, mask) < nr_cpu_ids)
3587             /* One of our CPUs online: restore mask */
3588             set_cpus_allowed_ptr(pgdat->kswapd, mask);
3589     }
3590     return 0;
3591 }
3592 
3593 /*
3594  * This kswapd start function will be called by init and node-hot-add.
3595  * On node-hot-add, kswapd will moved to proper cpus if cpus are hot-added.
3596  */
3597 int kswapd_run(int nid)
3598 {
3599     pg_data_t *pgdat = NODE_DATA(nid);
3600     int ret = 0;
3601 
3602     if (pgdat->kswapd)
3603         return 0;
3604 
3605     pgdat->kswapd = kthread_run(kswapd, pgdat, "kswapd%d", nid);
3606     if (IS_ERR(pgdat->kswapd)) {
3607         /* failure at boot is fatal */
3608         BUG_ON(system_state == SYSTEM_BOOTING);
3609         pr_err("Failed to start kswapd on node %d\n", nid);
3610         ret = PTR_ERR(pgdat->kswapd);
3611         pgdat->kswapd = NULL;
3612     }
3613     return ret;
3614 }
3615 
3616 /*
3617  * Called by memory hotplug when all memory in a node is offlined.  Caller must
3618  * hold mem_hotplug_begin/end().
3619  */
3620 void kswapd_stop(int nid)
3621 {
3622     struct task_struct *kswapd = NODE_DATA(nid)->kswapd;
3623 
3624     if (kswapd) {
3625         kthread_stop(kswapd);
3626         NODE_DATA(nid)->kswapd = NULL;
3627     }
3628 }
3629 
3630 static int __init kswapd_init(void)
3631 {
3632     int nid, ret;
3633 
3634     swap_setup();
3635     for_each_node_state(nid, N_MEMORY)
3636         kswapd_run(nid);
3637     ret = cpuhp_setup_state_nocalls(CPUHP_AP_ONLINE_DYN,
3638                     "mm/vmscan:online", kswapd_cpu_online,
3639                     NULL);
3640     WARN_ON(ret < 0);
3641     return 0;
3642 }
3643 
3644 module_init(kswapd_init)
3645 
3646 #ifdef CONFIG_NUMA
3647 /*
3648  * Node reclaim mode
3649  *
3650  * If non-zero call node_reclaim when the number of free pages falls below
3651  * the watermarks.
3652  */
3653 int node_reclaim_mode __read_mostly;
3654 
3655 #define RECLAIM_OFF 0
3656 #define RECLAIM_ZONE (1<<0) /* Run shrink_inactive_list on the zone */
3657 #define RECLAIM_WRITE (1<<1)    /* Writeout pages during reclaim */
3658 #define RECLAIM_UNMAP (1<<2)    /* Unmap pages during reclaim */
3659 
3660 /*
3661  * Priority for NODE_RECLAIM. This determines the fraction of pages
3662  * of a node considered for each zone_reclaim. 4 scans 1/16th of
3663  * a zone.
3664  */
3665 #define NODE_RECLAIM_PRIORITY 4
3666 
3667 /*
3668  * Percentage of pages in a zone that must be unmapped for node_reclaim to
3669  * occur.
3670  */
3671 int sysctl_min_unmapped_ratio = 1;
3672 
3673 /*
3674  * If the number of slab pages in a zone grows beyond this percentage then
3675  * slab reclaim needs to occur.
3676  */
3677 int sysctl_min_slab_ratio = 5;
3678 
3679 static inline unsigned long node_unmapped_file_pages(struct pglist_data *pgdat)
3680 {
3681     unsigned long file_mapped = node_page_state(pgdat, NR_FILE_MAPPED);
3682     unsigned long file_lru = node_page_state(pgdat, NR_INACTIVE_FILE) +
3683         node_page_state(pgdat, NR_ACTIVE_FILE);
3684 
3685     /*
3686      * It's possible for there to be more file mapped pages than
3687      * accounted for by the pages on the file LRU lists because
3688      * tmpfs pages accounted for as ANON can also be FILE_MAPPED
3689      */
3690     return (file_lru > file_mapped) ? (file_lru - file_mapped) : 0;
3691 }
3692 
3693 /* Work out how many page cache pages we can reclaim in this reclaim_mode */
3694 static unsigned long node_pagecache_reclaimable(struct pglist_data *pgdat)
3695 {
3696     unsigned long nr_pagecache_reclaimable;
3697     unsigned long delta = 0;
3698 
3699     /*
3700      * If RECLAIM_UNMAP is set, then all file pages are considered
3701      * potentially reclaimable. Otherwise, we have to worry about
3702      * pages like swapcache and node_unmapped_file_pages() provides
3703      * a better estimate
3704      */
3705     if (node_reclaim_mode & RECLAIM_UNMAP)
3706         nr_pagecache_reclaimable = node_page_state(pgdat, NR_FILE_PAGES);
3707     else
3708         nr_pagecache_reclaimable = node_unmapped_file_pages(pgdat);
3709 
3710     /* If we can't clean pages, remove dirty pages from consideration */
3711     if (!(node_reclaim_mode & RECLAIM_WRITE))
3712         delta += node_page_state(pgdat, NR_FILE_DIRTY);
3713 
3714     /* Watch for any possible underflows due to delta */
3715     if (unlikely(delta > nr_pagecache_reclaimable))
3716         delta = nr_pagecache_reclaimable;
3717 
3718     return nr_pagecache_reclaimable - delta;
3719 }
3720 
3721 /*
3722  * Try to free up some pages from this node through reclaim.
3723  */
3724 static int __node_reclaim(struct pglist_data *pgdat, gfp_t gfp_mask, unsigned int order)
3725 {
3726     /* Minimum pages needed in order to stay on node */
3727     const unsigned long nr_pages = 1 << order;
3728     struct task_struct *p = current;
3729     struct reclaim_state reclaim_state;
3730     int classzone_idx = gfp_zone(gfp_mask);
3731     struct scan_control sc = {
3732         .nr_to_reclaim = max(nr_pages, SWAP_CLUSTER_MAX),
3733         .gfp_mask = (gfp_mask = memalloc_noio_flags(gfp_mask)),
3734         .order = order,
3735         .priority = NODE_RECLAIM_PRIORITY,
3736         .may_writepage = !!(node_reclaim_mode & RECLAIM_WRITE),
3737         .may_unmap = !!(node_reclaim_mode & RECLAIM_UNMAP),
3738         .may_swap = 1,
3739         .reclaim_idx = classzone_idx,
3740     };
3741 
3742     cond_resched();
3743     /*
3744      * We need to be able to allocate from the reserves for RECLAIM_UNMAP
3745      * and we also need to be able to write out pages for RECLAIM_WRITE
3746      * and RECLAIM_UNMAP.
3747      */
3748     p->flags |= PF_MEMALLOC | PF_SWAPWRITE;
3749     lockdep_set_current_reclaim_state(gfp_mask);
3750     reclaim_state.reclaimed_slab = 0;
3751     p->reclaim_state = &reclaim_state;
3752 
3753     if (node_pagecache_reclaimable(pgdat) > pgdat->min_unmapped_pages) {
3754         /*
3755          * Free memory by calling shrink zone with increasing
3756          * priorities until we have enough memory freed.
3757          */
3758         do {
3759             shrink_node(pgdat, &sc);
3760         } while (sc.nr_reclaimed < nr_pages && --sc.priority >= 0);
3761     }
3762 
3763     p->reclaim_state = NULL;
3764     current->flags &= ~(PF_MEMALLOC | PF_SWAPWRITE);
3765     lockdep_clear_current_reclaim_state();
3766     return sc.nr_reclaimed >= nr_pages;
3767 }
3768 
3769 int node_reclaim(struct pglist_data *pgdat, gfp_t gfp_mask, unsigned int order)
3770 {
3771     int ret;
3772 
3773     /*
3774      * Node reclaim reclaims unmapped file backed pages and
3775      * slab pages if we are over the defined limits.
3776      *
3777      * A small portion of unmapped file backed pages is needed for
3778      * file I/O otherwise pages read by file I/O will be immediately
3779      * thrown out if the node is overallocated. So we do not reclaim
3780      * if less than a specified percentage of the node is used by
3781      * unmapped file backed pages.
3782      */
3783     if (node_pagecache_reclaimable(pgdat) <= pgdat->min_unmapped_pages &&
3784         sum_zone_node_page_state(pgdat->node_id, NR_SLAB_RECLAIMABLE) <= pgdat->min_slab_pages)
3785         return NODE_RECLAIM_FULL;
3786 
3787     if (!pgdat_reclaimable(pgdat))
3788         return NODE_RECLAIM_FULL;
3789 
3790     /*
3791      * Do not scan if the allocation should not be delayed.
3792      */
3793     if (!gfpflags_allow_blocking(gfp_mask) || (current->flags & PF_MEMALLOC))
3794         return NODE_RECLAIM_NOSCAN;
3795 
3796     /*
3797      * Only run node reclaim on the local node or on nodes that do not
3798      * have associated processors. This will favor the local processor
3799      * over remote processors and spread off node memory allocations
3800      * as wide as possible.
3801      */
3802     if (node_state(pgdat->node_id, N_CPU) && pgdat->node_id != numa_node_id())
3803         return NODE_RECLAIM_NOSCAN;
3804 
3805     if (test_and_set_bit(PGDAT_RECLAIM_LOCKED, &pgdat->flags))
3806         return NODE_RECLAIM_NOSCAN;
3807 
3808     ret = __node_reclaim(pgdat, gfp_mask, order);
3809     clear_bit(PGDAT_RECLAIM_LOCKED, &pgdat->flags);
3810 
3811     if (!ret)
3812         count_vm_event(PGSCAN_ZONE_RECLAIM_FAILED);
3813 
3814     return ret;
3815 }
3816 #endif
3817 
3818 /*
3819  * page_evictable - test whether a page is evictable
3820  * @page: the page to test
3821  *
3822  * Test whether page is evictable--i.e., should be placed on active/inactive
3823  * lists vs unevictable list.
3824  *
3825  * Reasons page might not be evictable:
3826  * (1) page's mapping marked unevictable
3827  * (2) page is part of an mlocked VMA
3828  *
3829  */
3830 int page_evictable(struct page *page)
3831 {
3832     return !mapping_unevictable(page_mapping(page)) && !PageMlocked(page);
3833 }
3834 
3835 #ifdef CONFIG_SHMEM
3836 /**
3837  * check_move_unevictable_pages - check pages for evictability and move to appropriate zone lru list
3838  * @pages:  array of pages to check
3839  * @nr_pages:   number of pages to check
3840  *
3841  * Checks pages for evictability and moves them to the appropriate lru list.
3842  *
3843  * This function is only used for SysV IPC SHM_UNLOCK.
3844  */
3845 void check_move_unevictable_pages(struct page **pages, int nr_pages)
3846 {
3847     struct lruvec *lruvec;
3848     struct pglist_data *pgdat = NULL;
3849     int pgscanned = 0;
3850     int pgrescued = 0;
3851     int i;
3852 
3853     for (i = 0; i < nr_pages; i++) {
3854         struct page *page = pages[i];
3855         struct pglist_data *pagepgdat = page_pgdat(page);
3856 
3857         pgscanned++;
3858         if (pagepgdat != pgdat) {
3859             if (pgdat)
3860                 spin_unlock_irq(&pgdat->lru_lock);
3861             pgdat = pagepgdat;
3862             spin_lock_irq(&pgdat->lru_lock);
3863         }
3864         lruvec = mem_cgroup_page_lruvec(page, pgdat);
3865 
3866         if (!PageLRU(page) || !PageUnevictable(page))
3867             continue;
3868 
3869         if (page_evictable(page)) {
3870             enum lru_list lru = page_lru_base_type(page);
3871 
3872             VM_BUG_ON_PAGE(PageActive(page), page);
3873             ClearPageUnevictable(page);
3874             del_page_from_lru_list(page, lruvec, LRU_UNEVICTABLE);
3875             add_page_to_lru_list(page, lruvec, lru);
3876             pgrescued++;
3877         }
3878     }
3879 
3880     if (pgdat) {
3881         __count_vm_events(UNEVICTABLE_PGRESCUED, pgrescued);
3882         __count_vm_events(UNEVICTABLE_PGSCANNED, pgscanned);
3883         spin_unlock_irq(&pgdat->lru_lock);
3884     }
3885 }
3886 #endif /* CONFIG_SHMEM */