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 // SPDX-License-Identifier: GPL-2.0
 /*
  * linux/mm/compaction.c
  *
  * Memory compaction for the reduction of external fragmentation. Note that
  * this heavily depends upon page migration to do all the real heavy
  * lifting
  *
  * Copyright IBM Corp. 2007-2010 Mel Gorman <mel@csn.ul.ie>
  */
 #include <linux/cpu.h>
 #include <linux/swap.h>
 #include <linux/migrate.h>
 #include <linux/compaction.h>
 #include <linux/mm_inline.h>
 #include <linux/sched/signal.h>
 #include <linux/backing-dev.h>
 #include <linux/sysctl.h>
 #include <linux/sysfs.h>
 #include <linux/page-isolation.h>
 #include <linux/kasan.h>
 #include <linux/kthread.h>
 #include <linux/freezer.h>
 #include <linux/page_owner.h>
 #include <linux/psi.h>
 #include "internal.h"
 
 #ifdef CONFIG_COMPACTION
 /*
  * Fragmentation score check interval for proactive compaction purposes.
  */
 #define HPAGE_FRAG_CHECK_INTERVAL_MSEC  (500)
 
 static inline void count_compact_event(enum vm_event_item item)
 {
     count_vm_event(item);
 }
 
 static inline void count_compact_events(enum vm_event_item item, long delta)
 {
     count_vm_events(item, delta);
 }
 #else
 #define count_compact_event(item) do { } while (0)
 #define count_compact_events(item, delta) do { } while (0)
 #endif
 
 #if defined CONFIG_COMPACTION || defined CONFIG_CMA
 
 #define CREATE_TRACE_POINTS
 #include <trace/events/compaction.h>
 
 #define block_start_pfn(pfn, order) round_down(pfn, 1UL << (order))
 #define block_end_pfn(pfn, order)   ALIGN((pfn) + 1, 1UL << (order))
 #define pageblock_start_pfn(pfn)    block_start_pfn(pfn, pageblock_order)
 #define pageblock_end_pfn(pfn)      block_end_pfn(pfn, pageblock_order)
 
 /*
  * Page order with-respect-to which proactive compaction
  * calculates external fragmentation, which is used as
  * the "fragmentation score" of a node/zone.
  */
 #if defined CONFIG_TRANSPARENT_HUGEPAGE
 #define COMPACTION_HPAGE_ORDER  HPAGE_PMD_ORDER
 #elif defined CONFIG_HUGETLBFS
 #define COMPACTION_HPAGE_ORDER  HUGETLB_PAGE_ORDER
 #else
 #define COMPACTION_HPAGE_ORDER  (PMD_SHIFT - PAGE_SHIFT)
 #endif
 
 static unsigned long release_freepages(struct list_head *freelist)
 {
     struct page *page, *next;
     unsigned long high_pfn = 0;
 
     list_for_each_entry_safe(page, next, freelist, lru) {
         unsigned long pfn = page_to_pfn(page);
         list_del(&page->lru);
         __free_page(page);
         if (pfn > high_pfn)
             high_pfn = pfn;
     }
 
     return high_pfn;
 }
 
 static void split_map_pages(struct list_head *list)
 {
     unsigned int i, order, nr_pages;
     struct page *page, *next;
     LIST_HEAD(tmp_list);
 
     list_for_each_entry_safe(page, next, list, lru) {
         list_del(&page->lru);
 
         order = page_private(page);
         nr_pages = 1 << order;
 
         post_alloc_hook(page, order, __GFP_MOVABLE);
         if (order)
             split_page(page, order);
 
         for (i = 0; i < nr_pages; i++) {
             list_add(&page->lru, &tmp_list);
             page++;
         }
     }
 
     list_splice(&tmp_list, list);
 }
 
 #ifdef CONFIG_COMPACTION
 bool PageMovable(struct page *page)
 {
     const struct movable_operations *mops;
 
     VM_BUG_ON_PAGE(!PageLocked(page), page);
     if (!__PageMovable(page))
         return false;
 
     mops = page_movable_ops(page);
     if (mops)
         return true;
 
     return false;
 }
 EXPORT_SYMBOL(PageMovable);
 
 void __SetPageMovable(struct page *page, const struct movable_operations *mops)
 {
     VM_BUG_ON_PAGE(!PageLocked(page), page);
     VM_BUG_ON_PAGE((unsigned long)mops & PAGE_MAPPING_MOVABLE, page);
     page->mapping = (void *)((unsigned long)mops | PAGE_MAPPING_MOVABLE);
 }
 EXPORT_SYMBOL(__SetPageMovable);
 
 void __ClearPageMovable(struct page *page)
 {
     VM_BUG_ON_PAGE(!PageMovable(page), page);
     /*
      * This page still has the type of a movable page, but it's
      * actually not movable any more.
      */
     page->mapping = (void *)PAGE_MAPPING_MOVABLE;
 }
 EXPORT_SYMBOL(__ClearPageMovable);
 
 /* Do not skip compaction more than 64 times */
 #define COMPACT_MAX_DEFER_SHIFT 6
 
 /*
  * Compaction is deferred when compaction fails to result in a page
  * allocation success. 1 << compact_defer_shift, compactions are skipped up
  * to a limit of 1 << COMPACT_MAX_DEFER_SHIFT
  */
 static void defer_compaction(struct zone *zone, int order)
 {
     zone->compact_considered = 0;
     zone->compact_defer_shift++;
 
     if (order < zone->compact_order_failed)
         zone->compact_order_failed = order;
 
     if (zone->compact_defer_shift > COMPACT_MAX_DEFER_SHIFT)
         zone->compact_defer_shift = COMPACT_MAX_DEFER_SHIFT;
 
     trace_mm_compaction_defer_compaction(zone, order);
 }
 
 /* Returns true if compaction should be skipped this time */
 static bool compaction_deferred(struct zone *zone, int order)
 {
     unsigned long defer_limit = 1UL << zone->compact_defer_shift;
 
     if (order < zone->compact_order_failed)
         return false;
 
     /* Avoid possible overflow */
     if (++zone->compact_considered >= defer_limit) {
         zone->compact_considered = defer_limit;
         return false;
     }
 
     trace_mm_compaction_deferred(zone, order);
 
     return true;
 }
 
 /*
  * Update defer tracking counters after successful compaction of given order,
  * which means an allocation either succeeded (alloc_success == true) or is
  * expected to succeed.
  */
 void compaction_defer_reset(struct zone *zone, int order,
         bool alloc_success)
 {
     if (alloc_success) {
         zone->compact_considered = 0;
         zone->compact_defer_shift = 0;
     }
     if (order >= zone->compact_order_failed)
         zone->compact_order_failed = order + 1;
 
     trace_mm_compaction_defer_reset(zone, order);
 }
 
 /* Returns true if restarting compaction after many failures */
 static bool compaction_restarting(struct zone *zone, int order)
 {
     if (order < zone->compact_order_failed)
         return false;
 
     return zone->compact_defer_shift == COMPACT_MAX_DEFER_SHIFT &&
         zone->compact_considered >= 1UL << zone->compact_defer_shift;
 }
 
 /* Returns true if the pageblock should be scanned for pages to isolate. */
 static inline bool isolation_suitable(struct compact_control *cc,
                     struct page *page)
 {
     if (cc->ignore_skip_hint)
         return true;
 
     return !get_pageblock_skip(page);
 }
 
 static void reset_cached_positions(struct zone *zone)
 {
     zone->compact_cached_migrate_pfn[0] = zone->zone_start_pfn;
     zone->compact_cached_migrate_pfn[1] = zone->zone_start_pfn;
     zone->compact_cached_free_pfn =
                 pageblock_start_pfn(zone_end_pfn(zone) - 1);
 }
 
 /*
  * Compound pages of >= pageblock_order should consistently be skipped until
  * released. It is always pointless to compact pages of such order (if they are
  * migratable), and the pageblocks they occupy cannot contain any free pages.
  */
 static bool pageblock_skip_persistent(struct page *page)
 {
     if (!PageCompound(page))
         return false;
 
     page = compound_head(page);
 
     if (compound_order(page) >= pageblock_order)
         return true;
 
     return false;
 }
 
 static bool
 __reset_isolation_pfn(struct zone *zone, unsigned long pfn, bool check_source,
                             bool check_target)
 {
     struct page *page = pfn_to_online_page(pfn);
     struct page *block_page;
     struct page *end_page;
     unsigned long block_pfn;
 
     if (!page)
         return false;
     if (zone != page_zone(page))
         return false;
     if (pageblock_skip_persistent(page))
         return false;
 
     /*
      * If skip is already cleared do no further checking once the
      * restart points have been set.
      */
     if (check_source && check_target && !get_pageblock_skip(page))
         return true;
 
     /*
      * If clearing skip for the target scanner, do not select a
      * non-movable pageblock as the starting point.
      */
     if (!check_source && check_target &&
         get_pageblock_migratetype(page) != MIGRATE_MOVABLE)
         return false;
 
     /* Ensure the start of the pageblock or zone is online and valid */
     block_pfn = pageblock_start_pfn(pfn);
     block_pfn = max(block_pfn, zone->zone_start_pfn);
     block_page = pfn_to_online_page(block_pfn);
     if (block_page) {
         page = block_page;
         pfn = block_pfn;
     }
 
     /* Ensure the end of the pageblock or zone is online and valid */
     block_pfn = pageblock_end_pfn(pfn) - 1;
     block_pfn = min(block_pfn, zone_end_pfn(zone) - 1);
     end_page = pfn_to_online_page(block_pfn);
     if (!end_page)
         return false;
 
     /*
      * Only clear the hint if a sample indicates there is either a
      * free page or an LRU page in the block. One or other condition
      * is necessary for the block to be a migration source/target.
      */
     do {
         if (check_source && PageLRU(page)) {
             clear_pageblock_skip(page);
             return true;
         }
 
         if (check_target && PageBuddy(page)) {
             clear_pageblock_skip(page);
             return true;
         }
 
         page += (1 << PAGE_ALLOC_COSTLY_ORDER);
     } while (page <= end_page);
 
     return false;
 }
 
 /*
  * This function is called to clear all cached information on pageblocks that
  * should be skipped for page isolation when the migrate and free page scanner
  * meet.
  */
 static void __reset_isolation_suitable(struct zone *zone)
 {
     unsigned long migrate_pfn = zone->zone_start_pfn;
     unsigned long free_pfn = zone_end_pfn(zone) - 1;
     unsigned long reset_migrate = free_pfn;
     unsigned long reset_free = migrate_pfn;
     bool source_set = false;
     bool free_set = false;
 
     if (!zone->compact_blockskip_flush)
         return;
 
     zone->compact_blockskip_flush = false;
 
     /*
      * Walk the zone and update pageblock skip information. Source looks
      * for PageLRU while target looks for PageBuddy. When the scanner
      * is found, both PageBuddy and PageLRU are checked as the pageblock
      * is suitable as both source and target.
      */
     for (; migrate_pfn < free_pfn; migrate_pfn += pageblock_nr_pages,
                     free_pfn -= pageblock_nr_pages) {
         cond_resched();
 
         /* Update the migrate PFN */
         if (__reset_isolation_pfn(zone, migrate_pfn, true, source_set) &&
             migrate_pfn < reset_migrate) {
             source_set = true;
             reset_migrate = migrate_pfn;
             zone->compact_init_migrate_pfn = reset_migrate;
             zone->compact_cached_migrate_pfn[0] = reset_migrate;
             zone->compact_cached_migrate_pfn[1] = reset_migrate;
         }
 
         /* Update the free PFN */
         if (__reset_isolation_pfn(zone, free_pfn, free_set, true) &&
             free_pfn > reset_free) {
             free_set = true;
             reset_free = free_pfn;
             zone->compact_init_free_pfn = reset_free;
             zone->compact_cached_free_pfn = reset_free;
         }
     }
 
     /* Leave no distance if no suitable block was reset */
     if (reset_migrate >= reset_free) {
         zone->compact_cached_migrate_pfn[0] = migrate_pfn;
         zone->compact_cached_migrate_pfn[1] = migrate_pfn;
         zone->compact_cached_free_pfn = free_pfn;
     }
 }
 
 void reset_isolation_suitable(pg_data_t *pgdat)
 {
     int zoneid;
 
     for (zoneid = 0; zoneid < MAX_NR_ZONES; zoneid++) {
         struct zone *zone = &pgdat->node_zones[zoneid];
         if (!populated_zone(zone))
             continue;
 
         /* Only flush if a full compaction finished recently */
         if (zone->compact_blockskip_flush)
             __reset_isolation_suitable(zone);
     }
 }
 
 /*
  * Sets the pageblock skip bit if it was clear. Note that this is a hint as
  * locks are not required for read/writers. Returns true if it was already set.
  */
 static bool test_and_set_skip(struct compact_control *cc, struct page *page,
                             unsigned long pfn)
 {
     bool skip;
 
     /* Do no update if skip hint is being ignored */
     if (cc->ignore_skip_hint)
         return false;
 
     if (!IS_ALIGNED(pfn, pageblock_nr_pages))
         return false;
 
     skip = get_pageblock_skip(page);
     if (!skip && !cc->no_set_skip_hint)
         set_pageblock_skip(page);
 
     return skip;
 }
 
 static void update_cached_migrate(struct compact_control *cc, unsigned long pfn)
 {
     struct zone *zone = cc->zone;
 
     pfn = pageblock_end_pfn(pfn);
 
     /* Set for isolation rather than compaction */
     if (cc->no_set_skip_hint)
         return;
 
     if (pfn > zone->compact_cached_migrate_pfn[0])
         zone->compact_cached_migrate_pfn[0] = pfn;
     if (cc->mode != MIGRATE_ASYNC &&
         pfn > zone->compact_cached_migrate_pfn[1])
         zone->compact_cached_migrate_pfn[1] = pfn;
 }
 
 /*
  * If no pages were isolated then mark this pageblock to be skipped in the
  * future. The information is later cleared by __reset_isolation_suitable().
  */
 static void update_pageblock_skip(struct compact_control *cc,
             struct page *page, unsigned long pfn)
 {
     struct zone *zone = cc->zone;
 
     if (cc->no_set_skip_hint)
         return;
 
     if (!page)
         return;
 
     set_pageblock_skip(page);
 
     /* Update where async and sync compaction should restart */
     if (pfn < zone->compact_cached_free_pfn)
         zone->compact_cached_free_pfn = pfn;
 }
 #else
 static inline bool isolation_suitable(struct compact_control *cc,
                     struct page *page)
 {
     return true;
 }
 
 static inline bool pageblock_skip_persistent(struct page *page)
 {
     return false;
 }
 
 static inline void update_pageblock_skip(struct compact_control *cc,
             struct page *page, unsigned long pfn)
 {
 }
 
 static void update_cached_migrate(struct compact_control *cc, unsigned long pfn)
 {
 }
 
 static bool test_and_set_skip(struct compact_control *cc, struct page *page,
                             unsigned long pfn)
 {
     return false;
 }
 #endif /* CONFIG_COMPACTION */
 
 /*
  * Compaction requires the taking of some coarse locks that are potentially
  * very heavily contended. For async compaction, trylock and record if the
  * lock is contended. The lock will still be acquired but compaction will
  * abort when the current block is finished regardless of success rate.
  * Sync compaction acquires the lock.
  *
  * Always returns true which makes it easier to track lock state in callers.
  */
 static bool compact_lock_irqsave(spinlock_t *lock, unsigned long *flags,
                         struct compact_control *cc)
     __acquires(lock)
 {
     /* Track if the lock is contended in async mode */
     if (cc->mode == MIGRATE_ASYNC && !cc->contended) {
         if (spin_trylock_irqsave(lock, *flags))
             return true;
 
         cc->contended = true;
     }
 
     spin_lock_irqsave(lock, *flags);
     return true;
 }
 
 /*
  * Compaction requires the taking of some coarse locks that are potentially
  * very heavily contended. The lock should be periodically unlocked to avoid
  * having disabled IRQs for a long time, even when there is nobody waiting on
  * the lock. It might also be that allowing the IRQs will result in
  * need_resched() becoming true. If scheduling is needed, compaction schedules.
  * Either compaction type will also abort if a fatal signal is pending.
  * In either case if the lock was locked, it is dropped and not regained.
  *
  * Returns true if compaction should abort due to fatal signal pending.
  * Returns false when compaction can continue.
  */
 static bool compact_unlock_should_abort(spinlock_t *lock,
         unsigned long flags, bool *locked, struct compact_control *cc)
 {
     if (*locked) {
         spin_unlock_irqrestore(lock, flags);
         *locked = false;
     }
 
     if (fatal_signal_pending(current)) {
         cc->contended = true;
         return true;
     }
 
     cond_resched();
 
     return false;
 }
 
 /*
  * Isolate free pages onto a private freelist. If @strict is true, will abort
  * returning 0 on any invalid PFNs or non-free pages inside of the pageblock
  * (even though it may still end up isolating some pages).
  */
 static unsigned long isolate_freepages_block(struct compact_control *cc,
                 unsigned long *start_pfn,
                 unsigned long end_pfn,
                 struct list_head *freelist,
                 unsigned int stride,
                 bool strict)
 {
     int nr_scanned = 0, total_isolated = 0;
     struct page *cursor;
     unsigned long flags = 0;
     bool locked = false;
     unsigned long blockpfn = *start_pfn;
     unsigned int order;
 
     /* Strict mode is for isolation, speed is secondary */
     if (strict)
         stride = 1;
 
     cursor = pfn_to_page(blockpfn);
 
     /* Isolate free pages. */
     for (; blockpfn < end_pfn; blockpfn += stride, cursor += stride) {
         int isolated;
         struct page *page = cursor;
 
         /*
          * Periodically drop the lock (if held) regardless of its
          * contention, to give chance to IRQs. Abort if fatal signal
          * pending.
          */
         if (!(blockpfn % COMPACT_CLUSTER_MAX)
             && compact_unlock_should_abort(&cc->zone->lock, flags,
                                 &locked, cc))
             break;
 
         nr_scanned++;
 
         /*
          * For compound pages such as THP and hugetlbfs, we can save
          * potentially a lot of iterations if we skip them at once.
          * The check is racy, but we can consider only valid values
          * and the only danger is skipping too much.
          */
         if (PageCompound(page)) {
             const unsigned int order = compound_order(page);
 
             if (likely(order < MAX_ORDER)) {
                 blockpfn += (1UL << order) - 1;
                 cursor += (1UL << order) - 1;
             }
             goto isolate_fail;
         }
 
         if (!PageBuddy(page))
             goto isolate_fail;
 
         /* If we already hold the lock, we can skip some rechecking. */
         if (!locked) {
             locked = compact_lock_irqsave(&cc->zone->lock,
                                 &flags, cc);
 
             /* Recheck this is a buddy page under lock */
             if (!PageBuddy(page))
                 goto isolate_fail;
         }
 
         /* Found a free page, will break it into order-0 pages */
         order = buddy_order(page);
         isolated = __isolate_free_page(page, order);
         if (!isolated)
             break;
         set_page_private(page, order);
 
         nr_scanned += isolated - 1;
         total_isolated += isolated;
         cc->nr_freepages += isolated;
         list_add_tail(&page->lru, freelist);
 
         if (!strict && cc->nr_migratepages <= cc->nr_freepages) {
             blockpfn += isolated;
             break;
         }
         /* Advance to the end of split page */
         blockpfn += isolated - 1;
         cursor += isolated - 1;
         continue;
 
 isolate_fail:
         if (strict)
             break;
         else
             continue;
 
     }
 
     if (locked)
         spin_unlock_irqrestore(&cc->zone->lock, flags);
 
     /*
      * There is a tiny chance that we have read bogus compound_order(),
      * so be careful to not go outside of the pageblock.
      */
     if (unlikely(blockpfn > end_pfn))
         blockpfn = end_pfn;
 
     trace_mm_compaction_isolate_freepages(*start_pfn, blockpfn,
                     nr_scanned, total_isolated);
 
     /* Record how far we have got within the block */
     *start_pfn = blockpfn;
 
     /*
      * If strict isolation is requested by CMA then check that all the
      * pages requested were isolated. If there were any failures, 0 is
      * returned and CMA will fail.
      */
     if (strict && blockpfn < end_pfn)
         total_isolated = 0;
 
     cc->total_free_scanned += nr_scanned;
     if (total_isolated)
         count_compact_events(COMPACTISOLATED, total_isolated);
     return total_isolated;
 }
 
 /**
  * isolate_freepages_range() - isolate free pages.
  * @cc:        Compaction control structure.
  * @start_pfn: The first PFN to start isolating.
  * @end_pfn:   The one-past-last PFN.
  *
  * Non-free pages, invalid PFNs, or zone boundaries within the
  * [start_pfn, end_pfn) range are considered errors, cause function to
  * undo its actions and return zero.
  *
  * Otherwise, function returns one-past-the-last PFN of isolated page
  * (which may be greater then end_pfn if end fell in a middle of
  * a free page).
  */
 unsigned long
 isolate_freepages_range(struct compact_control *cc,
             unsigned long start_pfn, unsigned long end_pfn)
 {
     unsigned long isolated, pfn, block_start_pfn, block_end_pfn;
     LIST_HEAD(freelist);
 
     pfn = start_pfn;
     block_start_pfn = pageblock_start_pfn(pfn);
     if (block_start_pfn < cc->zone->zone_start_pfn)
         block_start_pfn = cc->zone->zone_start_pfn;
     block_end_pfn = pageblock_end_pfn(pfn);
 
     for (; pfn < end_pfn; pfn += isolated,
                 block_start_pfn = block_end_pfn,
                 block_end_pfn += pageblock_nr_pages) {
         /* Protect pfn from changing by isolate_freepages_block */
         unsigned long isolate_start_pfn = pfn;
 
         block_end_pfn = min(block_end_pfn, end_pfn);
 
         /*
          * pfn could pass the block_end_pfn if isolated freepage
          * is more than pageblock order. In this case, we adjust
          * scanning range to right one.
          */
         if (pfn >= block_end_pfn) {
             block_start_pfn = pageblock_start_pfn(pfn);
             block_end_pfn = pageblock_end_pfn(pfn);
             block_end_pfn = min(block_end_pfn, end_pfn);
         }
 
         if (!pageblock_pfn_to_page(block_start_pfn,
                     block_end_pfn, cc->zone))
             break;
 
         isolated = isolate_freepages_block(cc, &isolate_start_pfn,
                     block_end_pfn, &freelist, 0, true);
 
         /*
          * In strict mode, isolate_freepages_block() returns 0 if
          * there are any holes in the block (ie. invalid PFNs or
          * non-free pages).
          */
         if (!isolated)
             break;
 
         /*
          * If we managed to isolate pages, it is always (1 << n) *
          * pageblock_nr_pages for some non-negative n.  (Max order
          * page may span two pageblocks).
          */
     }
 
     /* __isolate_free_page() does not map the pages */
     split_map_pages(&freelist);
 
     if (pfn < end_pfn) {
         /* Loop terminated early, cleanup. */
         release_freepages(&freelist);
         return 0;
     }
 
     /* We don't use freelists for anything. */
     return pfn;
 }
 
 /* Similar to reclaim, but different enough that they don't share logic */
 static bool too_many_isolated(pg_data_t *pgdat)
 {
     bool too_many;
 
     unsigned long active, inactive, isolated;
 
     inactive = node_page_state(pgdat, NR_INACTIVE_FILE) +
             node_page_state(pgdat, NR_INACTIVE_ANON);
     active = node_page_state(pgdat, NR_ACTIVE_FILE) +
             node_page_state(pgdat, NR_ACTIVE_ANON);
     isolated = node_page_state(pgdat, NR_ISOLATED_FILE) +
             node_page_state(pgdat, NR_ISOLATED_ANON);
 
     too_many = isolated > (inactive + active) / 2;
     if (!too_many)
         wake_throttle_isolated(pgdat);
 
     return too_many;
 }
 
 /**
  * isolate_migratepages_block() - isolate all migrate-able pages within
  *                a single pageblock
  * @cc:     Compaction control structure.
  * @low_pfn:    The first PFN to isolate
  * @end_pfn:    The one-past-the-last PFN to isolate, within same pageblock
  * @mode:   Isolation mode to be used.
  *
  * Isolate all pages that can be migrated from the range specified by
  * [low_pfn, end_pfn). The range is expected to be within same pageblock.
  * Returns errno, like -EAGAIN or -EINTR in case e.g signal pending or congestion,
  * -ENOMEM in case we could not allocate a page, or 0.
  * cc->migrate_pfn will contain the next pfn to scan.
  *
  * The pages are isolated on cc->migratepages list (not required to be empty),
  * and cc->nr_migratepages is updated accordingly.
  */
 static int
 isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
             unsigned long end_pfn, isolate_mode_t mode)
 {
     pg_data_t *pgdat = cc->zone->zone_pgdat;
     unsigned long nr_scanned = 0, nr_isolated = 0;
     struct lruvec *lruvec;
     unsigned long flags = 0;
     struct lruvec *locked = NULL;
     struct page *page = NULL, *valid_page = NULL;
     struct address_space *mapping;
     unsigned long start_pfn = low_pfn;
     bool skip_on_failure = false;
     unsigned long next_skip_pfn = 0;
     bool skip_updated = false;
     int ret = 0;
 
     cc->migrate_pfn = low_pfn;
 
     /*
      * Ensure that there are not too many pages isolated from the LRU
      * list by either parallel reclaimers or compaction. If there are,
      * delay for some time until fewer pages are isolated
      */
     while (unlikely(too_many_isolated(pgdat))) {
         /* stop isolation if there are still pages not migrated */
         if (cc->nr_migratepages)
             return -EAGAIN;
 
         /* async migration should just abort */
         if (cc->mode == MIGRATE_ASYNC)
             return -EAGAIN;
 
         reclaim_throttle(pgdat, VMSCAN_THROTTLE_ISOLATED);
 
         if (fatal_signal_pending(current))
             return -EINTR;
     }
 
     cond_resched();
 
     if (cc->direct_compaction && (cc->mode == MIGRATE_ASYNC)) {
         skip_on_failure = true;
         next_skip_pfn = block_end_pfn(low_pfn, cc->order);
     }
 
     /* Time to isolate some pages for migration */
     for (; low_pfn < end_pfn; low_pfn++) {
 
         if (skip_on_failure && low_pfn >= next_skip_pfn) {
             /*
              * We have isolated all migration candidates in the
              * previous order-aligned block, and did not skip it due
              * to failure. We should migrate the pages now and
              * hopefully succeed compaction.
              */
             if (nr_isolated)
                 break;
 
             /*
              * We failed to isolate in the previous order-aligned
              * block. Set the new boundary to the end of the
              * current block. Note we can't simply increase
              * next_skip_pfn by 1 << order, as low_pfn might have
              * been incremented by a higher number due to skipping
              * a compound or a high-order buddy page in the
              * previous loop iteration.
              */
             next_skip_pfn = block_end_pfn(low_pfn, cc->order);
         }
 
         /*
          * Periodically drop the lock (if held) regardless of its
          * contention, to give chance to IRQs. Abort completely if
          * a fatal signal is pending.
          */
         if (!(low_pfn % COMPACT_CLUSTER_MAX)) {
             if (locked) {
                 unlock_page_lruvec_irqrestore(locked, flags);
                 locked = NULL;
             }
 
             if (fatal_signal_pending(current)) {
                 cc->contended = true;
                 ret = -EINTR;
 
                 goto fatal_pending;
             }
 
             cond_resched();
         }
 
         nr_scanned++;
 
         page = pfn_to_page(low_pfn);
 
         /*
          * Check if the pageblock has already been marked skipped.
          * Only the aligned PFN is checked as the caller isolates
          * COMPACT_CLUSTER_MAX at a time so the second call must
          * not falsely conclude that the block should be skipped.
          */
         if (!valid_page && IS_ALIGNED(low_pfn, pageblock_nr_pages)) {
             if (!isolation_suitable(cc, page)) {
                 low_pfn = end_pfn;
                 page = NULL;
                 goto isolate_abort;
             }
             valid_page = page;
         }
 
         if (PageHuge(page) && cc->alloc_contig) {
             ret = isolate_or_dissolve_huge_page(page, &cc->migratepages);
 
             /*
              * Fail isolation in case isolate_or_dissolve_huge_page()
              * reports an error. In case of -ENOMEM, abort right away.
              */
             if (ret < 0) {
                  /* Do not report -EBUSY down the chain */
                 if (ret == -EBUSY)
                     ret = 0;
                 low_pfn += compound_nr(page) - 1;
                 goto isolate_fail;
             }
 
             if (PageHuge(page)) {
                 /*
                  * Hugepage was successfully isolated and placed
                  * on the cc->migratepages list.
                  */
                 low_pfn += compound_nr(page) - 1;
                 goto isolate_success_no_list;
             }
 
             /*
              * Ok, the hugepage was dissolved. Now these pages are
              * Buddy and cannot be re-allocated because they are
              * isolated. Fall-through as the check below handles
              * Buddy pages.
              */
         }
 
         /*
          * Skip if free. We read page order here without zone lock
          * which is generally unsafe, but the race window is small and
          * the worst thing that can happen is that we skip some
          * potential isolation targets.
          */
         if (PageBuddy(page)) {
             unsigned long freepage_order = buddy_order_unsafe(page);
 
             /*
              * Without lock, we cannot be sure that what we got is
              * a valid page order. Consider only values in the
              * valid order range to prevent low_pfn overflow.
              */
             if (freepage_order > 0 && freepage_order < MAX_ORDER)
                 low_pfn += (1UL << freepage_order) - 1;
             continue;
         }
 
         /*
          * Regardless of being on LRU, compound pages such as THP and
          * hugetlbfs are not to be compacted unless we are attempting
          * an allocation much larger than the huge page size (eg CMA).
          * We can potentially save a lot of iterations if we skip them
          * at once. The check is racy, but we can consider only valid
          * values and the only danger is skipping too much.
          */
         if (PageCompound(page) && !cc->alloc_contig) {
             const unsigned int order = compound_order(page);
 
             if (likely(order < MAX_ORDER))
                 low_pfn += (1UL << order) - 1;
             goto isolate_fail;
         }
 
         /*
          * Check may be lockless but that's ok as we recheck later.
          * It's possible to migrate LRU and non-lru movable pages.
          * Skip any other type of page
          */
         if (!PageLRU(page)) {
             /*
              * __PageMovable can return false positive so we need
              * to verify it under page_lock.
              */
             if (unlikely(__PageMovable(page)) &&
                     !PageIsolated(page)) {
                 if (locked) {
                     unlock_page_lruvec_irqrestore(locked, flags);
                     locked = NULL;
                 }
 
                 if (!isolate_movable_page(page, mode))
                     goto isolate_success;
             }
 
             goto isolate_fail;
         }
 
         /*
          * Migration will fail if an anonymous page is pinned in memory,
          * so avoid taking lru_lock and isolating it unnecessarily in an
          * admittedly racy check.
          */
         mapping = page_mapping(page);
         if (!mapping && page_count(page) > page_mapcount(page))
             goto isolate_fail;
 
         /*
          * Only allow to migrate anonymous pages in GFP_NOFS context
          * because those do not depend on fs locks.
          */
         if (!(cc->gfp_mask & __GFP_FS) && mapping)
             goto isolate_fail;
 
         /*
          * Be careful not to clear PageLRU until after we're
          * sure the page is not being freed elsewhere -- the
          * page release code relies on it.
          */
         if (unlikely(!get_page_unless_zero(page)))
             goto isolate_fail;
 
         /* Only take pages on LRU: a check now makes later tests safe */
         if (!PageLRU(page))
             goto isolate_fail_put;
 
         /* Compaction might skip unevictable pages but CMA takes them */
         if (!(mode & ISOLATE_UNEVICTABLE) && PageUnevictable(page))
             goto isolate_fail_put;
 
         /*
          * To minimise LRU disruption, the caller can indicate with
          * ISOLATE_ASYNC_MIGRATE that it only wants to isolate pages
          * it will be able to migrate without blocking - clean pages
          * for the most part.  PageWriteback would require blocking.
          */
         if ((mode & ISOLATE_ASYNC_MIGRATE) && PageWriteback(page))
             goto isolate_fail_put;
 
         if ((mode & ISOLATE_ASYNC_MIGRATE) && PageDirty(page)) {
             bool migrate_dirty;
 
             /*
              * Only pages without mappings or that have a
              * ->migrate_folio callback are possible to migrate
              * without blocking. However, we can be racing with
              * truncation so it's necessary to lock the page
              * to stabilise the mapping as truncation holds
              * the page lock until after the page is removed
              * from the page cache.
              */
             if (!trylock_page(page))
                 goto isolate_fail_put;
 
             mapping = page_mapping(page);
             migrate_dirty = !mapping ||
                     mapping->a_ops->migrate_folio;
             unlock_page(page);
             if (!migrate_dirty)
                 goto isolate_fail_put;
         }
 
         /* Try isolate the page */
         if (!TestClearPageLRU(page))
             goto isolate_fail_put;
 
         lruvec = folio_lruvec(page_folio(page));
 
         /* If we already hold the lock, we can skip some rechecking */
         if (lruvec != locked) {
             if (locked)
                 unlock_page_lruvec_irqrestore(locked, flags);
 
             compact_lock_irqsave(&lruvec->lru_lock, &flags, cc);
             locked = lruvec;
 
             lruvec_memcg_debug(lruvec, page_folio(page));
 
             /* Try get exclusive access under lock */
             if (!skip_updated) {
                 skip_updated = true;
                 if (test_and_set_skip(cc, page, low_pfn))
                     goto isolate_abort;
             }
 
             /*
              * Page become compound since the non-locked check,
              * and it's on LRU. It can only be a THP so the order
              * is safe to read and it's 0 for tail pages.
              */
             if (unlikely(PageCompound(page) && !cc->alloc_contig)) {
                 low_pfn += compound_nr(page) - 1;
                 SetPageLRU(page);
                 goto isolate_fail_put;
             }
         }
 
         /* The whole page is taken off the LRU; skip the tail pages. */
         if (PageCompound(page))
             low_pfn += compound_nr(page) - 1;
 
         /* Successfully isolated */
         del_page_from_lru_list(page, lruvec);
         mod_node_page_state(page_pgdat(page),
                 NR_ISOLATED_ANON + page_is_file_lru(page),
                 thp_nr_pages(page));
 
 isolate_success:
         list_add(&page->lru, &cc->migratepages);
 isolate_success_no_list:
         cc->nr_migratepages += compound_nr(page);
         nr_isolated += compound_nr(page);
         nr_scanned += compound_nr(page) - 1;
 
         /*
          * Avoid isolating too much unless this block is being
          * rescanned (e.g. dirty/writeback pages, parallel allocation)
          * or a lock is contended. For contention, isolate quickly to
          * potentially remove one source of contention.
          */
         if (cc->nr_migratepages >= COMPACT_CLUSTER_MAX &&
             !cc->rescan && !cc->contended) {
             ++low_pfn;
             break;
         }
 
         continue;
 
 isolate_fail_put:
         /* Avoid potential deadlock in freeing page under lru_lock */
         if (locked) {
             unlock_page_lruvec_irqrestore(locked, flags);
             locked = NULL;
         }
         put_page(page);
 
 isolate_fail:
         if (!skip_on_failure && ret != -ENOMEM)
             continue;
 
         /*
          * We have isolated some pages, but then failed. Release them
          * instead of migrating, as we cannot form the cc->order buddy
          * page anyway.
          */
         if (nr_isolated) {
             if (locked) {
                 unlock_page_lruvec_irqrestore(locked, flags);
                 locked = NULL;
             }
             putback_movable_pages(&cc->migratepages);
             cc->nr_migratepages = 0;
             nr_isolated = 0;
         }
 
         if (low_pfn < next_skip_pfn) {
             low_pfn = next_skip_pfn - 1;
             /*
              * The check near the loop beginning would have updated
              * next_skip_pfn too, but this is a bit simpler.
              */
             next_skip_pfn += 1UL << cc->order;
         }
 
         if (ret == -ENOMEM)
             break;
     }
 
     /*
      * The PageBuddy() check could have potentially brought us outside
      * the range to be scanned.
      */
     if (unlikely(low_pfn > end_pfn))
         low_pfn = end_pfn;
 
     page = NULL;
 
 isolate_abort:
     if (locked)
         unlock_page_lruvec_irqrestore(locked, flags);
     if (page) {
         SetPageLRU(page);
         put_page(page);
     }
 
     /*
      * Updated the cached scanner pfn once the pageblock has been scanned
      * Pages will either be migrated in which case there is no point
      * scanning in the near future or migration failed in which case the
      * failure reason may persist. The block is marked for skipping if
      * there were no pages isolated in the block or if the block is
      * rescanned twice in a row.
      */
     if (low_pfn == end_pfn && (!nr_isolated || cc->rescan)) {
         if (valid_page && !skip_updated)
             set_pageblock_skip(valid_page);
         update_cached_migrate(cc, low_pfn);
     }
 
     trace_mm_compaction_isolate_migratepages(start_pfn, low_pfn,
                         nr_scanned, nr_isolated);
 
 fatal_pending:
     cc->total_migrate_scanned += nr_scanned;
     if (nr_isolated)
         count_compact_events(COMPACTISOLATED, nr_isolated);
 
     cc->migrate_pfn = low_pfn;
 
     return ret;
 }
 
 /**
  * isolate_migratepages_range() - isolate migrate-able pages in a PFN range
  * @cc:        Compaction control structure.
  * @start_pfn: The first PFN to start isolating.
  * @end_pfn:   The one-past-last PFN.
  *
  * Returns -EAGAIN when contented, -EINTR in case of a signal pending, -ENOMEM
  * in case we could not allocate a page, or 0.
  */
 int
 isolate_migratepages_range(struct compact_control *cc, unsigned long start_pfn,
                             unsigned long end_pfn)
 {
     unsigned long pfn, block_start_pfn, block_end_pfn;
     int ret = 0;
 
     /* Scan block by block. First and last block may be incomplete */
     pfn = start_pfn;
     block_start_pfn = pageblock_start_pfn(pfn);
     if (block_start_pfn < cc->zone->zone_start_pfn)
         block_start_pfn = cc->zone->zone_start_pfn;
     block_end_pfn = pageblock_end_pfn(pfn);
 
     for (; pfn < end_pfn; pfn = block_end_pfn,
                 block_start_pfn = block_end_pfn,
                 block_end_pfn += pageblock_nr_pages) {
 
         block_end_pfn = min(block_end_pfn, end_pfn);
 
         if (!pageblock_pfn_to_page(block_start_pfn,
                     block_end_pfn, cc->zone))
             continue;
 
         ret = isolate_migratepages_block(cc, pfn, block_end_pfn,
                          ISOLATE_UNEVICTABLE);
 
         if (ret)
             break;
 
         if (cc->nr_migratepages >= COMPACT_CLUSTER_MAX)
             break;
     }
 
     return ret;
 }
 
 #endif /* CONFIG_COMPACTION || CONFIG_CMA */
 #ifdef CONFIG_COMPACTION
 
 static bool suitable_migration_source(struct compact_control *cc,
                             struct page *page)
 {
     int block_mt;
 
     if (pageblock_skip_persistent(page))
         return false;
 
     if ((cc->mode != MIGRATE_ASYNC) || !cc->direct_compaction)
         return true;
 
     block_mt = get_pageblock_migratetype(page);
 
     if (cc->migratetype == MIGRATE_MOVABLE)
         return is_migrate_movable(block_mt);
     else
         return block_mt == cc->migratetype;
 }
 
 /* Returns true if the page is within a block suitable for migration to */
 static bool suitable_migration_target(struct compact_control *cc,
                             struct page *page)
 {
     /* If the page is a large free page, then disallow migration */
     if (PageBuddy(page)) {
         /*
          * We are checking page_order without zone->lock taken. But
          * the only small danger is that we skip a potentially suitable
          * pageblock, so it's not worth to check order for valid range.
          */
         if (buddy_order_unsafe(page) >= pageblock_order)
             return false;
     }
 
     if (cc->ignore_block_suitable)
         return true;
 
     /* If the block is MIGRATE_MOVABLE or MIGRATE_CMA, allow migration */
     if (is_migrate_movable(get_pageblock_migratetype(page)))
         return true;
 
     /* Otherwise skip the block */
     return false;
 }
 
 static inline unsigned int
 freelist_scan_limit(struct compact_control *cc)
 {
     unsigned short shift = BITS_PER_LONG - 1;
 
     return (COMPACT_CLUSTER_MAX >> min(shift, cc->fast_search_fail)) + 1;
 }
 
 /*
  * Test whether the free scanner has reached the same or lower pageblock than
  * the migration scanner, and compaction should thus terminate.
  */
 static inline bool compact_scanners_met(struct compact_control *cc)
 {
     return (cc->free_pfn >> pageblock_order)
         <= (cc->migrate_pfn >> pageblock_order);
 }
 
 /*
  * Used when scanning for a suitable migration target which scans freelists
  * in reverse. Reorders the list such as the unscanned pages are scanned
  * first on the next iteration of the free scanner
  */
 static void
 move_freelist_head(struct list_head *freelist, struct page *freepage)
 {
     LIST_HEAD(sublist);
 
     if (!list_is_last(freelist, &freepage->lru)) {
         list_cut_before(&sublist, freelist, &freepage->lru);
         list_splice_tail(&sublist, freelist);
     }
 }
 
 /*
  * Similar to move_freelist_head except used by the migration scanner
  * when scanning forward. It's possible for these list operations to
  * move against each other if they search the free list exactly in
  * lockstep.
  */
 static void
 move_freelist_tail(struct list_head *freelist, struct page *freepage)
 {
     LIST_HEAD(sublist);
 
     if (!list_is_first(freelist, &freepage->lru)) {
         list_cut_position(&sublist, freelist, &freepage->lru);
         list_splice_tail(&sublist, freelist);
     }
 }
 
 static void
 fast_isolate_around(struct compact_control *cc, unsigned long pfn, unsigned long nr_isolated)
 {
     unsigned long start_pfn, end_pfn;
     struct page *page;
 
     /* Do not search around if there are enough pages already */
     if (cc->nr_freepages >= cc->nr_migratepages)
         return;
 
     /* Minimise scanning during async compaction */
     if (cc->direct_compaction && cc->mode == MIGRATE_ASYNC)
         return;
 
     /* Pageblock boundaries */
     start_pfn = max(pageblock_start_pfn(pfn), cc->zone->zone_start_pfn);
     end_pfn = min(pageblock_end_pfn(pfn), zone_end_pfn(cc->zone));
 
     page = pageblock_pfn_to_page(start_pfn, end_pfn, cc->zone);
     if (!page)
         return;
 
     /* Scan before */
     if (start_pfn != pfn) {
         isolate_freepages_block(cc, &start_pfn, pfn, &cc->freepages, 1, false);
         if (cc->nr_freepages >= cc->nr_migratepages)
             return;
     }
 
     /* Scan after */
     start_pfn = pfn + nr_isolated;
     if (start_pfn < end_pfn)
         isolate_freepages_block(cc, &start_pfn, end_pfn, &cc->freepages, 1, false);
 
     /* Skip this pageblock in the future as it's full or nearly full */
     if (cc->nr_freepages < cc->nr_migratepages)
         set_pageblock_skip(page);
 }
 
 /* Search orders in round-robin fashion */
 static int next_search_order(struct compact_control *cc, int order)
 {
     order--;
     if (order < 0)
         order = cc->order - 1;
 
     /* Search wrapped around? */
     if (order == cc->search_order) {
         cc->search_order--;
         if (cc->search_order < 0)
             cc->search_order = cc->order - 1;
         return -1;
     }
 
     return order;
 }
 
 static unsigned long
 fast_isolate_freepages(struct compact_control *cc)
 {
     unsigned int limit = max(1U, freelist_scan_limit(cc) >> 1);
     unsigned int nr_scanned = 0;
     unsigned long low_pfn, min_pfn, highest = 0;
     unsigned long nr_isolated = 0;
     unsigned long distance;
     struct page *page = NULL;
     bool scan_start = false;
     int order;
 
     /* Full compaction passes in a negative order */
     if (cc->order <= 0)
         return cc->free_pfn;
 
     /*
      * If starting the scan, use a deeper search and use the highest
      * PFN found if a suitable one is not found.
      */
     if (cc->free_pfn >= cc->zone->compact_init_free_pfn) {
         limit = pageblock_nr_pages >> 1;
         scan_start = true;
     }
 
     /*
      * Preferred point is in the top quarter of the scan space but take
      * a pfn from the top half if the search is problematic.
      */
     distance = (cc->free_pfn - cc->migrate_pfn);
     low_pfn = pageblock_start_pfn(cc->free_pfn - (distance >> 2));
     min_pfn = pageblock_start_pfn(cc->free_pfn - (distance >> 1));
 
     if (WARN_ON_ONCE(min_pfn > low_pfn))
         low_pfn = min_pfn;
 
     /*
      * Search starts from the last successful isolation order or the next
      * order to search after a previous failure
      */
     cc->search_order = min_t(unsigned int, cc->order - 1, cc->search_order);
 
     for (order = cc->search_order;
          !page && order >= 0;
          order = next_search_order(cc, order)) {
         struct free_area *area = &cc->zone->free_area[order];
         struct list_head *freelist;
         struct page *freepage;
         unsigned long flags;
         unsigned int order_scanned = 0;
         unsigned long high_pfn = 0;
 
         if (!area->nr_free)
             continue;
 
         spin_lock_irqsave(&cc->zone->lock, flags);
         freelist = &area->free_list[MIGRATE_MOVABLE];
         list_for_each_entry_reverse(freepage, freelist, lru) {
             unsigned long pfn;
 
             order_scanned++;
             nr_scanned++;
             pfn = page_to_pfn(freepage);
 
             if (pfn >= highest)
                 highest = max(pageblock_start_pfn(pfn),
                           cc->zone->zone_start_pfn);
 
             if (pfn >= low_pfn) {
                 cc->fast_search_fail = 0;
                 cc->search_order = order;
                 page = freepage;
                 break;
             }
 
             if (pfn >= min_pfn && pfn > high_pfn) {
                 high_pfn = pfn;
 
                 /* Shorten the scan if a candidate is found */
                 limit >>= 1;
             }
 
             if (order_scanned >= limit)
                 break;
         }
 
         /* Use a minimum pfn if a preferred one was not found */
         if (!page && high_pfn) {
             page = pfn_to_page(high_pfn);
 
             /* Update freepage for the list reorder below */
             freepage = page;
         }
 
         /* Reorder to so a future search skips recent pages */
         move_freelist_head(freelist, freepage);
 
         /* Isolate the page if available */
         if (page) {
             if (__isolate_free_page(page, order)) {
                 set_page_private(page, order);
                 nr_isolated = 1 << order;
                 nr_scanned += nr_isolated - 1;
                 cc->nr_freepages += nr_isolated;
                 list_add_tail(&page->lru, &cc->freepages);
                 count_compact_events(COMPACTISOLATED, nr_isolated);
             } else {
                 /* If isolation fails, abort the search */
                 order = cc->search_order + 1;
                 page = NULL;
             }
         }
 
         spin_unlock_irqrestore(&cc->zone->lock, flags);
 
         /*
          * Smaller scan on next order so the total scan is related
          * to freelist_scan_limit.
          */
         if (order_scanned >= limit)
             limit = max(1U, limit >> 1);
     }
 
     if (!page) {
         cc->fast_search_fail++;
         if (scan_start) {
             /*
              * Use the highest PFN found above min. If one was
              * not found, be pessimistic for direct compaction
              * and use the min mark.
              */
             if (highest >= min_pfn) {
                 page = pfn_to_page(highest);
                 cc->free_pfn = highest;
             } else {
                 if (cc->direct_compaction && pfn_valid(min_pfn)) {
                     page = pageblock_pfn_to_page(min_pfn,
                         min(pageblock_end_pfn(min_pfn),
                             zone_end_pfn(cc->zone)),
                         cc->zone);
                     cc->free_pfn = min_pfn;
                 }
             }
         }
     }
 
     if (highest && highest >= cc->zone->compact_cached_free_pfn) {
         highest -= pageblock_nr_pages;
         cc->zone->compact_cached_free_pfn = highest;
     }
 
     cc->total_free_scanned += nr_scanned;
     if (!page)
         return cc->free_pfn;
 
     low_pfn = page_to_pfn(page);
     fast_isolate_around(cc, low_pfn, nr_isolated);
     return low_pfn;
 }
 
 /*
  * Based on information in the current compact_control, find blocks
  * suitable for isolating free pages from and then isolate them.
  */
 static void isolate_freepages(struct compact_control *cc)
 {
     struct zone *zone = cc->zone;
     struct page *page;
     unsigned long block_start_pfn;  /* start of current pageblock */
     unsigned long isolate_start_pfn; /* exact pfn we start at */
     unsigned long block_end_pfn;    /* end of current pageblock */
     unsigned long low_pfn;       /* lowest pfn scanner is able to scan */
     struct list_head *freelist = &cc->freepages;
     unsigned int stride;
 
     /* Try a small search of the free lists for a candidate */
     fast_isolate_freepages(cc);
     if (cc->nr_freepages)
         goto splitmap;
 
     /*
      * Initialise the free scanner. The starting point is where we last
      * successfully isolated from, zone-cached value, or the end of the
      * zone when isolating for the first time. For looping we also need
      * this pfn aligned down to the pageblock boundary, because we do
      * block_start_pfn -= pageblock_nr_pages in the for loop.
      * For ending point, take care when isolating in last pageblock of a
      * zone which ends in the middle of a pageblock.
      * The low boundary is the end of the pageblock the migration scanner
      * is using.
      */
     isolate_start_pfn = cc->free_pfn;
     block_start_pfn = pageblock_start_pfn(isolate_start_pfn);
     block_end_pfn = min(block_start_pfn + pageblock_nr_pages,
                         zone_end_pfn(zone));
     low_pfn = pageblock_end_pfn(cc->migrate_pfn);
     stride = cc->mode == MIGRATE_ASYNC ? COMPACT_CLUSTER_MAX : 1;
 
     /*
      * Isolate free pages until enough are available to migrate the
      * pages on cc->migratepages. We stop searching if the migrate
      * and free page scanners meet or enough free pages are isolated.
      */
     for (; block_start_pfn >= low_pfn;
                 block_end_pfn = block_start_pfn,
                 block_start_pfn -= pageblock_nr_pages,
                 isolate_start_pfn = block_start_pfn) {
         unsigned long nr_isolated;
 
         /*
          * This can iterate a massively long zone without finding any
          * suitable migration targets, so periodically check resched.
          */
         if (!(block_start_pfn % (COMPACT_CLUSTER_MAX * pageblock_nr_pages)))
             cond_resched();
 
         page = pageblock_pfn_to_page(block_start_pfn, block_end_pfn,
                                     zone);
         if (!page)
             continue;
 
         /* Check the block is suitable for migration */
         if (!suitable_migration_target(cc, page))
             continue;
 
         /* If isolation recently failed, do not retry */
         if (!isolation_suitable(cc, page))
             continue;
 
         /* Found a block suitable for isolating free pages from. */
         nr_isolated = isolate_freepages_block(cc, &isolate_start_pfn,
                     block_end_pfn, freelist, stride, false);
 
         /* Update the skip hint if the full pageblock was scanned */
         if (isolate_start_pfn == block_end_pfn)
             update_pageblock_skip(cc, page, block_start_pfn);
 
         /* Are enough freepages isolated? */
         if (cc->nr_freepages >= cc->nr_migratepages) {
             if (isolate_start_pfn >= block_end_pfn) {
                 /*
                  * Restart at previous pageblock if more
                  * freepages can be isolated next time.
                  */
                 isolate_start_pfn =
                     block_start_pfn - pageblock_nr_pages;
             }
             break;
         } else if (isolate_start_pfn < block_end_pfn) {
             /*
              * If isolation failed early, do not continue
              * needlessly.
              */
             break;
         }
 
         /* Adjust stride depending on isolation */
         if (nr_isolated) {
             stride = 1;
             continue;
         }
         stride = min_t(unsigned int, COMPACT_CLUSTER_MAX, stride << 1);
     }
 
     /*
      * Record where the free scanner will restart next time. Either we
      * broke from the loop and set isolate_start_pfn based on the last
      * call to isolate_freepages_block(), or we met the migration scanner
      * and the loop terminated due to isolate_start_pfn < low_pfn
      */
     cc->free_pfn = isolate_start_pfn;
 
 splitmap:
     /* __isolate_free_page() does not map the pages */
     split_map_pages(freelist);
 }
 
 /*
  * This is a migrate-callback that "allocates" freepages by taking pages
  * from the isolated freelists in the block we are migrating to.
  */
 static struct page *compaction_alloc(struct page *migratepage,
                     unsigned long data)
 {
     struct compact_control *cc = (struct compact_control *)data;
     struct page *freepage;
 
     if (list_empty(&cc->freepages)) {
         isolate_freepages(cc);
 
         if (list_empty(&cc->freepages))
             return NULL;
     }
 
     freepage = list_entry(cc->freepages.next, struct page, lru);
     list_del(&freepage->lru);
     cc->nr_freepages--;
 
     return freepage;
 }
 
 /*
  * This is a migrate-callback that "frees" freepages back to the isolated
  * freelist.  All pages on the freelist are from the same zone, so there is no
  * special handling needed for NUMA.
  */
 static void compaction_free(struct page *page, unsigned long data)
 {
     struct compact_control *cc = (struct compact_control *)data;
 
     list_add(&page->lru, &cc->freepages);
     cc->nr_freepages++;
 }
 
 /* possible outcome of isolate_migratepages */
 typedef enum {
     ISOLATE_ABORT,      /* Abort compaction now */
     ISOLATE_NONE,       /* No pages isolated, continue scanning */
     ISOLATE_SUCCESS,    /* Pages isolated, migrate */
 } isolate_migrate_t;
 
 /*
  * Allow userspace to control policy on scanning the unevictable LRU for
  * compactable pages.
  */
 #ifdef CONFIG_PREEMPT_RT
 int sysctl_compact_unevictable_allowed __read_mostly = 0;
 #else
 int sysctl_compact_unevictable_allowed __read_mostly = 1;
 #endif
 
 static inline void
 update_fast_start_pfn(struct compact_control *cc, unsigned long pfn)
 {
     if (cc->fast_start_pfn == ULONG_MAX)
         return;
 
     if (!cc->fast_start_pfn)
         cc->fast_start_pfn = pfn;
 
     cc->fast_start_pfn = min(cc->fast_start_pfn, pfn);
 }
 
 static inline unsigned long
 reinit_migrate_pfn(struct compact_control *cc)
 {
     if (!cc->fast_start_pfn || cc->fast_start_pfn == ULONG_MAX)
         return cc->migrate_pfn;
 
     cc->migrate_pfn = cc->fast_start_pfn;
     cc->fast_start_pfn = ULONG_MAX;
 
     return cc->migrate_pfn;
 }
 
 /*
  * Briefly search the free lists for a migration source that already has
  * some free pages to reduce the number of pages that need migration
  * before a pageblock is free.
  */
 static unsigned long fast_find_migrateblock(struct compact_control *cc)
 {
     unsigned int limit = freelist_scan_limit(cc);
     unsigned int nr_scanned = 0;
     unsigned long distance;
     unsigned long pfn = cc->migrate_pfn;
     unsigned long high_pfn;
     int order;
     bool found_block = false;
 
     /* Skip hints are relied on to avoid repeats on the fast search */
     if (cc->ignore_skip_hint)
         return pfn;
 
     /*
      * If the migrate_pfn is not at the start of a zone or the start
      * of a pageblock then assume this is a continuation of a previous
      * scan restarted due to COMPACT_CLUSTER_MAX.
      */
     if (pfn != cc->zone->zone_start_pfn && pfn != pageblock_start_pfn(pfn))
         return pfn;
 
     /*
      * For smaller orders, just linearly scan as the number of pages
      * to migrate should be relatively small and does not necessarily
      * justify freeing up a large block for a small allocation.
      */
     if (cc->order <= PAGE_ALLOC_COSTLY_ORDER)
         return pfn;
 
     /*
      * Only allow kcompactd and direct requests for movable pages to
      * quickly clear out a MOVABLE pageblock for allocation. This
      * reduces the risk that a large movable pageblock is freed for
      * an unmovable/reclaimable small allocation.
      */
     if (cc->direct_compaction && cc->migratetype != MIGRATE_MOVABLE)
         return pfn;
 
     /*
      * When starting the migration scanner, pick any pageblock within the
      * first half of the search space. Otherwise try and pick a pageblock
      * within the first eighth to reduce the chances that a migration
      * target later becomes a source.
      */
     distance = (cc->free_pfn - cc->migrate_pfn) >> 1;
     if (cc->migrate_pfn != cc->zone->zone_start_pfn)
         distance >>= 2;
     high_pfn = pageblock_start_pfn(cc->migrate_pfn + distance);
 
     for (order = cc->order - 1;
          order >= PAGE_ALLOC_COSTLY_ORDER && !found_block && nr_scanned < limit;
          order--) {
         struct free_area *area = &cc->zone->free_area[order];
         struct list_head *freelist;
         unsigned long flags;
         struct page *freepage;
 
         if (!area->nr_free)
             continue;
 
         spin_lock_irqsave(&cc->zone->lock, flags);
         freelist = &area->free_list[MIGRATE_MOVABLE];
         list_for_each_entry(freepage, freelist, lru) {
             unsigned long free_pfn;
 
             if (nr_scanned++ >= limit) {
                 move_freelist_tail(freelist, freepage);
                 break;
             }
 
             free_pfn = page_to_pfn(freepage);
             if (free_pfn < high_pfn) {
                 /*
                  * Avoid if skipped recently. Ideally it would
                  * move to the tail but even safe iteration of
                  * the list assumes an entry is deleted, not
                  * reordered.
                  */
                 if (get_pageblock_skip(freepage))
                     continue;
 
                 /* Reorder to so a future search skips recent pages */
                 move_freelist_tail(freelist, freepage);
 
                 update_fast_start_pfn(cc, free_pfn);
                 pfn = pageblock_start_pfn(free_pfn);
                 if (pfn < cc->zone->zone_start_pfn)
                     pfn = cc->zone->zone_start_pfn;
                 cc->fast_search_fail = 0;
                 found_block = true;
                 set_pageblock_skip(freepage);
                 break;
             }
         }
         spin_unlock_irqrestore(&cc->zone->lock, flags);
     }
 
     cc->total_migrate_scanned += nr_scanned;
 
     /*
      * If fast scanning failed then use a cached entry for a page block
      * that had free pages as the basis for starting a linear scan.
      */
     if (!found_block) {
         cc->fast_search_fail++;
         pfn = reinit_migrate_pfn(cc);
     }
     return pfn;
 }
 
 /*
  * Isolate all pages that can be migrated from the first suitable block,
  * starting at the block pointed to by the migrate scanner pfn within
  * compact_control.
  */
 static isolate_migrate_t isolate_migratepages(struct compact_control *cc)
 {
     unsigned long block_start_pfn;
     unsigned long block_end_pfn;
     unsigned long low_pfn;
     struct page *page;
     const isolate_mode_t isolate_mode =
         (sysctl_compact_unevictable_allowed ? ISOLATE_UNEVICTABLE : 0) |
         (cc->mode != MIGRATE_SYNC ? ISOLATE_ASYNC_MIGRATE : 0);
     bool fast_find_block;
 
     /*
      * Start at where we last stopped, or beginning of the zone as
      * initialized by compact_zone(). The first failure will use
      * the lowest PFN as the starting point for linear scanning.
      */
     low_pfn = fast_find_migrateblock(cc);
     block_start_pfn = pageblock_start_pfn(low_pfn);
     if (block_start_pfn < cc->zone->zone_start_pfn)
         block_start_pfn = cc->zone->zone_start_pfn;
 
     /*
      * fast_find_migrateblock marks a pageblock skipped so to avoid
      * the isolation_suitable check below, check whether the fast
      * search was successful.
      */
     fast_find_block = low_pfn != cc->migrate_pfn && !cc->fast_search_fail;
 
     /* Only scan within a pageblock boundary */
     block_end_pfn = pageblock_end_pfn(low_pfn);
 
     /*
      * Iterate over whole pageblocks until we find the first suitable.
      * Do not cross the free scanner.
      */
     for (; block_end_pfn <= cc->free_pfn;
             fast_find_block = false,
             cc->migrate_pfn = low_pfn = block_end_pfn,
             block_start_pfn = block_end_pfn,
             block_end_pfn += pageblock_nr_pages) {
 
         /*
          * This can potentially iterate a massively long zone with
          * many pageblocks unsuitable, so periodically check if we
          * need to schedule.
          */
         if (!(low_pfn % (COMPACT_CLUSTER_MAX * pageblock_nr_pages)))
             cond_resched();
 
         page = pageblock_pfn_to_page(block_start_pfn,
                         block_end_pfn, cc->zone);
         if (!page)
             continue;
 
         /*
          * If isolation recently failed, do not retry. Only check the
          * pageblock once. COMPACT_CLUSTER_MAX causes a pageblock
          * to be visited multiple times. Assume skip was checked
          * before making it "skip" so other compaction instances do
          * not scan the same block.
          */
         if (IS_ALIGNED(low_pfn, pageblock_nr_pages) &&
             !fast_find_block && !isolation_suitable(cc, page))
             continue;
 
         /*
          * For async direct compaction, only scan the pageblocks of the
          * same migratetype without huge pages. Async direct compaction
          * is optimistic to see if the minimum amount of work satisfies
          * the allocation. The cached PFN is updated as it's possible
          * that all remaining blocks between source and target are
          * unsuitable and the compaction scanners fail to meet.
          */
         if (!suitable_migration_source(cc, page)) {
             update_cached_migrate(cc, block_end_pfn);
             continue;
         }
 
         /* Perform the isolation */
         if (isolate_migratepages_block(cc, low_pfn, block_end_pfn,
                         isolate_mode))
             return ISOLATE_ABORT;
 
         /*
          * Either we isolated something and proceed with migration. Or
          * we failed and compact_zone should decide if we should
          * continue or not.
          */
         break;
     }
 
     return cc->nr_migratepages ? ISOLATE_SUCCESS : ISOLATE_NONE;
 }
 
 /*
  * order == -1 is expected when compacting via
  * /proc/sys/vm/compact_memory
  */
 static inline bool is_via_compact_memory(int order)
 {
     return order == -1;
 }
 
 static bool kswapd_is_running(pg_data_t *pgdat)
 {
     return pgdat->kswapd && task_is_running(pgdat->kswapd);
 }
 
 /*
  * A zone's fragmentation score is the external fragmentation wrt to the
  * COMPACTION_HPAGE_ORDER. It returns a value in the range [0, 100].
  */
 static unsigned int fragmentation_score_zone(struct zone *zone)
 {
     return extfrag_for_order(zone, COMPACTION_HPAGE_ORDER);
 }
 
 /*
  * A weighted zone's fragmentation score is the external fragmentation
  * wrt to the COMPACTION_HPAGE_ORDER scaled by the zone's size. It
  * returns a value in the range [0, 100].
  *
  * The scaling factor ensures that proactive compaction focuses on larger
  * zones like ZONE_NORMAL, rather than smaller, specialized zones like
  * ZONE_DMA32. For smaller zones, the score value remains close to zero,
  * and thus never exceeds the high threshold for proactive compaction.
  */
 static unsigned int fragmentation_score_zone_weighted(struct zone *zone)
 {
     unsigned long score;
 
     score = zone->present_pages * fragmentation_score_zone(zone);
     return div64_ul(score, zone->zone_pgdat->node_present_pages + 1);
 }
 
 /*
  * The per-node proactive (background) compaction process is started by its
  * corresponding kcompactd thread when the node's fragmentation score
  * exceeds the high threshold. The compaction process remains active till
  * the node's score falls below the low threshold, or one of the back-off
  * conditions is met.
  */
 static unsigned int fragmentation_score_node(pg_data_t *pgdat)
 {
     unsigned int score = 0;
     int zoneid;
 
     for (zoneid = 0; zoneid < MAX_NR_ZONES; zoneid++) {
         struct zone *zone;
 
         zone = &pgdat->node_zones[zoneid];
         score += fragmentation_score_zone_weighted(zone);
     }
 
     return score;
 }
 
 static unsigned int fragmentation_score_wmark(pg_data_t *pgdat, bool low)
 {
     unsigned int wmark_low;
 
     /*
      * Cap the low watermark to avoid excessive compaction
      * activity in case a user sets the proactiveness tunable
      * close to 100 (maximum).
      */
     wmark_low = max(100U - sysctl_compaction_proactiveness, 5U);
     return low ? wmark_low : min(wmark_low + 10, 100U);
 }
 
 static bool should_proactive_compact_node(pg_data_t *pgdat)
 {
     int wmark_high;
 
     if (!sysctl_compaction_proactiveness || kswapd_is_running(pgdat))
         return false;
 
     wmark_high = fragmentation_score_wmark(pgdat, false);
     return fragmentation_score_node(pgdat) > wmark_high;
 }
 
 static enum compact_result __compact_finished(struct compact_control *cc)
 {
     unsigned int order;
     const int migratetype = cc->migratetype;
     int ret;
 
     /* Compaction run completes if the migrate and free scanner meet */
     if (compact_scanners_met(cc)) {
         /* Let the next compaction start anew. */
         reset_cached_positions(cc->zone);
 
         /*
          * Mark that the PG_migrate_skip information should be cleared
          * by kswapd when it goes to sleep. kcompactd does not set the
          * flag itself as the decision to be clear should be directly
          * based on an allocation request.
          */
         if (cc->direct_compaction)
             cc->zone->compact_blockskip_flush = true;
 
         if (cc->whole_zone)
             return COMPACT_COMPLETE;
         else
             return COMPACT_PARTIAL_SKIPPED;
     }
 
     if (cc->proactive_compaction) {
         int score, wmark_low;
         pg_data_t *pgdat;
 
         pgdat = cc->zone->zone_pgdat;
         if (kswapd_is_running(pgdat))
             return COMPACT_PARTIAL_SKIPPED;
 
         score = fragmentation_score_zone(cc->zone);
         wmark_low = fragmentation_score_wmark(pgdat, true);
 
         if (score > wmark_low)
             ret = COMPACT_CONTINUE;
         else
             ret = COMPACT_SUCCESS;
 
         goto out;
     }
 
     if (is_via_compact_memory(cc->order))
         return COMPACT_CONTINUE;
 
     /*
      * Always finish scanning a pageblock to reduce the possibility of
      * fallbacks in the future. This is particularly important when
      * migration source is unmovable/reclaimable but it's not worth
      * special casing.
      */
     if (!IS_ALIGNED(cc->migrate_pfn, pageblock_nr_pages))
         return COMPACT_CONTINUE;
 
     /* Direct compactor: Is a suitable page free? */
     ret = COMPACT_NO_SUITABLE_PAGE;
     for (order = cc->order; order < MAX_ORDER; order++) {
         struct free_area *area = &cc->zone->free_area[order];
         bool can_steal;
 
         /* Job done if page is free of the right migratetype */
         if (!free_area_empty(area, migratetype))
             return COMPACT_SUCCESS;
 
 #ifdef CONFIG_CMA
         /* MIGRATE_MOVABLE can fallback on MIGRATE_CMA */
         if (migratetype == MIGRATE_MOVABLE &&
             !free_area_empty(area, MIGRATE_CMA))
             return COMPACT_SUCCESS;
 #endif
         /*
          * Job done if allocation would steal freepages from
          * other migratetype buddy lists.
          */
         if (find_suitable_fallback(area, order, migratetype,
                         true, &can_steal) != -1)
             /*
              * Movable pages are OK in any pageblock. If we are
              * stealing for a non-movable allocation, make sure
              * we finish compacting the current pageblock first
              * (which is assured by the above migrate_pfn align
              * check) so it is as free as possible and we won't
              * have to steal another one soon.
              */
             return COMPACT_SUCCESS;
     }
 
 out:
     if (cc->contended || fatal_signal_pending(current))
         ret = COMPACT_CONTENDED;
 
     return ret;
 }
 
 static enum compact_result compact_finished(struct compact_control *cc)
 {
     int ret;
 
     ret = __compact_finished(cc);
     trace_mm_compaction_finished(cc->zone, cc->order, ret);
     if (ret == COMPACT_NO_SUITABLE_PAGE)
         ret = COMPACT_CONTINUE;
 
     return ret;
 }
 
 static enum compact_result __compaction_suitable(struct zone *zone, int order,
                     unsigned int alloc_flags,
                     int highest_zoneidx,
                     unsigned long wmark_target)
 {
     unsigned long watermark;
 
     if (is_via_compact_memory(order))
         return COMPACT_CONTINUE;
 
     watermark = wmark_pages(zone, alloc_flags & ALLOC_WMARK_MASK);
     /*
      * If watermarks for high-order allocation are already met, there
      * should be no need for compaction at all.
      */
     if (zone_watermark_ok(zone, order, watermark, highest_zoneidx,
                                 alloc_flags))
         return COMPACT_SUCCESS;
 
     /*
      * Watermarks for order-0 must be met for compaction to be able to
      * isolate free pages for migration targets. This means that the
      * watermark and alloc_flags have to match, or be more pessimistic than
      * the check in __isolate_free_page(). We don't use the direct
      * compactor's alloc_flags, as they are not relevant for freepage
      * isolation. We however do use the direct compactor's highest_zoneidx
      * to skip over zones where lowmem reserves would prevent allocation
      * even if compaction succeeds.
      * For costly orders, we require low watermark instead of min for
      * compaction to proceed to increase its chances.
      * ALLOC_CMA is used, as pages in CMA pageblocks are considered
      * suitable migration targets
      */
     watermark = (order > PAGE_ALLOC_COSTLY_ORDER) ?
                 low_wmark_pages(zone) : min_wmark_pages(zone);
     watermark += compact_gap(order);
     if (!__zone_watermark_ok(zone, 0, watermark, highest_zoneidx,
                         ALLOC_CMA, wmark_target))
         return COMPACT_SKIPPED;
 
     return COMPACT_CONTINUE;
 }
 
 /*
  * compaction_suitable: Is this suitable to run compaction on this zone now?
  * Returns
  *   COMPACT_SKIPPED  - If there are too few free pages for compaction
  *   COMPACT_SUCCESS  - If the allocation would succeed without compaction
  *   COMPACT_CONTINUE - If compaction should run now
  */
 enum compact_result compaction_suitable(struct zone *zone, int order,
                     unsigned int alloc_flags,
                     int highest_zoneidx)
 {
     enum compact_result ret;
     int fragindex;
 
     ret = __compaction_suitable(zone, order, alloc_flags, highest_zoneidx,
                     zone_page_state(zone, NR_FREE_PAGES));
     /*
      * fragmentation index determines if allocation failures are due to
      * low memory or external fragmentation
      *
      * index of -1000 would imply allocations might succeed depending on
      * watermarks, but we already failed the high-order watermark check
      * index towards 0 implies failure is due to lack of memory
      * index towards 1000 implies failure is due to fragmentation
      *
      * Only compact if a failure would be due to fragmentation. Also
      * ignore fragindex for non-costly orders where the alternative to
      * a successful reclaim/compaction is OOM. Fragindex and the
      * vm.extfrag_threshold sysctl is meant as a heuristic to prevent
      * excessive compaction for costly orders, but it should not be at the
      * expense of system stability.
      */
     if (ret == COMPACT_CONTINUE && (order > PAGE_ALLOC_COSTLY_ORDER)) {
         fragindex = fragmentation_index(zone, order);
         if (fragindex >= 0 && fragindex <= sysctl_extfrag_threshold)
             ret = COMPACT_NOT_SUITABLE_ZONE;
     }
 
     trace_mm_compaction_suitable(zone, order, ret);
     if (ret == COMPACT_NOT_SUITABLE_ZONE)
         ret = COMPACT_SKIPPED;
 
     return ret;
 }
 
 bool compaction_zonelist_suitable(struct alloc_context *ac, int order,
         int alloc_flags)
 {
     struct zone *zone;
     struct zoneref *z;
 
     /*
      * Make sure at least one zone would pass __compaction_suitable if we continue
      * retrying the reclaim.
      */
     for_each_zone_zonelist_nodemask(zone, z, ac->zonelist,
                 ac->highest_zoneidx, ac->nodemask) {
         unsigned long available;
         enum compact_result compact_result;
 
         /*
          * Do not consider all the reclaimable memory because we do not
          * want to trash just for a single high order allocation which
          * is even not guaranteed to appear even if __compaction_suitable
          * is happy about the watermark check.
          */
         available = zone_reclaimable_pages(zone) / order;
         available += zone_page_state_snapshot(zone, NR_FREE_PAGES);
         compact_result = __compaction_suitable(zone, order, alloc_flags,
                 ac->highest_zoneidx, available);
         if (compact_result == COMPACT_CONTINUE)
             return true;
     }
 
     return false;
 }
 
 static enum compact_result
 compact_zone(struct compact_control *cc, struct capture_control *capc)
 {
     enum compact_result ret;
     unsigned long start_pfn = cc->zone->zone_start_pfn;
     unsigned long end_pfn = zone_end_pfn(cc->zone);
     unsigned long last_migrated_pfn;
     const bool sync = cc->mode != MIGRATE_ASYNC;
     bool update_cached;
     unsigned int nr_succeeded = 0;
 
     /*
      * These counters track activities during zone compaction.  Initialize
      * them before compacting a new zone.
      */
     cc->total_migrate_scanned = 0;
     cc->total_free_scanned = 0;
     cc->nr_migratepages = 0;
     cc->nr_freepages = 0;
     INIT_LIST_HEAD(&cc->freepages);
     INIT_LIST_HEAD(&cc->migratepages);
 
     cc->migratetype = gfp_migratetype(cc->gfp_mask);
     ret = compaction_suitable(cc->zone, cc->order, cc->alloc_flags,
                             cc->highest_zoneidx);
     /* Compaction is likely to fail */
     if (ret == COMPACT_SUCCESS || ret == COMPACT_SKIPPED)
         return ret;
 
     /* huh, compaction_suitable is returning something unexpected */
     VM_BUG_ON(ret != COMPACT_CONTINUE);
 
     /*
      * Clear pageblock skip if there were failures recently and compaction
      * is about to be retried after being deferred.
      */
     if (compaction_restarting(cc->zone, cc->order))
         __reset_isolation_suitable(cc->zone);
 
     /*
      * Setup to move all movable pages to the end of the zone. Used cached
      * information on where the scanners should start (unless we explicitly
      * want to compact the whole zone), but check that it is initialised
      * by ensuring the values are within zone boundaries.
      */
     cc->fast_start_pfn = 0;
     if (cc->whole_zone) {
         cc->migrate_pfn = start_pfn;
         cc->free_pfn = pageblock_start_pfn(end_pfn - 1);
     } else {
         cc->migrate_pfn = cc->zone->compact_cached_migrate_pfn[sync];
         cc->free_pfn = cc->zone->compact_cached_free_pfn;
         if (cc->free_pfn < start_pfn || cc->free_pfn >= end_pfn) {
             cc->free_pfn = pageblock_start_pfn(end_pfn - 1);
             cc->zone->compact_cached_free_pfn = cc->free_pfn;
         }
         if (cc->migrate_pfn < start_pfn || cc->migrate_pfn >= end_pfn) {
             cc->migrate_pfn = start_pfn;
             cc->zone->compact_cached_migrate_pfn[0] = cc->migrate_pfn;
             cc->zone->compact_cached_migrate_pfn[1] = cc->migrate_pfn;
         }
 
         if (cc->migrate_pfn <= cc->zone->compact_init_migrate_pfn)
             cc->whole_zone = true;
     }
 
     last_migrated_pfn = 0;
 
     /*
      * Migrate has separate cached PFNs for ASYNC and SYNC* migration on
      * the basis that some migrations will fail in ASYNC mode. However,
      * if the cached PFNs match and pageblocks are skipped due to having
      * no isolation candidates, then the sync state does not matter.
      * Until a pageblock with isolation candidates is found, keep the
      * cached PFNs in sync to avoid revisiting the same blocks.
      */
     update_cached = !sync &&
         cc->zone->compact_cached_migrate_pfn[0] == cc->zone->compact_cached_migrate_pfn[1];
 
     trace_mm_compaction_begin(cc, start_pfn, end_pfn, sync);
 
     /* lru_add_drain_all could be expensive with involving other CPUs */
     lru_add_drain();
 
     while ((ret = compact_finished(cc)) == COMPACT_CONTINUE) {
         int err;
         unsigned long iteration_start_pfn = cc->migrate_pfn;
 
         /*
          * Avoid multiple rescans which can happen if a page cannot be
          * isolated (dirty/writeback in async mode) or if the migrated
          * pages are being allocated before the pageblock is cleared.
          * The first rescan will capture the entire pageblock for
          * migration. If it fails, it'll be marked skip and scanning
          * will proceed as normal.
          */
         cc->rescan = false;
         if (pageblock_start_pfn(last_migrated_pfn) ==
             pageblock_start_pfn(iteration_start_pfn)) {
             cc->rescan = true;
         }
 
         switch (isolate_migratepages(cc)) {
         case ISOLATE_ABORT:
             ret = COMPACT_CONTENDED;
             putback_movable_pages(&cc->migratepages);
             cc->nr_migratepages = 0;
             goto out;
         case ISOLATE_NONE:
             if (update_cached) {
                 cc->zone->compact_cached_migrate_pfn[1] =
                     cc->zone->compact_cached_migrate_pfn[0];
             }
 
             /*
              * We haven't isolated and migrated anything, but
              * there might still be unflushed migrations from
              * previous cc->order aligned block.
              */
             goto check_drain;
         case ISOLATE_SUCCESS:
             update_cached = false;
             last_migrated_pfn = iteration_start_pfn;
         }
 
         err = migrate_pages(&cc->migratepages, compaction_alloc,
                 compaction_free, (unsigned long)cc, cc->mode,
                 MR_COMPACTION, &nr_succeeded);
 
         trace_mm_compaction_migratepages(cc, nr_succeeded);
 
         /* All pages were either migrated or will be released */
         cc->nr_migratepages = 0;
         if (err) {
             putback_movable_pages(&cc->migratepages);
             /*
              * migrate_pages() may return -ENOMEM when scanners meet
              * and we want compact_finished() to detect it
              */
             if (err == -ENOMEM && !compact_scanners_met(cc)) {
                 ret = COMPACT_CONTENDED;
                 goto out;
             }
             /*
              * We failed to migrate at least one page in the current
              * order-aligned block, so skip the rest of it.
              */
             if (cc->direct_compaction &&
                         (cc->mode == MIGRATE_ASYNC)) {
                 cc->migrate_pfn = block_end_pfn(
                         cc->migrate_pfn - 1, cc->order);
                 /* Draining pcplists is useless in this case */
                 last_migrated_pfn = 0;
             }
         }
 
 check_drain:
         /*
          * Has the migration scanner moved away from the previous
          * cc->order aligned block where we migrated from? If yes,
          * flush the pages that were freed, so that they can merge and
          * compact_finished() can detect immediately if allocation
          * would succeed.
          */
         if (cc->order > 0 && last_migrated_pfn) {
             unsigned long current_block_start =
                 block_start_pfn(cc->migrate_pfn, cc->order);
 
             if (last_migrated_pfn < current_block_start) {
                 lru_add_drain_cpu_zone(cc->zone);
                 /* No more flushing until we migrate again */
                 last_migrated_pfn = 0;
             }
         }
 
         /* Stop if a page has been captured */
         if (capc && capc->page) {
             ret = COMPACT_SUCCESS;
             break;
         }
     }
 
 out:
     /*
      * Release free pages and update where the free scanner should restart,
      * so we don't leave any returned pages behind in the next attempt.
      */
     if (cc->nr_freepages > 0) {
         unsigned long free_pfn = release_freepages(&cc->freepages);
 
         cc->nr_freepages = 0;
         VM_BUG_ON(free_pfn == 0);
         /* The cached pfn is always the first in a pageblock */
         free_pfn = pageblock_start_pfn(free_pfn);
         /*
          * Only go back, not forward. The cached pfn might have been
          * already reset to zone end in compact_finished()
          */
         if (free_pfn > cc->zone->compact_cached_free_pfn)
             cc->zone->compact_cached_free_pfn = free_pfn;
     }
 
     count_compact_events(COMPACTMIGRATE_SCANNED, cc->total_migrate_scanned);
     count_compact_events(COMPACTFREE_SCANNED, cc->total_free_scanned);
 
     trace_mm_compaction_end(cc, start_pfn, end_pfn, sync, ret);
 
     return ret;
 }
 
 static enum compact_result compact_zone_order(struct zone *zone, int order,
         gfp_t gfp_mask, enum compact_priority prio,
         unsigned int alloc_flags, int highest_zoneidx,
         struct page **capture)
 {
     enum compact_result ret;
     struct compact_control cc = {
         .order = order,
         .search_order = order,
         .gfp_mask = gfp_mask,
         .zone = zone,
         .mode = (prio == COMPACT_PRIO_ASYNC) ?
                     MIGRATE_ASYNC : MIGRATE_SYNC_LIGHT,
         .alloc_flags = alloc_flags,
         .highest_zoneidx = highest_zoneidx,
         .direct_compaction = true,
         .whole_zone = (prio == MIN_COMPACT_PRIORITY),
         .ignore_skip_hint = (prio == MIN_COMPACT_PRIORITY),
         .ignore_block_suitable = (prio == MIN_COMPACT_PRIORITY)
     };
     struct capture_control capc = {
         .cc = &cc,
         .page = NULL,
     };
 
     /*
      * Make sure the structs are really initialized before we expose the
      * capture control, in case we are interrupted and the interrupt handler
      * frees a page.
      */
     barrier();
     WRITE_ONCE(current->capture_control, &capc);
 
     ret = compact_zone(&cc, &capc);
 
     VM_BUG_ON(!list_empty(&cc.freepages));
     VM_BUG_ON(!list_empty(&cc.migratepages));
 
     /*
      * Make sure we hide capture control first before we read the captured
      * page pointer, otherwise an interrupt could free and capture a page
      * and we would leak it.
      */
     WRITE_ONCE(current->capture_control, NULL);
     *capture = READ_ONCE(capc.page);
     /*
      * Technically, it is also possible that compaction is skipped but
      * the page is still captured out of luck(IRQ came and freed the page).
      * Returning COMPACT_SUCCESS in such cases helps in properly accounting
      * the COMPACT[STALL|FAIL] when compaction is skipped.
      */
     if (*capture)
         ret = COMPACT_SUCCESS;
 
     return ret;
 }
 
 int sysctl_extfrag_threshold = 500;
 
 /**
  * try_to_compact_pages - Direct compact to satisfy a high-order allocation
  * @gfp_mask: The GFP mask of the current allocation
  * @order: The order of the current allocation
  * @alloc_flags: The allocation flags of the current allocation
  * @ac: The context of current allocation
  * @prio: Determines how hard direct compaction should try to succeed
  * @capture: Pointer to free page created by compaction will be stored here
  *
  * This is the main entry point for direct page compaction.
  */
 enum compact_result try_to_compact_pages(gfp_t gfp_mask, unsigned int order,
         unsigned int alloc_flags, const struct alloc_context *ac,
         enum compact_priority prio, struct page **capture)
 {
     int may_perform_io = (__force int)(gfp_mask & __GFP_IO);
     struct zoneref *z;
     struct zone *zone;
     enum compact_result rc = COMPACT_SKIPPED;
 
     /*
      * Check if the GFP flags allow compaction - GFP_NOIO is really
      * tricky context because the migration might require IO
      */
     if (!may_perform_io)
         return COMPACT_SKIPPED;
 
     trace_mm_compaction_try_to_compact_pages(order, gfp_mask, prio);
 
     /* Compact each zone in the list */
     for_each_zone_zonelist_nodemask(zone, z, ac->zonelist,
                     ac->highest_zoneidx, ac->nodemask) {
         enum compact_result status;
 
         if (prio > MIN_COMPACT_PRIORITY
                     && compaction_deferred(zone, order)) {
             rc = max_t(enum compact_result, COMPACT_DEFERRED, rc);
             continue;
         }
 
         status = compact_zone_order(zone, order, gfp_mask, prio,
                 alloc_flags, ac->highest_zoneidx, capture);
         rc = max(status, rc);
 
         /* The allocation should succeed, stop compacting */
         if (status == COMPACT_SUCCESS) {
             /*
              * We think the allocation will succeed in this zone,
              * but it is not certain, hence the false. The caller
              * will repeat this with true if allocation indeed
              * succeeds in this zone.
              */
             compaction_defer_reset(zone, order, false);
 
             break;
         }
 
         if (prio != COMPACT_PRIO_ASYNC && (status == COMPACT_COMPLETE ||
                     status == COMPACT_PARTIAL_SKIPPED))
             /*
              * We think that allocation won't succeed in this zone
              * so we defer compaction there. If it ends up
              * succeeding after all, it will be reset.
              */
             defer_compaction(zone, order);
 
         /*
          * We might have stopped compacting due to need_resched() in
          * async compaction, or due to a fatal signal detected. In that
          * case do not try further zones
          */
         if ((prio == COMPACT_PRIO_ASYNC && need_resched())
                     || fatal_signal_pending(current))
             break;
     }
 
     return rc;
 }
 
 /*
  * Compact all zones within a node till each zone's fragmentation score
  * reaches within proactive compaction thresholds (as determined by the
  * proactiveness tunable).
  *
  * It is possible that the function returns before reaching score targets
  * due to various back-off conditions, such as, contention on per-node or
  * per-zone locks.
  */
 static void proactive_compact_node(pg_data_t *pgdat)
 {
     int zoneid;
     struct zone *zone;
     struct compact_control cc = {
         .order = -1,
         .mode = MIGRATE_SYNC_LIGHT,
         .ignore_skip_hint = true,
         .whole_zone = true,
         .gfp_mask = GFP_KERNEL,
         .proactive_compaction = true,
     };
 
     for (zoneid = 0; zoneid < MAX_NR_ZONES; zoneid++) {
         zone = &pgdat->node_zones[zoneid];
         if (!populated_zone(zone))
             continue;
 
         cc.zone = zone;
 
         compact_zone(&cc, NULL);
 
         VM_BUG_ON(!list_empty(&cc.freepages));
         VM_BUG_ON(!list_empty(&cc.migratepages));
     }
 }
 
 /* Compact all zones within a node */
 static void compact_node(int nid)
 {
     pg_data_t *pgdat = NODE_DATA(nid);
     int zoneid;
     struct zone *zone;
     struct compact_control cc = {
         .order = -1,
         .mode = MIGRATE_SYNC,
         .ignore_skip_hint = true,
         .whole_zone = true,
         .gfp_mask = GFP_KERNEL,
     };
 
 
     for (zoneid = 0; zoneid < MAX_NR_ZONES; zoneid++) {
 
         zone = &pgdat->node_zones[zoneid];
         if (!populated_zone(zone))
             continue;
 
         cc.zone = zone;
 
         compact_zone(&cc, NULL);
 
         VM_BUG_ON(!list_empty(&cc.freepages));
         VM_BUG_ON(!list_empty(&cc.migratepages));
     }
 }
 
 /* Compact all nodes in the system */
 static void compact_nodes(void)
 {
     int nid;
 
     /* Flush pending updates to the LRU lists */
     lru_add_drain_all();
 
     for_each_online_node(nid)
         compact_node(nid);
 }
 
 /*
  * Tunable for proactive compaction. It determines how
  * aggressively the kernel should compact memory in the
  * background. It takes values in the range [0, 100].
  */
 unsigned int __read_mostly sysctl_compaction_proactiveness = 20;
 
 int compaction_proactiveness_sysctl_handler(struct ctl_table *table, int write,
         void *buffer, size_t *length, loff_t *ppos)
 {
     int rc, nid;
 
     rc = proc_dointvec_minmax(table, write, buffer, length, ppos);
     if (rc)
         return rc;
 
     if (write && sysctl_compaction_proactiveness) {
         for_each_online_node(nid) {
             pg_data_t *pgdat = NODE_DATA(nid);
 
             if (pgdat->proactive_compact_trigger)
                 continue;
 
             pgdat->proactive_compact_trigger = true;
             wake_up_interruptible(&pgdat->kcompactd_wait);
         }
     }
 
     return 0;
 }
 
 /*
  * This is the entry point for compacting all nodes via
  * /proc/sys/vm/compact_memory
  */
 int sysctl_compaction_handler(struct ctl_table *table, int write,
             void *buffer, size_t *length, loff_t *ppos)
 {
     if (write)
         compact_nodes();
 
     return 0;
 }
 
 #if defined(CONFIG_SYSFS) && defined(CONFIG_NUMA)
 static ssize_t compact_store(struct device *dev,
                  struct device_attribute *attr,
                  const char *buf, size_t count)
 {
     int nid = dev->id;
 
     if (nid >= 0 && nid < nr_node_ids && node_online(nid)) {
         /* Flush pending updates to the LRU lists */
         lru_add_drain_all();
 
         compact_node(nid);
     }
 
     return count;
 }
 static DEVICE_ATTR_WO(compact);
 
 int compaction_register_node(struct node *node)
 {
     return device_create_file(&node->dev, &dev_attr_compact);
 }
 
 void compaction_unregister_node(struct node *node)
 {
     return device_remove_file(&node->dev, &dev_attr_compact);
 }
 #endif /* CONFIG_SYSFS && CONFIG_NUMA */
 
 static inline bool kcompactd_work_requested(pg_data_t *pgdat)
 {
     return pgdat->kcompactd_max_order > 0 || kthread_should_stop() ||
         pgdat->proactive_compact_trigger;
 }
 
 static bool kcompactd_node_suitable(pg_data_t *pgdat)
 {
     int zoneid;
     struct zone *zone;
     enum zone_type highest_zoneidx = pgdat->kcompactd_highest_zoneidx;
 
     for (zoneid = 0; zoneid <= highest_zoneidx; zoneid++) {
         zone = &pgdat->node_zones[zoneid];
 
         if (!populated_zone(zone))
             continue;
 
         if (compaction_suitable(zone, pgdat->kcompactd_max_order, 0,
                     highest_zoneidx) == COMPACT_CONTINUE)
             return true;
     }
 
     return false;
 }
 
 static void kcompactd_do_work(pg_data_t *pgdat)
 {
     /*
      * With no special task, compact all zones so that a page of requested
      * order is allocatable.
      */
     int zoneid;
     struct zone *zone;
     struct compact_control cc = {
         .order = pgdat->kcompactd_max_order,
         .search_order = pgdat->kcompactd_max_order,
         .highest_zoneidx = pgdat->kcompactd_highest_zoneidx,
         .mode = MIGRATE_SYNC_LIGHT,
         .ignore_skip_hint = false,
         .gfp_mask = GFP_KERNEL,
     };
     trace_mm_compaction_kcompactd_wake(pgdat->node_id, cc.order,
                             cc.highest_zoneidx);
     count_compact_event(KCOMPACTD_WAKE);
 
     for (zoneid = 0; zoneid <= cc.highest_zoneidx; zoneid++) {
         int status;
 
         zone = &pgdat->node_zones[zoneid];
         if (!populated_zone(zone))
             continue;
 
         if (compaction_deferred(zone, cc.order))
             continue;
 
         if (compaction_suitable(zone, cc.order, 0, zoneid) !=
                             COMPACT_CONTINUE)
             continue;
 
         if (kthread_should_stop())
             return;
 
         cc.zone = zone;
         status = compact_zone(&cc, NULL);
 
         if (status == COMPACT_SUCCESS) {
             compaction_defer_reset(zone, cc.order, false);
         } else if (status == COMPACT_PARTIAL_SKIPPED || status == COMPACT_COMPLETE) {
             /*
              * Buddy pages may become stranded on pcps that could
              * otherwise coalesce on the zone's free area for
              * order >= cc.order.  This is ratelimited by the
              * upcoming deferral.
              */
             drain_all_pages(zone);
 
             /*
              * We use sync migration mode here, so we defer like
              * sync direct compaction does.
              */
             defer_compaction(zone, cc.order);
         }
 
         count_compact_events(KCOMPACTD_MIGRATE_SCANNED,
                      cc.total_migrate_scanned);
         count_compact_events(KCOMPACTD_FREE_SCANNED,
                      cc.total_free_scanned);
 
         VM_BUG_ON(!list_empty(&cc.freepages));
         VM_BUG_ON(!list_empty(&cc.migratepages));
     }
 
     /*
      * Regardless of success, we are done until woken up next. But remember
      * the requested order/highest_zoneidx in case it was higher/tighter
      * than our current ones
      */
     if (pgdat->kcompactd_max_order <= cc.order)
         pgdat->kcompactd_max_order = 0;
     if (pgdat->kcompactd_highest_zoneidx >= cc.highest_zoneidx)
         pgdat->kcompactd_highest_zoneidx = pgdat->nr_zones - 1;
 }
 
 void wakeup_kcompactd(pg_data_t *pgdat, int order, int highest_zoneidx)
 {
     if (!order)
         return;
 
     if (pgdat->kcompactd_max_order < order)
         pgdat->kcompactd_max_order = order;
 
     if (pgdat->kcompactd_highest_zoneidx > highest_zoneidx)
         pgdat->kcompactd_highest_zoneidx = highest_zoneidx;
 
     /*
      * Pairs with implicit barrier in wait_event_freezable()
      * such that wakeups are not missed.
      */
     if (!wq_has_sleeper(&pgdat->kcompactd_wait))
         return;
 
     if (!kcompactd_node_suitable(pgdat))
         return;
 
     trace_mm_compaction_wakeup_kcompactd(pgdat->node_id, order,
                             highest_zoneidx);
     wake_up_interruptible(&pgdat->kcompactd_wait);
 }
 
 /*
  * The background compaction daemon, started as a kernel thread
  * from the init process.
  */
 static int kcompactd(void *p)
 {
     pg_data_t *pgdat = (pg_data_t *)p;
     struct task_struct *tsk = current;
     long default_timeout = msecs_to_jiffies(HPAGE_FRAG_CHECK_INTERVAL_MSEC);
     long timeout = default_timeout;
 
     const struct cpumask *cpumask = cpumask_of_node(pgdat->node_id);
 
     if (!cpumask_empty(cpumask))
         set_cpus_allowed_ptr(tsk, cpumask);
 
     set_freezable();
 
     pgdat->kcompactd_max_order = 0;
     pgdat->kcompactd_highest_zoneidx = pgdat->nr_zones - 1;
 
     while (!kthread_should_stop()) {
         unsigned long pflags;
 
         /*
          * Avoid the unnecessary wakeup for proactive compaction
          * when it is disabled.
          */
         if (!sysctl_compaction_proactiveness)
             timeout = MAX_SCHEDULE_TIMEOUT;
         trace_mm_compaction_kcompactd_sleep(pgdat->node_id);
         if (wait_event_freezable_timeout(pgdat->kcompactd_wait,
             kcompactd_work_requested(pgdat), timeout) &&
             !pgdat->proactive_compact_trigger) {
 
             psi_memstall_enter(&pflags);
             kcompactd_do_work(pgdat);
             psi_memstall_leave(&pflags);
             /*
              * Reset the timeout value. The defer timeout from
              * proactive compaction is lost here but that is fine
              * as the condition of the zone changing substantionally
              * then carrying on with the previous defer interval is
              * not useful.
              */
             timeout = default_timeout;
             continue;
         }
 
         /*
          * Start the proactive work with default timeout. Based
          * on the fragmentation score, this timeout is updated.
          */
         timeout = default_timeout;
         if (should_proactive_compact_node(pgdat)) {
             unsigned int prev_score, score;
 
             prev_score = fragmentation_score_node(pgdat);
             proactive_compact_node(pgdat);
             score = fragmentation_score_node(pgdat);
             /*
              * Defer proactive compaction if the fragmentation
              * score did not go down i.e. no progress made.
              */
             if (unlikely(score >= prev_score))
                 timeout =
                    default_timeout << COMPACT_MAX_DEFER_SHIFT;
         }
         if (unlikely(pgdat->proactive_compact_trigger))
             pgdat->proactive_compact_trigger = false;
     }
 
     return 0;
 }
 
 /*
  * This kcompactd start function will be called by init and node-hot-add.
  * On node-hot-add, kcompactd will moved to proper cpus if cpus are hot-added.
  */
 void kcompactd_run(int nid)
 {
     pg_data_t *pgdat = NODE_DATA(nid);
 
     if (pgdat->kcompactd)
         return;
 
     pgdat->kcompactd = kthread_run(kcompactd, pgdat, "kcompactd%d", nid);
     if (IS_ERR(pgdat->kcompactd)) {
         pr_err("Failed to start kcompactd on node %d\n", nid);
         pgdat->kcompactd = NULL;
     }
 }
 
 /*
  * Called by memory hotplug when all memory in a node is offlined. Caller must
  * be holding mem_hotplug_begin/done().
  */
 void kcompactd_stop(int nid)
 {
     struct task_struct *kcompactd = NODE_DATA(nid)->kcompactd;
 
     if (kcompactd) {
         kthread_stop(kcompactd);
         NODE_DATA(nid)->kcompactd = NULL;
     }
 }
 
 /*
  * It's optimal to keep kcompactd on the same CPUs as their memory, but
  * not required for correctness. So if the last cpu in a node goes
  * away, we get changed to run anywhere: as the first one comes back,
  * restore their cpu bindings.
  */
 static int kcompactd_cpu_online(unsigned int cpu)
 {
     int nid;
 
     for_each_node_state(nid, N_MEMORY) {
         pg_data_t *pgdat = NODE_DATA(nid);
         const struct cpumask *mask;
 
         mask = cpumask_of_node(pgdat->node_id);
 
         if (cpumask_any_and(cpu_online_mask, mask) < nr_cpu_ids)
             /* One of our CPUs online: restore mask */
             if (pgdat->kcompactd)
                 set_cpus_allowed_ptr(pgdat->kcompactd, mask);
     }
     return 0;
 }
 
 static int __init kcompactd_init(void)
 {
     int nid;
     int ret;
 
     ret = cpuhp_setup_state_nocalls(CPUHP_AP_ONLINE_DYN,
                     "mm/compaction:online",
                     kcompactd_cpu_online, NULL);
     if (ret < 0) {
         pr_err("kcompactd: failed to register hotplug callbacks.\n");
         return ret;
     }
 
     for_each_node_state(nid, N_MEMORY)
         kcompactd_run(nid);
     return 0;
 }
 subsys_initcall(kcompactd_init)
 
 #endif /* CONFIG_COMPACTION */