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0001 /*
0002  * mm/page-writeback.c
0003  *
0004  * Copyright (C) 2002, Linus Torvalds.
0005  * Copyright (C) 2007 Red Hat, Inc., Peter Zijlstra
0006  *
0007  * Contains functions related to writing back dirty pages at the
0008  * address_space level.
0009  *
0010  * 10Apr2002    Andrew Morton
0011  *      Initial version
0012  */
0013 
0014 #include <linux/kernel.h>
0015 #include <linux/export.h>
0016 #include <linux/spinlock.h>
0017 #include <linux/fs.h>
0018 #include <linux/mm.h>
0019 #include <linux/swap.h>
0020 #include <linux/slab.h>
0021 #include <linux/pagemap.h>
0022 #include <linux/writeback.h>
0023 #include <linux/init.h>
0024 #include <linux/backing-dev.h>
0025 #include <linux/task_io_accounting_ops.h>
0026 #include <linux/blkdev.h>
0027 #include <linux/mpage.h>
0028 #include <linux/rmap.h>
0029 #include <linux/percpu.h>
0030 #include <linux/notifier.h>
0031 #include <linux/smp.h>
0032 #include <linux/sysctl.h>
0033 #include <linux/cpu.h>
0034 #include <linux/syscalls.h>
0035 #include <linux/buffer_head.h> /* __set_page_dirty_buffers */
0036 #include <linux/pagevec.h>
0037 #include <linux/timer.h>
0038 #include <linux/sched/rt.h>
0039 #include <linux/mm_inline.h>
0040 #include <trace/events/writeback.h>
0041 
0042 #include "internal.h"
0043 
0044 /*
0045  * Sleep at most 200ms at a time in balance_dirty_pages().
0046  */
0047 #define MAX_PAUSE       max(HZ/5, 1)
0048 
0049 /*
0050  * Try to keep balance_dirty_pages() call intervals higher than this many pages
0051  * by raising pause time to max_pause when falls below it.
0052  */
0053 #define DIRTY_POLL_THRESH   (128 >> (PAGE_SHIFT - 10))
0054 
0055 /*
0056  * Estimate write bandwidth at 200ms intervals.
0057  */
0058 #define BANDWIDTH_INTERVAL  max(HZ/5, 1)
0059 
0060 #define RATELIMIT_CALC_SHIFT    10
0061 
0062 /*
0063  * After a CPU has dirtied this many pages, balance_dirty_pages_ratelimited
0064  * will look to see if it needs to force writeback or throttling.
0065  */
0066 static long ratelimit_pages = 32;
0067 
0068 /* The following parameters are exported via /proc/sys/vm */
0069 
0070 /*
0071  * Start background writeback (via writeback threads) at this percentage
0072  */
0073 int dirty_background_ratio = 10;
0074 
0075 /*
0076  * dirty_background_bytes starts at 0 (disabled) so that it is a function of
0077  * dirty_background_ratio * the amount of dirtyable memory
0078  */
0079 unsigned long dirty_background_bytes;
0080 
0081 /*
0082  * free highmem will not be subtracted from the total free memory
0083  * for calculating free ratios if vm_highmem_is_dirtyable is true
0084  */
0085 int vm_highmem_is_dirtyable;
0086 
0087 /*
0088  * The generator of dirty data starts writeback at this percentage
0089  */
0090 int vm_dirty_ratio = 20;
0091 
0092 /*
0093  * vm_dirty_bytes starts at 0 (disabled) so that it is a function of
0094  * vm_dirty_ratio * the amount of dirtyable memory
0095  */
0096 unsigned long vm_dirty_bytes;
0097 
0098 /*
0099  * The interval between `kupdate'-style writebacks
0100  */
0101 unsigned int dirty_writeback_interval = 5 * 100; /* centiseconds */
0102 
0103 EXPORT_SYMBOL_GPL(dirty_writeback_interval);
0104 
0105 /*
0106  * The longest time for which data is allowed to remain dirty
0107  */
0108 unsigned int dirty_expire_interval = 30 * 100; /* centiseconds */
0109 
0110 /*
0111  * Flag that makes the machine dump writes/reads and block dirtyings.
0112  */
0113 int block_dump;
0114 
0115 /*
0116  * Flag that puts the machine in "laptop mode". Doubles as a timeout in jiffies:
0117  * a full sync is triggered after this time elapses without any disk activity.
0118  */
0119 int laptop_mode;
0120 
0121 EXPORT_SYMBOL(laptop_mode);
0122 
0123 /* End of sysctl-exported parameters */
0124 
0125 struct wb_domain global_wb_domain;
0126 
0127 /* consolidated parameters for balance_dirty_pages() and its subroutines */
0128 struct dirty_throttle_control {
0129 #ifdef CONFIG_CGROUP_WRITEBACK
0130     struct wb_domain    *dom;
0131     struct dirty_throttle_control *gdtc;    /* only set in memcg dtc's */
0132 #endif
0133     struct bdi_writeback    *wb;
0134     struct fprop_local_percpu *wb_completions;
0135 
0136     unsigned long       avail;      /* dirtyable */
0137     unsigned long       dirty;      /* file_dirty + write + nfs */
0138     unsigned long       thresh;     /* dirty threshold */
0139     unsigned long       bg_thresh;  /* dirty background threshold */
0140 
0141     unsigned long       wb_dirty;   /* per-wb counterparts */
0142     unsigned long       wb_thresh;
0143     unsigned long       wb_bg_thresh;
0144 
0145     unsigned long       pos_ratio;
0146 };
0147 
0148 /*
0149  * Length of period for aging writeout fractions of bdis. This is an
0150  * arbitrarily chosen number. The longer the period, the slower fractions will
0151  * reflect changes in current writeout rate.
0152  */
0153 #define VM_COMPLETIONS_PERIOD_LEN (3*HZ)
0154 
0155 #ifdef CONFIG_CGROUP_WRITEBACK
0156 
0157 #define GDTC_INIT(__wb)     .wb = (__wb),               \
0158                 .dom = &global_wb_domain,       \
0159                 .wb_completions = &(__wb)->completions
0160 
0161 #define GDTC_INIT_NO_WB     .dom = &global_wb_domain
0162 
0163 #define MDTC_INIT(__wb, __gdtc) .wb = (__wb),               \
0164                 .dom = mem_cgroup_wb_domain(__wb),  \
0165                 .wb_completions = &(__wb)->memcg_completions, \
0166                 .gdtc = __gdtc
0167 
0168 static bool mdtc_valid(struct dirty_throttle_control *dtc)
0169 {
0170     return dtc->dom;
0171 }
0172 
0173 static struct wb_domain *dtc_dom(struct dirty_throttle_control *dtc)
0174 {
0175     return dtc->dom;
0176 }
0177 
0178 static struct dirty_throttle_control *mdtc_gdtc(struct dirty_throttle_control *mdtc)
0179 {
0180     return mdtc->gdtc;
0181 }
0182 
0183 static struct fprop_local_percpu *wb_memcg_completions(struct bdi_writeback *wb)
0184 {
0185     return &wb->memcg_completions;
0186 }
0187 
0188 static void wb_min_max_ratio(struct bdi_writeback *wb,
0189                  unsigned long *minp, unsigned long *maxp)
0190 {
0191     unsigned long this_bw = wb->avg_write_bandwidth;
0192     unsigned long tot_bw = atomic_long_read(&wb->bdi->tot_write_bandwidth);
0193     unsigned long long min = wb->bdi->min_ratio;
0194     unsigned long long max = wb->bdi->max_ratio;
0195 
0196     /*
0197      * @wb may already be clean by the time control reaches here and
0198      * the total may not include its bw.
0199      */
0200     if (this_bw < tot_bw) {
0201         if (min) {
0202             min *= this_bw;
0203             do_div(min, tot_bw);
0204         }
0205         if (max < 100) {
0206             max *= this_bw;
0207             do_div(max, tot_bw);
0208         }
0209     }
0210 
0211     *minp = min;
0212     *maxp = max;
0213 }
0214 
0215 #else   /* CONFIG_CGROUP_WRITEBACK */
0216 
0217 #define GDTC_INIT(__wb)     .wb = (__wb),                           \
0218                 .wb_completions = &(__wb)->completions
0219 #define GDTC_INIT_NO_WB
0220 #define MDTC_INIT(__wb, __gdtc)
0221 
0222 static bool mdtc_valid(struct dirty_throttle_control *dtc)
0223 {
0224     return false;
0225 }
0226 
0227 static struct wb_domain *dtc_dom(struct dirty_throttle_control *dtc)
0228 {
0229     return &global_wb_domain;
0230 }
0231 
0232 static struct dirty_throttle_control *mdtc_gdtc(struct dirty_throttle_control *mdtc)
0233 {
0234     return NULL;
0235 }
0236 
0237 static struct fprop_local_percpu *wb_memcg_completions(struct bdi_writeback *wb)
0238 {
0239     return NULL;
0240 }
0241 
0242 static void wb_min_max_ratio(struct bdi_writeback *wb,
0243                  unsigned long *minp, unsigned long *maxp)
0244 {
0245     *minp = wb->bdi->min_ratio;
0246     *maxp = wb->bdi->max_ratio;
0247 }
0248 
0249 #endif  /* CONFIG_CGROUP_WRITEBACK */
0250 
0251 /*
0252  * In a memory zone, there is a certain amount of pages we consider
0253  * available for the page cache, which is essentially the number of
0254  * free and reclaimable pages, minus some zone reserves to protect
0255  * lowmem and the ability to uphold the zone's watermarks without
0256  * requiring writeback.
0257  *
0258  * This number of dirtyable pages is the base value of which the
0259  * user-configurable dirty ratio is the effictive number of pages that
0260  * are allowed to be actually dirtied.  Per individual zone, or
0261  * globally by using the sum of dirtyable pages over all zones.
0262  *
0263  * Because the user is allowed to specify the dirty limit globally as
0264  * absolute number of bytes, calculating the per-zone dirty limit can
0265  * require translating the configured limit into a percentage of
0266  * global dirtyable memory first.
0267  */
0268 
0269 /**
0270  * node_dirtyable_memory - number of dirtyable pages in a node
0271  * @pgdat: the node
0272  *
0273  * Returns the node's number of pages potentially available for dirty
0274  * page cache.  This is the base value for the per-node dirty limits.
0275  */
0276 static unsigned long node_dirtyable_memory(struct pglist_data *pgdat)
0277 {
0278     unsigned long nr_pages = 0;
0279     int z;
0280 
0281     for (z = 0; z < MAX_NR_ZONES; z++) {
0282         struct zone *zone = pgdat->node_zones + z;
0283 
0284         if (!populated_zone(zone))
0285             continue;
0286 
0287         nr_pages += zone_page_state(zone, NR_FREE_PAGES);
0288     }
0289 
0290     /*
0291      * Pages reserved for the kernel should not be considered
0292      * dirtyable, to prevent a situation where reclaim has to
0293      * clean pages in order to balance the zones.
0294      */
0295     nr_pages -= min(nr_pages, pgdat->totalreserve_pages);
0296 
0297     nr_pages += node_page_state(pgdat, NR_INACTIVE_FILE);
0298     nr_pages += node_page_state(pgdat, NR_ACTIVE_FILE);
0299 
0300     return nr_pages;
0301 }
0302 
0303 static unsigned long highmem_dirtyable_memory(unsigned long total)
0304 {
0305 #ifdef CONFIG_HIGHMEM
0306     int node;
0307     unsigned long x = 0;
0308     int i;
0309 
0310     for_each_node_state(node, N_HIGH_MEMORY) {
0311         for (i = ZONE_NORMAL + 1; i < MAX_NR_ZONES; i++) {
0312             struct zone *z;
0313             unsigned long nr_pages;
0314 
0315             if (!is_highmem_idx(i))
0316                 continue;
0317 
0318             z = &NODE_DATA(node)->node_zones[i];
0319             if (!populated_zone(z))
0320                 continue;
0321 
0322             nr_pages = zone_page_state(z, NR_FREE_PAGES);
0323             /* watch for underflows */
0324             nr_pages -= min(nr_pages, high_wmark_pages(z));
0325             nr_pages += zone_page_state(z, NR_ZONE_INACTIVE_FILE);
0326             nr_pages += zone_page_state(z, NR_ZONE_ACTIVE_FILE);
0327             x += nr_pages;
0328         }
0329     }
0330 
0331     /*
0332      * Unreclaimable memory (kernel memory or anonymous memory
0333      * without swap) can bring down the dirtyable pages below
0334      * the zone's dirty balance reserve and the above calculation
0335      * will underflow.  However we still want to add in nodes
0336      * which are below threshold (negative values) to get a more
0337      * accurate calculation but make sure that the total never
0338      * underflows.
0339      */
0340     if ((long)x < 0)
0341         x = 0;
0342 
0343     /*
0344      * Make sure that the number of highmem pages is never larger
0345      * than the number of the total dirtyable memory. This can only
0346      * occur in very strange VM situations but we want to make sure
0347      * that this does not occur.
0348      */
0349     return min(x, total);
0350 #else
0351     return 0;
0352 #endif
0353 }
0354 
0355 /**
0356  * global_dirtyable_memory - number of globally dirtyable pages
0357  *
0358  * Returns the global number of pages potentially available for dirty
0359  * page cache.  This is the base value for the global dirty limits.
0360  */
0361 static unsigned long global_dirtyable_memory(void)
0362 {
0363     unsigned long x;
0364 
0365     x = global_page_state(NR_FREE_PAGES);
0366     /*
0367      * Pages reserved for the kernel should not be considered
0368      * dirtyable, to prevent a situation where reclaim has to
0369      * clean pages in order to balance the zones.
0370      */
0371     x -= min(x, totalreserve_pages);
0372 
0373     x += global_node_page_state(NR_INACTIVE_FILE);
0374     x += global_node_page_state(NR_ACTIVE_FILE);
0375 
0376     if (!vm_highmem_is_dirtyable)
0377         x -= highmem_dirtyable_memory(x);
0378 
0379     return x + 1;   /* Ensure that we never return 0 */
0380 }
0381 
0382 /**
0383  * domain_dirty_limits - calculate thresh and bg_thresh for a wb_domain
0384  * @dtc: dirty_throttle_control of interest
0385  *
0386  * Calculate @dtc->thresh and ->bg_thresh considering
0387  * vm_dirty_{bytes|ratio} and dirty_background_{bytes|ratio}.  The caller
0388  * must ensure that @dtc->avail is set before calling this function.  The
0389  * dirty limits will be lifted by 1/4 for PF_LESS_THROTTLE (ie. nfsd) and
0390  * real-time tasks.
0391  */
0392 static void domain_dirty_limits(struct dirty_throttle_control *dtc)
0393 {
0394     const unsigned long available_memory = dtc->avail;
0395     struct dirty_throttle_control *gdtc = mdtc_gdtc(dtc);
0396     unsigned long bytes = vm_dirty_bytes;
0397     unsigned long bg_bytes = dirty_background_bytes;
0398     /* convert ratios to per-PAGE_SIZE for higher precision */
0399     unsigned long ratio = (vm_dirty_ratio * PAGE_SIZE) / 100;
0400     unsigned long bg_ratio = (dirty_background_ratio * PAGE_SIZE) / 100;
0401     unsigned long thresh;
0402     unsigned long bg_thresh;
0403     struct task_struct *tsk;
0404 
0405     /* gdtc is !NULL iff @dtc is for memcg domain */
0406     if (gdtc) {
0407         unsigned long global_avail = gdtc->avail;
0408 
0409         /*
0410          * The byte settings can't be applied directly to memcg
0411          * domains.  Convert them to ratios by scaling against
0412          * globally available memory.  As the ratios are in
0413          * per-PAGE_SIZE, they can be obtained by dividing bytes by
0414          * number of pages.
0415          */
0416         if (bytes)
0417             ratio = min(DIV_ROUND_UP(bytes, global_avail),
0418                     PAGE_SIZE);
0419         if (bg_bytes)
0420             bg_ratio = min(DIV_ROUND_UP(bg_bytes, global_avail),
0421                        PAGE_SIZE);
0422         bytes = bg_bytes = 0;
0423     }
0424 
0425     if (bytes)
0426         thresh = DIV_ROUND_UP(bytes, PAGE_SIZE);
0427     else
0428         thresh = (ratio * available_memory) / PAGE_SIZE;
0429 
0430     if (bg_bytes)
0431         bg_thresh = DIV_ROUND_UP(bg_bytes, PAGE_SIZE);
0432     else
0433         bg_thresh = (bg_ratio * available_memory) / PAGE_SIZE;
0434 
0435     if (bg_thresh >= thresh)
0436         bg_thresh = thresh / 2;
0437     tsk = current;
0438     if (tsk->flags & PF_LESS_THROTTLE || rt_task(tsk)) {
0439         bg_thresh += bg_thresh / 4 + global_wb_domain.dirty_limit / 32;
0440         thresh += thresh / 4 + global_wb_domain.dirty_limit / 32;
0441     }
0442     dtc->thresh = thresh;
0443     dtc->bg_thresh = bg_thresh;
0444 
0445     /* we should eventually report the domain in the TP */
0446     if (!gdtc)
0447         trace_global_dirty_state(bg_thresh, thresh);
0448 }
0449 
0450 /**
0451  * global_dirty_limits - background-writeback and dirty-throttling thresholds
0452  * @pbackground: out parameter for bg_thresh
0453  * @pdirty: out parameter for thresh
0454  *
0455  * Calculate bg_thresh and thresh for global_wb_domain.  See
0456  * domain_dirty_limits() for details.
0457  */
0458 void global_dirty_limits(unsigned long *pbackground, unsigned long *pdirty)
0459 {
0460     struct dirty_throttle_control gdtc = { GDTC_INIT_NO_WB };
0461 
0462     gdtc.avail = global_dirtyable_memory();
0463     domain_dirty_limits(&gdtc);
0464 
0465     *pbackground = gdtc.bg_thresh;
0466     *pdirty = gdtc.thresh;
0467 }
0468 
0469 /**
0470  * node_dirty_limit - maximum number of dirty pages allowed in a node
0471  * @pgdat: the node
0472  *
0473  * Returns the maximum number of dirty pages allowed in a node, based
0474  * on the node's dirtyable memory.
0475  */
0476 static unsigned long node_dirty_limit(struct pglist_data *pgdat)
0477 {
0478     unsigned long node_memory = node_dirtyable_memory(pgdat);
0479     struct task_struct *tsk = current;
0480     unsigned long dirty;
0481 
0482     if (vm_dirty_bytes)
0483         dirty = DIV_ROUND_UP(vm_dirty_bytes, PAGE_SIZE) *
0484             node_memory / global_dirtyable_memory();
0485     else
0486         dirty = vm_dirty_ratio * node_memory / 100;
0487 
0488     if (tsk->flags & PF_LESS_THROTTLE || rt_task(tsk))
0489         dirty += dirty / 4;
0490 
0491     return dirty;
0492 }
0493 
0494 /**
0495  * node_dirty_ok - tells whether a node is within its dirty limits
0496  * @pgdat: the node to check
0497  *
0498  * Returns %true when the dirty pages in @pgdat are within the node's
0499  * dirty limit, %false if the limit is exceeded.
0500  */
0501 bool node_dirty_ok(struct pglist_data *pgdat)
0502 {
0503     unsigned long limit = node_dirty_limit(pgdat);
0504     unsigned long nr_pages = 0;
0505 
0506     nr_pages += node_page_state(pgdat, NR_FILE_DIRTY);
0507     nr_pages += node_page_state(pgdat, NR_UNSTABLE_NFS);
0508     nr_pages += node_page_state(pgdat, NR_WRITEBACK);
0509 
0510     return nr_pages <= limit;
0511 }
0512 
0513 int dirty_background_ratio_handler(struct ctl_table *table, int write,
0514         void __user *buffer, size_t *lenp,
0515         loff_t *ppos)
0516 {
0517     int ret;
0518 
0519     ret = proc_dointvec_minmax(table, write, buffer, lenp, ppos);
0520     if (ret == 0 && write)
0521         dirty_background_bytes = 0;
0522     return ret;
0523 }
0524 
0525 int dirty_background_bytes_handler(struct ctl_table *table, int write,
0526         void __user *buffer, size_t *lenp,
0527         loff_t *ppos)
0528 {
0529     int ret;
0530 
0531     ret = proc_doulongvec_minmax(table, write, buffer, lenp, ppos);
0532     if (ret == 0 && write)
0533         dirty_background_ratio = 0;
0534     return ret;
0535 }
0536 
0537 int dirty_ratio_handler(struct ctl_table *table, int write,
0538         void __user *buffer, size_t *lenp,
0539         loff_t *ppos)
0540 {
0541     int old_ratio = vm_dirty_ratio;
0542     int ret;
0543 
0544     ret = proc_dointvec_minmax(table, write, buffer, lenp, ppos);
0545     if (ret == 0 && write && vm_dirty_ratio != old_ratio) {
0546         writeback_set_ratelimit();
0547         vm_dirty_bytes = 0;
0548     }
0549     return ret;
0550 }
0551 
0552 int dirty_bytes_handler(struct ctl_table *table, int write,
0553         void __user *buffer, size_t *lenp,
0554         loff_t *ppos)
0555 {
0556     unsigned long old_bytes = vm_dirty_bytes;
0557     int ret;
0558 
0559     ret = proc_doulongvec_minmax(table, write, buffer, lenp, ppos);
0560     if (ret == 0 && write && vm_dirty_bytes != old_bytes) {
0561         writeback_set_ratelimit();
0562         vm_dirty_ratio = 0;
0563     }
0564     return ret;
0565 }
0566 
0567 static unsigned long wp_next_time(unsigned long cur_time)
0568 {
0569     cur_time += VM_COMPLETIONS_PERIOD_LEN;
0570     /* 0 has a special meaning... */
0571     if (!cur_time)
0572         return 1;
0573     return cur_time;
0574 }
0575 
0576 static void wb_domain_writeout_inc(struct wb_domain *dom,
0577                    struct fprop_local_percpu *completions,
0578                    unsigned int max_prop_frac)
0579 {
0580     __fprop_inc_percpu_max(&dom->completions, completions,
0581                    max_prop_frac);
0582     /* First event after period switching was turned off? */
0583     if (!unlikely(dom->period_time)) {
0584         /*
0585          * We can race with other __bdi_writeout_inc calls here but
0586          * it does not cause any harm since the resulting time when
0587          * timer will fire and what is in writeout_period_time will be
0588          * roughly the same.
0589          */
0590         dom->period_time = wp_next_time(jiffies);
0591         mod_timer(&dom->period_timer, dom->period_time);
0592     }
0593 }
0594 
0595 /*
0596  * Increment @wb's writeout completion count and the global writeout
0597  * completion count. Called from test_clear_page_writeback().
0598  */
0599 static inline void __wb_writeout_inc(struct bdi_writeback *wb)
0600 {
0601     struct wb_domain *cgdom;
0602 
0603     __inc_wb_stat(wb, WB_WRITTEN);
0604     wb_domain_writeout_inc(&global_wb_domain, &wb->completions,
0605                    wb->bdi->max_prop_frac);
0606 
0607     cgdom = mem_cgroup_wb_domain(wb);
0608     if (cgdom)
0609         wb_domain_writeout_inc(cgdom, wb_memcg_completions(wb),
0610                        wb->bdi->max_prop_frac);
0611 }
0612 
0613 void wb_writeout_inc(struct bdi_writeback *wb)
0614 {
0615     unsigned long flags;
0616 
0617     local_irq_save(flags);
0618     __wb_writeout_inc(wb);
0619     local_irq_restore(flags);
0620 }
0621 EXPORT_SYMBOL_GPL(wb_writeout_inc);
0622 
0623 /*
0624  * On idle system, we can be called long after we scheduled because we use
0625  * deferred timers so count with missed periods.
0626  */
0627 static void writeout_period(unsigned long t)
0628 {
0629     struct wb_domain *dom = (void *)t;
0630     int miss_periods = (jiffies - dom->period_time) /
0631                          VM_COMPLETIONS_PERIOD_LEN;
0632 
0633     if (fprop_new_period(&dom->completions, miss_periods + 1)) {
0634         dom->period_time = wp_next_time(dom->period_time +
0635                 miss_periods * VM_COMPLETIONS_PERIOD_LEN);
0636         mod_timer(&dom->period_timer, dom->period_time);
0637     } else {
0638         /*
0639          * Aging has zeroed all fractions. Stop wasting CPU on period
0640          * updates.
0641          */
0642         dom->period_time = 0;
0643     }
0644 }
0645 
0646 int wb_domain_init(struct wb_domain *dom, gfp_t gfp)
0647 {
0648     memset(dom, 0, sizeof(*dom));
0649 
0650     spin_lock_init(&dom->lock);
0651 
0652     init_timer_deferrable(&dom->period_timer);
0653     dom->period_timer.function = writeout_period;
0654     dom->period_timer.data = (unsigned long)dom;
0655 
0656     dom->dirty_limit_tstamp = jiffies;
0657 
0658     return fprop_global_init(&dom->completions, gfp);
0659 }
0660 
0661 #ifdef CONFIG_CGROUP_WRITEBACK
0662 void wb_domain_exit(struct wb_domain *dom)
0663 {
0664     del_timer_sync(&dom->period_timer);
0665     fprop_global_destroy(&dom->completions);
0666 }
0667 #endif
0668 
0669 /*
0670  * bdi_min_ratio keeps the sum of the minimum dirty shares of all
0671  * registered backing devices, which, for obvious reasons, can not
0672  * exceed 100%.
0673  */
0674 static unsigned int bdi_min_ratio;
0675 
0676 int bdi_set_min_ratio(struct backing_dev_info *bdi, unsigned int min_ratio)
0677 {
0678     int ret = 0;
0679 
0680     spin_lock_bh(&bdi_lock);
0681     if (min_ratio > bdi->max_ratio) {
0682         ret = -EINVAL;
0683     } else {
0684         min_ratio -= bdi->min_ratio;
0685         if (bdi_min_ratio + min_ratio < 100) {
0686             bdi_min_ratio += min_ratio;
0687             bdi->min_ratio += min_ratio;
0688         } else {
0689             ret = -EINVAL;
0690         }
0691     }
0692     spin_unlock_bh(&bdi_lock);
0693 
0694     return ret;
0695 }
0696 
0697 int bdi_set_max_ratio(struct backing_dev_info *bdi, unsigned max_ratio)
0698 {
0699     int ret = 0;
0700 
0701     if (max_ratio > 100)
0702         return -EINVAL;
0703 
0704     spin_lock_bh(&bdi_lock);
0705     if (bdi->min_ratio > max_ratio) {
0706         ret = -EINVAL;
0707     } else {
0708         bdi->max_ratio = max_ratio;
0709         bdi->max_prop_frac = (FPROP_FRAC_BASE * max_ratio) / 100;
0710     }
0711     spin_unlock_bh(&bdi_lock);
0712 
0713     return ret;
0714 }
0715 EXPORT_SYMBOL(bdi_set_max_ratio);
0716 
0717 static unsigned long dirty_freerun_ceiling(unsigned long thresh,
0718                        unsigned long bg_thresh)
0719 {
0720     return (thresh + bg_thresh) / 2;
0721 }
0722 
0723 static unsigned long hard_dirty_limit(struct wb_domain *dom,
0724                       unsigned long thresh)
0725 {
0726     return max(thresh, dom->dirty_limit);
0727 }
0728 
0729 /*
0730  * Memory which can be further allocated to a memcg domain is capped by
0731  * system-wide clean memory excluding the amount being used in the domain.
0732  */
0733 static void mdtc_calc_avail(struct dirty_throttle_control *mdtc,
0734                 unsigned long filepages, unsigned long headroom)
0735 {
0736     struct dirty_throttle_control *gdtc = mdtc_gdtc(mdtc);
0737     unsigned long clean = filepages - min(filepages, mdtc->dirty);
0738     unsigned long global_clean = gdtc->avail - min(gdtc->avail, gdtc->dirty);
0739     unsigned long other_clean = global_clean - min(global_clean, clean);
0740 
0741     mdtc->avail = filepages + min(headroom, other_clean);
0742 }
0743 
0744 /**
0745  * __wb_calc_thresh - @wb's share of dirty throttling threshold
0746  * @dtc: dirty_throttle_context of interest
0747  *
0748  * Returns @wb's dirty limit in pages. The term "dirty" in the context of
0749  * dirty balancing includes all PG_dirty, PG_writeback and NFS unstable pages.
0750  *
0751  * Note that balance_dirty_pages() will only seriously take it as a hard limit
0752  * when sleeping max_pause per page is not enough to keep the dirty pages under
0753  * control. For example, when the device is completely stalled due to some error
0754  * conditions, or when there are 1000 dd tasks writing to a slow 10MB/s USB key.
0755  * In the other normal situations, it acts more gently by throttling the tasks
0756  * more (rather than completely block them) when the wb dirty pages go high.
0757  *
0758  * It allocates high/low dirty limits to fast/slow devices, in order to prevent
0759  * - starving fast devices
0760  * - piling up dirty pages (that will take long time to sync) on slow devices
0761  *
0762  * The wb's share of dirty limit will be adapting to its throughput and
0763  * bounded by the bdi->min_ratio and/or bdi->max_ratio parameters, if set.
0764  */
0765 static unsigned long __wb_calc_thresh(struct dirty_throttle_control *dtc)
0766 {
0767     struct wb_domain *dom = dtc_dom(dtc);
0768     unsigned long thresh = dtc->thresh;
0769     u64 wb_thresh;
0770     long numerator, denominator;
0771     unsigned long wb_min_ratio, wb_max_ratio;
0772 
0773     /*
0774      * Calculate this BDI's share of the thresh ratio.
0775      */
0776     fprop_fraction_percpu(&dom->completions, dtc->wb_completions,
0777                   &numerator, &denominator);
0778 
0779     wb_thresh = (thresh * (100 - bdi_min_ratio)) / 100;
0780     wb_thresh *= numerator;
0781     do_div(wb_thresh, denominator);
0782 
0783     wb_min_max_ratio(dtc->wb, &wb_min_ratio, &wb_max_ratio);
0784 
0785     wb_thresh += (thresh * wb_min_ratio) / 100;
0786     if (wb_thresh > (thresh * wb_max_ratio) / 100)
0787         wb_thresh = thresh * wb_max_ratio / 100;
0788 
0789     return wb_thresh;
0790 }
0791 
0792 unsigned long wb_calc_thresh(struct bdi_writeback *wb, unsigned long thresh)
0793 {
0794     struct dirty_throttle_control gdtc = { GDTC_INIT(wb),
0795                            .thresh = thresh };
0796     return __wb_calc_thresh(&gdtc);
0797 }
0798 
0799 /*
0800  *                           setpoint - dirty 3
0801  *        f(dirty) := 1.0 + (----------------)
0802  *                           limit - setpoint
0803  *
0804  * it's a 3rd order polynomial that subjects to
0805  *
0806  * (1) f(freerun)  = 2.0 => rampup dirty_ratelimit reasonably fast
0807  * (2) f(setpoint) = 1.0 => the balance point
0808  * (3) f(limit)    = 0   => the hard limit
0809  * (4) df/dx      <= 0   => negative feedback control
0810  * (5) the closer to setpoint, the smaller |df/dx| (and the reverse)
0811  *     => fast response on large errors; small oscillation near setpoint
0812  */
0813 static long long pos_ratio_polynom(unsigned long setpoint,
0814                       unsigned long dirty,
0815                       unsigned long limit)
0816 {
0817     long long pos_ratio;
0818     long x;
0819 
0820     x = div64_s64(((s64)setpoint - (s64)dirty) << RATELIMIT_CALC_SHIFT,
0821               (limit - setpoint) | 1);
0822     pos_ratio = x;
0823     pos_ratio = pos_ratio * x >> RATELIMIT_CALC_SHIFT;
0824     pos_ratio = pos_ratio * x >> RATELIMIT_CALC_SHIFT;
0825     pos_ratio += 1 << RATELIMIT_CALC_SHIFT;
0826 
0827     return clamp(pos_ratio, 0LL, 2LL << RATELIMIT_CALC_SHIFT);
0828 }
0829 
0830 /*
0831  * Dirty position control.
0832  *
0833  * (o) global/bdi setpoints
0834  *
0835  * We want the dirty pages be balanced around the global/wb setpoints.
0836  * When the number of dirty pages is higher/lower than the setpoint, the
0837  * dirty position control ratio (and hence task dirty ratelimit) will be
0838  * decreased/increased to bring the dirty pages back to the setpoint.
0839  *
0840  *     pos_ratio = 1 << RATELIMIT_CALC_SHIFT
0841  *
0842  *     if (dirty < setpoint) scale up   pos_ratio
0843  *     if (dirty > setpoint) scale down pos_ratio
0844  *
0845  *     if (wb_dirty < wb_setpoint) scale up   pos_ratio
0846  *     if (wb_dirty > wb_setpoint) scale down pos_ratio
0847  *
0848  *     task_ratelimit = dirty_ratelimit * pos_ratio >> RATELIMIT_CALC_SHIFT
0849  *
0850  * (o) global control line
0851  *
0852  *     ^ pos_ratio
0853  *     |
0854  *     |            |<===== global dirty control scope ======>|
0855  * 2.0 .............*
0856  *     |            .*
0857  *     |            . *
0858  *     |            .   *
0859  *     |            .     *
0860  *     |            .        *
0861  *     |            .            *
0862  * 1.0 ................................*
0863  *     |            .                  .     *
0864  *     |            .                  .          *
0865  *     |            .                  .              *
0866  *     |            .                  .                 *
0867  *     |            .                  .                    *
0868  *   0 +------------.------------------.----------------------*------------->
0869  *           freerun^          setpoint^                 limit^   dirty pages
0870  *
0871  * (o) wb control line
0872  *
0873  *     ^ pos_ratio
0874  *     |
0875  *     |            *
0876  *     |              *
0877  *     |                *
0878  *     |                  *
0879  *     |                    * |<=========== span ============>|
0880  * 1.0 .......................*
0881  *     |                      . *
0882  *     |                      .   *
0883  *     |                      .     *
0884  *     |                      .       *
0885  *     |                      .         *
0886  *     |                      .           *
0887  *     |                      .             *
0888  *     |                      .               *
0889  *     |                      .                 *
0890  *     |                      .                   *
0891  *     |                      .                     *
0892  * 1/4 ...............................................* * * * * * * * * * * *
0893  *     |                      .                         .
0894  *     |                      .                           .
0895  *     |                      .                             .
0896  *   0 +----------------------.-------------------------------.------------->
0897  *                wb_setpoint^                    x_intercept^
0898  *
0899  * The wb control line won't drop below pos_ratio=1/4, so that wb_dirty can
0900  * be smoothly throttled down to normal if it starts high in situations like
0901  * - start writing to a slow SD card and a fast disk at the same time. The SD
0902  *   card's wb_dirty may rush to many times higher than wb_setpoint.
0903  * - the wb dirty thresh drops quickly due to change of JBOD workload
0904  */
0905 static void wb_position_ratio(struct dirty_throttle_control *dtc)
0906 {
0907     struct bdi_writeback *wb = dtc->wb;
0908     unsigned long write_bw = wb->avg_write_bandwidth;
0909     unsigned long freerun = dirty_freerun_ceiling(dtc->thresh, dtc->bg_thresh);
0910     unsigned long limit = hard_dirty_limit(dtc_dom(dtc), dtc->thresh);
0911     unsigned long wb_thresh = dtc->wb_thresh;
0912     unsigned long x_intercept;
0913     unsigned long setpoint;     /* dirty pages' target balance point */
0914     unsigned long wb_setpoint;
0915     unsigned long span;
0916     long long pos_ratio;        /* for scaling up/down the rate limit */
0917     long x;
0918 
0919     dtc->pos_ratio = 0;
0920 
0921     if (unlikely(dtc->dirty >= limit))
0922         return;
0923 
0924     /*
0925      * global setpoint
0926      *
0927      * See comment for pos_ratio_polynom().
0928      */
0929     setpoint = (freerun + limit) / 2;
0930     pos_ratio = pos_ratio_polynom(setpoint, dtc->dirty, limit);
0931 
0932     /*
0933      * The strictlimit feature is a tool preventing mistrusted filesystems
0934      * from growing a large number of dirty pages before throttling. For
0935      * such filesystems balance_dirty_pages always checks wb counters
0936      * against wb limits. Even if global "nr_dirty" is under "freerun".
0937      * This is especially important for fuse which sets bdi->max_ratio to
0938      * 1% by default. Without strictlimit feature, fuse writeback may
0939      * consume arbitrary amount of RAM because it is accounted in
0940      * NR_WRITEBACK_TEMP which is not involved in calculating "nr_dirty".
0941      *
0942      * Here, in wb_position_ratio(), we calculate pos_ratio based on
0943      * two values: wb_dirty and wb_thresh. Let's consider an example:
0944      * total amount of RAM is 16GB, bdi->max_ratio is equal to 1%, global
0945      * limits are set by default to 10% and 20% (background and throttle).
0946      * Then wb_thresh is 1% of 20% of 16GB. This amounts to ~8K pages.
0947      * wb_calc_thresh(wb, bg_thresh) is about ~4K pages. wb_setpoint is
0948      * about ~6K pages (as the average of background and throttle wb
0949      * limits). The 3rd order polynomial will provide positive feedback if
0950      * wb_dirty is under wb_setpoint and vice versa.
0951      *
0952      * Note, that we cannot use global counters in these calculations
0953      * because we want to throttle process writing to a strictlimit wb
0954      * much earlier than global "freerun" is reached (~23MB vs. ~2.3GB
0955      * in the example above).
0956      */
0957     if (unlikely(wb->bdi->capabilities & BDI_CAP_STRICTLIMIT)) {
0958         long long wb_pos_ratio;
0959 
0960         if (dtc->wb_dirty < 8) {
0961             dtc->pos_ratio = min_t(long long, pos_ratio * 2,
0962                        2 << RATELIMIT_CALC_SHIFT);
0963             return;
0964         }
0965 
0966         if (dtc->wb_dirty >= wb_thresh)
0967             return;
0968 
0969         wb_setpoint = dirty_freerun_ceiling(wb_thresh,
0970                             dtc->wb_bg_thresh);
0971 
0972         if (wb_setpoint == 0 || wb_setpoint == wb_thresh)
0973             return;
0974 
0975         wb_pos_ratio = pos_ratio_polynom(wb_setpoint, dtc->wb_dirty,
0976                          wb_thresh);
0977 
0978         /*
0979          * Typically, for strictlimit case, wb_setpoint << setpoint
0980          * and pos_ratio >> wb_pos_ratio. In the other words global
0981          * state ("dirty") is not limiting factor and we have to
0982          * make decision based on wb counters. But there is an
0983          * important case when global pos_ratio should get precedence:
0984          * global limits are exceeded (e.g. due to activities on other
0985          * wb's) while given strictlimit wb is below limit.
0986          *
0987          * "pos_ratio * wb_pos_ratio" would work for the case above,
0988          * but it would look too non-natural for the case of all
0989          * activity in the system coming from a single strictlimit wb
0990          * with bdi->max_ratio == 100%.
0991          *
0992          * Note that min() below somewhat changes the dynamics of the
0993          * control system. Normally, pos_ratio value can be well over 3
0994          * (when globally we are at freerun and wb is well below wb
0995          * setpoint). Now the maximum pos_ratio in the same situation
0996          * is 2. We might want to tweak this if we observe the control
0997          * system is too slow to adapt.
0998          */
0999         dtc->pos_ratio = min(pos_ratio, wb_pos_ratio);
1000         return;
1001     }
1002 
1003     /*
1004      * We have computed basic pos_ratio above based on global situation. If
1005      * the wb is over/under its share of dirty pages, we want to scale
1006      * pos_ratio further down/up. That is done by the following mechanism.
1007      */
1008 
1009     /*
1010      * wb setpoint
1011      *
1012      *        f(wb_dirty) := 1.0 + k * (wb_dirty - wb_setpoint)
1013      *
1014      *                        x_intercept - wb_dirty
1015      *                     := --------------------------
1016      *                        x_intercept - wb_setpoint
1017      *
1018      * The main wb control line is a linear function that subjects to
1019      *
1020      * (1) f(wb_setpoint) = 1.0
1021      * (2) k = - 1 / (8 * write_bw)  (in single wb case)
1022      *     or equally: x_intercept = wb_setpoint + 8 * write_bw
1023      *
1024      * For single wb case, the dirty pages are observed to fluctuate
1025      * regularly within range
1026      *        [wb_setpoint - write_bw/2, wb_setpoint + write_bw/2]
1027      * for various filesystems, where (2) can yield in a reasonable 12.5%
1028      * fluctuation range for pos_ratio.
1029      *
1030      * For JBOD case, wb_thresh (not wb_dirty!) could fluctuate up to its
1031      * own size, so move the slope over accordingly and choose a slope that
1032      * yields 100% pos_ratio fluctuation on suddenly doubled wb_thresh.
1033      */
1034     if (unlikely(wb_thresh > dtc->thresh))
1035         wb_thresh = dtc->thresh;
1036     /*
1037      * It's very possible that wb_thresh is close to 0 not because the
1038      * device is slow, but that it has remained inactive for long time.
1039      * Honour such devices a reasonable good (hopefully IO efficient)
1040      * threshold, so that the occasional writes won't be blocked and active
1041      * writes can rampup the threshold quickly.
1042      */
1043     wb_thresh = max(wb_thresh, (limit - dtc->dirty) / 8);
1044     /*
1045      * scale global setpoint to wb's:
1046      *  wb_setpoint = setpoint * wb_thresh / thresh
1047      */
1048     x = div_u64((u64)wb_thresh << 16, dtc->thresh | 1);
1049     wb_setpoint = setpoint * (u64)x >> 16;
1050     /*
1051      * Use span=(8*write_bw) in single wb case as indicated by
1052      * (thresh - wb_thresh ~= 0) and transit to wb_thresh in JBOD case.
1053      *
1054      *        wb_thresh                    thresh - wb_thresh
1055      * span = --------- * (8 * write_bw) + ------------------ * wb_thresh
1056      *         thresh                           thresh
1057      */
1058     span = (dtc->thresh - wb_thresh + 8 * write_bw) * (u64)x >> 16;
1059     x_intercept = wb_setpoint + span;
1060 
1061     if (dtc->wb_dirty < x_intercept - span / 4) {
1062         pos_ratio = div64_u64(pos_ratio * (x_intercept - dtc->wb_dirty),
1063                       (x_intercept - wb_setpoint) | 1);
1064     } else
1065         pos_ratio /= 4;
1066 
1067     /*
1068      * wb reserve area, safeguard against dirty pool underrun and disk idle
1069      * It may push the desired control point of global dirty pages higher
1070      * than setpoint.
1071      */
1072     x_intercept = wb_thresh / 2;
1073     if (dtc->wb_dirty < x_intercept) {
1074         if (dtc->wb_dirty > x_intercept / 8)
1075             pos_ratio = div_u64(pos_ratio * x_intercept,
1076                         dtc->wb_dirty);
1077         else
1078             pos_ratio *= 8;
1079     }
1080 
1081     dtc->pos_ratio = pos_ratio;
1082 }
1083 
1084 static void wb_update_write_bandwidth(struct bdi_writeback *wb,
1085                       unsigned long elapsed,
1086                       unsigned long written)
1087 {
1088     const unsigned long period = roundup_pow_of_two(3 * HZ);
1089     unsigned long avg = wb->avg_write_bandwidth;
1090     unsigned long old = wb->write_bandwidth;
1091     u64 bw;
1092 
1093     /*
1094      * bw = written * HZ / elapsed
1095      *
1096      *                   bw * elapsed + write_bandwidth * (period - elapsed)
1097      * write_bandwidth = ---------------------------------------------------
1098      *                                          period
1099      *
1100      * @written may have decreased due to account_page_redirty().
1101      * Avoid underflowing @bw calculation.
1102      */
1103     bw = written - min(written, wb->written_stamp);
1104     bw *= HZ;
1105     if (unlikely(elapsed > period)) {
1106         do_div(bw, elapsed);
1107         avg = bw;
1108         goto out;
1109     }
1110     bw += (u64)wb->write_bandwidth * (period - elapsed);
1111     bw >>= ilog2(period);
1112 
1113     /*
1114      * one more level of smoothing, for filtering out sudden spikes
1115      */
1116     if (avg > old && old >= (unsigned long)bw)
1117         avg -= (avg - old) >> 3;
1118 
1119     if (avg < old && old <= (unsigned long)bw)
1120         avg += (old - avg) >> 3;
1121 
1122 out:
1123     /* keep avg > 0 to guarantee that tot > 0 if there are dirty wbs */
1124     avg = max(avg, 1LU);
1125     if (wb_has_dirty_io(wb)) {
1126         long delta = avg - wb->avg_write_bandwidth;
1127         WARN_ON_ONCE(atomic_long_add_return(delta,
1128                     &wb->bdi->tot_write_bandwidth) <= 0);
1129     }
1130     wb->write_bandwidth = bw;
1131     wb->avg_write_bandwidth = avg;
1132 }
1133 
1134 static void update_dirty_limit(struct dirty_throttle_control *dtc)
1135 {
1136     struct wb_domain *dom = dtc_dom(dtc);
1137     unsigned long thresh = dtc->thresh;
1138     unsigned long limit = dom->dirty_limit;
1139 
1140     /*
1141      * Follow up in one step.
1142      */
1143     if (limit < thresh) {
1144         limit = thresh;
1145         goto update;
1146     }
1147 
1148     /*
1149      * Follow down slowly. Use the higher one as the target, because thresh
1150      * may drop below dirty. This is exactly the reason to introduce
1151      * dom->dirty_limit which is guaranteed to lie above the dirty pages.
1152      */
1153     thresh = max(thresh, dtc->dirty);
1154     if (limit > thresh) {
1155         limit -= (limit - thresh) >> 5;
1156         goto update;
1157     }
1158     return;
1159 update:
1160     dom->dirty_limit = limit;
1161 }
1162 
1163 static void domain_update_bandwidth(struct dirty_throttle_control *dtc,
1164                     unsigned long now)
1165 {
1166     struct wb_domain *dom = dtc_dom(dtc);
1167 
1168     /*
1169      * check locklessly first to optimize away locking for the most time
1170      */
1171     if (time_before(now, dom->dirty_limit_tstamp + BANDWIDTH_INTERVAL))
1172         return;
1173 
1174     spin_lock(&dom->lock);
1175     if (time_after_eq(now, dom->dirty_limit_tstamp + BANDWIDTH_INTERVAL)) {
1176         update_dirty_limit(dtc);
1177         dom->dirty_limit_tstamp = now;
1178     }
1179     spin_unlock(&dom->lock);
1180 }
1181 
1182 /*
1183  * Maintain wb->dirty_ratelimit, the base dirty throttle rate.
1184  *
1185  * Normal wb tasks will be curbed at or below it in long term.
1186  * Obviously it should be around (write_bw / N) when there are N dd tasks.
1187  */
1188 static void wb_update_dirty_ratelimit(struct dirty_throttle_control *dtc,
1189                       unsigned long dirtied,
1190                       unsigned long elapsed)
1191 {
1192     struct bdi_writeback *wb = dtc->wb;
1193     unsigned long dirty = dtc->dirty;
1194     unsigned long freerun = dirty_freerun_ceiling(dtc->thresh, dtc->bg_thresh);
1195     unsigned long limit = hard_dirty_limit(dtc_dom(dtc), dtc->thresh);
1196     unsigned long setpoint = (freerun + limit) / 2;
1197     unsigned long write_bw = wb->avg_write_bandwidth;
1198     unsigned long dirty_ratelimit = wb->dirty_ratelimit;
1199     unsigned long dirty_rate;
1200     unsigned long task_ratelimit;
1201     unsigned long balanced_dirty_ratelimit;
1202     unsigned long step;
1203     unsigned long x;
1204     unsigned long shift;
1205 
1206     /*
1207      * The dirty rate will match the writeout rate in long term, except
1208      * when dirty pages are truncated by userspace or re-dirtied by FS.
1209      */
1210     dirty_rate = (dirtied - wb->dirtied_stamp) * HZ / elapsed;
1211 
1212     /*
1213      * task_ratelimit reflects each dd's dirty rate for the past 200ms.
1214      */
1215     task_ratelimit = (u64)dirty_ratelimit *
1216                     dtc->pos_ratio >> RATELIMIT_CALC_SHIFT;
1217     task_ratelimit++; /* it helps rampup dirty_ratelimit from tiny values */
1218 
1219     /*
1220      * A linear estimation of the "balanced" throttle rate. The theory is,
1221      * if there are N dd tasks, each throttled at task_ratelimit, the wb's
1222      * dirty_rate will be measured to be (N * task_ratelimit). So the below
1223      * formula will yield the balanced rate limit (write_bw / N).
1224      *
1225      * Note that the expanded form is not a pure rate feedback:
1226      *  rate_(i+1) = rate_(i) * (write_bw / dirty_rate)          (1)
1227      * but also takes pos_ratio into account:
1228      *  rate_(i+1) = rate_(i) * (write_bw / dirty_rate) * pos_ratio  (2)
1229      *
1230      * (1) is not realistic because pos_ratio also takes part in balancing
1231      * the dirty rate.  Consider the state
1232      *  pos_ratio = 0.5                          (3)
1233      *  rate = 2 * (write_bw / N)                    (4)
1234      * If (1) is used, it will stuck in that state! Because each dd will
1235      * be throttled at
1236      *  task_ratelimit = pos_ratio * rate = (write_bw / N)       (5)
1237      * yielding
1238      *  dirty_rate = N * task_ratelimit = write_bw           (6)
1239      * put (6) into (1) we get
1240      *  rate_(i+1) = rate_(i)                        (7)
1241      *
1242      * So we end up using (2) to always keep
1243      *  rate_(i+1) ~= (write_bw / N)                     (8)
1244      * regardless of the value of pos_ratio. As long as (8) is satisfied,
1245      * pos_ratio is able to drive itself to 1.0, which is not only where
1246      * the dirty count meet the setpoint, but also where the slope of
1247      * pos_ratio is most flat and hence task_ratelimit is least fluctuated.
1248      */
1249     balanced_dirty_ratelimit = div_u64((u64)task_ratelimit * write_bw,
1250                        dirty_rate | 1);
1251     /*
1252      * balanced_dirty_ratelimit ~= (write_bw / N) <= write_bw
1253      */
1254     if (unlikely(balanced_dirty_ratelimit > write_bw))
1255         balanced_dirty_ratelimit = write_bw;
1256 
1257     /*
1258      * We could safely do this and return immediately:
1259      *
1260      *  wb->dirty_ratelimit = balanced_dirty_ratelimit;
1261      *
1262      * However to get a more stable dirty_ratelimit, the below elaborated
1263      * code makes use of task_ratelimit to filter out singular points and
1264      * limit the step size.
1265      *
1266      * The below code essentially only uses the relative value of
1267      *
1268      *  task_ratelimit - dirty_ratelimit
1269      *  = (pos_ratio - 1) * dirty_ratelimit
1270      *
1271      * which reflects the direction and size of dirty position error.
1272      */
1273 
1274     /*
1275      * dirty_ratelimit will follow balanced_dirty_ratelimit iff
1276      * task_ratelimit is on the same side of dirty_ratelimit, too.
1277      * For example, when
1278      * - dirty_ratelimit > balanced_dirty_ratelimit
1279      * - dirty_ratelimit > task_ratelimit (dirty pages are above setpoint)
1280      * lowering dirty_ratelimit will help meet both the position and rate
1281      * control targets. Otherwise, don't update dirty_ratelimit if it will
1282      * only help meet the rate target. After all, what the users ultimately
1283      * feel and care are stable dirty rate and small position error.
1284      *
1285      * |task_ratelimit - dirty_ratelimit| is used to limit the step size
1286      * and filter out the singular points of balanced_dirty_ratelimit. Which
1287      * keeps jumping around randomly and can even leap far away at times
1288      * due to the small 200ms estimation period of dirty_rate (we want to
1289      * keep that period small to reduce time lags).
1290      */
1291     step = 0;
1292 
1293     /*
1294      * For strictlimit case, calculations above were based on wb counters
1295      * and limits (starting from pos_ratio = wb_position_ratio() and up to
1296      * balanced_dirty_ratelimit = task_ratelimit * write_bw / dirty_rate).
1297      * Hence, to calculate "step" properly, we have to use wb_dirty as
1298      * "dirty" and wb_setpoint as "setpoint".
1299      *
1300      * We rampup dirty_ratelimit forcibly if wb_dirty is low because
1301      * it's possible that wb_thresh is close to zero due to inactivity
1302      * of backing device.
1303      */
1304     if (unlikely(wb->bdi->capabilities & BDI_CAP_STRICTLIMIT)) {
1305         dirty = dtc->wb_dirty;
1306         if (dtc->wb_dirty < 8)
1307             setpoint = dtc->wb_dirty + 1;
1308         else
1309             setpoint = (dtc->wb_thresh + dtc->wb_bg_thresh) / 2;
1310     }
1311 
1312     if (dirty < setpoint) {
1313         x = min3(wb->balanced_dirty_ratelimit,
1314              balanced_dirty_ratelimit, task_ratelimit);
1315         if (dirty_ratelimit < x)
1316             step = x - dirty_ratelimit;
1317     } else {
1318         x = max3(wb->balanced_dirty_ratelimit,
1319              balanced_dirty_ratelimit, task_ratelimit);
1320         if (dirty_ratelimit > x)
1321             step = dirty_ratelimit - x;
1322     }
1323 
1324     /*
1325      * Don't pursue 100% rate matching. It's impossible since the balanced
1326      * rate itself is constantly fluctuating. So decrease the track speed
1327      * when it gets close to the target. Helps eliminate pointless tremors.
1328      */
1329     shift = dirty_ratelimit / (2 * step + 1);
1330     if (shift < BITS_PER_LONG)
1331         step = DIV_ROUND_UP(step >> shift, 8);
1332     else
1333         step = 0;
1334 
1335     if (dirty_ratelimit < balanced_dirty_ratelimit)
1336         dirty_ratelimit += step;
1337     else
1338         dirty_ratelimit -= step;
1339 
1340     wb->dirty_ratelimit = max(dirty_ratelimit, 1UL);
1341     wb->balanced_dirty_ratelimit = balanced_dirty_ratelimit;
1342 
1343     trace_bdi_dirty_ratelimit(wb, dirty_rate, task_ratelimit);
1344 }
1345 
1346 static void __wb_update_bandwidth(struct dirty_throttle_control *gdtc,
1347                   struct dirty_throttle_control *mdtc,
1348                   unsigned long start_time,
1349                   bool update_ratelimit)
1350 {
1351     struct bdi_writeback *wb = gdtc->wb;
1352     unsigned long now = jiffies;
1353     unsigned long elapsed = now - wb->bw_time_stamp;
1354     unsigned long dirtied;
1355     unsigned long written;
1356 
1357     lockdep_assert_held(&wb->list_lock);
1358 
1359     /*
1360      * rate-limit, only update once every 200ms.
1361      */
1362     if (elapsed < BANDWIDTH_INTERVAL)
1363         return;
1364 
1365     dirtied = percpu_counter_read(&wb->stat[WB_DIRTIED]);
1366     written = percpu_counter_read(&wb->stat[WB_WRITTEN]);
1367 
1368     /*
1369      * Skip quiet periods when disk bandwidth is under-utilized.
1370      * (at least 1s idle time between two flusher runs)
1371      */
1372     if (elapsed > HZ && time_before(wb->bw_time_stamp, start_time))
1373         goto snapshot;
1374 
1375     if (update_ratelimit) {
1376         domain_update_bandwidth(gdtc, now);
1377         wb_update_dirty_ratelimit(gdtc, dirtied, elapsed);
1378 
1379         /*
1380          * @mdtc is always NULL if !CGROUP_WRITEBACK but the
1381          * compiler has no way to figure that out.  Help it.
1382          */
1383         if (IS_ENABLED(CONFIG_CGROUP_WRITEBACK) && mdtc) {
1384             domain_update_bandwidth(mdtc, now);
1385             wb_update_dirty_ratelimit(mdtc, dirtied, elapsed);
1386         }
1387     }
1388     wb_update_write_bandwidth(wb, elapsed, written);
1389 
1390 snapshot:
1391     wb->dirtied_stamp = dirtied;
1392     wb->written_stamp = written;
1393     wb->bw_time_stamp = now;
1394 }
1395 
1396 void wb_update_bandwidth(struct bdi_writeback *wb, unsigned long start_time)
1397 {
1398     struct dirty_throttle_control gdtc = { GDTC_INIT(wb) };
1399 
1400     __wb_update_bandwidth(&gdtc, NULL, start_time, false);
1401 }
1402 
1403 /*
1404  * After a task dirtied this many pages, balance_dirty_pages_ratelimited()
1405  * will look to see if it needs to start dirty throttling.
1406  *
1407  * If dirty_poll_interval is too low, big NUMA machines will call the expensive
1408  * global_page_state() too often. So scale it near-sqrt to the safety margin
1409  * (the number of pages we may dirty without exceeding the dirty limits).
1410  */
1411 static unsigned long dirty_poll_interval(unsigned long dirty,
1412                      unsigned long thresh)
1413 {
1414     if (thresh > dirty)
1415         return 1UL << (ilog2(thresh - dirty) >> 1);
1416 
1417     return 1;
1418 }
1419 
1420 static unsigned long wb_max_pause(struct bdi_writeback *wb,
1421                   unsigned long wb_dirty)
1422 {
1423     unsigned long bw = wb->avg_write_bandwidth;
1424     unsigned long t;
1425 
1426     /*
1427      * Limit pause time for small memory systems. If sleeping for too long
1428      * time, a small pool of dirty/writeback pages may go empty and disk go
1429      * idle.
1430      *
1431      * 8 serves as the safety ratio.
1432      */
1433     t = wb_dirty / (1 + bw / roundup_pow_of_two(1 + HZ / 8));
1434     t++;
1435 
1436     return min_t(unsigned long, t, MAX_PAUSE);
1437 }
1438 
1439 static long wb_min_pause(struct bdi_writeback *wb,
1440              long max_pause,
1441              unsigned long task_ratelimit,
1442              unsigned long dirty_ratelimit,
1443              int *nr_dirtied_pause)
1444 {
1445     long hi = ilog2(wb->avg_write_bandwidth);
1446     long lo = ilog2(wb->dirty_ratelimit);
1447     long t;     /* target pause */
1448     long pause; /* estimated next pause */
1449     int pages;  /* target nr_dirtied_pause */
1450 
1451     /* target for 10ms pause on 1-dd case */
1452     t = max(1, HZ / 100);
1453 
1454     /*
1455      * Scale up pause time for concurrent dirtiers in order to reduce CPU
1456      * overheads.
1457      *
1458      * (N * 10ms) on 2^N concurrent tasks.
1459      */
1460     if (hi > lo)
1461         t += (hi - lo) * (10 * HZ) / 1024;
1462 
1463     /*
1464      * This is a bit convoluted. We try to base the next nr_dirtied_pause
1465      * on the much more stable dirty_ratelimit. However the next pause time
1466      * will be computed based on task_ratelimit and the two rate limits may
1467      * depart considerably at some time. Especially if task_ratelimit goes
1468      * below dirty_ratelimit/2 and the target pause is max_pause, the next
1469      * pause time will be max_pause*2 _trimmed down_ to max_pause.  As a
1470      * result task_ratelimit won't be executed faithfully, which could
1471      * eventually bring down dirty_ratelimit.
1472      *
1473      * We apply two rules to fix it up:
1474      * 1) try to estimate the next pause time and if necessary, use a lower
1475      *    nr_dirtied_pause so as not to exceed max_pause. When this happens,
1476      *    nr_dirtied_pause will be "dancing" with task_ratelimit.
1477      * 2) limit the target pause time to max_pause/2, so that the normal
1478      *    small fluctuations of task_ratelimit won't trigger rule (1) and
1479      *    nr_dirtied_pause will remain as stable as dirty_ratelimit.
1480      */
1481     t = min(t, 1 + max_pause / 2);
1482     pages = dirty_ratelimit * t / roundup_pow_of_two(HZ);
1483 
1484     /*
1485      * Tiny nr_dirtied_pause is found to hurt I/O performance in the test
1486      * case fio-mmap-randwrite-64k, which does 16*{sync read, async write}.
1487      * When the 16 consecutive reads are often interrupted by some dirty
1488      * throttling pause during the async writes, cfq will go into idles
1489      * (deadline is fine). So push nr_dirtied_pause as high as possible
1490      * until reaches DIRTY_POLL_THRESH=32 pages.
1491      */
1492     if (pages < DIRTY_POLL_THRESH) {
1493         t = max_pause;
1494         pages = dirty_ratelimit * t / roundup_pow_of_two(HZ);
1495         if (pages > DIRTY_POLL_THRESH) {
1496             pages = DIRTY_POLL_THRESH;
1497             t = HZ * DIRTY_POLL_THRESH / dirty_ratelimit;
1498         }
1499     }
1500 
1501     pause = HZ * pages / (task_ratelimit + 1);
1502     if (pause > max_pause) {
1503         t = max_pause;
1504         pages = task_ratelimit * t / roundup_pow_of_two(HZ);
1505     }
1506 
1507     *nr_dirtied_pause = pages;
1508     /*
1509      * The minimal pause time will normally be half the target pause time.
1510      */
1511     return pages >= DIRTY_POLL_THRESH ? 1 + t / 2 : t;
1512 }
1513 
1514 static inline void wb_dirty_limits(struct dirty_throttle_control *dtc)
1515 {
1516     struct bdi_writeback *wb = dtc->wb;
1517     unsigned long wb_reclaimable;
1518 
1519     /*
1520      * wb_thresh is not treated as some limiting factor as
1521      * dirty_thresh, due to reasons
1522      * - in JBOD setup, wb_thresh can fluctuate a lot
1523      * - in a system with HDD and USB key, the USB key may somehow
1524      *   go into state (wb_dirty >> wb_thresh) either because
1525      *   wb_dirty starts high, or because wb_thresh drops low.
1526      *   In this case we don't want to hard throttle the USB key
1527      *   dirtiers for 100 seconds until wb_dirty drops under
1528      *   wb_thresh. Instead the auxiliary wb control line in
1529      *   wb_position_ratio() will let the dirtier task progress
1530      *   at some rate <= (write_bw / 2) for bringing down wb_dirty.
1531      */
1532     dtc->wb_thresh = __wb_calc_thresh(dtc);
1533     dtc->wb_bg_thresh = dtc->thresh ?
1534         div_u64((u64)dtc->wb_thresh * dtc->bg_thresh, dtc->thresh) : 0;
1535 
1536     /*
1537      * In order to avoid the stacked BDI deadlock we need
1538      * to ensure we accurately count the 'dirty' pages when
1539      * the threshold is low.
1540      *
1541      * Otherwise it would be possible to get thresh+n pages
1542      * reported dirty, even though there are thresh-m pages
1543      * actually dirty; with m+n sitting in the percpu
1544      * deltas.
1545      */
1546     if (dtc->wb_thresh < 2 * wb_stat_error(wb)) {
1547         wb_reclaimable = wb_stat_sum(wb, WB_RECLAIMABLE);
1548         dtc->wb_dirty = wb_reclaimable + wb_stat_sum(wb, WB_WRITEBACK);
1549     } else {
1550         wb_reclaimable = wb_stat(wb, WB_RECLAIMABLE);
1551         dtc->wb_dirty = wb_reclaimable + wb_stat(wb, WB_WRITEBACK);
1552     }
1553 }
1554 
1555 /*
1556  * balance_dirty_pages() must be called by processes which are generating dirty
1557  * data.  It looks at the number of dirty pages in the machine and will force
1558  * the caller to wait once crossing the (background_thresh + dirty_thresh) / 2.
1559  * If we're over `background_thresh' then the writeback threads are woken to
1560  * perform some writeout.
1561  */
1562 static void balance_dirty_pages(struct address_space *mapping,
1563                 struct bdi_writeback *wb,
1564                 unsigned long pages_dirtied)
1565 {
1566     struct dirty_throttle_control gdtc_stor = { GDTC_INIT(wb) };
1567     struct dirty_throttle_control mdtc_stor = { MDTC_INIT(wb, &gdtc_stor) };
1568     struct dirty_throttle_control * const gdtc = &gdtc_stor;
1569     struct dirty_throttle_control * const mdtc = mdtc_valid(&mdtc_stor) ?
1570                              &mdtc_stor : NULL;
1571     struct dirty_throttle_control *sdtc;
1572     unsigned long nr_reclaimable;   /* = file_dirty + unstable_nfs */
1573     long period;
1574     long pause;
1575     long max_pause;
1576     long min_pause;
1577     int nr_dirtied_pause;
1578     bool dirty_exceeded = false;
1579     unsigned long task_ratelimit;
1580     unsigned long dirty_ratelimit;
1581     struct backing_dev_info *bdi = wb->bdi;
1582     bool strictlimit = bdi->capabilities & BDI_CAP_STRICTLIMIT;
1583     unsigned long start_time = jiffies;
1584 
1585     for (;;) {
1586         unsigned long now = jiffies;
1587         unsigned long dirty, thresh, bg_thresh;
1588         unsigned long m_dirty = 0;  /* stop bogus uninit warnings */
1589         unsigned long m_thresh = 0;
1590         unsigned long m_bg_thresh = 0;
1591 
1592         /*
1593          * Unstable writes are a feature of certain networked
1594          * filesystems (i.e. NFS) in which data may have been
1595          * written to the server's write cache, but has not yet
1596          * been flushed to permanent storage.
1597          */
1598         nr_reclaimable = global_node_page_state(NR_FILE_DIRTY) +
1599                     global_node_page_state(NR_UNSTABLE_NFS);
1600         gdtc->avail = global_dirtyable_memory();
1601         gdtc->dirty = nr_reclaimable + global_node_page_state(NR_WRITEBACK);
1602 
1603         domain_dirty_limits(gdtc);
1604 
1605         if (unlikely(strictlimit)) {
1606             wb_dirty_limits(gdtc);
1607 
1608             dirty = gdtc->wb_dirty;
1609             thresh = gdtc->wb_thresh;
1610             bg_thresh = gdtc->wb_bg_thresh;
1611         } else {
1612             dirty = gdtc->dirty;
1613             thresh = gdtc->thresh;
1614             bg_thresh = gdtc->bg_thresh;
1615         }
1616 
1617         if (mdtc) {
1618             unsigned long filepages, headroom, writeback;
1619 
1620             /*
1621              * If @wb belongs to !root memcg, repeat the same
1622              * basic calculations for the memcg domain.
1623              */
1624             mem_cgroup_wb_stats(wb, &filepages, &headroom,
1625                         &mdtc->dirty, &writeback);
1626             mdtc->dirty += writeback;
1627             mdtc_calc_avail(mdtc, filepages, headroom);
1628 
1629             domain_dirty_limits(mdtc);
1630 
1631             if (unlikely(strictlimit)) {
1632                 wb_dirty_limits(mdtc);
1633                 m_dirty = mdtc->wb_dirty;
1634                 m_thresh = mdtc->wb_thresh;
1635                 m_bg_thresh = mdtc->wb_bg_thresh;
1636             } else {
1637                 m_dirty = mdtc->dirty;
1638                 m_thresh = mdtc->thresh;
1639                 m_bg_thresh = mdtc->bg_thresh;
1640             }
1641         }
1642 
1643         /*
1644          * Throttle it only when the background writeback cannot
1645          * catch-up. This avoids (excessively) small writeouts
1646          * when the wb limits are ramping up in case of !strictlimit.
1647          *
1648          * In strictlimit case make decision based on the wb counters
1649          * and limits. Small writeouts when the wb limits are ramping
1650          * up are the price we consciously pay for strictlimit-ing.
1651          *
1652          * If memcg domain is in effect, @dirty should be under
1653          * both global and memcg freerun ceilings.
1654          */
1655         if (dirty <= dirty_freerun_ceiling(thresh, bg_thresh) &&
1656             (!mdtc ||
1657              m_dirty <= dirty_freerun_ceiling(m_thresh, m_bg_thresh))) {
1658             unsigned long intv = dirty_poll_interval(dirty, thresh);
1659             unsigned long m_intv = ULONG_MAX;
1660 
1661             current->dirty_paused_when = now;
1662             current->nr_dirtied = 0;
1663             if (mdtc)
1664                 m_intv = dirty_poll_interval(m_dirty, m_thresh);
1665             current->nr_dirtied_pause = min(intv, m_intv);
1666             break;
1667         }
1668 
1669         if (unlikely(!writeback_in_progress(wb)))
1670             wb_start_background_writeback(wb);
1671 
1672         /*
1673          * Calculate global domain's pos_ratio and select the
1674          * global dtc by default.
1675          */
1676         if (!strictlimit)
1677             wb_dirty_limits(gdtc);
1678 
1679         dirty_exceeded = (gdtc->wb_dirty > gdtc->wb_thresh) &&
1680             ((gdtc->dirty > gdtc->thresh) || strictlimit);
1681 
1682         wb_position_ratio(gdtc);
1683         sdtc = gdtc;
1684 
1685         if (mdtc) {
1686             /*
1687              * If memcg domain is in effect, calculate its
1688              * pos_ratio.  @wb should satisfy constraints from
1689              * both global and memcg domains.  Choose the one
1690              * w/ lower pos_ratio.
1691              */
1692             if (!strictlimit)
1693                 wb_dirty_limits(mdtc);
1694 
1695             dirty_exceeded |= (mdtc->wb_dirty > mdtc->wb_thresh) &&
1696                 ((mdtc->dirty > mdtc->thresh) || strictlimit);
1697 
1698             wb_position_ratio(mdtc);
1699             if (mdtc->pos_ratio < gdtc->pos_ratio)
1700                 sdtc = mdtc;
1701         }
1702 
1703         if (dirty_exceeded && !wb->dirty_exceeded)
1704             wb->dirty_exceeded = 1;
1705 
1706         if (time_is_before_jiffies(wb->bw_time_stamp +
1707                        BANDWIDTH_INTERVAL)) {
1708             spin_lock(&wb->list_lock);
1709             __wb_update_bandwidth(gdtc, mdtc, start_time, true);
1710             spin_unlock(&wb->list_lock);
1711         }
1712 
1713         /* throttle according to the chosen dtc */
1714         dirty_ratelimit = wb->dirty_ratelimit;
1715         task_ratelimit = ((u64)dirty_ratelimit * sdtc->pos_ratio) >>
1716                             RATELIMIT_CALC_SHIFT;
1717         max_pause = wb_max_pause(wb, sdtc->wb_dirty);
1718         min_pause = wb_min_pause(wb, max_pause,
1719                      task_ratelimit, dirty_ratelimit,
1720                      &nr_dirtied_pause);
1721 
1722         if (unlikely(task_ratelimit == 0)) {
1723             period = max_pause;
1724             pause = max_pause;
1725             goto pause;
1726         }
1727         period = HZ * pages_dirtied / task_ratelimit;
1728         pause = period;
1729         if (current->dirty_paused_when)
1730             pause -= now - current->dirty_paused_when;
1731         /*
1732          * For less than 1s think time (ext3/4 may block the dirtier
1733          * for up to 800ms from time to time on 1-HDD; so does xfs,
1734          * however at much less frequency), try to compensate it in
1735          * future periods by updating the virtual time; otherwise just
1736          * do a reset, as it may be a light dirtier.
1737          */
1738         if (pause < min_pause) {
1739             trace_balance_dirty_pages(wb,
1740                           sdtc->thresh,
1741                           sdtc->bg_thresh,
1742                           sdtc->dirty,
1743                           sdtc->wb_thresh,
1744                           sdtc->wb_dirty,
1745                           dirty_ratelimit,
1746                           task_ratelimit,
1747                           pages_dirtied,
1748                           period,
1749                           min(pause, 0L),
1750                           start_time);
1751             if (pause < -HZ) {
1752                 current->dirty_paused_when = now;
1753                 current->nr_dirtied = 0;
1754             } else if (period) {
1755                 current->dirty_paused_when += period;
1756                 current->nr_dirtied = 0;
1757             } else if (current->nr_dirtied_pause <= pages_dirtied)
1758                 current->nr_dirtied_pause += pages_dirtied;
1759             break;
1760         }
1761         if (unlikely(pause > max_pause)) {
1762             /* for occasional dropped task_ratelimit */
1763             now += min(pause - max_pause, max_pause);
1764             pause = max_pause;
1765         }
1766 
1767 pause:
1768         trace_balance_dirty_pages(wb,
1769                       sdtc->thresh,
1770                       sdtc->bg_thresh,
1771                       sdtc->dirty,
1772                       sdtc->wb_thresh,
1773                       sdtc->wb_dirty,
1774                       dirty_ratelimit,
1775                       task_ratelimit,
1776                       pages_dirtied,
1777                       period,
1778                       pause,
1779                       start_time);
1780         __set_current_state(TASK_KILLABLE);
1781         wb->dirty_sleep = now;
1782         io_schedule_timeout(pause);
1783 
1784         current->dirty_paused_when = now + pause;
1785         current->nr_dirtied = 0;
1786         current->nr_dirtied_pause = nr_dirtied_pause;
1787 
1788         /*
1789          * This is typically equal to (dirty < thresh) and can also
1790          * keep "1000+ dd on a slow USB stick" under control.
1791          */
1792         if (task_ratelimit)
1793             break;
1794 
1795         /*
1796          * In the case of an unresponding NFS server and the NFS dirty
1797          * pages exceeds dirty_thresh, give the other good wb's a pipe
1798          * to go through, so that tasks on them still remain responsive.
1799          *
1800          * In theory 1 page is enough to keep the comsumer-producer
1801          * pipe going: the flusher cleans 1 page => the task dirties 1
1802          * more page. However wb_dirty has accounting errors.  So use
1803          * the larger and more IO friendly wb_stat_error.
1804          */
1805         if (sdtc->wb_dirty <= wb_stat_error(wb))
1806             break;
1807 
1808         if (fatal_signal_pending(current))
1809             break;
1810     }
1811 
1812     if (!dirty_exceeded && wb->dirty_exceeded)
1813         wb->dirty_exceeded = 0;
1814 
1815     if (writeback_in_progress(wb))
1816         return;
1817 
1818     /*
1819      * In laptop mode, we wait until hitting the higher threshold before
1820      * starting background writeout, and then write out all the way down
1821      * to the lower threshold.  So slow writers cause minimal disk activity.
1822      *
1823      * In normal mode, we start background writeout at the lower
1824      * background_thresh, to keep the amount of dirty memory low.
1825      */
1826     if (laptop_mode)
1827         return;
1828 
1829     if (nr_reclaimable > gdtc->bg_thresh)
1830         wb_start_background_writeback(wb);
1831 }
1832 
1833 static DEFINE_PER_CPU(int, bdp_ratelimits);
1834 
1835 /*
1836  * Normal tasks are throttled by
1837  *  loop {
1838  *      dirty tsk->nr_dirtied_pause pages;
1839  *      take a snap in balance_dirty_pages();
1840  *  }
1841  * However there is a worst case. If every task exit immediately when dirtied
1842  * (tsk->nr_dirtied_pause - 1) pages, balance_dirty_pages() will never be
1843  * called to throttle the page dirties. The solution is to save the not yet
1844  * throttled page dirties in dirty_throttle_leaks on task exit and charge them
1845  * randomly into the running tasks. This works well for the above worst case,
1846  * as the new task will pick up and accumulate the old task's leaked dirty
1847  * count and eventually get throttled.
1848  */
1849 DEFINE_PER_CPU(int, dirty_throttle_leaks) = 0;
1850 
1851 /**
1852  * balance_dirty_pages_ratelimited - balance dirty memory state
1853  * @mapping: address_space which was dirtied
1854  *
1855  * Processes which are dirtying memory should call in here once for each page
1856  * which was newly dirtied.  The function will periodically check the system's
1857  * dirty state and will initiate writeback if needed.
1858  *
1859  * On really big machines, get_writeback_state is expensive, so try to avoid
1860  * calling it too often (ratelimiting).  But once we're over the dirty memory
1861  * limit we decrease the ratelimiting by a lot, to prevent individual processes
1862  * from overshooting the limit by (ratelimit_pages) each.
1863  */
1864 void balance_dirty_pages_ratelimited(struct address_space *mapping)
1865 {
1866     struct inode *inode = mapping->host;
1867     struct backing_dev_info *bdi = inode_to_bdi(inode);
1868     struct bdi_writeback *wb = NULL;
1869     int ratelimit;
1870     int *p;
1871 
1872     if (!bdi_cap_account_dirty(bdi))
1873         return;
1874 
1875     if (inode_cgwb_enabled(inode))
1876         wb = wb_get_create_current(bdi, GFP_KERNEL);
1877     if (!wb)
1878         wb = &bdi->wb;
1879 
1880     ratelimit = current->nr_dirtied_pause;
1881     if (wb->dirty_exceeded)
1882         ratelimit = min(ratelimit, 32 >> (PAGE_SHIFT - 10));
1883 
1884     preempt_disable();
1885     /*
1886      * This prevents one CPU to accumulate too many dirtied pages without
1887      * calling into balance_dirty_pages(), which can happen when there are
1888      * 1000+ tasks, all of them start dirtying pages at exactly the same
1889      * time, hence all honoured too large initial task->nr_dirtied_pause.
1890      */
1891     p =  this_cpu_ptr(&bdp_ratelimits);
1892     if (unlikely(current->nr_dirtied >= ratelimit))
1893         *p = 0;
1894     else if (unlikely(*p >= ratelimit_pages)) {
1895         *p = 0;
1896         ratelimit = 0;
1897     }
1898     /*
1899      * Pick up the dirtied pages by the exited tasks. This avoids lots of
1900      * short-lived tasks (eg. gcc invocations in a kernel build) escaping
1901      * the dirty throttling and livelock other long-run dirtiers.
1902      */
1903     p = this_cpu_ptr(&dirty_throttle_leaks);
1904     if (*p > 0 && current->nr_dirtied < ratelimit) {
1905         unsigned long nr_pages_dirtied;
1906         nr_pages_dirtied = min(*p, ratelimit - current->nr_dirtied);
1907         *p -= nr_pages_dirtied;
1908         current->nr_dirtied += nr_pages_dirtied;
1909     }
1910     preempt_enable();
1911 
1912     if (unlikely(current->nr_dirtied >= ratelimit))
1913         balance_dirty_pages(mapping, wb, current->nr_dirtied);
1914 
1915     wb_put(wb);
1916 }
1917 EXPORT_SYMBOL(balance_dirty_pages_ratelimited);
1918 
1919 /**
1920  * wb_over_bg_thresh - does @wb need to be written back?
1921  * @wb: bdi_writeback of interest
1922  *
1923  * Determines whether background writeback should keep writing @wb or it's
1924  * clean enough.  Returns %true if writeback should continue.
1925  */
1926 bool wb_over_bg_thresh(struct bdi_writeback *wb)
1927 {
1928     struct dirty_throttle_control gdtc_stor = { GDTC_INIT(wb) };
1929     struct dirty_throttle_control mdtc_stor = { MDTC_INIT(wb, &gdtc_stor) };
1930     struct dirty_throttle_control * const gdtc = &gdtc_stor;
1931     struct dirty_throttle_control * const mdtc = mdtc_valid(&mdtc_stor) ?
1932                              &mdtc_stor : NULL;
1933 
1934     /*
1935      * Similar to balance_dirty_pages() but ignores pages being written
1936      * as we're trying to decide whether to put more under writeback.
1937      */
1938     gdtc->avail = global_dirtyable_memory();
1939     gdtc->dirty = global_node_page_state(NR_FILE_DIRTY) +
1940               global_node_page_state(NR_UNSTABLE_NFS);
1941     domain_dirty_limits(gdtc);
1942 
1943     if (gdtc->dirty > gdtc->bg_thresh)
1944         return true;
1945 
1946     if (wb_stat(wb, WB_RECLAIMABLE) >
1947         wb_calc_thresh(gdtc->wb, gdtc->bg_thresh))
1948         return true;
1949 
1950     if (mdtc) {
1951         unsigned long filepages, headroom, writeback;
1952 
1953         mem_cgroup_wb_stats(wb, &filepages, &headroom, &mdtc->dirty,
1954                     &writeback);
1955         mdtc_calc_avail(mdtc, filepages, headroom);
1956         domain_dirty_limits(mdtc);  /* ditto, ignore writeback */
1957 
1958         if (mdtc->dirty > mdtc->bg_thresh)
1959             return true;
1960 
1961         if (wb_stat(wb, WB_RECLAIMABLE) >
1962             wb_calc_thresh(mdtc->wb, mdtc->bg_thresh))
1963             return true;
1964     }
1965 
1966     return false;
1967 }
1968 
1969 /*
1970  * sysctl handler for /proc/sys/vm/dirty_writeback_centisecs
1971  */
1972 int dirty_writeback_centisecs_handler(struct ctl_table *table, int write,
1973     void __user *buffer, size_t *length, loff_t *ppos)
1974 {
1975     proc_dointvec(table, write, buffer, length, ppos);
1976     return 0;
1977 }
1978 
1979 #ifdef CONFIG_BLOCK
1980 void laptop_mode_timer_fn(unsigned long data)
1981 {
1982     struct request_queue *q = (struct request_queue *)data;
1983     int nr_pages = global_node_page_state(NR_FILE_DIRTY) +
1984         global_node_page_state(NR_UNSTABLE_NFS);
1985     struct bdi_writeback *wb;
1986 
1987     /*
1988      * We want to write everything out, not just down to the dirty
1989      * threshold
1990      */
1991     if (!bdi_has_dirty_io(&q->backing_dev_info))
1992         return;
1993 
1994     rcu_read_lock();
1995     list_for_each_entry_rcu(wb, &q->backing_dev_info.wb_list, bdi_node)
1996         if (wb_has_dirty_io(wb))
1997             wb_start_writeback(wb, nr_pages, true,
1998                        WB_REASON_LAPTOP_TIMER);
1999     rcu_read_unlock();
2000 }
2001 
2002 /*
2003  * We've spun up the disk and we're in laptop mode: schedule writeback
2004  * of all dirty data a few seconds from now.  If the flush is already scheduled
2005  * then push it back - the user is still using the disk.
2006  */
2007 void laptop_io_completion(struct backing_dev_info *info)
2008 {
2009     mod_timer(&info->laptop_mode_wb_timer, jiffies + laptop_mode);
2010 }
2011 
2012 /*
2013  * We're in laptop mode and we've just synced. The sync's writes will have
2014  * caused another writeback to be scheduled by laptop_io_completion.
2015  * Nothing needs to be written back anymore, so we unschedule the writeback.
2016  */
2017 void laptop_sync_completion(void)
2018 {
2019     struct backing_dev_info *bdi;
2020 
2021     rcu_read_lock();
2022 
2023     list_for_each_entry_rcu(bdi, &bdi_list, bdi_list)
2024         del_timer(&bdi->laptop_mode_wb_timer);
2025 
2026     rcu_read_unlock();
2027 }
2028 #endif
2029 
2030 /*
2031  * If ratelimit_pages is too high then we can get into dirty-data overload
2032  * if a large number of processes all perform writes at the same time.
2033  * If it is too low then SMP machines will call the (expensive)
2034  * get_writeback_state too often.
2035  *
2036  * Here we set ratelimit_pages to a level which ensures that when all CPUs are
2037  * dirtying in parallel, we cannot go more than 3% (1/32) over the dirty memory
2038  * thresholds.
2039  */
2040 
2041 void writeback_set_ratelimit(void)
2042 {
2043     struct wb_domain *dom = &global_wb_domain;
2044     unsigned long background_thresh;
2045     unsigned long dirty_thresh;
2046 
2047     global_dirty_limits(&background_thresh, &dirty_thresh);
2048     dom->dirty_limit = dirty_thresh;
2049     ratelimit_pages = dirty_thresh / (num_online_cpus() * 32);
2050     if (ratelimit_pages < 16)
2051         ratelimit_pages = 16;
2052 }
2053 
2054 static int page_writeback_cpu_online(unsigned int cpu)
2055 {
2056     writeback_set_ratelimit();
2057     return 0;
2058 }
2059 
2060 /*
2061  * Called early on to tune the page writeback dirty limits.
2062  *
2063  * We used to scale dirty pages according to how total memory
2064  * related to pages that could be allocated for buffers (by
2065  * comparing nr_free_buffer_pages() to vm_total_pages.
2066  *
2067  * However, that was when we used "dirty_ratio" to scale with
2068  * all memory, and we don't do that any more. "dirty_ratio"
2069  * is now applied to total non-HIGHPAGE memory (by subtracting
2070  * totalhigh_pages from vm_total_pages), and as such we can't
2071  * get into the old insane situation any more where we had
2072  * large amounts of dirty pages compared to a small amount of
2073  * non-HIGHMEM memory.
2074  *
2075  * But we might still want to scale the dirty_ratio by how
2076  * much memory the box has..
2077  */
2078 void __init page_writeback_init(void)
2079 {
2080     BUG_ON(wb_domain_init(&global_wb_domain, GFP_KERNEL));
2081 
2082     cpuhp_setup_state(CPUHP_AP_ONLINE_DYN, "mm/writeback:online",
2083               page_writeback_cpu_online, NULL);
2084     cpuhp_setup_state(CPUHP_MM_WRITEBACK_DEAD, "mm/writeback:dead", NULL,
2085               page_writeback_cpu_online);
2086 }
2087 
2088 /**
2089  * tag_pages_for_writeback - tag pages to be written by write_cache_pages
2090  * @mapping: address space structure to write
2091  * @start: starting page index
2092  * @end: ending page index (inclusive)
2093  *
2094  * This function scans the page range from @start to @end (inclusive) and tags
2095  * all pages that have DIRTY tag set with a special TOWRITE tag. The idea is
2096  * that write_cache_pages (or whoever calls this function) will then use
2097  * TOWRITE tag to identify pages eligible for writeback.  This mechanism is
2098  * used to avoid livelocking of writeback by a process steadily creating new
2099  * dirty pages in the file (thus it is important for this function to be quick
2100  * so that it can tag pages faster than a dirtying process can create them).
2101  */
2102 /*
2103  * We tag pages in batches of WRITEBACK_TAG_BATCH to reduce tree_lock latency.
2104  */
2105 void tag_pages_for_writeback(struct address_space *mapping,
2106                  pgoff_t start, pgoff_t end)
2107 {
2108 #define WRITEBACK_TAG_BATCH 4096
2109     unsigned long tagged = 0;
2110     struct radix_tree_iter iter;
2111     void **slot;
2112 
2113     spin_lock_irq(&mapping->tree_lock);
2114     radix_tree_for_each_tagged(slot, &mapping->page_tree, &iter, start,
2115                             PAGECACHE_TAG_DIRTY) {
2116         if (iter.index > end)
2117             break;
2118         radix_tree_iter_tag_set(&mapping->page_tree, &iter,
2119                             PAGECACHE_TAG_TOWRITE);
2120         tagged++;
2121         if ((tagged % WRITEBACK_TAG_BATCH) != 0)
2122             continue;
2123         slot = radix_tree_iter_resume(slot, &iter);
2124         spin_unlock_irq(&mapping->tree_lock);
2125         cond_resched();
2126         spin_lock_irq(&mapping->tree_lock);
2127     }
2128     spin_unlock_irq(&mapping->tree_lock);
2129 }
2130 EXPORT_SYMBOL(tag_pages_for_writeback);
2131 
2132 /**
2133  * write_cache_pages - walk the list of dirty pages of the given address space and write all of them.
2134  * @mapping: address space structure to write
2135  * @wbc: subtract the number of written pages from *@wbc->nr_to_write
2136  * @writepage: function called for each page
2137  * @data: data passed to writepage function
2138  *
2139  * If a page is already under I/O, write_cache_pages() skips it, even
2140  * if it's dirty.  This is desirable behaviour for memory-cleaning writeback,
2141  * but it is INCORRECT for data-integrity system calls such as fsync().  fsync()
2142  * and msync() need to guarantee that all the data which was dirty at the time
2143  * the call was made get new I/O started against them.  If wbc->sync_mode is
2144  * WB_SYNC_ALL then we were called for data integrity and we must wait for
2145  * existing IO to complete.
2146  *
2147  * To avoid livelocks (when other process dirties new pages), we first tag
2148  * pages which should be written back with TOWRITE tag and only then start
2149  * writing them. For data-integrity sync we have to be careful so that we do
2150  * not miss some pages (e.g., because some other process has cleared TOWRITE
2151  * tag we set). The rule we follow is that TOWRITE tag can be cleared only
2152  * by the process clearing the DIRTY tag (and submitting the page for IO).
2153  */
2154 int write_cache_pages(struct address_space *mapping,
2155               struct writeback_control *wbc, writepage_t writepage,
2156               void *data)
2157 {
2158     int ret = 0;
2159     int done = 0;
2160     struct pagevec pvec;
2161     int nr_pages;
2162     pgoff_t uninitialized_var(writeback_index);
2163     pgoff_t index;
2164     pgoff_t end;        /* Inclusive */
2165     pgoff_t done_index;
2166     int cycled;
2167     int range_whole = 0;
2168     int tag;
2169 
2170     pagevec_init(&pvec, 0);
2171     if (wbc->range_cyclic) {
2172         writeback_index = mapping->writeback_index; /* prev offset */
2173         index = writeback_index;
2174         if (index == 0)
2175             cycled = 1;
2176         else
2177             cycled = 0;
2178         end = -1;
2179     } else {
2180         index = wbc->range_start >> PAGE_SHIFT;
2181         end = wbc->range_end >> PAGE_SHIFT;
2182         if (wbc->range_start == 0 && wbc->range_end == LLONG_MAX)
2183             range_whole = 1;
2184         cycled = 1; /* ignore range_cyclic tests */
2185     }
2186     if (wbc->sync_mode == WB_SYNC_ALL || wbc->tagged_writepages)
2187         tag = PAGECACHE_TAG_TOWRITE;
2188     else
2189         tag = PAGECACHE_TAG_DIRTY;
2190 retry:
2191     if (wbc->sync_mode == WB_SYNC_ALL || wbc->tagged_writepages)
2192         tag_pages_for_writeback(mapping, index, end);
2193     done_index = index;
2194     while (!done && (index <= end)) {
2195         int i;
2196 
2197         nr_pages = pagevec_lookup_tag(&pvec, mapping, &index, tag,
2198                   min(end - index, (pgoff_t)PAGEVEC_SIZE-1) + 1);
2199         if (nr_pages == 0)
2200             break;
2201 
2202         for (i = 0; i < nr_pages; i++) {
2203             struct page *page = pvec.pages[i];
2204 
2205             /*
2206              * At this point, the page may be truncated or
2207              * invalidated (changing page->mapping to NULL), or
2208              * even swizzled back from swapper_space to tmpfs file
2209              * mapping. However, page->index will not change
2210              * because we have a reference on the page.
2211              */
2212             if (page->index > end) {
2213                 /*
2214                  * can't be range_cyclic (1st pass) because
2215                  * end == -1 in that case.
2216                  */
2217                 done = 1;
2218                 break;
2219             }
2220 
2221             done_index = page->index;
2222 
2223             lock_page(page);
2224 
2225             /*
2226              * Page truncated or invalidated. We can freely skip it
2227              * then, even for data integrity operations: the page
2228              * has disappeared concurrently, so there could be no
2229              * real expectation of this data interity operation
2230              * even if there is now a new, dirty page at the same
2231              * pagecache address.
2232              */
2233             if (unlikely(page->mapping != mapping)) {
2234 continue_unlock:
2235                 unlock_page(page);
2236                 continue;
2237             }
2238 
2239             if (!PageDirty(page)) {
2240                 /* someone wrote it for us */
2241                 goto continue_unlock;
2242             }
2243 
2244             if (PageWriteback(page)) {
2245                 if (wbc->sync_mode != WB_SYNC_NONE)
2246                     wait_on_page_writeback(page);
2247                 else
2248                     goto continue_unlock;
2249             }
2250 
2251             BUG_ON(PageWriteback(page));
2252             if (!clear_page_dirty_for_io(page))
2253                 goto continue_unlock;
2254 
2255             trace_wbc_writepage(wbc, inode_to_bdi(mapping->host));
2256             ret = (*writepage)(page, wbc, data);
2257             if (unlikely(ret)) {
2258                 if (ret == AOP_WRITEPAGE_ACTIVATE) {
2259                     unlock_page(page);
2260                     ret = 0;
2261                 } else {
2262                     /*
2263                      * done_index is set past this page,
2264                      * so media errors will not choke
2265                      * background writeout for the entire
2266                      * file. This has consequences for
2267                      * range_cyclic semantics (ie. it may
2268                      * not be suitable for data integrity
2269                      * writeout).
2270                      */
2271                     done_index = page->index + 1;
2272                     done = 1;
2273                     break;
2274                 }
2275             }
2276 
2277             /*
2278              * We stop writing back only if we are not doing
2279              * integrity sync. In case of integrity sync we have to
2280              * keep going until we have written all the pages
2281              * we tagged for writeback prior to entering this loop.
2282              */
2283             if (--wbc->nr_to_write <= 0 &&
2284                 wbc->sync_mode == WB_SYNC_NONE) {
2285                 done = 1;
2286                 break;
2287             }
2288         }
2289         pagevec_release(&pvec);
2290         cond_resched();
2291     }
2292     if (!cycled && !done) {
2293         /*
2294          * range_cyclic:
2295          * We hit the last page and there is more work to be done: wrap
2296          * back to the start of the file
2297          */
2298         cycled = 1;
2299         index = 0;
2300         end = writeback_index - 1;
2301         goto retry;
2302     }
2303     if (wbc->range_cyclic || (range_whole && wbc->nr_to_write > 0))
2304         mapping->writeback_index = done_index;
2305 
2306     return ret;
2307 }
2308 EXPORT_SYMBOL(write_cache_pages);
2309 
2310 /*
2311  * Function used by generic_writepages to call the real writepage
2312  * function and set the mapping flags on error
2313  */
2314 static int __writepage(struct page *page, struct writeback_control *wbc,
2315                void *data)
2316 {
2317     struct address_space *mapping = data;
2318     int ret = mapping->a_ops->writepage(page, wbc);
2319     mapping_set_error(mapping, ret);
2320     return ret;
2321 }
2322 
2323 /**
2324  * generic_writepages - walk the list of dirty pages of the given address space and writepage() all of them.
2325  * @mapping: address space structure to write
2326  * @wbc: subtract the number of written pages from *@wbc->nr_to_write
2327  *
2328  * This is a library function, which implements the writepages()
2329  * address_space_operation.
2330  */
2331 int generic_writepages(struct address_space *mapping,
2332                struct writeback_control *wbc)
2333 {
2334     struct blk_plug plug;
2335     int ret;
2336 
2337     /* deal with chardevs and other special file */
2338     if (!mapping->a_ops->writepage)
2339         return 0;
2340 
2341     blk_start_plug(&plug);
2342     ret = write_cache_pages(mapping, wbc, __writepage, mapping);
2343     blk_finish_plug(&plug);
2344     return ret;
2345 }
2346 
2347 EXPORT_SYMBOL(generic_writepages);
2348 
2349 int do_writepages(struct address_space *mapping, struct writeback_control *wbc)
2350 {
2351     int ret;
2352 
2353     if (wbc->nr_to_write <= 0)
2354         return 0;
2355     if (mapping->a_ops->writepages)
2356         ret = mapping->a_ops->writepages(mapping, wbc);
2357     else
2358         ret = generic_writepages(mapping, wbc);
2359     return ret;
2360 }
2361 
2362 /**
2363  * write_one_page - write out a single page and optionally wait on I/O
2364  * @page: the page to write
2365  * @wait: if true, wait on writeout
2366  *
2367  * The page must be locked by the caller and will be unlocked upon return.
2368  *
2369  * write_one_page() returns a negative error code if I/O failed.
2370  */
2371 int write_one_page(struct page *page, int wait)
2372 {
2373     struct address_space *mapping = page->mapping;
2374     int ret = 0;
2375     struct writeback_control wbc = {
2376         .sync_mode = WB_SYNC_ALL,
2377         .nr_to_write = 1,
2378     };
2379 
2380     BUG_ON(!PageLocked(page));
2381 
2382     if (wait)
2383         wait_on_page_writeback(page);
2384 
2385     if (clear_page_dirty_for_io(page)) {
2386         get_page(page);
2387         ret = mapping->a_ops->writepage(page, &wbc);
2388         if (ret == 0 && wait) {
2389             wait_on_page_writeback(page);
2390             if (PageError(page))
2391                 ret = -EIO;
2392         }
2393         put_page(page);
2394     } else {
2395         unlock_page(page);
2396     }
2397     return ret;
2398 }
2399 EXPORT_SYMBOL(write_one_page);
2400 
2401 /*
2402  * For address_spaces which do not use buffers nor write back.
2403  */
2404 int __set_page_dirty_no_writeback(struct page *page)
2405 {
2406     if (!PageDirty(page))
2407         return !TestSetPageDirty(page);
2408     return 0;
2409 }
2410 
2411 /*
2412  * Helper function for set_page_dirty family.
2413  *
2414  * Caller must hold lock_page_memcg().
2415  *
2416  * NOTE: This relies on being atomic wrt interrupts.
2417  */
2418 void account_page_dirtied(struct page *page, struct address_space *mapping)
2419 {
2420     struct inode *inode = mapping->host;
2421 
2422     trace_writeback_dirty_page(page, mapping);
2423 
2424     if (mapping_cap_account_dirty(mapping)) {
2425         struct bdi_writeback *wb;
2426 
2427         inode_attach_wb(inode, page);
2428         wb = inode_to_wb(inode);
2429 
2430         mem_cgroup_inc_page_stat(page, MEM_CGROUP_STAT_DIRTY);
2431         __inc_node_page_state(page, NR_FILE_DIRTY);
2432         __inc_zone_page_state(page, NR_ZONE_WRITE_PENDING);
2433         __inc_node_page_state(page, NR_DIRTIED);
2434         __inc_wb_stat(wb, WB_RECLAIMABLE);
2435         __inc_wb_stat(wb, WB_DIRTIED);
2436         task_io_account_write(PAGE_SIZE);
2437         current->nr_dirtied++;
2438         this_cpu_inc(bdp_ratelimits);
2439     }
2440 }
2441 EXPORT_SYMBOL(account_page_dirtied);
2442 
2443 /*
2444  * Helper function for deaccounting dirty page without writeback.
2445  *
2446  * Caller must hold lock_page_memcg().
2447  */
2448 void account_page_cleaned(struct page *page, struct address_space *mapping,
2449               struct bdi_writeback *wb)
2450 {
2451     if (mapping_cap_account_dirty(mapping)) {
2452         mem_cgroup_dec_page_stat(page, MEM_CGROUP_STAT_DIRTY);
2453         dec_node_page_state(page, NR_FILE_DIRTY);
2454         dec_zone_page_state(page, NR_ZONE_WRITE_PENDING);
2455         dec_wb_stat(wb, WB_RECLAIMABLE);
2456         task_io_account_cancelled_write(PAGE_SIZE);
2457     }
2458 }
2459 
2460 /*
2461  * For address_spaces which do not use buffers.  Just tag the page as dirty in
2462  * its radix tree.
2463  *
2464  * This is also used when a single buffer is being dirtied: we want to set the
2465  * page dirty in that case, but not all the buffers.  This is a "bottom-up"
2466  * dirtying, whereas __set_page_dirty_buffers() is a "top-down" dirtying.
2467  *
2468  * The caller must ensure this doesn't race with truncation.  Most will simply
2469  * hold the page lock, but e.g. zap_pte_range() calls with the page mapped and
2470  * the pte lock held, which also locks out truncation.
2471  */
2472 int __set_page_dirty_nobuffers(struct page *page)
2473 {
2474     lock_page_memcg(page);
2475     if (!TestSetPageDirty(page)) {
2476         struct address_space *mapping = page_mapping(page);
2477         unsigned long flags;
2478 
2479         if (!mapping) {
2480             unlock_page_memcg(page);
2481             return 1;
2482         }
2483 
2484         spin_lock_irqsave(&mapping->tree_lock, flags);
2485         BUG_ON(page_mapping(page) != mapping);
2486         WARN_ON_ONCE(!PagePrivate(page) && !PageUptodate(page));
2487         account_page_dirtied(page, mapping);
2488         radix_tree_tag_set(&mapping->page_tree, page_index(page),
2489                    PAGECACHE_TAG_DIRTY);
2490         spin_unlock_irqrestore(&mapping->tree_lock, flags);
2491         unlock_page_memcg(page);
2492 
2493         if (mapping->host) {
2494             /* !PageAnon && !swapper_space */
2495             __mark_inode_dirty(mapping->host, I_DIRTY_PAGES);
2496         }
2497         return 1;
2498     }
2499     unlock_page_memcg(page);
2500     return 0;
2501 }
2502 EXPORT_SYMBOL(__set_page_dirty_nobuffers);
2503 
2504 /*
2505  * Call this whenever redirtying a page, to de-account the dirty counters
2506  * (NR_DIRTIED, BDI_DIRTIED, tsk->nr_dirtied), so that they match the written
2507  * counters (NR_WRITTEN, BDI_WRITTEN) in long term. The mismatches will lead to
2508  * systematic errors in balanced_dirty_ratelimit and the dirty pages position
2509  * control.
2510  */
2511 void account_page_redirty(struct page *page)
2512 {
2513     struct address_space *mapping = page->mapping;
2514 
2515     if (mapping && mapping_cap_account_dirty(mapping)) {
2516         struct inode *inode = mapping->host;
2517         struct bdi_writeback *wb;
2518         bool locked;
2519 
2520         wb = unlocked_inode_to_wb_begin(inode, &locked);
2521         current->nr_dirtied--;
2522         dec_node_page_state(page, NR_DIRTIED);
2523         dec_wb_stat(wb, WB_DIRTIED);
2524         unlocked_inode_to_wb_end(inode, locked);
2525     }
2526 }
2527 EXPORT_SYMBOL(account_page_redirty);
2528 
2529 /*
2530  * When a writepage implementation decides that it doesn't want to write this
2531  * page for some reason, it should redirty the locked page via
2532  * redirty_page_for_writepage() and it should then unlock the page and return 0
2533  */
2534 int redirty_page_for_writepage(struct writeback_control *wbc, struct page *page)
2535 {
2536     int ret;
2537 
2538     wbc->pages_skipped++;
2539     ret = __set_page_dirty_nobuffers(page);
2540     account_page_redirty(page);
2541     return ret;
2542 }
2543 EXPORT_SYMBOL(redirty_page_for_writepage);
2544 
2545 /*
2546  * Dirty a page.
2547  *
2548  * For pages with a mapping this should be done under the page lock
2549  * for the benefit of asynchronous memory errors who prefer a consistent
2550  * dirty state. This rule can be broken in some special cases,
2551  * but should be better not to.
2552  *
2553  * If the mapping doesn't provide a set_page_dirty a_op, then
2554  * just fall through and assume that it wants buffer_heads.
2555  */
2556 int set_page_dirty(struct page *page)
2557 {
2558     struct address_space *mapping = page_mapping(page);
2559 
2560     page = compound_head(page);
2561     if (likely(mapping)) {
2562         int (*spd)(struct page *) = mapping->a_ops->set_page_dirty;
2563         /*
2564          * readahead/lru_deactivate_page could remain
2565          * PG_readahead/PG_reclaim due to race with end_page_writeback
2566          * About readahead, if the page is written, the flags would be
2567          * reset. So no problem.
2568          * About lru_deactivate_page, if the page is redirty, the flag
2569          * will be reset. So no problem. but if the page is used by readahead
2570          * it will confuse readahead and make it restart the size rampup
2571          * process. But it's a trivial problem.
2572          */
2573         if (PageReclaim(page))
2574             ClearPageReclaim(page);
2575 #ifdef CONFIG_BLOCK
2576         if (!spd)
2577             spd = __set_page_dirty_buffers;
2578 #endif
2579         return (*spd)(page);
2580     }
2581     if (!PageDirty(page)) {
2582         if (!TestSetPageDirty(page))
2583             return 1;
2584     }
2585     return 0;
2586 }
2587 EXPORT_SYMBOL(set_page_dirty);
2588 
2589 /*
2590  * set_page_dirty() is racy if the caller has no reference against
2591  * page->mapping->host, and if the page is unlocked.  This is because another
2592  * CPU could truncate the page off the mapping and then free the mapping.
2593  *
2594  * Usually, the page _is_ locked, or the caller is a user-space process which
2595  * holds a reference on the inode by having an open file.
2596  *
2597  * In other cases, the page should be locked before running set_page_dirty().
2598  */
2599 int set_page_dirty_lock(struct page *page)
2600 {
2601     int ret;
2602 
2603     lock_page(page);
2604     ret = set_page_dirty(page);
2605     unlock_page(page);
2606     return ret;
2607 }
2608 EXPORT_SYMBOL(set_page_dirty_lock);
2609 
2610 /*
2611  * This cancels just the dirty bit on the kernel page itself, it does NOT
2612  * actually remove dirty bits on any mmap's that may be around. It also
2613  * leaves the page tagged dirty, so any sync activity will still find it on
2614  * the dirty lists, and in particular, clear_page_dirty_for_io() will still
2615  * look at the dirty bits in the VM.
2616  *
2617  * Doing this should *normally* only ever be done when a page is truncated,
2618  * and is not actually mapped anywhere at all. However, fs/buffer.c does
2619  * this when it notices that somebody has cleaned out all the buffers on a
2620  * page without actually doing it through the VM. Can you say "ext3 is
2621  * horribly ugly"? Thought you could.
2622  */
2623 void cancel_dirty_page(struct page *page)
2624 {
2625     struct address_space *mapping = page_mapping(page);
2626 
2627     if (mapping_cap_account_dirty(mapping)) {
2628         struct inode *inode = mapping->host;
2629         struct bdi_writeback *wb;
2630         bool locked;
2631 
2632         lock_page_memcg(page);
2633         wb = unlocked_inode_to_wb_begin(inode, &locked);
2634 
2635         if (TestClearPageDirty(page))
2636             account_page_cleaned(page, mapping, wb);
2637 
2638         unlocked_inode_to_wb_end(inode, locked);
2639         unlock_page_memcg(page);
2640     } else {
2641         ClearPageDirty(page);
2642     }
2643 }
2644 EXPORT_SYMBOL(cancel_dirty_page);
2645 
2646 /*
2647  * Clear a page's dirty flag, while caring for dirty memory accounting.
2648  * Returns true if the page was previously dirty.
2649  *
2650  * This is for preparing to put the page under writeout.  We leave the page
2651  * tagged as dirty in the radix tree so that a concurrent write-for-sync
2652  * can discover it via a PAGECACHE_TAG_DIRTY walk.  The ->writepage
2653  * implementation will run either set_page_writeback() or set_page_dirty(),
2654  * at which stage we bring the page's dirty flag and radix-tree dirty tag
2655  * back into sync.
2656  *
2657  * This incoherency between the page's dirty flag and radix-tree tag is
2658  * unfortunate, but it only exists while the page is locked.
2659  */
2660 int clear_page_dirty_for_io(struct page *page)
2661 {
2662     struct address_space *mapping = page_mapping(page);
2663     int ret = 0;
2664 
2665     BUG_ON(!PageLocked(page));
2666 
2667     if (mapping && mapping_cap_account_dirty(mapping)) {
2668         struct inode *inode = mapping->host;
2669         struct bdi_writeback *wb;
2670         bool locked;
2671 
2672         /*
2673          * Yes, Virginia, this is indeed insane.
2674          *
2675          * We use this sequence to make sure that
2676          *  (a) we account for dirty stats properly
2677          *  (b) we tell the low-level filesystem to
2678          *      mark the whole page dirty if it was
2679          *      dirty in a pagetable. Only to then
2680          *  (c) clean the page again and return 1 to
2681          *      cause the writeback.
2682          *
2683          * This way we avoid all nasty races with the
2684          * dirty bit in multiple places and clearing
2685          * them concurrently from different threads.
2686          *
2687          * Note! Normally the "set_page_dirty(page)"
2688          * has no effect on the actual dirty bit - since
2689          * that will already usually be set. But we
2690          * need the side effects, and it can help us
2691          * avoid races.
2692          *
2693          * We basically use the page "master dirty bit"
2694          * as a serialization point for all the different
2695          * threads doing their things.
2696          */
2697         if (page_mkclean(page))
2698             set_page_dirty(page);
2699         /*
2700          * We carefully synchronise fault handlers against
2701          * installing a dirty pte and marking the page dirty
2702          * at this point.  We do this by having them hold the
2703          * page lock while dirtying the page, and pages are
2704          * always locked coming in here, so we get the desired
2705          * exclusion.
2706          */
2707         wb = unlocked_inode_to_wb_begin(inode, &locked);
2708         if (TestClearPageDirty(page)) {
2709             mem_cgroup_dec_page_stat(page, MEM_CGROUP_STAT_DIRTY);
2710             dec_node_page_state(page, NR_FILE_DIRTY);
2711             dec_zone_page_state(page, NR_ZONE_WRITE_PENDING);
2712             dec_wb_stat(wb, WB_RECLAIMABLE);
2713             ret = 1;
2714         }
2715         unlocked_inode_to_wb_end(inode, locked);
2716         return ret;
2717     }
2718     return TestClearPageDirty(page);
2719 }
2720 EXPORT_SYMBOL(clear_page_dirty_for_io);
2721 
2722 int test_clear_page_writeback(struct page *page)
2723 {
2724     struct address_space *mapping = page_mapping(page);
2725     int ret;
2726 
2727     lock_page_memcg(page);
2728     if (mapping && mapping_use_writeback_tags(mapping)) {
2729         struct inode *inode = mapping->host;
2730         struct backing_dev_info *bdi = inode_to_bdi(inode);
2731         unsigned long flags;
2732 
2733         spin_lock_irqsave(&mapping->tree_lock, flags);
2734         ret = TestClearPageWriteback(page);
2735         if (ret) {
2736             radix_tree_tag_clear(&mapping->page_tree,
2737                         page_index(page),
2738                         PAGECACHE_TAG_WRITEBACK);
2739             if (bdi_cap_account_writeback(bdi)) {
2740                 struct bdi_writeback *wb = inode_to_wb(inode);
2741 
2742                 __dec_wb_stat(wb, WB_WRITEBACK);
2743                 __wb_writeout_inc(wb);
2744             }
2745         }
2746 
2747         if (mapping->host && !mapping_tagged(mapping,
2748                              PAGECACHE_TAG_WRITEBACK))
2749             sb_clear_inode_writeback(mapping->host);
2750 
2751         spin_unlock_irqrestore(&mapping->tree_lock, flags);
2752     } else {
2753         ret = TestClearPageWriteback(page);
2754     }
2755     if (ret) {
2756         mem_cgroup_dec_page_stat(page, MEM_CGROUP_STAT_WRITEBACK);
2757         dec_node_page_state(page, NR_WRITEBACK);
2758         dec_zone_page_state(page, NR_ZONE_WRITE_PENDING);
2759         inc_node_page_state(page, NR_WRITTEN);
2760     }
2761     unlock_page_memcg(page);
2762     return ret;
2763 }
2764 
2765 int __test_set_page_writeback(struct page *page, bool keep_write)
2766 {
2767     struct address_space *mapping = page_mapping(page);
2768     int ret;
2769 
2770     lock_page_memcg(page);
2771     if (mapping && mapping_use_writeback_tags(mapping)) {
2772         struct inode *inode = mapping->host;
2773         struct backing_dev_info *bdi = inode_to_bdi(inode);
2774         unsigned long flags;
2775 
2776         spin_lock_irqsave(&mapping->tree_lock, flags);
2777         ret = TestSetPageWriteback(page);
2778         if (!ret) {
2779             bool on_wblist;
2780 
2781             on_wblist = mapping_tagged(mapping,
2782                            PAGECACHE_TAG_WRITEBACK);
2783 
2784             radix_tree_tag_set(&mapping->page_tree,
2785                         page_index(page),
2786                         PAGECACHE_TAG_WRITEBACK);
2787             if (bdi_cap_account_writeback(bdi))
2788                 __inc_wb_stat(inode_to_wb(inode), WB_WRITEBACK);
2789 
2790             /*
2791              * We can come through here when swapping anonymous
2792              * pages, so we don't necessarily have an inode to track
2793              * for sync.
2794              */
2795             if (mapping->host && !on_wblist)
2796                 sb_mark_inode_writeback(mapping->host);
2797         }
2798         if (!PageDirty(page))
2799             radix_tree_tag_clear(&mapping->page_tree,
2800                         page_index(page),
2801                         PAGECACHE_TAG_DIRTY);
2802         if (!keep_write)
2803             radix_tree_tag_clear(&mapping->page_tree,
2804                         page_index(page),
2805                         PAGECACHE_TAG_TOWRITE);
2806         spin_unlock_irqrestore(&mapping->tree_lock, flags);
2807     } else {
2808         ret = TestSetPageWriteback(page);
2809     }
2810     if (!ret) {
2811         mem_cgroup_inc_page_stat(page, MEM_CGROUP_STAT_WRITEBACK);
2812         inc_node_page_state(page, NR_WRITEBACK);
2813         inc_zone_page_state(page, NR_ZONE_WRITE_PENDING);
2814     }
2815     unlock_page_memcg(page);
2816     return ret;
2817 
2818 }
2819 EXPORT_SYMBOL(__test_set_page_writeback);
2820 
2821 /*
2822  * Return true if any of the pages in the mapping are marked with the
2823  * passed tag.
2824  */
2825 int mapping_tagged(struct address_space *mapping, int tag)
2826 {
2827     return radix_tree_tagged(&mapping->page_tree, tag);
2828 }
2829 EXPORT_SYMBOL(mapping_tagged);
2830 
2831 /**
2832  * wait_for_stable_page() - wait for writeback to finish, if necessary.
2833  * @page:   The page to wait on.
2834  *
2835  * This function determines if the given page is related to a backing device
2836  * that requires page contents to be held stable during writeback.  If so, then
2837  * it will wait for any pending writeback to complete.
2838  */
2839 void wait_for_stable_page(struct page *page)
2840 {
2841     if (bdi_cap_stable_pages_required(inode_to_bdi(page->mapping->host)))
2842         wait_on_page_writeback(page);
2843 }
2844 EXPORT_SYMBOL_GPL(wait_for_stable_page);