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 // SPDX-License-Identifier: GPL-2.0-only
 /*
  * mm/page-writeback.c
  *
  * Copyright (C) 2002, Linus Torvalds.
  * Copyright (C) 2007 Red Hat, Inc., Peter Zijlstra
  *
  * Contains functions related to writing back dirty pages at the
  * address_space level.
  *
  * 10Apr2002    Andrew Morton
  *      Initial version
  */
 
 #include <linux/kernel.h>
 #include <linux/export.h>
 #include <linux/spinlock.h>
 #include <linux/fs.h>
 #include <linux/mm.h>
 #include <linux/swap.h>
 #include <linux/slab.h>
 #include <linux/pagemap.h>
 #include <linux/writeback.h>
 #include <linux/init.h>
 #include <linux/backing-dev.h>
 #include <linux/task_io_accounting_ops.h>
 #include <linux/blkdev.h>
 #include <linux/mpage.h>
 #include <linux/rmap.h>
 #include <linux/percpu.h>
 #include <linux/smp.h>
 #include <linux/sysctl.h>
 #include <linux/cpu.h>
 #include <linux/syscalls.h>
 #include <linux/pagevec.h>
 #include <linux/timer.h>
 #include <linux/sched/rt.h>
 #include <linux/sched/signal.h>
 #include <linux/mm_inline.h>
 #include <trace/events/writeback.h>
 
 #include "internal.h"
 
 /*
  * Sleep at most 200ms at a time in balance_dirty_pages().
  */
 #define MAX_PAUSE       max(HZ/5, 1)
 
 /*
  * Try to keep balance_dirty_pages() call intervals higher than this many pages
  * by raising pause time to max_pause when falls below it.
  */
 #define DIRTY_POLL_THRESH   (128 >> (PAGE_SHIFT - 10))
 
 /*
  * Estimate write bandwidth at 200ms intervals.
  */
 #define BANDWIDTH_INTERVAL  max(HZ/5, 1)
 
 #define RATELIMIT_CALC_SHIFT    10
 
 /*
  * After a CPU has dirtied this many pages, balance_dirty_pages_ratelimited
  * will look to see if it needs to force writeback or throttling.
  */
 static long ratelimit_pages = 32;
 
 /* The following parameters are exported via /proc/sys/vm */
 
 /*
  * Start background writeback (via writeback threads) at this percentage
  */
 static int dirty_background_ratio = 10;
 
 /*
  * dirty_background_bytes starts at 0 (disabled) so that it is a function of
  * dirty_background_ratio * the amount of dirtyable memory
  */
 static unsigned long dirty_background_bytes;
 
 /*
  * free highmem will not be subtracted from the total free memory
  * for calculating free ratios if vm_highmem_is_dirtyable is true
  */
 static int vm_highmem_is_dirtyable;
 
 /*
  * The generator of dirty data starts writeback at this percentage
  */
 static int vm_dirty_ratio = 20;
 
 /*
  * vm_dirty_bytes starts at 0 (disabled) so that it is a function of
  * vm_dirty_ratio * the amount of dirtyable memory
  */
 static unsigned long vm_dirty_bytes;
 
 /*
  * The interval between `kupdate'-style writebacks
  */
 unsigned int dirty_writeback_interval = 5 * 100; /* centiseconds */
 
 EXPORT_SYMBOL_GPL(dirty_writeback_interval);
 
 /*
  * The longest time for which data is allowed to remain dirty
  */
 unsigned int dirty_expire_interval = 30 * 100; /* centiseconds */
 
 /*
  * Flag that puts the machine in "laptop mode". Doubles as a timeout in jiffies:
  * a full sync is triggered after this time elapses without any disk activity.
  */
 int laptop_mode;
 
 EXPORT_SYMBOL(laptop_mode);
 
 /* End of sysctl-exported parameters */
 
 struct wb_domain global_wb_domain;
 
 /* consolidated parameters for balance_dirty_pages() and its subroutines */
 struct dirty_throttle_control {
 #ifdef CONFIG_CGROUP_WRITEBACK
     struct wb_domain    *dom;
     struct dirty_throttle_control *gdtc;    /* only set in memcg dtc's */
 #endif
     struct bdi_writeback    *wb;
     struct fprop_local_percpu *wb_completions;
 
     unsigned long       avail;      /* dirtyable */
     unsigned long       dirty;      /* file_dirty + write + nfs */
     unsigned long       thresh;     /* dirty threshold */
     unsigned long       bg_thresh;  /* dirty background threshold */
 
     unsigned long       wb_dirty;   /* per-wb counterparts */
     unsigned long       wb_thresh;
     unsigned long       wb_bg_thresh;
 
     unsigned long       pos_ratio;
 };
 
 /*
  * Length of period for aging writeout fractions of bdis. This is an
  * arbitrarily chosen number. The longer the period, the slower fractions will
  * reflect changes in current writeout rate.
  */
 #define VM_COMPLETIONS_PERIOD_LEN (3*HZ)
 
 #ifdef CONFIG_CGROUP_WRITEBACK
 
 #define GDTC_INIT(__wb)     .wb = (__wb),               \
                 .dom = &global_wb_domain,       \
                 .wb_completions = &(__wb)->completions
 
 #define GDTC_INIT_NO_WB     .dom = &global_wb_domain
 
 #define MDTC_INIT(__wb, __gdtc) .wb = (__wb),               \
                 .dom = mem_cgroup_wb_domain(__wb),  \
                 .wb_completions = &(__wb)->memcg_completions, \
                 .gdtc = __gdtc
 
 static bool mdtc_valid(struct dirty_throttle_control *dtc)
 {
     return dtc->dom;
 }
 
 static struct wb_domain *dtc_dom(struct dirty_throttle_control *dtc)
 {
     return dtc->dom;
 }
 
 static struct dirty_throttle_control *mdtc_gdtc(struct dirty_throttle_control *mdtc)
 {
     return mdtc->gdtc;
 }
 
 static struct fprop_local_percpu *wb_memcg_completions(struct bdi_writeback *wb)
 {
     return &wb->memcg_completions;
 }
 
 static void wb_min_max_ratio(struct bdi_writeback *wb,
                  unsigned long *minp, unsigned long *maxp)
 {
     unsigned long this_bw = READ_ONCE(wb->avg_write_bandwidth);
     unsigned long tot_bw = atomic_long_read(&wb->bdi->tot_write_bandwidth);
     unsigned long long min = wb->bdi->min_ratio;
     unsigned long long max = wb->bdi->max_ratio;
 
     /*
      * @wb may already be clean by the time control reaches here and
      * the total may not include its bw.
      */
     if (this_bw < tot_bw) {
         if (min) {
             min *= this_bw;
             min = div64_ul(min, tot_bw);
         }
         if (max < 100) {
             max *= this_bw;
             max = div64_ul(max, tot_bw);
         }
     }
 
     *minp = min;
     *maxp = max;
 }
 
 #else   /* CONFIG_CGROUP_WRITEBACK */
 
 #define GDTC_INIT(__wb)     .wb = (__wb),                           \
                 .wb_completions = &(__wb)->completions
 #define GDTC_INIT_NO_WB
 #define MDTC_INIT(__wb, __gdtc)
 
 static bool mdtc_valid(struct dirty_throttle_control *dtc)
 {
     return false;
 }
 
 static struct wb_domain *dtc_dom(struct dirty_throttle_control *dtc)
 {
     return &global_wb_domain;
 }
 
 static struct dirty_throttle_control *mdtc_gdtc(struct dirty_throttle_control *mdtc)
 {
     return NULL;
 }
 
 static struct fprop_local_percpu *wb_memcg_completions(struct bdi_writeback *wb)
 {
     return NULL;
 }
 
 static void wb_min_max_ratio(struct bdi_writeback *wb,
                  unsigned long *minp, unsigned long *maxp)
 {
     *minp = wb->bdi->min_ratio;
     *maxp = wb->bdi->max_ratio;
 }
 
 #endif  /* CONFIG_CGROUP_WRITEBACK */
 
 /*
  * In a memory zone, there is a certain amount of pages we consider
  * available for the page cache, which is essentially the number of
  * free and reclaimable pages, minus some zone reserves to protect
  * lowmem and the ability to uphold the zone's watermarks without
  * requiring writeback.
  *
  * This number of dirtyable pages is the base value of which the
  * user-configurable dirty ratio is the effective number of pages that
  * are allowed to be actually dirtied.  Per individual zone, or
  * globally by using the sum of dirtyable pages over all zones.
  *
  * Because the user is allowed to specify the dirty limit globally as
  * absolute number of bytes, calculating the per-zone dirty limit can
  * require translating the configured limit into a percentage of
  * global dirtyable memory first.
  */
 
 /**
  * node_dirtyable_memory - number of dirtyable pages in a node
  * @pgdat: the node
  *
  * Return: the node's number of pages potentially available for dirty
  * page cache.  This is the base value for the per-node dirty limits.
  */
 static unsigned long node_dirtyable_memory(struct pglist_data *pgdat)
 {
     unsigned long nr_pages = 0;
     int z;
 
     for (z = 0; z < MAX_NR_ZONES; z++) {
         struct zone *zone = pgdat->node_zones + z;
 
         if (!populated_zone(zone))
             continue;
 
         nr_pages += zone_page_state(zone, NR_FREE_PAGES);
     }
 
     /*
      * Pages reserved for the kernel should not be considered
      * dirtyable, to prevent a situation where reclaim has to
      * clean pages in order to balance the zones.
      */
     nr_pages -= min(nr_pages, pgdat->totalreserve_pages);
 
     nr_pages += node_page_state(pgdat, NR_INACTIVE_FILE);
     nr_pages += node_page_state(pgdat, NR_ACTIVE_FILE);
 
     return nr_pages;
 }
 
 static unsigned long highmem_dirtyable_memory(unsigned long total)
 {
 #ifdef CONFIG_HIGHMEM
     int node;
     unsigned long x = 0;
     int i;
 
     for_each_node_state(node, N_HIGH_MEMORY) {
         for (i = ZONE_NORMAL + 1; i < MAX_NR_ZONES; i++) {
             struct zone *z;
             unsigned long nr_pages;
 
             if (!is_highmem_idx(i))
                 continue;
 
             z = &NODE_DATA(node)->node_zones[i];
             if (!populated_zone(z))
                 continue;
 
             nr_pages = zone_page_state(z, NR_FREE_PAGES);
             /* watch for underflows */
             nr_pages -= min(nr_pages, high_wmark_pages(z));
             nr_pages += zone_page_state(z, NR_ZONE_INACTIVE_FILE);
             nr_pages += zone_page_state(z, NR_ZONE_ACTIVE_FILE);
             x += nr_pages;
         }
     }
 
     /*
      * Make sure that the number of highmem pages is never larger
      * than the number of the total dirtyable memory. This can only
      * occur in very strange VM situations but we want to make sure
      * that this does not occur.
      */
     return min(x, total);
 #else
     return 0;
 #endif
 }
 
 /**
  * global_dirtyable_memory - number of globally dirtyable pages
  *
  * Return: the global number of pages potentially available for dirty
  * page cache.  This is the base value for the global dirty limits.
  */
 static unsigned long global_dirtyable_memory(void)
 {
     unsigned long x;
 
     x = global_zone_page_state(NR_FREE_PAGES);
     /*
      * Pages reserved for the kernel should not be considered
      * dirtyable, to prevent a situation where reclaim has to
      * clean pages in order to balance the zones.
      */
     x -= min(x, totalreserve_pages);
 
     x += global_node_page_state(NR_INACTIVE_FILE);
     x += global_node_page_state(NR_ACTIVE_FILE);
 
     if (!vm_highmem_is_dirtyable)
         x -= highmem_dirtyable_memory(x);
 
     return x + 1;   /* Ensure that we never return 0 */
 }
 
 /**
  * domain_dirty_limits - calculate thresh and bg_thresh for a wb_domain
  * @dtc: dirty_throttle_control of interest
  *
  * Calculate @dtc->thresh and ->bg_thresh considering
  * vm_dirty_{bytes|ratio} and dirty_background_{bytes|ratio}.  The caller
  * must ensure that @dtc->avail is set before calling this function.  The
  * dirty limits will be lifted by 1/4 for real-time tasks.
  */
 static void domain_dirty_limits(struct dirty_throttle_control *dtc)
 {
     const unsigned long available_memory = dtc->avail;
     struct dirty_throttle_control *gdtc = mdtc_gdtc(dtc);
     unsigned long bytes = vm_dirty_bytes;
     unsigned long bg_bytes = dirty_background_bytes;
     /* convert ratios to per-PAGE_SIZE for higher precision */
     unsigned long ratio = (vm_dirty_ratio * PAGE_SIZE) / 100;
     unsigned long bg_ratio = (dirty_background_ratio * PAGE_SIZE) / 100;
     unsigned long thresh;
     unsigned long bg_thresh;
     struct task_struct *tsk;
 
     /* gdtc is !NULL iff @dtc is for memcg domain */
     if (gdtc) {
         unsigned long global_avail = gdtc->avail;
 
         /*
          * The byte settings can't be applied directly to memcg
          * domains.  Convert them to ratios by scaling against
          * globally available memory.  As the ratios are in
          * per-PAGE_SIZE, they can be obtained by dividing bytes by
          * number of pages.
          */
         if (bytes)
             ratio = min(DIV_ROUND_UP(bytes, global_avail),
                     PAGE_SIZE);
         if (bg_bytes)
             bg_ratio = min(DIV_ROUND_UP(bg_bytes, global_avail),
                        PAGE_SIZE);
         bytes = bg_bytes = 0;
     }
 
     if (bytes)
         thresh = DIV_ROUND_UP(bytes, PAGE_SIZE);
     else
         thresh = (ratio * available_memory) / PAGE_SIZE;
 
     if (bg_bytes)
         bg_thresh = DIV_ROUND_UP(bg_bytes, PAGE_SIZE);
     else
         bg_thresh = (bg_ratio * available_memory) / PAGE_SIZE;
 
     if (bg_thresh >= thresh)
         bg_thresh = thresh / 2;
     tsk = current;
     if (rt_task(tsk)) {
         bg_thresh += bg_thresh / 4 + global_wb_domain.dirty_limit / 32;
         thresh += thresh / 4 + global_wb_domain.dirty_limit / 32;
     }
     dtc->thresh = thresh;
     dtc->bg_thresh = bg_thresh;
 
     /* we should eventually report the domain in the TP */
     if (!gdtc)
         trace_global_dirty_state(bg_thresh, thresh);
 }
 
 /**
  * global_dirty_limits - background-writeback and dirty-throttling thresholds
  * @pbackground: out parameter for bg_thresh
  * @pdirty: out parameter for thresh
  *
  * Calculate bg_thresh and thresh for global_wb_domain.  See
  * domain_dirty_limits() for details.
  */
 void global_dirty_limits(unsigned long *pbackground, unsigned long *pdirty)
 {
     struct dirty_throttle_control gdtc = { GDTC_INIT_NO_WB };
 
     gdtc.avail = global_dirtyable_memory();
     domain_dirty_limits(&gdtc);
 
     *pbackground = gdtc.bg_thresh;
     *pdirty = gdtc.thresh;
 }
 
 /**
  * node_dirty_limit - maximum number of dirty pages allowed in a node
  * @pgdat: the node
  *
  * Return: the maximum number of dirty pages allowed in a node, based
  * on the node's dirtyable memory.
  */
 static unsigned long node_dirty_limit(struct pglist_data *pgdat)
 {
     unsigned long node_memory = node_dirtyable_memory(pgdat);
     struct task_struct *tsk = current;
     unsigned long dirty;
 
     if (vm_dirty_bytes)
         dirty = DIV_ROUND_UP(vm_dirty_bytes, PAGE_SIZE) *
             node_memory / global_dirtyable_memory();
     else
         dirty = vm_dirty_ratio * node_memory / 100;
 
     if (rt_task(tsk))
         dirty += dirty / 4;
 
     return dirty;
 }
 
 /**
  * node_dirty_ok - tells whether a node is within its dirty limits
  * @pgdat: the node to check
  *
  * Return: %true when the dirty pages in @pgdat are within the node's
  * dirty limit, %false if the limit is exceeded.
  */
 bool node_dirty_ok(struct pglist_data *pgdat)
 {
     unsigned long limit = node_dirty_limit(pgdat);
     unsigned long nr_pages = 0;
 
     nr_pages += node_page_state(pgdat, NR_FILE_DIRTY);
     nr_pages += node_page_state(pgdat, NR_WRITEBACK);
 
     return nr_pages <= limit;
 }
 
 #ifdef CONFIG_SYSCTL
 static int dirty_background_ratio_handler(struct ctl_table *table, int write,
         void *buffer, size_t *lenp, loff_t *ppos)
 {
     int ret;
 
     ret = proc_dointvec_minmax(table, write, buffer, lenp, ppos);
     if (ret == 0 && write)
         dirty_background_bytes = 0;
     return ret;
 }
 
 static int dirty_background_bytes_handler(struct ctl_table *table, int write,
         void *buffer, size_t *lenp, loff_t *ppos)
 {
     int ret;
 
     ret = proc_doulongvec_minmax(table, write, buffer, lenp, ppos);
     if (ret == 0 && write)
         dirty_background_ratio = 0;
     return ret;
 }
 
 static int dirty_ratio_handler(struct ctl_table *table, int write, void *buffer,
         size_t *lenp, loff_t *ppos)
 {
     int old_ratio = vm_dirty_ratio;
     int ret;
 
     ret = proc_dointvec_minmax(table, write, buffer, lenp, ppos);
     if (ret == 0 && write && vm_dirty_ratio != old_ratio) {
         writeback_set_ratelimit();
         vm_dirty_bytes = 0;
     }
     return ret;
 }
 
 static int dirty_bytes_handler(struct ctl_table *table, int write,
         void *buffer, size_t *lenp, loff_t *ppos)
 {
     unsigned long old_bytes = vm_dirty_bytes;
     int ret;
 
     ret = proc_doulongvec_minmax(table, write, buffer, lenp, ppos);
     if (ret == 0 && write && vm_dirty_bytes != old_bytes) {
         writeback_set_ratelimit();
         vm_dirty_ratio = 0;
     }
     return ret;
 }
 #endif
 
 static unsigned long wp_next_time(unsigned long cur_time)
 {
     cur_time += VM_COMPLETIONS_PERIOD_LEN;
     /* 0 has a special meaning... */
     if (!cur_time)
         return 1;
     return cur_time;
 }
 
 static void wb_domain_writeout_add(struct wb_domain *dom,
                    struct fprop_local_percpu *completions,
                    unsigned int max_prop_frac, long nr)
 {
     __fprop_add_percpu_max(&dom->completions, completions,
                    max_prop_frac, nr);
     /* First event after period switching was turned off? */
     if (unlikely(!dom->period_time)) {
         /*
          * We can race with other __bdi_writeout_inc calls here but
          * it does not cause any harm since the resulting time when
          * timer will fire and what is in writeout_period_time will be
          * roughly the same.
          */
         dom->period_time = wp_next_time(jiffies);
         mod_timer(&dom->period_timer, dom->period_time);
     }
 }
 
 /*
  * Increment @wb's writeout completion count and the global writeout
  * completion count. Called from __folio_end_writeback().
  */
 static inline void __wb_writeout_add(struct bdi_writeback *wb, long nr)
 {
     struct wb_domain *cgdom;
 
     wb_stat_mod(wb, WB_WRITTEN, nr);
     wb_domain_writeout_add(&global_wb_domain, &wb->completions,
                    wb->bdi->max_prop_frac, nr);
 
     cgdom = mem_cgroup_wb_domain(wb);
     if (cgdom)
         wb_domain_writeout_add(cgdom, wb_memcg_completions(wb),
                        wb->bdi->max_prop_frac, nr);
 }
 
 void wb_writeout_inc(struct bdi_writeback *wb)
 {
     unsigned long flags;
 
     local_irq_save(flags);
     __wb_writeout_add(wb, 1);
     local_irq_restore(flags);
 }
 EXPORT_SYMBOL_GPL(wb_writeout_inc);
 
 /*
  * On idle system, we can be called long after we scheduled because we use
  * deferred timers so count with missed periods.
  */
 static void writeout_period(struct timer_list *t)
 {
     struct wb_domain *dom = from_timer(dom, t, period_timer);
     int miss_periods = (jiffies - dom->period_time) /
                          VM_COMPLETIONS_PERIOD_LEN;
 
     if (fprop_new_period(&dom->completions, miss_periods + 1)) {
         dom->period_time = wp_next_time(dom->period_time +
                 miss_periods * VM_COMPLETIONS_PERIOD_LEN);
         mod_timer(&dom->period_timer, dom->period_time);
     } else {
         /*
          * Aging has zeroed all fractions. Stop wasting CPU on period
          * updates.
          */
         dom->period_time = 0;
     }
 }
 
 int wb_domain_init(struct wb_domain *dom, gfp_t gfp)
 {
     memset(dom, 0, sizeof(*dom));
 
     spin_lock_init(&dom->lock);
 
     timer_setup(&dom->period_timer, writeout_period, TIMER_DEFERRABLE);
 
     dom->dirty_limit_tstamp = jiffies;
 
     return fprop_global_init(&dom->completions, gfp);
 }
 
 #ifdef CONFIG_CGROUP_WRITEBACK
 void wb_domain_exit(struct wb_domain *dom)
 {
     del_timer_sync(&dom->period_timer);
     fprop_global_destroy(&dom->completions);
 }
 #endif
 
 /*
  * bdi_min_ratio keeps the sum of the minimum dirty shares of all
  * registered backing devices, which, for obvious reasons, can not
  * exceed 100%.
  */
 static unsigned int bdi_min_ratio;
 
 int bdi_set_min_ratio(struct backing_dev_info *bdi, unsigned int min_ratio)
 {
     unsigned int delta;
     int ret = 0;
 
     spin_lock_bh(&bdi_lock);
     if (min_ratio > bdi->max_ratio) {
         ret = -EINVAL;
     } else {
         if (min_ratio < bdi->min_ratio) {
             delta = bdi->min_ratio - min_ratio;
             bdi_min_ratio -= delta;
             bdi->min_ratio = min_ratio;
         } else {
             delta = min_ratio - bdi->min_ratio;
             if (bdi_min_ratio + delta < 100) {
                 bdi_min_ratio += delta;
                 bdi->min_ratio = min_ratio;
             } else {
                 ret = -EINVAL;
             }
         }
     }
     spin_unlock_bh(&bdi_lock);
 
     return ret;
 }
 
 int bdi_set_max_ratio(struct backing_dev_info *bdi, unsigned max_ratio)
 {
     int ret = 0;
 
     if (max_ratio > 100)
         return -EINVAL;
 
     spin_lock_bh(&bdi_lock);
     if (bdi->min_ratio > max_ratio) {
         ret = -EINVAL;
     } else {
         bdi->max_ratio = max_ratio;
         bdi->max_prop_frac = (FPROP_FRAC_BASE * max_ratio) / 100;
     }
     spin_unlock_bh(&bdi_lock);
 
     return ret;
 }
 EXPORT_SYMBOL(bdi_set_max_ratio);
 
 static unsigned long dirty_freerun_ceiling(unsigned long thresh,
                        unsigned long bg_thresh)
 {
     return (thresh + bg_thresh) / 2;
 }
 
 static unsigned long hard_dirty_limit(struct wb_domain *dom,
                       unsigned long thresh)
 {
     return max(thresh, dom->dirty_limit);
 }
 
 /*
  * Memory which can be further allocated to a memcg domain is capped by
  * system-wide clean memory excluding the amount being used in the domain.
  */
 static void mdtc_calc_avail(struct dirty_throttle_control *mdtc,
                 unsigned long filepages, unsigned long headroom)
 {
     struct dirty_throttle_control *gdtc = mdtc_gdtc(mdtc);
     unsigned long clean = filepages - min(filepages, mdtc->dirty);
     unsigned long global_clean = gdtc->avail - min(gdtc->avail, gdtc->dirty);
     unsigned long other_clean = global_clean - min(global_clean, clean);
 
     mdtc->avail = filepages + min(headroom, other_clean);
 }
 
 /**
  * __wb_calc_thresh - @wb's share of dirty throttling threshold
  * @dtc: dirty_throttle_context of interest
  *
  * Note that balance_dirty_pages() will only seriously take it as a hard limit
  * when sleeping max_pause per page is not enough to keep the dirty pages under
  * control. For example, when the device is completely stalled due to some error
  * conditions, or when there are 1000 dd tasks writing to a slow 10MB/s USB key.
  * In the other normal situations, it acts more gently by throttling the tasks
  * more (rather than completely block them) when the wb dirty pages go high.
  *
  * It allocates high/low dirty limits to fast/slow devices, in order to prevent
  * - starving fast devices
  * - piling up dirty pages (that will take long time to sync) on slow devices
  *
  * The wb's share of dirty limit will be adapting to its throughput and
  * bounded by the bdi->min_ratio and/or bdi->max_ratio parameters, if set.
  *
  * Return: @wb's dirty limit in pages. The term "dirty" in the context of
  * dirty balancing includes all PG_dirty and PG_writeback pages.
  */
 static unsigned long __wb_calc_thresh(struct dirty_throttle_control *dtc)
 {
     struct wb_domain *dom = dtc_dom(dtc);
     unsigned long thresh = dtc->thresh;
     u64 wb_thresh;
     unsigned long numerator, denominator;
     unsigned long wb_min_ratio, wb_max_ratio;
 
     /*
      * Calculate this BDI's share of the thresh ratio.
      */
     fprop_fraction_percpu(&dom->completions, dtc->wb_completions,
                   &numerator, &denominator);
 
     wb_thresh = (thresh * (100 - bdi_min_ratio)) / 100;
     wb_thresh *= numerator;
     wb_thresh = div64_ul(wb_thresh, denominator);
 
     wb_min_max_ratio(dtc->wb, &wb_min_ratio, &wb_max_ratio);
 
     wb_thresh += (thresh * wb_min_ratio) / 100;
     if (wb_thresh > (thresh * wb_max_ratio) / 100)
         wb_thresh = thresh * wb_max_ratio / 100;
 
     return wb_thresh;
 }
 
 unsigned long wb_calc_thresh(struct bdi_writeback *wb, unsigned long thresh)
 {
     struct dirty_throttle_control gdtc = { GDTC_INIT(wb),
                            .thresh = thresh };
     return __wb_calc_thresh(&gdtc);
 }
 
 /*
  *                           setpoint - dirty 3
  *        f(dirty) := 1.0 + (----------------)
  *                           limit - setpoint
  *
  * it's a 3rd order polynomial that subjects to
  *
  * (1) f(freerun)  = 2.0 => rampup dirty_ratelimit reasonably fast
  * (2) f(setpoint) = 1.0 => the balance point
  * (3) f(limit)    = 0   => the hard limit
  * (4) df/dx      <= 0   => negative feedback control
  * (5) the closer to setpoint, the smaller |df/dx| (and the reverse)
  *     => fast response on large errors; small oscillation near setpoint
  */
 static long long pos_ratio_polynom(unsigned long setpoint,
                       unsigned long dirty,
                       unsigned long limit)
 {
     long long pos_ratio;
     long x;
 
     x = div64_s64(((s64)setpoint - (s64)dirty) << RATELIMIT_CALC_SHIFT,
               (limit - setpoint) | 1);
     pos_ratio = x;
     pos_ratio = pos_ratio * x >> RATELIMIT_CALC_SHIFT;
     pos_ratio = pos_ratio * x >> RATELIMIT_CALC_SHIFT;
     pos_ratio += 1 << RATELIMIT_CALC_SHIFT;
 
     return clamp(pos_ratio, 0LL, 2LL << RATELIMIT_CALC_SHIFT);
 }
 
 /*
  * Dirty position control.
  *
  * (o) global/bdi setpoints
  *
  * We want the dirty pages be balanced around the global/wb setpoints.
  * When the number of dirty pages is higher/lower than the setpoint, the
  * dirty position control ratio (and hence task dirty ratelimit) will be
  * decreased/increased to bring the dirty pages back to the setpoint.
  *
  *     pos_ratio = 1 << RATELIMIT_CALC_SHIFT
  *
  *     if (dirty < setpoint) scale up   pos_ratio
  *     if (dirty > setpoint) scale down pos_ratio
  *
  *     if (wb_dirty < wb_setpoint) scale up   pos_ratio
  *     if (wb_dirty > wb_setpoint) scale down pos_ratio
  *
  *     task_ratelimit = dirty_ratelimit * pos_ratio >> RATELIMIT_CALC_SHIFT
  *
  * (o) global control line
  *
  *     ^ pos_ratio
  *     |
  *     |            |<===== global dirty control scope ======>|
  * 2.0  * * * * * * *
  *     |            .*
  *     |            . *
  *     |            .   *
  *     |            .     *
  *     |            .        *
  *     |            .            *
  * 1.0 ................................*
  *     |            .                  .     *
  *     |            .                  .          *
  *     |            .                  .              *
  *     |            .                  .                 *
  *     |            .                  .                    *
  *   0 +------------.------------------.----------------------*------------->
  *           freerun^          setpoint^                 limit^   dirty pages
  *
  * (o) wb control line
  *
  *     ^ pos_ratio
  *     |
  *     |            *
  *     |              *
  *     |                *
  *     |                  *
  *     |                    * |<=========== span ============>|
  * 1.0 .......................*
  *     |                      . *
  *     |                      .   *
  *     |                      .     *
  *     |                      .       *
  *     |                      .         *
  *     |                      .           *
  *     |                      .             *
  *     |                      .               *
  *     |                      .                 *
  *     |                      .                   *
  *     |                      .                     *
  * 1/4 ...............................................* * * * * * * * * * * *
  *     |                      .                         .
  *     |                      .                           .
  *     |                      .                             .
  *   0 +----------------------.-------------------------------.------------->
  *                wb_setpoint^                    x_intercept^
  *
  * The wb control line won't drop below pos_ratio=1/4, so that wb_dirty can
  * be smoothly throttled down to normal if it starts high in situations like
  * - start writing to a slow SD card and a fast disk at the same time. The SD
  *   card's wb_dirty may rush to many times higher than wb_setpoint.
  * - the wb dirty thresh drops quickly due to change of JBOD workload
  */
 static void wb_position_ratio(struct dirty_throttle_control *dtc)
 {
     struct bdi_writeback *wb = dtc->wb;
     unsigned long write_bw = READ_ONCE(wb->avg_write_bandwidth);
     unsigned long freerun = dirty_freerun_ceiling(dtc->thresh, dtc->bg_thresh);
     unsigned long limit = hard_dirty_limit(dtc_dom(dtc), dtc->thresh);
     unsigned long wb_thresh = dtc->wb_thresh;
     unsigned long x_intercept;
     unsigned long setpoint;     /* dirty pages' target balance point */
     unsigned long wb_setpoint;
     unsigned long span;
     long long pos_ratio;        /* for scaling up/down the rate limit */
     long x;
 
     dtc->pos_ratio = 0;
 
     if (unlikely(dtc->dirty >= limit))
         return;
 
     /*
      * global setpoint
      *
      * See comment for pos_ratio_polynom().
      */
     setpoint = (freerun + limit) / 2;
     pos_ratio = pos_ratio_polynom(setpoint, dtc->dirty, limit);
 
     /*
      * The strictlimit feature is a tool preventing mistrusted filesystems
      * from growing a large number of dirty pages before throttling. For
      * such filesystems balance_dirty_pages always checks wb counters
      * against wb limits. Even if global "nr_dirty" is under "freerun".
      * This is especially important for fuse which sets bdi->max_ratio to
      * 1% by default. Without strictlimit feature, fuse writeback may
      * consume arbitrary amount of RAM because it is accounted in
      * NR_WRITEBACK_TEMP which is not involved in calculating "nr_dirty".
      *
      * Here, in wb_position_ratio(), we calculate pos_ratio based on
      * two values: wb_dirty and wb_thresh. Let's consider an example:
      * total amount of RAM is 16GB, bdi->max_ratio is equal to 1%, global
      * limits are set by default to 10% and 20% (background and throttle).
      * Then wb_thresh is 1% of 20% of 16GB. This amounts to ~8K pages.
      * wb_calc_thresh(wb, bg_thresh) is about ~4K pages. wb_setpoint is
      * about ~6K pages (as the average of background and throttle wb
      * limits). The 3rd order polynomial will provide positive feedback if
      * wb_dirty is under wb_setpoint and vice versa.
      *
      * Note, that we cannot use global counters in these calculations
      * because we want to throttle process writing to a strictlimit wb
      * much earlier than global "freerun" is reached (~23MB vs. ~2.3GB
      * in the example above).
      */
     if (unlikely(wb->bdi->capabilities & BDI_CAP_STRICTLIMIT)) {
         long long wb_pos_ratio;
 
         if (dtc->wb_dirty < 8) {
             dtc->pos_ratio = min_t(long long, pos_ratio * 2,
                        2 << RATELIMIT_CALC_SHIFT);
             return;
         }
 
         if (dtc->wb_dirty >= wb_thresh)
             return;
 
         wb_setpoint = dirty_freerun_ceiling(wb_thresh,
                             dtc->wb_bg_thresh);
 
         if (wb_setpoint == 0 || wb_setpoint == wb_thresh)
             return;
 
         wb_pos_ratio = pos_ratio_polynom(wb_setpoint, dtc->wb_dirty,
                          wb_thresh);
 
         /*
          * Typically, for strictlimit case, wb_setpoint << setpoint
          * and pos_ratio >> wb_pos_ratio. In the other words global
          * state ("dirty") is not limiting factor and we have to
          * make decision based on wb counters. But there is an
          * important case when global pos_ratio should get precedence:
          * global limits are exceeded (e.g. due to activities on other
          * wb's) while given strictlimit wb is below limit.
          *
          * "pos_ratio * wb_pos_ratio" would work for the case above,
          * but it would look too non-natural for the case of all
          * activity in the system coming from a single strictlimit wb
          * with bdi->max_ratio == 100%.
          *
          * Note that min() below somewhat changes the dynamics of the
          * control system. Normally, pos_ratio value can be well over 3
          * (when globally we are at freerun and wb is well below wb
          * setpoint). Now the maximum pos_ratio in the same situation
          * is 2. We might want to tweak this if we observe the control
          * system is too slow to adapt.
          */
         dtc->pos_ratio = min(pos_ratio, wb_pos_ratio);
         return;
     }
 
     /*
      * We have computed basic pos_ratio above based on global situation. If
      * the wb is over/under its share of dirty pages, we want to scale
      * pos_ratio further down/up. That is done by the following mechanism.
      */
 
     /*
      * wb setpoint
      *
      *        f(wb_dirty) := 1.0 + k * (wb_dirty - wb_setpoint)
      *
      *                        x_intercept - wb_dirty
      *                     := --------------------------
      *                        x_intercept - wb_setpoint
      *
      * The main wb control line is a linear function that subjects to
      *
      * (1) f(wb_setpoint) = 1.0
      * (2) k = - 1 / (8 * write_bw)  (in single wb case)
      *     or equally: x_intercept = wb_setpoint + 8 * write_bw
      *
      * For single wb case, the dirty pages are observed to fluctuate
      * regularly within range
      *        [wb_setpoint - write_bw/2, wb_setpoint + write_bw/2]
      * for various filesystems, where (2) can yield in a reasonable 12.5%
      * fluctuation range for pos_ratio.
      *
      * For JBOD case, wb_thresh (not wb_dirty!) could fluctuate up to its
      * own size, so move the slope over accordingly and choose a slope that
      * yields 100% pos_ratio fluctuation on suddenly doubled wb_thresh.
      */
     if (unlikely(wb_thresh > dtc->thresh))
         wb_thresh = dtc->thresh;
     /*
      * It's very possible that wb_thresh is close to 0 not because the
      * device is slow, but that it has remained inactive for long time.
      * Honour such devices a reasonable good (hopefully IO efficient)
      * threshold, so that the occasional writes won't be blocked and active
      * writes can rampup the threshold quickly.
      */
     wb_thresh = max(wb_thresh, (limit - dtc->dirty) / 8);
     /*
      * scale global setpoint to wb's:
      *  wb_setpoint = setpoint * wb_thresh / thresh
      */
     x = div_u64((u64)wb_thresh << 16, dtc->thresh | 1);
     wb_setpoint = setpoint * (u64)x >> 16;
     /*
      * Use span=(8*write_bw) in single wb case as indicated by
      * (thresh - wb_thresh ~= 0) and transit to wb_thresh in JBOD case.
      *
      *        wb_thresh                    thresh - wb_thresh
      * span = --------- * (8 * write_bw) + ------------------ * wb_thresh
      *         thresh                           thresh
      */
     span = (dtc->thresh - wb_thresh + 8 * write_bw) * (u64)x >> 16;
     x_intercept = wb_setpoint + span;
 
     if (dtc->wb_dirty < x_intercept - span / 4) {
         pos_ratio = div64_u64(pos_ratio * (x_intercept - dtc->wb_dirty),
                       (x_intercept - wb_setpoint) | 1);
     } else
         pos_ratio /= 4;
 
     /*
      * wb reserve area, safeguard against dirty pool underrun and disk idle
      * It may push the desired control point of global dirty pages higher
      * than setpoint.
      */
     x_intercept = wb_thresh / 2;
     if (dtc->wb_dirty < x_intercept) {
         if (dtc->wb_dirty > x_intercept / 8)
             pos_ratio = div_u64(pos_ratio * x_intercept,
                         dtc->wb_dirty);
         else
             pos_ratio *= 8;
     }
 
     dtc->pos_ratio = pos_ratio;
 }
 
 static void wb_update_write_bandwidth(struct bdi_writeback *wb,
                       unsigned long elapsed,
                       unsigned long written)
 {
     const unsigned long period = roundup_pow_of_two(3 * HZ);
     unsigned long avg = wb->avg_write_bandwidth;
     unsigned long old = wb->write_bandwidth;
     u64 bw;
 
     /*
      * bw = written * HZ / elapsed
      *
      *                   bw * elapsed + write_bandwidth * (period - elapsed)
      * write_bandwidth = ---------------------------------------------------
      *                                          period
      *
      * @written may have decreased due to folio_account_redirty().
      * Avoid underflowing @bw calculation.
      */
     bw = written - min(written, wb->written_stamp);
     bw *= HZ;
     if (unlikely(elapsed > period)) {
         bw = div64_ul(bw, elapsed);
         avg = bw;
         goto out;
     }
     bw += (u64)wb->write_bandwidth * (period - elapsed);
     bw >>= ilog2(period);
 
     /*
      * one more level of smoothing, for filtering out sudden spikes
      */
     if (avg > old && old >= (unsigned long)bw)
         avg -= (avg - old) >> 3;
 
     if (avg < old && old <= (unsigned long)bw)
         avg += (old - avg) >> 3;
 
 out:
     /* keep avg > 0 to guarantee that tot > 0 if there are dirty wbs */
     avg = max(avg, 1LU);
     if (wb_has_dirty_io(wb)) {
         long delta = avg - wb->avg_write_bandwidth;
         WARN_ON_ONCE(atomic_long_add_return(delta,
                     &wb->bdi->tot_write_bandwidth) <= 0);
     }
     wb->write_bandwidth = bw;
     WRITE_ONCE(wb->avg_write_bandwidth, avg);
 }
 
 static void update_dirty_limit(struct dirty_throttle_control *dtc)
 {
     struct wb_domain *dom = dtc_dom(dtc);
     unsigned long thresh = dtc->thresh;
     unsigned long limit = dom->dirty_limit;
 
     /*
      * Follow up in one step.
      */
     if (limit < thresh) {
         limit = thresh;
         goto update;
     }
 
     /*
      * Follow down slowly. Use the higher one as the target, because thresh
      * may drop below dirty. This is exactly the reason to introduce
      * dom->dirty_limit which is guaranteed to lie above the dirty pages.
      */
     thresh = max(thresh, dtc->dirty);
     if (limit > thresh) {
         limit -= (limit - thresh) >> 5;
         goto update;
     }
     return;
 update:
     dom->dirty_limit = limit;
 }
 
 static void domain_update_dirty_limit(struct dirty_throttle_control *dtc,
                       unsigned long now)
 {
     struct wb_domain *dom = dtc_dom(dtc);
 
     /*
      * check locklessly first to optimize away locking for the most time
      */
     if (time_before(now, dom->dirty_limit_tstamp + BANDWIDTH_INTERVAL))
         return;
 
     spin_lock(&dom->lock);
     if (time_after_eq(now, dom->dirty_limit_tstamp + BANDWIDTH_INTERVAL)) {
         update_dirty_limit(dtc);
         dom->dirty_limit_tstamp = now;
     }
     spin_unlock(&dom->lock);
 }
 
 /*
  * Maintain wb->dirty_ratelimit, the base dirty throttle rate.
  *
  * Normal wb tasks will be curbed at or below it in long term.
  * Obviously it should be around (write_bw / N) when there are N dd tasks.
  */
 static void wb_update_dirty_ratelimit(struct dirty_throttle_control *dtc,
                       unsigned long dirtied,
                       unsigned long elapsed)
 {
     struct bdi_writeback *wb = dtc->wb;
     unsigned long dirty = dtc->dirty;
     unsigned long freerun = dirty_freerun_ceiling(dtc->thresh, dtc->bg_thresh);
     unsigned long limit = hard_dirty_limit(dtc_dom(dtc), dtc->thresh);
     unsigned long setpoint = (freerun + limit) / 2;
     unsigned long write_bw = wb->avg_write_bandwidth;
     unsigned long dirty_ratelimit = wb->dirty_ratelimit;
     unsigned long dirty_rate;
     unsigned long task_ratelimit;
     unsigned long balanced_dirty_ratelimit;
     unsigned long step;
     unsigned long x;
     unsigned long shift;
 
     /*
      * The dirty rate will match the writeout rate in long term, except
      * when dirty pages are truncated by userspace or re-dirtied by FS.
      */
     dirty_rate = (dirtied - wb->dirtied_stamp) * HZ / elapsed;
 
     /*
      * task_ratelimit reflects each dd's dirty rate for the past 200ms.
      */
     task_ratelimit = (u64)dirty_ratelimit *
                     dtc->pos_ratio >> RATELIMIT_CALC_SHIFT;
     task_ratelimit++; /* it helps rampup dirty_ratelimit from tiny values */
 
     /*
      * A linear estimation of the "balanced" throttle rate. The theory is,
      * if there are N dd tasks, each throttled at task_ratelimit, the wb's
      * dirty_rate will be measured to be (N * task_ratelimit). So the below
      * formula will yield the balanced rate limit (write_bw / N).
      *
      * Note that the expanded form is not a pure rate feedback:
      *  rate_(i+1) = rate_(i) * (write_bw / dirty_rate)          (1)
      * but also takes pos_ratio into account:
      *  rate_(i+1) = rate_(i) * (write_bw / dirty_rate) * pos_ratio  (2)
      *
      * (1) is not realistic because pos_ratio also takes part in balancing
      * the dirty rate.  Consider the state
      *  pos_ratio = 0.5                          (3)
      *  rate = 2 * (write_bw / N)                    (4)
      * If (1) is used, it will stuck in that state! Because each dd will
      * be throttled at
      *  task_ratelimit = pos_ratio * rate = (write_bw / N)       (5)
      * yielding
      *  dirty_rate = N * task_ratelimit = write_bw           (6)
      * put (6) into (1) we get
      *  rate_(i+1) = rate_(i)                        (7)
      *
      * So we end up using (2) to always keep
      *  rate_(i+1) ~= (write_bw / N)                     (8)
      * regardless of the value of pos_ratio. As long as (8) is satisfied,
      * pos_ratio is able to drive itself to 1.0, which is not only where
      * the dirty count meet the setpoint, but also where the slope of
      * pos_ratio is most flat and hence task_ratelimit is least fluctuated.
      */
     balanced_dirty_ratelimit = div_u64((u64)task_ratelimit * write_bw,
                        dirty_rate | 1);
     /*
      * balanced_dirty_ratelimit ~= (write_bw / N) <= write_bw
      */
     if (unlikely(balanced_dirty_ratelimit > write_bw))
         balanced_dirty_ratelimit = write_bw;
 
     /*
      * We could safely do this and return immediately:
      *
      *  wb->dirty_ratelimit = balanced_dirty_ratelimit;
      *
      * However to get a more stable dirty_ratelimit, the below elaborated
      * code makes use of task_ratelimit to filter out singular points and
      * limit the step size.
      *
      * The below code essentially only uses the relative value of
      *
      *  task_ratelimit - dirty_ratelimit
      *  = (pos_ratio - 1) * dirty_ratelimit
      *
      * which reflects the direction and size of dirty position error.
      */
 
     /*
      * dirty_ratelimit will follow balanced_dirty_ratelimit iff
      * task_ratelimit is on the same side of dirty_ratelimit, too.
      * For example, when
      * - dirty_ratelimit > balanced_dirty_ratelimit
      * - dirty_ratelimit > task_ratelimit (dirty pages are above setpoint)
      * lowering dirty_ratelimit will help meet both the position and rate
      * control targets. Otherwise, don't update dirty_ratelimit if it will
      * only help meet the rate target. After all, what the users ultimately
      * feel and care are stable dirty rate and small position error.
      *
      * |task_ratelimit - dirty_ratelimit| is used to limit the step size
      * and filter out the singular points of balanced_dirty_ratelimit. Which
      * keeps jumping around randomly and can even leap far away at times
      * due to the small 200ms estimation period of dirty_rate (we want to
      * keep that period small to reduce time lags).
      */
     step = 0;
 
     /*
      * For strictlimit case, calculations above were based on wb counters
      * and limits (starting from pos_ratio = wb_position_ratio() and up to
      * balanced_dirty_ratelimit = task_ratelimit * write_bw / dirty_rate).
      * Hence, to calculate "step" properly, we have to use wb_dirty as
      * "dirty" and wb_setpoint as "setpoint".
      *
      * We rampup dirty_ratelimit forcibly if wb_dirty is low because
      * it's possible that wb_thresh is close to zero due to inactivity
      * of backing device.
      */
     if (unlikely(wb->bdi->capabilities & BDI_CAP_STRICTLIMIT)) {
         dirty = dtc->wb_dirty;
         if (dtc->wb_dirty < 8)
             setpoint = dtc->wb_dirty + 1;
         else
             setpoint = (dtc->wb_thresh + dtc->wb_bg_thresh) / 2;
     }
 
     if (dirty < setpoint) {
         x = min3(wb->balanced_dirty_ratelimit,
              balanced_dirty_ratelimit, task_ratelimit);
         if (dirty_ratelimit < x)
             step = x - dirty_ratelimit;
     } else {
         x = max3(wb->balanced_dirty_ratelimit,
              balanced_dirty_ratelimit, task_ratelimit);
         if (dirty_ratelimit > x)
             step = dirty_ratelimit - x;
     }
 
     /*
      * Don't pursue 100% rate matching. It's impossible since the balanced
      * rate itself is constantly fluctuating. So decrease the track speed
      * when it gets close to the target. Helps eliminate pointless tremors.
      */
     shift = dirty_ratelimit / (2 * step + 1);
     if (shift < BITS_PER_LONG)
         step = DIV_ROUND_UP(step >> shift, 8);
     else
         step = 0;
 
     if (dirty_ratelimit < balanced_dirty_ratelimit)
         dirty_ratelimit += step;
     else
         dirty_ratelimit -= step;
 
     WRITE_ONCE(wb->dirty_ratelimit, max(dirty_ratelimit, 1UL));
     wb->balanced_dirty_ratelimit = balanced_dirty_ratelimit;
 
     trace_bdi_dirty_ratelimit(wb, dirty_rate, task_ratelimit);
 }
 
 static void __wb_update_bandwidth(struct dirty_throttle_control *gdtc,
                   struct dirty_throttle_control *mdtc,
                   bool update_ratelimit)
 {
     struct bdi_writeback *wb = gdtc->wb;
     unsigned long now = jiffies;
     unsigned long elapsed;
     unsigned long dirtied;
     unsigned long written;
 
     spin_lock(&wb->list_lock);
 
     /*
      * Lockless checks for elapsed time are racy and delayed update after
      * IO completion doesn't do it at all (to make sure written pages are
      * accounted reasonably quickly). Make sure elapsed >= 1 to avoid
      * division errors.
      */
     elapsed = max(now - wb->bw_time_stamp, 1UL);
     dirtied = percpu_counter_read(&wb->stat[WB_DIRTIED]);
     written = percpu_counter_read(&wb->stat[WB_WRITTEN]);
 
     if (update_ratelimit) {
         domain_update_dirty_limit(gdtc, now);
         wb_update_dirty_ratelimit(gdtc, dirtied, elapsed);
 
         /*
          * @mdtc is always NULL if !CGROUP_WRITEBACK but the
          * compiler has no way to figure that out.  Help it.
          */
         if (IS_ENABLED(CONFIG_CGROUP_WRITEBACK) && mdtc) {
             domain_update_dirty_limit(mdtc, now);
             wb_update_dirty_ratelimit(mdtc, dirtied, elapsed);
         }
     }
     wb_update_write_bandwidth(wb, elapsed, written);
 
     wb->dirtied_stamp = dirtied;
     wb->written_stamp = written;
     WRITE_ONCE(wb->bw_time_stamp, now);
     spin_unlock(&wb->list_lock);
 }
 
 void wb_update_bandwidth(struct bdi_writeback *wb)
 {
     struct dirty_throttle_control gdtc = { GDTC_INIT(wb) };
 
     __wb_update_bandwidth(&gdtc, NULL, false);
 }
 
 /* Interval after which we consider wb idle and don't estimate bandwidth */
 #define WB_BANDWIDTH_IDLE_JIF (HZ)
 
 static void wb_bandwidth_estimate_start(struct bdi_writeback *wb)
 {
     unsigned long now = jiffies;
     unsigned long elapsed = now - READ_ONCE(wb->bw_time_stamp);
 
     if (elapsed > WB_BANDWIDTH_IDLE_JIF &&
         !atomic_read(&wb->writeback_inodes)) {
         spin_lock(&wb->list_lock);
         wb->dirtied_stamp = wb_stat(wb, WB_DIRTIED);
         wb->written_stamp = wb_stat(wb, WB_WRITTEN);
         WRITE_ONCE(wb->bw_time_stamp, now);
         spin_unlock(&wb->list_lock);
     }
 }
 
 /*
  * After a task dirtied this many pages, balance_dirty_pages_ratelimited()
  * will look to see if it needs to start dirty throttling.
  *
  * If dirty_poll_interval is too low, big NUMA machines will call the expensive
  * global_zone_page_state() too often. So scale it near-sqrt to the safety margin
  * (the number of pages we may dirty without exceeding the dirty limits).
  */
 static unsigned long dirty_poll_interval(unsigned long dirty,
                      unsigned long thresh)
 {
     if (thresh > dirty)
         return 1UL << (ilog2(thresh - dirty) >> 1);
 
     return 1;
 }
 
 static unsigned long wb_max_pause(struct bdi_writeback *wb,
                   unsigned long wb_dirty)
 {
     unsigned long bw = READ_ONCE(wb->avg_write_bandwidth);
     unsigned long t;
 
     /*
      * Limit pause time for small memory systems. If sleeping for too long
      * time, a small pool of dirty/writeback pages may go empty and disk go
      * idle.
      *
      * 8 serves as the safety ratio.
      */
     t = wb_dirty / (1 + bw / roundup_pow_of_two(1 + HZ / 8));
     t++;
 
     return min_t(unsigned long, t, MAX_PAUSE);
 }
 
 static long wb_min_pause(struct bdi_writeback *wb,
              long max_pause,
              unsigned long task_ratelimit,
              unsigned long dirty_ratelimit,
              int *nr_dirtied_pause)
 {
     long hi = ilog2(READ_ONCE(wb->avg_write_bandwidth));
     long lo = ilog2(READ_ONCE(wb->dirty_ratelimit));
     long t;     /* target pause */
     long pause; /* estimated next pause */
     int pages;  /* target nr_dirtied_pause */
 
     /* target for 10ms pause on 1-dd case */
     t = max(1, HZ / 100);
 
     /*
      * Scale up pause time for concurrent dirtiers in order to reduce CPU
      * overheads.
      *
      * (N * 10ms) on 2^N concurrent tasks.
      */
     if (hi > lo)
         t += (hi - lo) * (10 * HZ) / 1024;
 
     /*
      * This is a bit convoluted. We try to base the next nr_dirtied_pause
      * on the much more stable dirty_ratelimit. However the next pause time
      * will be computed based on task_ratelimit and the two rate limits may
      * depart considerably at some time. Especially if task_ratelimit goes
      * below dirty_ratelimit/2 and the target pause is max_pause, the next
      * pause time will be max_pause*2 _trimmed down_ to max_pause.  As a
      * result task_ratelimit won't be executed faithfully, which could
      * eventually bring down dirty_ratelimit.
      *
      * We apply two rules to fix it up:
      * 1) try to estimate the next pause time and if necessary, use a lower
      *    nr_dirtied_pause so as not to exceed max_pause. When this happens,
      *    nr_dirtied_pause will be "dancing" with task_ratelimit.
      * 2) limit the target pause time to max_pause/2, so that the normal
      *    small fluctuations of task_ratelimit won't trigger rule (1) and
      *    nr_dirtied_pause will remain as stable as dirty_ratelimit.
      */
     t = min(t, 1 + max_pause / 2);
     pages = dirty_ratelimit * t / roundup_pow_of_two(HZ);
 
     /*
      * Tiny nr_dirtied_pause is found to hurt I/O performance in the test
      * case fio-mmap-randwrite-64k, which does 16*{sync read, async write}.
      * When the 16 consecutive reads are often interrupted by some dirty
      * throttling pause during the async writes, cfq will go into idles
      * (deadline is fine). So push nr_dirtied_pause as high as possible
      * until reaches DIRTY_POLL_THRESH=32 pages.
      */
     if (pages < DIRTY_POLL_THRESH) {
         t = max_pause;
         pages = dirty_ratelimit * t / roundup_pow_of_two(HZ);
         if (pages > DIRTY_POLL_THRESH) {
             pages = DIRTY_POLL_THRESH;
             t = HZ * DIRTY_POLL_THRESH / dirty_ratelimit;
         }
     }
 
     pause = HZ * pages / (task_ratelimit + 1);
     if (pause > max_pause) {
         t = max_pause;
         pages = task_ratelimit * t / roundup_pow_of_two(HZ);
     }
 
     *nr_dirtied_pause = pages;
     /*
      * The minimal pause time will normally be half the target pause time.
      */
     return pages >= DIRTY_POLL_THRESH ? 1 + t / 2 : t;
 }
 
 static inline void wb_dirty_limits(struct dirty_throttle_control *dtc)
 {
     struct bdi_writeback *wb = dtc->wb;
     unsigned long wb_reclaimable;
 
     /*
      * wb_thresh is not treated as some limiting factor as
      * dirty_thresh, due to reasons
      * - in JBOD setup, wb_thresh can fluctuate a lot
      * - in a system with HDD and USB key, the USB key may somehow
      *   go into state (wb_dirty >> wb_thresh) either because
      *   wb_dirty starts high, or because wb_thresh drops low.
      *   In this case we don't want to hard throttle the USB key
      *   dirtiers for 100 seconds until wb_dirty drops under
      *   wb_thresh. Instead the auxiliary wb control line in
      *   wb_position_ratio() will let the dirtier task progress
      *   at some rate <= (write_bw / 2) for bringing down wb_dirty.
      */
     dtc->wb_thresh = __wb_calc_thresh(dtc);
     dtc->wb_bg_thresh = dtc->thresh ?
         div_u64((u64)dtc->wb_thresh * dtc->bg_thresh, dtc->thresh) : 0;
 
     /*
      * In order to avoid the stacked BDI deadlock we need
      * to ensure we accurately count the 'dirty' pages when
      * the threshold is low.
      *
      * Otherwise it would be possible to get thresh+n pages
      * reported dirty, even though there are thresh-m pages
      * actually dirty; with m+n sitting in the percpu
      * deltas.
      */
     if (dtc->wb_thresh < 2 * wb_stat_error()) {
         wb_reclaimable = wb_stat_sum(wb, WB_RECLAIMABLE);
         dtc->wb_dirty = wb_reclaimable + wb_stat_sum(wb, WB_WRITEBACK);
     } else {
         wb_reclaimable = wb_stat(wb, WB_RECLAIMABLE);
         dtc->wb_dirty = wb_reclaimable + wb_stat(wb, WB_WRITEBACK);
     }
 }
 
 /*
  * balance_dirty_pages() must be called by processes which are generating dirty
  * data.  It looks at the number of dirty pages in the machine and will force
  * the caller to wait once crossing the (background_thresh + dirty_thresh) / 2.
  * If we're over `background_thresh' then the writeback threads are woken to
  * perform some writeout.
  */
 static int balance_dirty_pages(struct bdi_writeback *wb,
                    unsigned long pages_dirtied, unsigned int flags)
 {
     struct dirty_throttle_control gdtc_stor = { GDTC_INIT(wb) };
     struct dirty_throttle_control mdtc_stor = { MDTC_INIT(wb, &gdtc_stor) };
     struct dirty_throttle_control * const gdtc = &gdtc_stor;
     struct dirty_throttle_control * const mdtc = mdtc_valid(&mdtc_stor) ?
                              &mdtc_stor : NULL;
     struct dirty_throttle_control *sdtc;
     unsigned long nr_reclaimable;   /* = file_dirty */
     long period;
     long pause;
     long max_pause;
     long min_pause;
     int nr_dirtied_pause;
     bool dirty_exceeded = false;
     unsigned long task_ratelimit;
     unsigned long dirty_ratelimit;
     struct backing_dev_info *bdi = wb->bdi;
     bool strictlimit = bdi->capabilities & BDI_CAP_STRICTLIMIT;
     unsigned long start_time = jiffies;
     int ret = 0;
 
     for (;;) {
         unsigned long now = jiffies;
         unsigned long dirty, thresh, bg_thresh;
         unsigned long m_dirty = 0;  /* stop bogus uninit warnings */
         unsigned long m_thresh = 0;
         unsigned long m_bg_thresh = 0;
 
         nr_reclaimable = global_node_page_state(NR_FILE_DIRTY);
         gdtc->avail = global_dirtyable_memory();
         gdtc->dirty = nr_reclaimable + global_node_page_state(NR_WRITEBACK);
 
         domain_dirty_limits(gdtc);
 
         if (unlikely(strictlimit)) {
             wb_dirty_limits(gdtc);
 
             dirty = gdtc->wb_dirty;
             thresh = gdtc->wb_thresh;
             bg_thresh = gdtc->wb_bg_thresh;
         } else {
             dirty = gdtc->dirty;
             thresh = gdtc->thresh;
             bg_thresh = gdtc->bg_thresh;
         }
 
         if (mdtc) {
             unsigned long filepages, headroom, writeback;
 
             /*
              * If @wb belongs to !root memcg, repeat the same
              * basic calculations for the memcg domain.
              */
             mem_cgroup_wb_stats(wb, &filepages, &headroom,
                         &mdtc->dirty, &writeback);
             mdtc->dirty += writeback;
             mdtc_calc_avail(mdtc, filepages, headroom);
 
             domain_dirty_limits(mdtc);
 
             if (unlikely(strictlimit)) {
                 wb_dirty_limits(mdtc);
                 m_dirty = mdtc->wb_dirty;
                 m_thresh = mdtc->wb_thresh;
                 m_bg_thresh = mdtc->wb_bg_thresh;
             } else {
                 m_dirty = mdtc->dirty;
                 m_thresh = mdtc->thresh;
                 m_bg_thresh = mdtc->bg_thresh;
             }
         }
 
         /*
          * In laptop mode, we wait until hitting the higher threshold
          * before starting background writeout, and then write out all
          * the way down to the lower threshold.  So slow writers cause
          * minimal disk activity.
          *
          * In normal mode, we start background writeout at the lower
          * background_thresh, to keep the amount of dirty memory low.
          */
         if (!laptop_mode && nr_reclaimable > gdtc->bg_thresh &&
             !writeback_in_progress(wb))
             wb_start_background_writeback(wb);
 
         /*
          * Throttle it only when the background writeback cannot
          * catch-up. This avoids (excessively) small writeouts
          * when the wb limits are ramping up in case of !strictlimit.
          *
          * In strictlimit case make decision based on the wb counters
          * and limits. Small writeouts when the wb limits are ramping
          * up are the price we consciously pay for strictlimit-ing.
          *
          * If memcg domain is in effect, @dirty should be under
          * both global and memcg freerun ceilings.
          */
         if (dirty <= dirty_freerun_ceiling(thresh, bg_thresh) &&
             (!mdtc ||
              m_dirty <= dirty_freerun_ceiling(m_thresh, m_bg_thresh))) {
             unsigned long intv;
             unsigned long m_intv;
 
 free_running:
             intv = dirty_poll_interval(dirty, thresh);
             m_intv = ULONG_MAX;
 
             current->dirty_paused_when = now;
             current->nr_dirtied = 0;
             if (mdtc)
                 m_intv = dirty_poll_interval(m_dirty, m_thresh);
             current->nr_dirtied_pause = min(intv, m_intv);
             break;
         }
 
         /* Start writeback even when in laptop mode */
         if (unlikely(!writeback_in_progress(wb)))
             wb_start_background_writeback(wb);
 
         mem_cgroup_flush_foreign(wb);
 
         /*
          * Calculate global domain's pos_ratio and select the
          * global dtc by default.
          */
         if (!strictlimit) {
             wb_dirty_limits(gdtc);
 
             if ((current->flags & PF_LOCAL_THROTTLE) &&
                 gdtc->wb_dirty <
                 dirty_freerun_ceiling(gdtc->wb_thresh,
                           gdtc->wb_bg_thresh))
                 /*
                  * LOCAL_THROTTLE tasks must not be throttled
                  * when below the per-wb freerun ceiling.
                  */
                 goto free_running;
         }
 
         dirty_exceeded = (gdtc->wb_dirty > gdtc->wb_thresh) &&
             ((gdtc->dirty > gdtc->thresh) || strictlimit);
 
         wb_position_ratio(gdtc);
         sdtc = gdtc;
 
         if (mdtc) {
             /*
              * If memcg domain is in effect, calculate its
              * pos_ratio.  @wb should satisfy constraints from
              * both global and memcg domains.  Choose the one
              * w/ lower pos_ratio.
              */
             if (!strictlimit) {
                 wb_dirty_limits(mdtc);
 
                 if ((current->flags & PF_LOCAL_THROTTLE) &&
                     mdtc->wb_dirty <
                     dirty_freerun_ceiling(mdtc->wb_thresh,
                               mdtc->wb_bg_thresh))
                     /*
                      * LOCAL_THROTTLE tasks must not be
                      * throttled when below the per-wb
                      * freerun ceiling.
                      */
                     goto free_running;
             }
             dirty_exceeded |= (mdtc->wb_dirty > mdtc->wb_thresh) &&
                 ((mdtc->dirty > mdtc->thresh) || strictlimit);
 
             wb_position_ratio(mdtc);
             if (mdtc->pos_ratio < gdtc->pos_ratio)
                 sdtc = mdtc;
         }
 
         if (dirty_exceeded != wb->dirty_exceeded)
             wb->dirty_exceeded = dirty_exceeded;
 
         if (time_is_before_jiffies(READ_ONCE(wb->bw_time_stamp) +
                        BANDWIDTH_INTERVAL))
             __wb_update_bandwidth(gdtc, mdtc, true);
 
         /* throttle according to the chosen dtc */
         dirty_ratelimit = READ_ONCE(wb->dirty_ratelimit);
         task_ratelimit = ((u64)dirty_ratelimit * sdtc->pos_ratio) >>
                             RATELIMIT_CALC_SHIFT;
         max_pause = wb_max_pause(wb, sdtc->wb_dirty);
         min_pause = wb_min_pause(wb, max_pause,
                      task_ratelimit, dirty_ratelimit,
                      &nr_dirtied_pause);
 
         if (unlikely(task_ratelimit == 0)) {
             period = max_pause;
             pause = max_pause;
             goto pause;
         }
         period = HZ * pages_dirtied / task_ratelimit;
         pause = period;
         if (current->dirty_paused_when)
             pause -= now - current->dirty_paused_when;
         /*
          * For less than 1s think time (ext3/4 may block the dirtier
          * for up to 800ms from time to time on 1-HDD; so does xfs,
          * however at much less frequency), try to compensate it in
          * future periods by updating the virtual time; otherwise just
          * do a reset, as it may be a light dirtier.
          */
         if (pause < min_pause) {
             trace_balance_dirty_pages(wb,
                           sdtc->thresh,
                           sdtc->bg_thresh,
                           sdtc->dirty,
                           sdtc->wb_thresh,
                           sdtc->wb_dirty,
                           dirty_ratelimit,
                           task_ratelimit,
                           pages_dirtied,
                           period,
                           min(pause, 0L),
                           start_time);
             if (pause < -HZ) {
                 current->dirty_paused_when = now;
                 current->nr_dirtied = 0;
             } else if (period) {
                 current->dirty_paused_when += period;
                 current->nr_dirtied = 0;
             } else if (current->nr_dirtied_pause <= pages_dirtied)
                 current->nr_dirtied_pause += pages_dirtied;
             break;
         }
         if (unlikely(pause > max_pause)) {
             /* for occasional dropped task_ratelimit */
             now += min(pause - max_pause, max_pause);
             pause = max_pause;
         }
 
 pause:
         trace_balance_dirty_pages(wb,
                       sdtc->thresh,
                       sdtc->bg_thresh,
                       sdtc->dirty,
                       sdtc->wb_thresh,
                       sdtc->wb_dirty,
                       dirty_ratelimit,
                       task_ratelimit,
                       pages_dirtied,
                       period,
                       pause,
                       start_time);
         if (flags & BDP_ASYNC) {
             ret = -EAGAIN;
             break;
         }
         __set_current_state(TASK_KILLABLE);
         wb->dirty_sleep = now;
         io_schedule_timeout(pause);
 
         current->dirty_paused_when = now + pause;
         current->nr_dirtied = 0;
         current->nr_dirtied_pause = nr_dirtied_pause;
 
         /*
          * This is typically equal to (dirty < thresh) and can also
          * keep "1000+ dd on a slow USB stick" under control.
          */
         if (task_ratelimit)
             break;
 
         /*
          * In the case of an unresponsive NFS server and the NFS dirty
          * pages exceeds dirty_thresh, give the other good wb's a pipe
          * to go through, so that tasks on them still remain responsive.
          *
          * In theory 1 page is enough to keep the consumer-producer
          * pipe going: the flusher cleans 1 page => the task dirties 1
          * more page. However wb_dirty has accounting errors.  So use
          * the larger and more IO friendly wb_stat_error.
          */
         if (sdtc->wb_dirty <= wb_stat_error())
             break;
 
         if (fatal_signal_pending(current))
             break;
     }
     return ret;
 }
 
 static DEFINE_PER_CPU(int, bdp_ratelimits);
 
 /*
  * Normal tasks are throttled by
  *  loop {
  *      dirty tsk->nr_dirtied_pause pages;
  *      take a snap in balance_dirty_pages();
  *  }
  * However there is a worst case. If every task exit immediately when dirtied
  * (tsk->nr_dirtied_pause - 1) pages, balance_dirty_pages() will never be
  * called to throttle the page dirties. The solution is to save the not yet
  * throttled page dirties in dirty_throttle_leaks on task exit and charge them
  * randomly into the running tasks. This works well for the above worst case,
  * as the new task will pick up and accumulate the old task's leaked dirty
  * count and eventually get throttled.
  */
 DEFINE_PER_CPU(int, dirty_throttle_leaks) = 0;
 
 /**
  * balance_dirty_pages_ratelimited_flags - Balance dirty memory state.
  * @mapping: address_space which was dirtied.
  * @flags: BDP flags.
  *
  * Processes which are dirtying memory should call in here once for each page
  * which was newly dirtied.  The function will periodically check the system's
  * dirty state and will initiate writeback if needed.
  *
  * See balance_dirty_pages_ratelimited() for details.
  *
  * Return: If @flags contains BDP_ASYNC, it may return -EAGAIN to
  * indicate that memory is out of balance and the caller must wait
  * for I/O to complete.  Otherwise, it will return 0 to indicate
  * that either memory was already in balance, or it was able to sleep
  * until the amount of dirty memory returned to balance.
  */
 int balance_dirty_pages_ratelimited_flags(struct address_space *mapping,
                     unsigned int flags)
 {
     struct inode *inode = mapping->host;
     struct backing_dev_info *bdi = inode_to_bdi(inode);
     struct bdi_writeback *wb = NULL;
     int ratelimit;
     int ret = 0;
     int *p;
 
     if (!(bdi->capabilities & BDI_CAP_WRITEBACK))
         return ret;
 
     if (inode_cgwb_enabled(inode))
         wb = wb_get_create_current(bdi, GFP_KERNEL);
     if (!wb)
         wb = &bdi->wb;
 
     ratelimit = current->nr_dirtied_pause;
     if (wb->dirty_exceeded)
         ratelimit = min(ratelimit, 32 >> (PAGE_SHIFT - 10));
 
     preempt_disable();
     /*
      * This prevents one CPU to accumulate too many dirtied pages without
      * calling into balance_dirty_pages(), which can happen when there are
      * 1000+ tasks, all of them start dirtying pages at exactly the same
      * time, hence all honoured too large initial task->nr_dirtied_pause.
      */
     p =  this_cpu_ptr(&bdp_ratelimits);
     if (unlikely(current->nr_dirtied >= ratelimit))
         *p = 0;
     else if (unlikely(*p >= ratelimit_pages)) {
         *p = 0;
         ratelimit = 0;
     }
     /*
      * Pick up the dirtied pages by the exited tasks. This avoids lots of
      * short-lived tasks (eg. gcc invocations in a kernel build) escaping
      * the dirty throttling and livelock other long-run dirtiers.
      */
     p = this_cpu_ptr(&dirty_throttle_leaks);
     if (*p > 0 && current->nr_dirtied < ratelimit) {
         unsigned long nr_pages_dirtied;
         nr_pages_dirtied = min(*p, ratelimit - current->nr_dirtied);
         *p -= nr_pages_dirtied;
         current->nr_dirtied += nr_pages_dirtied;
     }
     preempt_enable();
 
     if (unlikely(current->nr_dirtied >= ratelimit))
         ret = balance_dirty_pages(wb, current->nr_dirtied, flags);
 
     wb_put(wb);
     return ret;
 }
 
 /**
  * balance_dirty_pages_ratelimited - balance dirty memory state.
  * @mapping: address_space which was dirtied.
  *
  * Processes which are dirtying memory should call in here once for each page
  * which was newly dirtied.  The function will periodically check the system's
  * dirty state and will initiate writeback if needed.
  *
  * Once we're over the dirty memory limit we decrease the ratelimiting
  * by a lot, to prevent individual processes from overshooting the limit
  * by (ratelimit_pages) each.
  */
 void balance_dirty_pages_ratelimited(struct address_space *mapping)
 {
     balance_dirty_pages_ratelimited_flags(mapping, 0);
 }
 EXPORT_SYMBOL(balance_dirty_pages_ratelimited);
 
 /**
  * wb_over_bg_thresh - does @wb need to be written back?
  * @wb: bdi_writeback of interest
  *
  * Determines whether background writeback should keep writing @wb or it's
  * clean enough.
  *
  * Return: %true if writeback should continue.
  */
 bool wb_over_bg_thresh(struct bdi_writeback *wb)
 {
     struct dirty_throttle_control gdtc_stor = { GDTC_INIT(wb) };
     struct dirty_throttle_control mdtc_stor = { MDTC_INIT(wb, &gdtc_stor) };
     struct dirty_throttle_control * const gdtc = &gdtc_stor;
     struct dirty_throttle_control * const mdtc = mdtc_valid(&mdtc_stor) ?
                              &mdtc_stor : NULL;
     unsigned long reclaimable;
     unsigned long thresh;
 
     /*
      * Similar to balance_dirty_pages() but ignores pages being written
      * as we're trying to decide whether to put more under writeback.
      */
     gdtc->avail = global_dirtyable_memory();
     gdtc->dirty = global_node_page_state(NR_FILE_DIRTY);
     domain_dirty_limits(gdtc);
 
     if (gdtc->dirty > gdtc->bg_thresh)
         return true;
 
     thresh = wb_calc_thresh(gdtc->wb, gdtc->bg_thresh);
     if (thresh < 2 * wb_stat_error())
         reclaimable = wb_stat_sum(wb, WB_RECLAIMABLE);
     else
         reclaimable = wb_stat(wb, WB_RECLAIMABLE);
 
     if (reclaimable > thresh)
         return true;
 
     if (mdtc) {
         unsigned long filepages, headroom, writeback;
 
         mem_cgroup_wb_stats(wb, &filepages, &headroom, &mdtc->dirty,
                     &writeback);
         mdtc_calc_avail(mdtc, filepages, headroom);
         domain_dirty_limits(mdtc);  /* ditto, ignore writeback */
 
         if (mdtc->dirty > mdtc->bg_thresh)
             return true;
 
         thresh = wb_calc_thresh(mdtc->wb, mdtc->bg_thresh);
         if (thresh < 2 * wb_stat_error())
             reclaimable = wb_stat_sum(wb, WB_RECLAIMABLE);
         else
             reclaimable = wb_stat(wb, WB_RECLAIMABLE);
 
         if (reclaimable > thresh)
             return true;
     }
 
     return false;
 }
 
 #ifdef CONFIG_SYSCTL
 /*
  * sysctl handler for /proc/sys/vm/dirty_writeback_centisecs
  */
 static int dirty_writeback_centisecs_handler(struct ctl_table *table, int write,
         void *buffer, size_t *length, loff_t *ppos)
 {
     unsigned int old_interval = dirty_writeback_interval;
     int ret;
 
     ret = proc_dointvec(table, write, buffer, length, ppos);
 
     /*
      * Writing 0 to dirty_writeback_interval will disable periodic writeback
      * and a different non-zero value will wakeup the writeback threads.
      * wb_wakeup_delayed() would be more appropriate, but it's a pain to
      * iterate over all bdis and wbs.
      * The reason we do this is to make the change take effect immediately.
      */
     if (!ret && write && dirty_writeback_interval &&
         dirty_writeback_interval != old_interval)
         wakeup_flusher_threads(WB_REASON_PERIODIC);
 
     return ret;
 }
 #endif
 
 void laptop_mode_timer_fn(struct timer_list *t)
 {
     struct backing_dev_info *backing_dev_info =
         from_timer(backing_dev_info, t, laptop_mode_wb_timer);
 
     wakeup_flusher_threads_bdi(backing_dev_info, WB_REASON_LAPTOP_TIMER);
 }
 
 /*
  * We've spun up the disk and we're in laptop mode: schedule writeback
  * of all dirty data a few seconds from now.  If the flush is already scheduled
  * then push it back - the user is still using the disk.
  */
 void laptop_io_completion(struct backing_dev_info *info)
 {
     mod_timer(&info->laptop_mode_wb_timer, jiffies + laptop_mode);
 }
 
 /*
  * We're in laptop mode and we've just synced. The sync's writes will have
  * caused another writeback to be scheduled by laptop_io_completion.
  * Nothing needs to be written back anymore, so we unschedule the writeback.
  */
 void laptop_sync_completion(void)
 {
     struct backing_dev_info *bdi;
 
     rcu_read_lock();
 
     list_for_each_entry_rcu(bdi, &bdi_list, bdi_list)
         del_timer(&bdi->laptop_mode_wb_timer);
 
     rcu_read_unlock();
 }
 
 /*
  * If ratelimit_pages is too high then we can get into dirty-data overload
  * if a large number of processes all perform writes at the same time.
  *
  * Here we set ratelimit_pages to a level which ensures that when all CPUs are
  * dirtying in parallel, we cannot go more than 3% (1/32) over the dirty memory
  * thresholds.
  */
 
 void writeback_set_ratelimit(void)
 {
     struct wb_domain *dom = &global_wb_domain;
     unsigned long background_thresh;
     unsigned long dirty_thresh;
 
     global_dirty_limits(&background_thresh, &dirty_thresh);
     dom->dirty_limit = dirty_thresh;
     ratelimit_pages = dirty_thresh / (num_online_cpus() * 32);
     if (ratelimit_pages < 16)
         ratelimit_pages = 16;
 }
 
 static int page_writeback_cpu_online(unsigned int cpu)
 {
     writeback_set_ratelimit();
     return 0;
 }
 
 #ifdef CONFIG_SYSCTL
 
 /* this is needed for the proc_doulongvec_minmax of vm_dirty_bytes */
 static const unsigned long dirty_bytes_min = 2 * PAGE_SIZE;
 
 static struct ctl_table vm_page_writeback_sysctls[] = {
     {
         .procname   = "dirty_background_ratio",
         .data       = &dirty_background_ratio,
         .maxlen     = sizeof(dirty_background_ratio),
         .mode       = 0644,
         .proc_handler   = dirty_background_ratio_handler,
         .extra1     = SYSCTL_ZERO,
         .extra2     = SYSCTL_ONE_HUNDRED,
     },
     {
         .procname   = "dirty_background_bytes",
         .data       = &dirty_background_bytes,
         .maxlen     = sizeof(dirty_background_bytes),
         .mode       = 0644,
         .proc_handler   = dirty_background_bytes_handler,
         .extra1     = SYSCTL_LONG_ONE,
     },
     {
         .procname   = "dirty_ratio",
         .data       = &vm_dirty_ratio,
         .maxlen     = sizeof(vm_dirty_ratio),
         .mode       = 0644,
         .proc_handler   = dirty_ratio_handler,
         .extra1     = SYSCTL_ZERO,
         .extra2     = SYSCTL_ONE_HUNDRED,
     },
     {
         .procname   = "dirty_bytes",
         .data       = &vm_dirty_bytes,
         .maxlen     = sizeof(vm_dirty_bytes),
         .mode       = 0644,
         .proc_handler   = dirty_bytes_handler,
         .extra1     = (void *)&dirty_bytes_min,
     },
     {
         .procname   = "dirty_writeback_centisecs",
         .data       = &dirty_writeback_interval,
         .maxlen     = sizeof(dirty_writeback_interval),
         .mode       = 0644,
         .proc_handler   = dirty_writeback_centisecs_handler,
     },
     {
         .procname   = "dirty_expire_centisecs",
         .data       = &dirty_expire_interval,
         .maxlen     = sizeof(dirty_expire_interval),
         .mode       = 0644,
         .proc_handler   = proc_dointvec_minmax,
         .extra1     = SYSCTL_ZERO,
     },
 #ifdef CONFIG_HIGHMEM
     {
         .procname   = "highmem_is_dirtyable",
         .data       = &vm_highmem_is_dirtyable,
         .maxlen     = sizeof(vm_highmem_is_dirtyable),
         .mode       = 0644,
         .proc_handler   = proc_dointvec_minmax,
         .extra1     = SYSCTL_ZERO,
         .extra2     = SYSCTL_ONE,
     },
 #endif
     {
         .procname   = "laptop_mode",
         .data       = &laptop_mode,
         .maxlen     = sizeof(laptop_mode),
         .mode       = 0644,
         .proc_handler   = proc_dointvec_jiffies,
     },
     {}
 };
 #endif
 
 /*
  * Called early on to tune the page writeback dirty limits.
  *
  * We used to scale dirty pages according to how total memory
  * related to pages that could be allocated for buffers.
  *
  * However, that was when we used "dirty_ratio" to scale with
  * all memory, and we don't do that any more. "dirty_ratio"
  * is now applied to total non-HIGHPAGE memory, and as such we can't
  * get into the old insane situation any more where we had
  * large amounts of dirty pages compared to a small amount of
  * non-HIGHMEM memory.
  *
  * But we might still want to scale the dirty_ratio by how
  * much memory the box has..
  */
 void __init page_writeback_init(void)
 {
     BUG_ON(wb_domain_init(&global_wb_domain, GFP_KERNEL));
 
     cpuhp_setup_state(CPUHP_AP_ONLINE_DYN, "mm/writeback:online",
               page_writeback_cpu_online, NULL);
     cpuhp_setup_state(CPUHP_MM_WRITEBACK_DEAD, "mm/writeback:dead", NULL,
               page_writeback_cpu_online);
 #ifdef CONFIG_SYSCTL
     register_sysctl_init("vm", vm_page_writeback_sysctls);
 #endif
 }
 
 /**
  * tag_pages_for_writeback - tag pages to be written by write_cache_pages
  * @mapping: address space structure to write
  * @start: starting page index
  * @end: ending page index (inclusive)
  *
  * This function scans the page range from @start to @end (inclusive) and tags
  * all pages that have DIRTY tag set with a special TOWRITE tag. The idea is
  * that write_cache_pages (or whoever calls this function) will then use
  * TOWRITE tag to identify pages eligible for writeback.  This mechanism is
  * used to avoid livelocking of writeback by a process steadily creating new
  * dirty pages in the file (thus it is important for this function to be quick
  * so that it can tag pages faster than a dirtying process can create them).
  */
 void tag_pages_for_writeback(struct address_space *mapping,
                  pgoff_t start, pgoff_t end)
 {
     XA_STATE(xas, &mapping->i_pages, start);
     unsigned int tagged = 0;
     void *page;
 
     xas_lock_irq(&xas);
     xas_for_each_marked(&xas, page, end, PAGECACHE_TAG_DIRTY) {
         xas_set_mark(&xas, PAGECACHE_TAG_TOWRITE);
         if (++tagged % XA_CHECK_SCHED)
             continue;
 
         xas_pause(&xas);
         xas_unlock_irq(&xas);
         cond_resched();
         xas_lock_irq(&xas);
     }
     xas_unlock_irq(&xas);
 }
 EXPORT_SYMBOL(tag_pages_for_writeback);
 
 /**
  * write_cache_pages - walk the list of dirty pages of the given address space and write all of them.
  * @mapping: address space structure to write
  * @wbc: subtract the number of written pages from *@wbc->nr_to_write
  * @writepage: function called for each page
  * @data: data passed to writepage function
  *
  * If a page is already under I/O, write_cache_pages() skips it, even
  * if it's dirty.  This is desirable behaviour for memory-cleaning writeback,
  * but it is INCORRECT for data-integrity system calls such as fsync().  fsync()
  * and msync() need to guarantee that all the data which was dirty at the time
  * the call was made get new I/O started against them.  If wbc->sync_mode is
  * WB_SYNC_ALL then we were called for data integrity and we must wait for
  * existing IO to complete.
  *
  * To avoid livelocks (when other process dirties new pages), we first tag
  * pages which should be written back with TOWRITE tag and only then start
  * writing them. For data-integrity sync we have to be careful so that we do
  * not miss some pages (e.g., because some other process has cleared TOWRITE
  * tag we set). The rule we follow is that TOWRITE tag can be cleared only
  * by the process clearing the DIRTY tag (and submitting the page for IO).
  *
  * To avoid deadlocks between range_cyclic writeback and callers that hold
  * pages in PageWriteback to aggregate IO until write_cache_pages() returns,
  * we do not loop back to the start of the file. Doing so causes a page
  * lock/page writeback access order inversion - we should only ever lock
  * multiple pages in ascending page->index order, and looping back to the start
  * of the file violates that rule and causes deadlocks.
  *
  * Return: %0 on success, negative error code otherwise
  */
 int write_cache_pages(struct address_space *mapping,
               struct writeback_control *wbc, writepage_t writepage,
               void *data)
 {
     int ret = 0;
     int done = 0;
     int error;
     struct pagevec pvec;
     int nr_pages;
     pgoff_t index;
     pgoff_t end;        /* Inclusive */
     pgoff_t done_index;
     int range_whole = 0;
     xa_mark_t tag;
 
     pagevec_init(&pvec);
     if (wbc->range_cyclic) {
         index = mapping->writeback_index; /* prev offset */
         end = -1;
     } else {
         index = wbc->range_start >> PAGE_SHIFT;
         end = wbc->range_end >> PAGE_SHIFT;
         if (wbc->range_start == 0 && wbc->range_end == LLONG_MAX)
             range_whole = 1;
     }
     if (wbc->sync_mode == WB_SYNC_ALL || wbc->tagged_writepages) {
         tag_pages_for_writeback(mapping, index, end);
         tag = PAGECACHE_TAG_TOWRITE;
     } else {
         tag = PAGECACHE_TAG_DIRTY;
     }
     done_index = index;
     while (!done && (index <= end)) {
         int i;
 
         nr_pages = pagevec_lookup_range_tag(&pvec, mapping, &index, end,
                 tag);
         if (nr_pages == 0)
             break;
 
         for (i = 0; i < nr_pages; i++) {
             struct page *page = pvec.pages[i];
 
             done_index = page->index;
 
             lock_page(page);
 
             /*
              * Page truncated or invalidated. We can freely skip it
              * then, even for data integrity operations: the page
              * has disappeared concurrently, so there could be no
              * real expectation of this data integrity operation
              * even if there is now a new, dirty page at the same
              * pagecache address.
              */
             if (unlikely(page->mapping != mapping)) {
 continue_unlock:
                 unlock_page(page);
                 continue;
             }
 
             if (!PageDirty(page)) {
                 /* someone wrote it for us */
                 goto continue_unlock;
             }
 
             if (PageWriteback(page)) {
                 if (wbc->sync_mode != WB_SYNC_NONE)
                     wait_on_page_writeback(page);
                 else
                     goto continue_unlock;
             }
 
             BUG_ON(PageWriteback(page));
             if (!clear_page_dirty_for_io(page))
                 goto continue_unlock;
 
             trace_wbc_writepage(wbc, inode_to_bdi(mapping->host));
             error = (*writepage)(page, wbc, data);
             if (unlikely(error)) {
                 /*
                  * Handle errors according to the type of
                  * writeback. There's no need to continue for
                  * background writeback. Just push done_index
                  * past this page so media errors won't choke
                  * writeout for the entire file. For integrity
                  * writeback, we must process the entire dirty
                  * set regardless of errors because the fs may
                  * still have state to clear for each page. In
                  * that case we continue processing and return
                  * the first error.
                  */
                 if (error == AOP_WRITEPAGE_ACTIVATE) {
                     unlock_page(page);
                     error = 0;
                 } else if (wbc->sync_mode != WB_SYNC_ALL) {
                     ret = error;
                     done_index = page->index + 1;
                     done = 1;
                     break;
                 }
                 if (!ret)
                     ret = error;
             }
 
             /*
              * We stop writing back only if we are not doing
              * integrity sync. In case of integrity sync we have to
              * keep going until we have written all the pages
              * we tagged for writeback prior to entering this loop.
              */
             if (--wbc->nr_to_write <= 0 &&
                 wbc->sync_mode == WB_SYNC_NONE) {
                 done = 1;
                 break;
             }
         }
         pagevec_release(&pvec);
         cond_resched();
     }
 
     /*
      * If we hit the last page and there is more work to be done: wrap
      * back the index back to the start of the file for the next
      * time we are called.
      */
     if (wbc->range_cyclic && !done)
         done_index = 0;
     if (wbc->range_cyclic || (range_whole && wbc->nr_to_write > 0))
         mapping->writeback_index = done_index;
 
     return ret;
 }
 EXPORT_SYMBOL(write_cache_pages);
 
 /*
  * Function used by generic_writepages to call the real writepage
  * function and set the mapping flags on error
  */
 static int __writepage(struct page *page, struct writeback_control *wbc,
                void *data)
 {
     struct address_space *mapping = data;
     int ret = mapping->a_ops->writepage(page, wbc);
     mapping_set_error(mapping, ret);
     return ret;
 }
 
 /**
  * generic_writepages - walk the list of dirty pages of the given address space and writepage() all of them.
  * @mapping: address space structure to write
  * @wbc: subtract the number of written pages from *@wbc->nr_to_write
  *
  * This is a library function, which implements the writepages()
  * address_space_operation.
  *
  * Return: %0 on success, negative error code otherwise
  */
 int generic_writepages(struct address_space *mapping,
                struct writeback_control *wbc)
 {
     struct blk_plug plug;
     int ret;
 
     /* deal with chardevs and other special file */
     if (!mapping->a_ops->writepage)
         return 0;
 
     blk_start_plug(&plug);
     ret = write_cache_pages(mapping, wbc, __writepage, mapping);
     blk_finish_plug(&plug);
     return ret;
 }
 
 EXPORT_SYMBOL(generic_writepages);
 
 int do_writepages(struct address_space *mapping, struct writeback_control *wbc)
 {
     int ret;
     struct bdi_writeback *wb;
 
     if (wbc->nr_to_write <= 0)
         return 0;
     wb = inode_to_wb_wbc(mapping->host, wbc);
     wb_bandwidth_estimate_start(wb);
     while (1) {
         if (mapping->a_ops->writepages)
             ret = mapping->a_ops->writepages(mapping, wbc);
         else
             ret = generic_writepages(mapping, wbc);
         if ((ret != -ENOMEM) || (wbc->sync_mode != WB_SYNC_ALL))
             break;
 
         /*
          * Lacking an allocation context or the locality or writeback
          * state of any of the inode's pages, throttle based on
          * writeback activity on the local node. It's as good a
          * guess as any.
          */
         reclaim_throttle(NODE_DATA(numa_node_id()),
             VMSCAN_THROTTLE_WRITEBACK);
     }
     /*
      * Usually few pages are written by now from those we've just submitted
      * but if there's constant writeback being submitted, this makes sure
      * writeback bandwidth is updated once in a while.
      */
     if (time_is_before_jiffies(READ_ONCE(wb->bw_time_stamp) +
                    BANDWIDTH_INTERVAL))
         wb_update_bandwidth(wb);
     return ret;
 }
 
 /**
  * folio_write_one - write out a single folio and wait on I/O.
  * @folio: The folio to write.
  *
  * The folio must be locked by the caller and will be unlocked upon return.
  *
  * Note that the mapping's AS_EIO/AS_ENOSPC flags will be cleared when this
  * function returns.
  *
  * Return: %0 on success, negative error code otherwise
  */
 int folio_write_one(struct folio *folio)
 {
     struct address_space *mapping = folio->mapping;
     int ret = 0;
     struct writeback_control wbc = {
         .sync_mode = WB_SYNC_ALL,
         .nr_to_write = folio_nr_pages(folio),
     };
 
     BUG_ON(!folio_test_locked(folio));
 
     folio_wait_writeback(folio);
 
     if (folio_clear_dirty_for_io(folio)) {
         folio_get(folio);
         ret = mapping->a_ops->writepage(&folio->page, &wbc);
         if (ret == 0)
             folio_wait_writeback(folio);
         folio_put(folio);
     } else {
         folio_unlock(folio);
     }
 
     if (!ret)
         ret = filemap_check_errors(mapping);
     return ret;
 }
 EXPORT_SYMBOL(folio_write_one);
 
 /*
  * For address_spaces which do not use buffers nor write back.
  */
 bool noop_dirty_folio(struct address_space *mapping, struct folio *folio)
 {
     if (!folio_test_dirty(folio))
         return !folio_test_set_dirty(folio);
     return false;
 }
 EXPORT_SYMBOL(noop_dirty_folio);
 
 /*
  * Helper function for set_page_dirty family.
  *
  * Caller must hold lock_page_memcg().
  *
  * NOTE: This relies on being atomic wrt interrupts.
  */
 static void folio_account_dirtied(struct folio *folio,
         struct address_space *mapping)
 {
     struct inode *inode = mapping->host;
 
     trace_writeback_dirty_folio(folio, mapping);
 
     if (mapping_can_writeback(mapping)) {
         struct bdi_writeback *wb;
         long nr = folio_nr_pages(folio);
 
         inode_attach_wb(inode, &folio->page);
         wb = inode_to_wb(inode);
 
         __lruvec_stat_mod_folio(folio, NR_FILE_DIRTY, nr);
         __zone_stat_mod_folio(folio, NR_ZONE_WRITE_PENDING, nr);
         __node_stat_mod_folio(folio, NR_DIRTIED, nr);
         wb_stat_mod(wb, WB_RECLAIMABLE, nr);
         wb_stat_mod(wb, WB_DIRTIED, nr);
         task_io_account_write(nr * PAGE_SIZE);
         current->nr_dirtied += nr;
         __this_cpu_add(bdp_ratelimits, nr);
 
         mem_cgroup_track_foreign_dirty(folio, wb);
     }
 }
 
 /*
  * Helper function for deaccounting dirty page without writeback.
  *
  * Caller must hold lock_page_memcg().
  */
 void folio_account_cleaned(struct folio *folio, struct bdi_writeback *wb)
 {
     long nr = folio_nr_pages(folio);
 
     lruvec_stat_mod_folio(folio, NR_FILE_DIRTY, -nr);
     zone_stat_mod_folio(folio, NR_ZONE_WRITE_PENDING, -nr);
     wb_stat_mod(wb, WB_RECLAIMABLE, -nr);
     task_io_account_cancelled_write(nr * PAGE_SIZE);
 }
 
 /*
  * Mark the folio dirty, and set it dirty in the page cache, and mark
  * the inode dirty.
  *
  * If warn is true, then emit a warning if the folio is not uptodate and has
  * not been truncated.
  *
  * The caller must hold lock_page_memcg().  Most callers have the folio
  * locked.  A few have the folio blocked from truncation through other
  * means (eg zap_page_range() has it mapped and is holding the page table
  * lock).  This can also be called from mark_buffer_dirty(), which I
  * cannot prove is always protected against truncate.
  */
 void __folio_mark_dirty(struct folio *folio, struct address_space *mapping,
                  int warn)
 {
     unsigned long flags;
 
     xa_lock_irqsave(&mapping->i_pages, flags);
     if (folio->mapping) {   /* Race with truncate? */
         WARN_ON_ONCE(warn && !folio_test_uptodate(folio));
         folio_account_dirtied(folio, mapping);
         __xa_set_mark(&mapping->i_pages, folio_index(folio),
                 PAGECACHE_TAG_DIRTY);
     }
     xa_unlock_irqrestore(&mapping->i_pages, flags);
 }
 
 /**
  * filemap_dirty_folio - Mark a folio dirty for filesystems which do not use buffer_heads.
  * @mapping: Address space this folio belongs to.
  * @folio: Folio to be marked as dirty.
  *
  * Filesystems which do not use buffer heads should call this function
  * from their set_page_dirty address space operation.  It ignores the
  * contents of folio_get_private(), so if the filesystem marks individual
  * blocks as dirty, the filesystem should handle that itself.
  *
  * This is also sometimes used by filesystems which use buffer_heads when
  * a single buffer is being dirtied: we want to set the folio dirty in
  * that case, but not all the buffers.  This is a "bottom-up" dirtying,
  * whereas block_dirty_folio() is a "top-down" dirtying.
  *
  * The caller must ensure this doesn't race with truncation.  Most will
  * simply hold the folio lock, but e.g. zap_pte_range() calls with the
  * folio mapped and the pte lock held, which also locks out truncation.
  */
 bool filemap_dirty_folio(struct address_space *mapping, struct folio *folio)
 {
     folio_memcg_lock(folio);
     if (folio_test_set_dirty(folio)) {
         folio_memcg_unlock(folio);
         return false;
     }
 
     __folio_mark_dirty(folio, mapping, !folio_test_private(folio));
     folio_memcg_unlock(folio);
 
     if (mapping->host) {
         /* !PageAnon && !swapper_space */
         __mark_inode_dirty(mapping->host, I_DIRTY_PAGES);
     }
     return true;
 }
 EXPORT_SYMBOL(filemap_dirty_folio);
 
 /**
  * folio_account_redirty - Manually account for redirtying a page.
  * @folio: The folio which is being redirtied.
  *
  * Most filesystems should call folio_redirty_for_writepage() instead
  * of this fuction.  If your filesystem is doing writeback outside the
  * context of a writeback_control(), it can call this when redirtying
  * a folio, to de-account the dirty counters (NR_DIRTIED, WB_DIRTIED,
  * tsk->nr_dirtied), so that they match the written counters (NR_WRITTEN,
  * WB_WRITTEN) in long term. The mismatches will lead to systematic errors
  * in balanced_dirty_ratelimit and the dirty pages position control.
  */
 void folio_account_redirty(struct folio *folio)
 {
     struct address_space *mapping = folio->mapping;
 
     if (mapping && mapping_can_writeback(mapping)) {
         struct inode *inode = mapping->host;
         struct bdi_writeback *wb;
         struct wb_lock_cookie cookie = {};
         long nr = folio_nr_pages(folio);
 
         wb = unlocked_inode_to_wb_begin(inode, &cookie);
         current->nr_dirtied -= nr;
         node_stat_mod_folio(folio, NR_DIRTIED, -nr);
         wb_stat_mod(wb, WB_DIRTIED, -nr);
         unlocked_inode_to_wb_end(inode, &cookie);
     }
 }
 EXPORT_SYMBOL(folio_account_redirty);
 
 /**
  * folio_redirty_for_writepage - Decline to write a dirty folio.
  * @wbc: The writeback control.
  * @folio: The folio.
  *
  * When a writepage implementation decides that it doesn't want to write
  * @folio for some reason, it should call this function, unlock @folio and
  * return 0.
  *
  * Return: True if we redirtied the folio.  False if someone else dirtied
  * it first.
  */
 bool folio_redirty_for_writepage(struct writeback_control *wbc,
         struct folio *folio)
 {
     bool ret;
     long nr = folio_nr_pages(folio);
 
     wbc->pages_skipped += nr;
     ret = filemap_dirty_folio(folio->mapping, folio);
     folio_account_redirty(folio);
 
     return ret;
 }
 EXPORT_SYMBOL(folio_redirty_for_writepage);
 
 /**
  * folio_mark_dirty - Mark a folio as being modified.
  * @folio: The folio.
  *
  * The folio may not be truncated while this function is running.
  * Holding the folio lock is sufficient to prevent truncation, but some
  * callers cannot acquire a sleeping lock.  These callers instead hold
  * the page table lock for a page table which contains at least one page
  * in this folio.  Truncation will block on the page table lock as it
  * unmaps pages before removing the folio from its mapping.
  *
  * Return: True if the folio was newly dirtied, false if it was already dirty.
  */
 bool folio_mark_dirty(struct folio *folio)
 {
     struct address_space *mapping = folio_mapping(folio);
 
     if (likely(mapping)) {
         /*
          * readahead/lru_deactivate_page could remain
          * PG_readahead/PG_reclaim due to race with folio_end_writeback
          * About readahead, if the folio is written, the flags would be
          * reset. So no problem.
          * About lru_deactivate_page, if the folio is redirtied,
          * the flag will be reset. So no problem. but if the
          * folio is used by readahead it will confuse readahead
          * and make it restart the size rampup process. But it's
          * a trivial problem.
          */
         if (folio_test_reclaim(folio))
             folio_clear_reclaim(folio);
         return mapping->a_ops->dirty_folio(mapping, folio);
     }
 
     return noop_dirty_folio(mapping, folio);
 }
 EXPORT_SYMBOL(folio_mark_dirty);
 
 /*
  * set_page_dirty() is racy if the caller has no reference against
  * page->mapping->host, and if the page is unlocked.  This is because another
  * CPU could truncate the page off the mapping and then free the mapping.
  *
  * Usually, the page _is_ locked, or the caller is a user-space process which
  * holds a reference on the inode by having an open file.
  *
  * In other cases, the page should be locked before running set_page_dirty().
  */
 int set_page_dirty_lock(struct page *page)
 {
     int ret;
 
     lock_page(page);
     ret = set_page_dirty(page);
     unlock_page(page);
     return ret;
 }
 EXPORT_SYMBOL(set_page_dirty_lock);
 
 /*
  * This cancels just the dirty bit on the kernel page itself, it does NOT
  * actually remove dirty bits on any mmap's that may be around. It also
  * leaves the page tagged dirty, so any sync activity will still find it on
  * the dirty lists, and in particular, clear_page_dirty_for_io() will still
  * look at the dirty bits in the VM.
  *
  * Doing this should *normally* only ever be done when a page is truncated,
  * and is not actually mapped anywhere at all. However, fs/buffer.c does
  * this when it notices that somebody has cleaned out all the buffers on a
  * page without actually doing it through the VM. Can you say "ext3 is
  * horribly ugly"? Thought you could.
  */
 void __folio_cancel_dirty(struct folio *folio)
 {
     struct address_space *mapping = folio_mapping(folio);
 
     if (mapping_can_writeback(mapping)) {
         struct inode *inode = mapping->host;
         struct bdi_writeback *wb;
         struct wb_lock_cookie cookie = {};
 
         folio_memcg_lock(folio);
         wb = unlocked_inode_to_wb_begin(inode, &cookie);
 
         if (folio_test_clear_dirty(folio))
             folio_account_cleaned(folio, wb);
 
         unlocked_inode_to_wb_end(inode, &cookie);
         folio_memcg_unlock(folio);
     } else {
         folio_clear_dirty(folio);
     }
 }
 EXPORT_SYMBOL(__folio_cancel_dirty);
 
 /*
  * Clear a folio's dirty flag, while caring for dirty memory accounting.
  * Returns true if the folio was previously dirty.
  *
  * This is for preparing to put the folio under writeout.  We leave
  * the folio tagged as dirty in the xarray so that a concurrent
  * write-for-sync can discover it via a PAGECACHE_TAG_DIRTY walk.
  * The ->writepage implementation will run either folio_start_writeback()
  * or folio_mark_dirty(), at which stage we bring the folio's dirty flag
  * and xarray dirty tag back into sync.
  *
  * This incoherency between the folio's dirty flag and xarray tag is
  * unfortunate, but it only exists while the folio is locked.
  */
 bool folio_clear_dirty_for_io(struct folio *folio)
 {
     struct address_space *mapping = folio_mapping(folio);
     bool ret = false;
 
     VM_BUG_ON_FOLIO(!folio_test_locked(folio), folio);
 
     if (mapping && mapping_can_writeback(mapping)) {
         struct inode *inode = mapping->host;
         struct bdi_writeback *wb;
         struct wb_lock_cookie cookie = {};
 
         /*
          * Yes, Virginia, this is indeed insane.
          *
          * We use this sequence to make sure that
          *  (a) we account for dirty stats properly
          *  (b) we tell the low-level filesystem to
          *      mark the whole folio dirty if it was
          *      dirty in a pagetable. Only to then
          *  (c) clean the folio again and return 1 to
          *      cause the writeback.
          *
          * This way we avoid all nasty races with the
          * dirty bit in multiple places and clearing
          * them concurrently from different threads.
          *
          * Note! Normally the "folio_mark_dirty(folio)"
          * has no effect on the actual dirty bit - since
          * that will already usually be set. But we
          * need the side effects, and it can help us
          * avoid races.
          *
          * We basically use the folio "master dirty bit"
          * as a serialization point for all the different
          * threads doing their things.
          */
         if (folio_mkclean(folio))
             folio_mark_dirty(folio);
         /*
          * We carefully synchronise fault handlers against
          * installing a dirty pte and marking the folio dirty
          * at this point.  We do this by having them hold the
          * page lock while dirtying the folio, and folios are
          * always locked coming in here, so we get the desired
          * exclusion.
          */
         wb = unlocked_inode_to_wb_begin(inode, &cookie);
         if (folio_test_clear_dirty(folio)) {
             long nr = folio_nr_pages(folio);
             lruvec_stat_mod_folio(folio, NR_FILE_DIRTY, -nr);
             zone_stat_mod_folio(folio, NR_ZONE_WRITE_PENDING, -nr);
             wb_stat_mod(wb, WB_RECLAIMABLE, -nr);
             ret = true;
         }
         unlocked_inode_to_wb_end(inode, &cookie);
         return ret;
     }
     return folio_test_clear_dirty(folio);
 }
 EXPORT_SYMBOL(folio_clear_dirty_for_io);
 
 static void wb_inode_writeback_start(struct bdi_writeback *wb)
 {
     atomic_inc(&wb->writeback_inodes);
 }
 
 static void wb_inode_writeback_end(struct bdi_writeback *wb)
 {
     unsigned long flags;
     atomic_dec(&wb->writeback_inodes);
     /*
      * Make sure estimate of writeback throughput gets updated after
      * writeback completed. We delay the update by BANDWIDTH_INTERVAL
      * (which is the interval other bandwidth updates use for batching) so
      * that if multiple inodes end writeback at a similar time, they get
      * batched into one bandwidth update.
      */
     spin_lock_irqsave(&wb->work_lock, flags);
     if (test_bit(WB_registered, &wb->state))
         queue_delayed_work(bdi_wq, &wb->bw_dwork, BANDWIDTH_INTERVAL);
     spin_unlock_irqrestore(&wb->work_lock, flags);
 }
 
 bool __folio_end_writeback(struct folio *folio)
 {
     long nr = folio_nr_pages(folio);
     struct address_space *mapping = folio_mapping(folio);
     bool ret;
 
     folio_memcg_lock(folio);
     if (mapping && mapping_use_writeback_tags(mapping)) {
         struct inode *inode = mapping->host;
         struct backing_dev_info *bdi = inode_to_bdi(inode);
         unsigned long flags;
 
         xa_lock_irqsave(&mapping->i_pages, flags);
         ret = folio_test_clear_writeback(folio);
         if (ret) {
             __xa_clear_mark(&mapping->i_pages, folio_index(folio),
                         PAGECACHE_TAG_WRITEBACK);
             if (bdi->capabilities & BDI_CAP_WRITEBACK_ACCT) {
                 struct bdi_writeback *wb = inode_to_wb(inode);
 
                 wb_stat_mod(wb, WB_WRITEBACK, -nr);
                 __wb_writeout_add(wb, nr);
                 if (!mapping_tagged(mapping,
                             PAGECACHE_TAG_WRITEBACK))
                     wb_inode_writeback_end(wb);
             }
         }
 
         if (mapping->host && !mapping_tagged(mapping,
                              PAGECACHE_TAG_WRITEBACK))
             sb_clear_inode_writeback(mapping->host);
 
         xa_unlock_irqrestore(&mapping->i_pages, flags);
     } else {
         ret = folio_test_clear_writeback(folio);
     }
     if (ret) {
         lruvec_stat_mod_folio(folio, NR_WRITEBACK, -nr);
         zone_stat_mod_folio(folio, NR_ZONE_WRITE_PENDING, -nr);
         node_stat_mod_folio(folio, NR_WRITTEN, nr);
     }
     folio_memcg_unlock(folio);
     return ret;
 }
 
 bool __folio_start_writeback(struct folio *folio, bool keep_write)
 {
     long nr = folio_nr_pages(folio);
     struct address_space *mapping = folio_mapping(folio);
     bool ret;
     int access_ret;
 
     folio_memcg_lock(folio);
     if (mapping && mapping_use_writeback_tags(mapping)) {
         XA_STATE(xas, &mapping->i_pages, folio_index(folio));
         struct inode *inode = mapping->host;
         struct backing_dev_info *bdi = inode_to_bdi(inode);
         unsigned long flags;
 
         xas_lock_irqsave(&xas, flags);
         xas_load(&xas);
         ret = folio_test_set_writeback(folio);
         if (!ret) {
             bool on_wblist;
 
             on_wblist = mapping_tagged(mapping,
                            PAGECACHE_TAG_WRITEBACK);
 
             xas_set_mark(&xas, PAGECACHE_TAG_WRITEBACK);
             if (bdi->capabilities & BDI_CAP_WRITEBACK_ACCT) {
                 struct bdi_writeback *wb = inode_to_wb(inode);
 
                 wb_stat_mod(wb, WB_WRITEBACK, nr);
                 if (!on_wblist)
                     wb_inode_writeback_start(wb);
             }
 
             /*
              * We can come through here when swapping
              * anonymous folios, so we don't necessarily
              * have an inode to track for sync.
              */
             if (mapping->host && !on_wblist)
                 sb_mark_inode_writeback(mapping->host);
         }
         if (!folio_test_dirty(folio))
             xas_clear_mark(&xas, PAGECACHE_TAG_DIRTY);
         if (!keep_write)
             xas_clear_mark(&xas, PAGECACHE_TAG_TOWRITE);
         xas_unlock_irqrestore(&xas, flags);
     } else {
         ret = folio_test_set_writeback(folio);
     }
     if (!ret) {
         lruvec_stat_mod_folio(folio, NR_WRITEBACK, nr);
         zone_stat_mod_folio(folio, NR_ZONE_WRITE_PENDING, nr);
     }
     folio_memcg_unlock(folio);
     access_ret = arch_make_folio_accessible(folio);
     /*
      * If writeback has been triggered on a page that cannot be made
      * accessible, it is too late to recover here.
      */
     VM_BUG_ON_FOLIO(access_ret != 0, folio);
 
     return ret;
 }
 EXPORT_SYMBOL(__folio_start_writeback);
 
 /**
  * folio_wait_writeback - Wait for a folio to finish writeback.
  * @folio: The folio to wait for.
  *
  * If the folio is currently being written back to storage, wait for the
  * I/O to complete.
  *
  * Context: Sleeps.  Must be called in process context and with
  * no spinlocks held.  Caller should hold a reference on the folio.
  * If the folio is not locked, writeback may start again after writeback
  * has finished.
  */
 void folio_wait_writeback(struct folio *folio)
 {
     while (folio_test_writeback(folio)) {
         trace_folio_wait_writeback(folio, folio_mapping(folio));
         folio_wait_bit(folio, PG_writeback);
     }
 }
 EXPORT_SYMBOL_GPL(folio_wait_writeback);
 
 /**
  * folio_wait_writeback_killable - Wait for a folio to finish writeback.
  * @folio: The folio to wait for.
  *
  * If the folio is currently being written back to storage, wait for the
  * I/O to complete or a fatal signal to arrive.
  *
  * Context: Sleeps.  Must be called in process context and with
  * no spinlocks held.  Caller should hold a reference on the folio.
  * If the folio is not locked, writeback may start again after writeback
  * has finished.
  * Return: 0 on success, -EINTR if we get a fatal signal while waiting.
  */
 int folio_wait_writeback_killable(struct folio *folio)
 {
     while (folio_test_writeback(folio)) {
         trace_folio_wait_writeback(folio, folio_mapping(folio));
         if (folio_wait_bit_killable(folio, PG_writeback))
             return -EINTR;
     }
 
     return 0;
 }
 EXPORT_SYMBOL_GPL(folio_wait_writeback_killable);
 
 /**
  * folio_wait_stable() - wait for writeback to finish, if necessary.
  * @folio: The folio to wait on.
  *
  * This function determines if the given folio is related to a backing
  * device that requires folio contents to be held stable during writeback.
  * If so, then it will wait for any pending writeback to complete.
  *
  * Context: Sleeps.  Must be called in process context and with
  * no spinlocks held.  Caller should hold a reference on the folio.
  * If the folio is not locked, writeback may start again after writeback
  * has finished.
  */
 void folio_wait_stable(struct folio *folio)
 {
     if (folio_inode(folio)->i_sb->s_iflags & SB_I_STABLE_WRITES)
         folio_wait_writeback(folio);
 }
 EXPORT_SYMBOL_GPL(folio_wait_stable);