FreeBSD/Linux Kernel Cross Reference
sys/mm/page-writeback.c

     /*
      * mm/page-writeback.c
      *
      * Copyright (C) 2002, Linus Torvalds.
      * Copyright (C) 2007 Red Hat, Inc., Peter Zijlstra <pzijlstr@redhat.com>
      *
      * 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/notifier.h>
    #include <linux/smp.h>
    #include <linux/sysctl.h>
    #include <linux/cpu.h>
    #include <linux/syscalls.h>
    #include <linux/buffer_head.h> /* __set_page_dirty_buffers */
    #include <linux/pagevec.h>
    #include <linux/timer.h>
    #include <trace/events/writeback.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
     */
    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
     */
    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
     */
    int vm_highmem_is_dirtyable;
    
    /*
     * The generator of dirty data starts writeback at this percentage
     */
    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
     */
    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 makes the machine dump writes/reads and block dirtyings.
    */
   int block_dump;
   
   /*
    * 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 */
   
   unsigned long global_dirty_limit;
   
   /*
    * Scale the writeback cache size proportional to the relative writeout speeds.
    *
    * We do this by keeping a floating proportion between BDIs, based on page
    * writeback completions [end_page_writeback()]. Those devices that write out
    * pages fastest will get the larger share, while the slower will get a smaller
    * share.
    *
    * We use page writeout completions because we are interested in getting rid of
    * dirty pages. Having them written out is the primary goal.
    *
    * We introduce a concept of time, a period over which we measure these events,
    * because demand can/will vary over time. The length of this period itself is
    * measured in page writeback completions.
    *
    */
   static struct fprop_global writeout_completions;
   
   static void writeout_period(unsigned long t);
   /* Timer for aging of writeout_completions */
   static struct timer_list writeout_period_timer =
                   TIMER_DEFERRED_INITIALIZER(writeout_period, 0, 0);
   static unsigned long writeout_period_time = 0;
   
   /*
    * 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)
   
   /*
    * Work out the current dirty-memory clamping and background writeout
    * thresholds.
    *
    * The main aim here is to lower them aggressively if there is a lot of mapped
    * memory around.  To avoid stressing page reclaim with lots of unreclaimable
    * pages.  It is better to clamp down on writers than to start swapping, and
    * performing lots of scanning.
    *
    * We only allow 1/2 of the currently-unmapped memory to be dirtied.
    *
    * We don't permit the clamping level to fall below 5% - that is getting rather
    * excessive.
    *
    * We make sure that the background writeout level is below the adjusted
    * clamping level.
    */
   
   /*
    * 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 effictive 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.
    */
   
   static unsigned long highmem_dirtyable_memory(unsigned long total)
   {
   #ifdef CONFIG_HIGHMEM
           int node;
           unsigned long x = 0;
   
           for_each_node_state(node, N_HIGH_MEMORY) {
                   struct zone *z =
                           &NODE_DATA(node)->node_zones[ZONE_HIGHMEM];
   
                   x += zone_page_state(z, NR_FREE_PAGES) +
                        zone_reclaimable_pages(z) - z->dirty_balance_reserve;
           }
           /*
            * Unreclaimable memory (kernel memory or anonymous memory
            * without swap) can bring down the dirtyable pages below
            * the zone's dirty balance reserve and the above calculation
            * will underflow.  However we still want to add in nodes
            * which are below threshold (negative values) to get a more
            * accurate calculation but make sure that the total never
            * underflows.
            */
           if ((long)x < 0)
                   x = 0;
   
           /*
            * 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
    *
    * Returns 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_page_state(NR_FREE_PAGES) + global_reclaimable_pages();
           x -= min(x, dirty_balance_reserve);
   
           if (!vm_highmem_is_dirtyable)
                   x -= highmem_dirtyable_memory(x);
   
           return x + 1;   /* Ensure that we never return 0 */
   }
   
   /*
    * global_dirty_limits - background-writeback and dirty-throttling thresholds
    *
    * Calculate the dirty thresholds based on sysctl parameters
    * - vm.dirty_background_ratio  or  vm.dirty_background_bytes
    * - vm.dirty_ratio             or  vm.dirty_bytes
    * The dirty limits will be lifted by 1/4 for PF_LESS_THROTTLE (ie. nfsd) and
    * real-time tasks.
    */
   void global_dirty_limits(unsigned long *pbackground, unsigned long *pdirty)
   {
           unsigned long background;
           unsigned long dirty;
           unsigned long uninitialized_var(available_memory);
           struct task_struct *tsk;
   
           if (!vm_dirty_bytes || !dirty_background_bytes)
                   available_memory = global_dirtyable_memory();
   
           if (vm_dirty_bytes)
                   dirty = DIV_ROUND_UP(vm_dirty_bytes, PAGE_SIZE);
           else
                   dirty = (vm_dirty_ratio * available_memory) / 100;
   
           if (dirty_background_bytes)
                   background = DIV_ROUND_UP(dirty_background_bytes, PAGE_SIZE);
           else
                   background = (dirty_background_ratio * available_memory) / 100;
   
           if (background >= dirty)
                   background = dirty / 2;
           tsk = current;
           if (tsk->flags & PF_LESS_THROTTLE || rt_task(tsk)) {
                   background += background / 4;
                   dirty += dirty / 4;
           }
           *pbackground = background;
           *pdirty = dirty;
           trace_global_dirty_state(background, dirty);
   }
   
   /**
    * zone_dirtyable_memory - number of dirtyable pages in a zone
    * @zone: the zone
    *
    * Returns the zone's number of pages potentially available for dirty
    * page cache.  This is the base value for the per-zone dirty limits.
    */
   static unsigned long zone_dirtyable_memory(struct zone *zone)
   {
           /*
            * The effective global number of dirtyable pages may exclude
            * highmem as a big-picture measure to keep the ratio between
            * dirty memory and lowmem reasonable.
            *
            * But this function is purely about the individual zone and a
            * highmem zone can hold its share of dirty pages, so we don't
            * care about vm_highmem_is_dirtyable here.
            */
           unsigned long nr_pages = zone_page_state(zone, NR_FREE_PAGES) +
                   zone_reclaimable_pages(zone);
   
           /* don't allow this to underflow */
           nr_pages -= min(nr_pages, zone->dirty_balance_reserve);
           return nr_pages;
   }
   
   /**
    * zone_dirty_limit - maximum number of dirty pages allowed in a zone
    * @zone: the zone
    *
    * Returns the maximum number of dirty pages allowed in a zone, based
    * on the zone's dirtyable memory.
    */
   static unsigned long zone_dirty_limit(struct zone *zone)
   {
           unsigned long zone_memory = zone_dirtyable_memory(zone);
           struct task_struct *tsk = current;
           unsigned long dirty;
   
           if (vm_dirty_bytes)
                   dirty = DIV_ROUND_UP(vm_dirty_bytes, PAGE_SIZE) *
                           zone_memory / global_dirtyable_memory();
           else
                   dirty = vm_dirty_ratio * zone_memory / 100;
   
           if (tsk->flags & PF_LESS_THROTTLE || rt_task(tsk))
                   dirty += dirty / 4;
   
           return dirty;
   }
   
   /**
    * zone_dirty_ok - tells whether a zone is within its dirty limits
    * @zone: the zone to check
    *
    * Returns %true when the dirty pages in @zone are within the zone's
    * dirty limit, %false if the limit is exceeded.
    */
   bool zone_dirty_ok(struct zone *zone)
   {
           unsigned long limit = zone_dirty_limit(zone);
   
           return zone_page_state(zone, NR_FILE_DIRTY) +
                  zone_page_state(zone, NR_UNSTABLE_NFS) +
                  zone_page_state(zone, NR_WRITEBACK) <= limit;
   }
   
   int dirty_background_ratio_handler(struct ctl_table *table, int write,
                   void __user *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;
   }
   
   int dirty_background_bytes_handler(struct ctl_table *table, int write,
                   void __user *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;
   }
   
   int dirty_ratio_handler(struct ctl_table *table, int write,
                   void __user *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;
   }
   
   int dirty_bytes_handler(struct ctl_table *table, int write,
                   void __user *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;
   }
   
   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;
   }
   
   /*
    * Increment the BDI's writeout completion count and the global writeout
    * completion count. Called from test_clear_page_writeback().
    */
   static inline void __bdi_writeout_inc(struct backing_dev_info *bdi)
   {
           __inc_bdi_stat(bdi, BDI_WRITTEN);
           __fprop_inc_percpu_max(&writeout_completions, &bdi->completions,
                                  bdi->max_prop_frac);
           /* First event after period switching was turned off? */
           if (!unlikely(writeout_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.
                    */
                   writeout_period_time = wp_next_time(jiffies);
                   mod_timer(&writeout_period_timer, writeout_period_time);
           }
   }
   
   void bdi_writeout_inc(struct backing_dev_info *bdi)
   {
           unsigned long flags;
   
           local_irq_save(flags);
           __bdi_writeout_inc(bdi);
           local_irq_restore(flags);
   }
   EXPORT_SYMBOL_GPL(bdi_writeout_inc);
   
   /*
    * Obtain an accurate fraction of the BDI's portion.
    */
   static void bdi_writeout_fraction(struct backing_dev_info *bdi,
                   long *numerator, long *denominator)
   {
           fprop_fraction_percpu(&writeout_completions, &bdi->completions,
                                   numerator, denominator);
   }
   
   /*
    * 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(unsigned long t)
   {
           int miss_periods = (jiffies - writeout_period_time) /
                                                    VM_COMPLETIONS_PERIOD_LEN;
   
           if (fprop_new_period(&writeout_completions, miss_periods + 1)) {
                   writeout_period_time = wp_next_time(writeout_period_time +
                                   miss_periods * VM_COMPLETIONS_PERIOD_LEN);
                   mod_timer(&writeout_period_timer, writeout_period_time);
           } else {
                   /*
                    * Aging has zeroed all fractions. Stop wasting CPU on period
                    * updates.
                    */
                   writeout_period_time = 0;
           }
   }
   
   /*
    * 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)
   {
           int ret = 0;
   
           spin_lock_bh(&bdi_lock);
           if (min_ratio > bdi->max_ratio) {
                   ret = -EINVAL;
           } else {
                   min_ratio -= bdi->min_ratio;
                   if (bdi_min_ratio + min_ratio < 100) {
                           bdi_min_ratio += min_ratio;
                           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(unsigned long thresh)
   {
           return max(thresh, global_dirty_limit);
   }
   
   /**
    * bdi_dirty_limit - @bdi's share of dirty throttling threshold
    * @bdi: the backing_dev_info to query
    * @dirty: global dirty limit in pages
    *
    * Returns @bdi's dirty limit in pages. The term "dirty" in the context of
    * dirty balancing includes all PG_dirty, PG_writeback and NFS unstable pages.
    *
    * 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 bdi 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 bdi'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.
    */
   unsigned long bdi_dirty_limit(struct backing_dev_info *bdi, unsigned long dirty)
   {
           u64 bdi_dirty;
           long numerator, denominator;
   
           /*
            * Calculate this BDI's share of the dirty ratio.
            */
           bdi_writeout_fraction(bdi, &numerator, &denominator);
   
           bdi_dirty = (dirty * (100 - bdi_min_ratio)) / 100;
           bdi_dirty *= numerator;
           do_div(bdi_dirty, denominator);
   
           bdi_dirty += (dirty * bdi->min_ratio) / 100;
           if (bdi_dirty > (dirty * bdi->max_ratio) / 100)
                   bdi_dirty = dirty * bdi->max_ratio / 100;
   
           return bdi_dirty;
   }
   
   /*
    * Dirty position control.
    *
    * (o) global/bdi setpoints
    *
    * We want the dirty pages be balanced around the global/bdi 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 (bdi_dirty < bdi_setpoint) scale up   pos_ratio
    *     if (bdi_dirty > bdi_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) bdi control line
    *
    *     ^ pos_ratio
    *     |
    *     |            *
    *     |              *
    *     |                *
    *     |                  *
    *     |                    * |<=========== span ============>|
    * 1.0 .......................*
    *     |                      . *
    *     |                      .   *
    *     |                      .     *
    *     |                      .       *
    *     |                      .         *
    *     |                      .           *
    *     |                      .             *
    *     |                      .               *
    *     |                      .                 *
    *     |                      .                   *
    *     |                      .                     *
    * 1/4 ...............................................* * * * * * * * * * * *
    *     |                      .                         .
    *     |                      .                           .
    *     |                      .                             .
    *   0 +----------------------.-------------------------------.------------->
    *                bdi_setpoint^                    x_intercept^
    *
    * The bdi control line won't drop below pos_ratio=1/4, so that bdi_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 bdi_dirty may rush to many times higher than bdi_setpoint.
    * - the bdi dirty thresh drops quickly due to change of JBOD workload
    */
   static unsigned long bdi_position_ratio(struct backing_dev_info *bdi,
                                           unsigned long thresh,
                                           unsigned long bg_thresh,
                                           unsigned long dirty,
                                           unsigned long bdi_thresh,
                                           unsigned long bdi_dirty)
   {
           unsigned long write_bw = bdi->avg_write_bandwidth;
           unsigned long freerun = dirty_freerun_ceiling(thresh, bg_thresh);
           unsigned long limit = hard_dirty_limit(thresh);
           unsigned long x_intercept;
           unsigned long setpoint;         /* dirty pages' target balance point */
           unsigned long bdi_setpoint;
           unsigned long span;
           long long pos_ratio;            /* for scaling up/down the rate limit */
           long x;
   
           if (unlikely(dirty >= limit))
                   return 0;
   
           /*
            * global setpoint
            *
            *                           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
            */
           setpoint = (freerun + limit) / 2;
           x = div_s64((setpoint - 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;
   
           /*
            * We have computed basic pos_ratio above based on global situation. If
            * the bdi is over/under its share of dirty pages, we want to scale
            * pos_ratio further down/up. That is done by the following mechanism.
            */
   
           /*
            * bdi setpoint
            *
            *        f(bdi_dirty) := 1.0 + k * (bdi_dirty - bdi_setpoint)
            *
            *                        x_intercept - bdi_dirty
            *                     := --------------------------
            *                        x_intercept - bdi_setpoint
            *
            * The main bdi control line is a linear function that subjects to
            *
            * (1) f(bdi_setpoint) = 1.0
            * (2) k = - 1 / (8 * write_bw)  (in single bdi case)
            *     or equally: x_intercept = bdi_setpoint + 8 * write_bw
            *
            * For single bdi case, the dirty pages are observed to fluctuate
            * regularly within range
            *        [bdi_setpoint - write_bw/2, bdi_setpoint + write_bw/2]
            * for various filesystems, where (2) can yield in a reasonable 12.5%
            * fluctuation range for pos_ratio.
            *
            * For JBOD case, bdi_thresh (not bdi_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 bdi_thresh.
            */
           if (unlikely(bdi_thresh > thresh))
                   bdi_thresh = thresh;
           /*
            * It's very possible that bdi_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.
            */
           bdi_thresh = max(bdi_thresh, (limit - dirty) / 8);
           /*
            * scale global setpoint to bdi's:
            *      bdi_setpoint = setpoint * bdi_thresh / thresh
            */
           x = div_u64((u64)bdi_thresh << 16, thresh + 1);
           bdi_setpoint = setpoint * (u64)x >> 16;
           /*
            * Use span=(8*write_bw) in single bdi case as indicated by
            * (thresh - bdi_thresh ~= 0) and transit to bdi_thresh in JBOD case.
            *
            *        bdi_thresh                    thresh - bdi_thresh
            * span = ---------- * (8 * write_bw) + ------------------- * bdi_thresh
            *          thresh                            thresh
            */
           span = (thresh - bdi_thresh + 8 * write_bw) * (u64)x >> 16;
           x_intercept = bdi_setpoint + span;
   
           if (bdi_dirty < x_intercept - span / 4) {
                   pos_ratio = div_u64(pos_ratio * (x_intercept - bdi_dirty),
                                       x_intercept - bdi_setpoint + 1);
           } else
                   pos_ratio /= 4;
   
           /*
            * bdi 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 = bdi_thresh / 2;
           if (bdi_dirty < x_intercept) {
                   if (bdi_dirty > x_intercept / 8)
                           pos_ratio = div_u64(pos_ratio * x_intercept, bdi_dirty);
                   else
                           pos_ratio *= 8;
           }
   
           return pos_ratio;
   }
   
   static void bdi_update_write_bandwidth(struct backing_dev_info *bdi,
                                          unsigned long elapsed,
                                          unsigned long written)
   {
           const unsigned long period = roundup_pow_of_two(3 * HZ);
           unsigned long avg = bdi->avg_write_bandwidth;
           unsigned long old = bdi->write_bandwidth;
           u64 bw;
   
           /*
            * bw = written * HZ / elapsed
            *
            *                   bw * elapsed + write_bandwidth * (period - elapsed)
            * write_bandwidth = ---------------------------------------------------
            *                                          period
            */
           bw = written - bdi->written_stamp;
           bw *= HZ;
           if (unlikely(elapsed > period)) {
                   do_div(bw, elapsed);
                   avg = bw;
                   goto out;
           }
           bw += (u64)bdi->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:
           bdi->write_bandwidth = bw;
           bdi->avg_write_bandwidth = avg;
   }
   
   /*
    * The global dirtyable memory and dirty threshold could be suddenly knocked
    * down by a large amount (eg. on the startup of KVM in a swapless system).
    * This may throw the system into deep dirty exceeded state and throttle
    * heavy/light dirtiers alike. To retain good responsiveness, maintain
    * global_dirty_limit for tracking slowly down to the knocked down dirty
    * threshold.
    */
   static void update_dirty_limit(unsigned long thresh, unsigned long dirty)
   {
           unsigned long limit = global_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
            * global_dirty_limit which is guaranteed to lie above the dirty pages.
            */
           thresh = max(thresh, dirty);
           if (limit > thresh) {
                   limit -= (limit - thresh) >> 5;
                   goto update;
           }
           return;
   update:
           global_dirty_limit = limit;
   }
   
   static void global_update_bandwidth(unsigned long thresh,
                                       unsigned long dirty,
                                       unsigned long now)
   {
           static DEFINE_SPINLOCK(dirty_lock);
           static unsigned long update_time;
   
           /*
            * check locklessly first to optimize away locking for the most time
            */
           if (time_before(now, update_time + BANDWIDTH_INTERVAL))
                   return;
   
           spin_lock(&dirty_lock);
           if (time_after_eq(now, update_time + BANDWIDTH_INTERVAL)) {
                   update_dirty_limit(thresh, dirty);
                   update_time = now;
           }
           spin_unlock(&dirty_lock);
   }
   
   /*
    * Maintain bdi->dirty_ratelimit, the base dirty throttle rate.
    *
    * Normal bdi 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 bdi_update_dirty_ratelimit(struct backing_dev_info *bdi,
                                          unsigned long thresh,
                                          unsigned long bg_thresh,
                                          unsigned long dirty,
                                          unsigned long bdi_thresh,
                                          unsigned long bdi_dirty,
                                          unsigned long dirtied,
                                          unsigned long elapsed)
   {
           unsigned long freerun = dirty_freerun_ceiling(thresh, bg_thresh);
           unsigned long limit = hard_dirty_limit(thresh);
           unsigned long setpoint = (freerun + limit) / 2;
           unsigned long write_bw = bdi->avg_write_bandwidth;
           unsigned long dirty_ratelimit = bdi->dirty_ratelimit;
           unsigned long dirty_rate;
           unsigned long task_ratelimit;
           unsigned long balanced_dirty_ratelimit;
           unsigned long pos_ratio;
           unsigned long step;
           unsigned long x;
   
           /*
            * 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 - bdi->dirtied_stamp) * HZ / elapsed;
   
           pos_ratio = bdi_position_ratio(bdi, thresh, bg_thresh, dirty,
                                          bdi_thresh, bdi_dirty);
           /*
            * task_ratelimit reflects each dd's dirty rate for the past 200ms.
            */
           task_ratelimit = (u64)dirty_ratelimit *
                                           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 bdi'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:
            *
            *      bdi->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;
           if (dirty < setpoint) {
                   x = min(bdi->balanced_dirty_ratelimit,
                            min(balanced_dirty_ratelimit, task_ratelimit));
                   if (dirty_ratelimit < x)
                           step = x - dirty_ratelimit;
           } else {
                   x = max(bdi->balanced_dirty_ratelimit,
                           max(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.
           */
          step >>= dirty_ratelimit / (2 * step + 1);
          /*
           * Limit the tracking speed to avoid overshooting.
           */
          step = (step + 7) / 8;
  
          if (dirty_ratelimit < balanced_dirty_ratelimit)
                  dirty_ratelimit += step;
          else
                  dirty_ratelimit -= step;
  
          bdi->dirty_ratelimit = max(dirty_ratelimit, 1UL);
          bdi->balanced_dirty_ratelimit = balanced_dirty_ratelimit;
  
          trace_bdi_dirty_ratelimit(bdi, dirty_rate, task_ratelimit);
  }
  
  void __bdi_update_bandwidth(struct backing_dev_info *bdi,
                              unsigned long thresh,
                              unsigned long bg_thresh,
                              unsigned long dirty,
                              unsigned long bdi_thresh,
                              unsigned long bdi_dirty,
                              unsigned long start_time)
  {
          unsigned long now = jiffies;
          unsigned long elapsed = now - bdi->bw_time_stamp;
          unsigned long dirtied;
          unsigned long written;
  
          /*
           * rate-limit, only update once every 200ms.
           */
          if (elapsed < BANDWIDTH_INTERVAL)
                  return;
  
          dirtied = percpu_counter_read(&bdi->bdi_stat[BDI_DIRTIED]);
          written = percpu_counter_read(&bdi->bdi_stat[BDI_WRITTEN]);
  
          /*
           * Skip quiet periods when disk bandwidth is under-utilized.
           * (at least 1s idle time between two flusher runs)
           */
          if (elapsed > HZ && time_before(bdi->bw_time_stamp, start_time))
                  goto snapshot;
  
          if (thresh) {
                  global_update_bandwidth(thresh, dirty, now);
                  bdi_update_dirty_ratelimit(bdi, thresh, bg_thresh, dirty,
                                             bdi_thresh, bdi_dirty,
                                             dirtied, elapsed);
          }
          bdi_update_write_bandwidth(bdi, elapsed, written);
  
  snapshot:
          bdi->dirtied_stamp = dirtied;
          bdi->written_stamp = written;
          bdi->bw_time_stamp = now;
  }
  
  static void bdi_update_bandwidth(struct backing_dev_info *bdi,
                                   unsigned long thresh,
                                   unsigned long bg_thresh,
                                   unsigned long dirty,
                                   unsigned long bdi_thresh,
                                   unsigned long bdi_dirty,
                                   unsigned long start_time)
  {
          if (time_is_after_eq_jiffies(bdi->bw_time_stamp + BANDWIDTH_INTERVAL))
                  return;
          spin_lock(&bdi->wb.list_lock);
          __bdi_update_bandwidth(bdi, thresh, bg_thresh, dirty,
                                 bdi_thresh, bdi_dirty, start_time);
          spin_unlock(&bdi->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_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 long bdi_max_pause(struct backing_dev_info *bdi,
                            unsigned long bdi_dirty)
  {
          long bw = bdi->avg_write_bandwidth;
          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 = bdi_dirty / (1 + bw / roundup_pow_of_two(1 + HZ / 8));
          t++;
  
          return min_t(long, t, MAX_PAUSE);
  }
  
  static long bdi_min_pause(struct backing_dev_info *bdi,
                            long max_pause,
                            unsigned long task_ratelimit,
                            unsigned long dirty_ratelimit,
                            int *nr_dirtied_pause)
  {
          long hi = ilog2(bdi->avg_write_bandwidth);
          long lo = ilog2(bdi->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;
  }
  
  /*
   * 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 void balance_dirty_pages(struct address_space *mapping,
                                  unsigned long pages_dirtied)
  {
          unsigned long nr_reclaimable;   /* = file_dirty + unstable_nfs */
          unsigned long bdi_reclaimable;
          unsigned long nr_dirty;  /* = file_dirty + writeback + unstable_nfs */
          unsigned long bdi_dirty;
          unsigned long freerun;
          unsigned long background_thresh;
          unsigned long dirty_thresh;
          unsigned long bdi_thresh;
          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;
          unsigned long pos_ratio;
          struct backing_dev_info *bdi = mapping->backing_dev_info;
          unsigned long start_time = jiffies;
  
          for (;;) {
                  unsigned long now = jiffies;
  
                  /*
                   * Unstable writes are a feature of certain networked
                   * filesystems (i.e. NFS) in which data may have been
                   * written to the server's write cache, but has not yet
                   * been flushed to permanent storage.
                   */
                  nr_reclaimable = global_page_state(NR_FILE_DIRTY) +
                                          global_page_state(NR_UNSTABLE_NFS);
                  nr_dirty = nr_reclaimable + global_page_state(NR_WRITEBACK);
  
                  global_dirty_limits(&background_thresh, &dirty_thresh);
  
                  /*
                   * Throttle it only when the background writeback cannot
                   * catch-up. This avoids (excessively) small writeouts
                   * when the bdi limits are ramping up.
                   */
                  freerun = dirty_freerun_ceiling(dirty_thresh,
                                                  background_thresh);
                  if (nr_dirty <= freerun) {
                          current->dirty_paused_when = now;
                          current->nr_dirtied = 0;
                          current->nr_dirtied_pause =
                                  dirty_poll_interval(nr_dirty, dirty_thresh);
                          break;
                  }
  
                  if (unlikely(!writeback_in_progress(bdi)))
                          bdi_start_background_writeback(bdi);
  
                  /*
                   * bdi_thresh is not treated as some limiting factor as
                   * dirty_thresh, due to reasons
                   * - in JBOD setup, bdi_thresh can fluctuate a lot
                   * - in a system with HDD and USB key, the USB key may somehow
                   *   go into state (bdi_dirty >> bdi_thresh) either because
                   *   bdi_dirty starts high, or because bdi_thresh drops low.
                   *   In this case we don't want to hard throttle the USB key
                   *   dirtiers for 100 seconds until bdi_dirty drops under
                   *   bdi_thresh. Instead the auxiliary bdi control line in
                   *   bdi_position_ratio() will let the dirtier task progress
                   *   at some rate <= (write_bw / 2) for bringing down bdi_dirty.
                   */
                  bdi_thresh = bdi_dirty_limit(bdi, dirty_thresh);
  
                  /*
                   * 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 (bdi_thresh < 2 * bdi_stat_error(bdi)) {
                          bdi_reclaimable = bdi_stat_sum(bdi, BDI_RECLAIMABLE);
                          bdi_dirty = bdi_reclaimable +
                                      bdi_stat_sum(bdi, BDI_WRITEBACK);
                  } else {
                          bdi_reclaimable = bdi_stat(bdi, BDI_RECLAIMABLE);
                          bdi_dirty = bdi_reclaimable +
                                      bdi_stat(bdi, BDI_WRITEBACK);
                  }
  
                  dirty_exceeded = (bdi_dirty > bdi_thresh) &&
                                    (nr_dirty > dirty_thresh);
                  if (dirty_exceeded && !bdi->dirty_exceeded)
                          bdi->dirty_exceeded = 1;
  
                  bdi_update_bandwidth(bdi, dirty_thresh, background_thresh,
                                       nr_dirty, bdi_thresh, bdi_dirty,
                                       start_time);
  
                  dirty_ratelimit = bdi->dirty_ratelimit;
                  pos_ratio = bdi_position_ratio(bdi, dirty_thresh,
                                                 background_thresh, nr_dirty,
                                                 bdi_thresh, bdi_dirty);
                  task_ratelimit = ((u64)dirty_ratelimit * pos_ratio) >>
                                                          RATELIMIT_CALC_SHIFT;
                  max_pause = bdi_max_pause(bdi, bdi_dirty);
                  min_pause = bdi_min_pause(bdi, 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(bdi,
                                                    dirty_thresh,
                                                    background_thresh,
                                                    nr_dirty,
                                                    bdi_thresh,
                                                    bdi_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(bdi,
                                            dirty_thresh,
                                            background_thresh,
                                            nr_dirty,
                                            bdi_thresh,
                                            bdi_dirty,
                                            dirty_ratelimit,
                                            task_ratelimit,
                                            pages_dirtied,
                                            period,
                                            pause,
                                            start_time);
                  __set_current_state(TASK_KILLABLE);
                  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 (nr_dirty < dirty_thresh) and can
                   * also keep "1000+ dd on a slow USB stick" under control.
                   */
                  if (task_ratelimit)
                          break;
  
                  /*
                   * In the case of an unresponding NFS server and the NFS dirty
                   * pages exceeds dirty_thresh, give the other good bdi's a pipe
                   * to go through, so that tasks on them still remain responsive.
                   *
                   * In theory 1 page is enough to keep the comsumer-producer
                   * pipe going: the flusher cleans 1 page => the task dirties 1
                   * more page. However bdi_dirty has accounting errors.  So use
                   * the larger and more IO friendly bdi_stat_error.
                   */
                  if (bdi_dirty <= bdi_stat_error(bdi))
                          break;
  
                  if (fatal_signal_pending(current))
                          break;
          }
  
          if (!dirty_exceeded && bdi->dirty_exceeded)
                  bdi->dirty_exceeded = 0;
  
          if (writeback_in_progress(bdi))
                  return;
  
          /*
           * 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)
                  return;
  
          if (nr_reclaimable > background_thresh)
                  bdi_start_background_writeback(bdi);
  }
  
  void set_page_dirty_balance(struct page *page, int page_mkwrite)
  {
          if (set_page_dirty(page) || page_mkwrite) {
                  struct address_space *mapping = page_mapping(page);
  
                  if (mapping)
                          balance_dirty_pages_ratelimited(mapping);
          }
  }
  
  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 - 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.
   *
   * On really big machines, get_writeback_state is expensive, so try to avoid
   * calling it too often (ratelimiting).  But 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)
  {
          struct backing_dev_info *bdi = mapping->backing_dev_info;
          int ratelimit;
          int *p;
  
          if (!bdi_cap_account_dirty(bdi))
                  return;
  
          ratelimit = current->nr_dirtied_pause;
          if (bdi->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 =  &__get_cpu_var(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 = &__get_cpu_var(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))
                  balance_dirty_pages(mapping, current->nr_dirtied);
  }
  EXPORT_SYMBOL(balance_dirty_pages_ratelimited);
  
  void throttle_vm_writeout(gfp_t gfp_mask)
  {
          unsigned long background_thresh;
          unsigned long dirty_thresh;
  
          for ( ; ; ) {
                  global_dirty_limits(&background_thresh, &dirty_thresh);
                  dirty_thresh = hard_dirty_limit(dirty_thresh);
  
                  /*
                   * Boost the allowable dirty threshold a bit for page
                   * allocators so they don't get DoS'ed by heavy writers
                   */
                  dirty_thresh += dirty_thresh / 10;      /* wheeee... */
  
                  if (global_page_state(NR_UNSTABLE_NFS) +
                          global_page_state(NR_WRITEBACK) <= dirty_thresh)
                                  break;
                  congestion_wait(BLK_RW_ASYNC, HZ/10);
  
                  /*
                   * The caller might hold locks which can prevent IO completion
                   * or progress in the filesystem.  So we cannot just sit here
                   * waiting for IO to complete.
                   */
                  if ((gfp_mask & (__GFP_FS|__GFP_IO)) != (__GFP_FS|__GFP_IO))
                          break;
          }
  }
  
  /*
   * sysctl handler for /proc/sys/vm/dirty_writeback_centisecs
   */
  int dirty_writeback_centisecs_handler(ctl_table *table, int write,
          void __user *buffer, size_t *length, loff_t *ppos)
  {
          proc_dointvec(table, write, buffer, length, ppos);
          return 0;
  }
  
  #ifdef CONFIG_BLOCK
  void laptop_mode_timer_fn(unsigned long data)
  {
          struct request_queue *q = (struct request_queue *)data;
          int nr_pages = global_page_state(NR_FILE_DIRTY) +
                  global_page_state(NR_UNSTABLE_NFS);
  
          /*
           * We want to write everything out, not just down to the dirty
           * threshold
           */
          if (bdi_has_dirty_io(&q->backing_dev_info))
                  bdi_start_writeback(&q->backing_dev_info, nr_pages,
                                          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();
  }
  #endif
  
  /*
   * 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.
   * If it is too low then SMP machines will call the (expensive)
   * get_writeback_state too often.
   *
   * 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)
  {
          unsigned long background_thresh;
          unsigned long dirty_thresh;
          global_dirty_limits(&background_thresh, &dirty_thresh);
          global_dirty_limit = dirty_thresh;
          ratelimit_pages = dirty_thresh / (num_online_cpus() * 32);
          if (ratelimit_pages < 16)
                  ratelimit_pages = 16;
  }
  
  static int __cpuinit
  ratelimit_handler(struct notifier_block *self, unsigned long action,
                    void *hcpu)
  {
  
          switch (action & ~CPU_TASKS_FROZEN) {
          case CPU_ONLINE:
          case CPU_DEAD:
                  writeback_set_ratelimit();
                  return NOTIFY_OK;
          default:
                  return NOTIFY_DONE;
          }
  }
  
  static struct notifier_block __cpuinitdata ratelimit_nb = {
          .notifier_call  = ratelimit_handler,
          .next           = NULL,
  };
  
  /*
   * 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 (by
   * comparing nr_free_buffer_pages() to vm_total_pages.
   *
   * 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 (by subtracting
   * totalhigh_pages from vm_total_pages), 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)
  {
          writeback_set_ratelimit();
          register_cpu_notifier(&ratelimit_nb);
  
          fprop_global_init(&writeout_completions);
  }
  
  /**
   * 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).
   */
  /*
   * We tag pages in batches of WRITEBACK_TAG_BATCH to reduce tree_lock latency.
   */
  void tag_pages_for_writeback(struct address_space *mapping,
                               pgoff_t start, pgoff_t end)
  {
  #define WRITEBACK_TAG_BATCH 4096
          unsigned long tagged;
  
          do {
                  spin_lock_irq(&mapping->tree_lock);
                  tagged = radix_tree_range_tag_if_tagged(&mapping->page_tree,
                                  &start, end, WRITEBACK_TAG_BATCH,
                                  PAGECACHE_TAG_DIRTY, PAGECACHE_TAG_TOWRITE);
                  spin_unlock_irq(&mapping->tree_lock);
                  WARN_ON_ONCE(tagged > WRITEBACK_TAG_BATCH);
                  cond_resched();
                  /* We check 'start' to handle wrapping when end == ~0UL */
          } while (tagged >= WRITEBACK_TAG_BATCH && start);
  }
  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).
   */
  int write_cache_pages(struct address_space *mapping,
                        struct writeback_control *wbc, writepage_t writepage,
                        void *data)
  {
          int ret = 0;
          int done = 0;
          struct pagevec pvec;
          int nr_pages;
          pgoff_t uninitialized_var(writeback_index);
          pgoff_t index;
          pgoff_t end;            /* Inclusive */
          pgoff_t done_index;
          int cycled;
          int range_whole = 0;
          int tag;
  
          pagevec_init(&pvec, 0);
          if (wbc->range_cyclic) {
                  writeback_index = mapping->writeback_index; /* prev offset */
                  index = writeback_index;
                  if (index == 0)
                          cycled = 1;
                  else
                          cycled = 0;
                  end = -1;
          } else {
                  index = wbc->range_start >> PAGE_CACHE_SHIFT;
                  end = wbc->range_end >> PAGE_CACHE_SHIFT;
                  if (wbc->range_start == 0 && wbc->range_end == LLONG_MAX)
                          range_whole = 1;
                  cycled = 1; /* ignore range_cyclic tests */
          }
          if (wbc->sync_mode == WB_SYNC_ALL || wbc->tagged_writepages)
                  tag = PAGECACHE_TAG_TOWRITE;
          else
                  tag = PAGECACHE_TAG_DIRTY;
  retry:
          if (wbc->sync_mode == WB_SYNC_ALL || wbc->tagged_writepages)
                  tag_pages_for_writeback(mapping, index, end);
          done_index = index;
          while (!done && (index <= end)) {
                  int i;
  
                  nr_pages = pagevec_lookup_tag(&pvec, mapping, &index, tag,
                                min(end - index, (pgoff_t)PAGEVEC_SIZE-1) + 1);
                  if (nr_pages == 0)
                          break;
  
                  for (i = 0; i < nr_pages; i++) {
                          struct page *page = pvec.pages[i];
  
                          /*
                           * At this point, the page may be truncated or
                           * invalidated (changing page->mapping to NULL), or
                           * even swizzled back from swapper_space to tmpfs file
                           * mapping. However, page->index will not change
                           * because we have a reference on the page.
                           */
                          if (page->index > end) {
                                  /*
                                   * can't be range_cyclic (1st pass) because
                                   * end == -1 in that case.
                                   */
                                  done = 1;
                                  break;
                          }
  
                          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 interity 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, mapping->backing_dev_info);
                          ret = (*writepage)(page, wbc, data);
                          if (unlikely(ret)) {
                                  if (ret == AOP_WRITEPAGE_ACTIVATE) {
                                          unlock_page(page);
                                          ret = 0;
                                  } else {
                                          /*
                                           * done_index is set past this page,
                                           * so media errors will not choke
                                           * background writeout for the entire
                                           * file. This has consequences for
                                           * range_cyclic semantics (ie. it may
                                           * not be suitable for data integrity
                                           * writeout).
                                           */
                                          done_index = page->index + 1;
                                          done = 1;
                                          break;
                                  }
                          }
  
                          /*
                           * 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 (!cycled && !done) {
                  /*
                   * range_cyclic:
                   * We hit the last page and there is more work to be done: wrap
                   * back to the start of the file
                   */
                  cycled = 1;
                  index = 0;
                  end = writeback_index - 1;
                  goto retry;
          }
          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.
   */
  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;
  
          if (wbc->nr_to_write <= 0)
                  return 0;
          if (mapping->a_ops->writepages)
                  ret = mapping->a_ops->writepages(mapping, wbc);
          else
                  ret = generic_writepages(mapping, wbc);
          return ret;
  }
  
  /**
   * write_one_page - write out a single page and optionally wait on I/O
   * @page: the page to write
   * @wait: if true, wait on writeout
   *
   * The page must be locked by the caller and will be unlocked upon return.
   *
   * write_one_page() returns a negative error code if I/O failed.
   */
  int write_one_page(struct page *page, int wait)
  {
          struct address_space *mapping = page->mapping;
          int ret = 0;
          struct writeback_control wbc = {
                  .sync_mode = WB_SYNC_ALL,
                  .nr_to_write = 1,
          };
  
          BUG_ON(!PageLocked(page));
  
          if (wait)
                  wait_on_page_writeback(page);
  
          if (clear_page_dirty_for_io(page)) {
                  page_cache_get(page);
                  ret = mapping->a_ops->writepage(page, &wbc);
                  if (ret == 0 && wait) {
                          wait_on_page_writeback(page);
                          if (PageError(page))
                                  ret = -EIO;
                  }
                  page_cache_release(page);
          } else {
                  unlock_page(page);
          }
          return ret;
  }
  EXPORT_SYMBOL(write_one_page);
  
  /*
   * For address_spaces which do not use buffers nor write back.
   */
  int __set_page_dirty_no_writeback(struct page *page)
  {
          if (!PageDirty(page))
                  return !TestSetPageDirty(page);
          return 0;
  }
  
  /*
   * Helper function for set_page_dirty family.
   * NOTE: This relies on being atomic wrt interrupts.
   */
  void account_page_dirtied(struct page *page, struct address_space *mapping)
  {
          if (mapping_cap_account_dirty(mapping)) {
                  __inc_zone_page_state(page, NR_FILE_DIRTY);
                  __inc_zone_page_state(page, NR_DIRTIED);
                  __inc_bdi_stat(mapping->backing_dev_info, BDI_RECLAIMABLE);
                  __inc_bdi_stat(mapping->backing_dev_info, BDI_DIRTIED);
                  task_io_account_write(PAGE_CACHE_SIZE);
                  current->nr_dirtied++;
                  this_cpu_inc(bdp_ratelimits);
          }
  }
  EXPORT_SYMBOL(account_page_dirtied);
  
  /*
   * Helper function for set_page_writeback family.
   * NOTE: Unlike account_page_dirtied this does not rely on being atomic
   * wrt interrupts.
   */
  void account_page_writeback(struct page *page)
  {
          inc_zone_page_state(page, NR_WRITEBACK);
  }
  EXPORT_SYMBOL(account_page_writeback);
  
  /*
   * For address_spaces which do not use buffers.  Just tag the page as dirty in
   * its radix tree.
   *
   * This is also used when a single buffer is being dirtied: we want to set the
   * page dirty in that case, but not all the buffers.  This is a "bottom-up"
   * dirtying, whereas __set_page_dirty_buffers() is a "top-down" dirtying.
   *
   * Most callers have locked the page, which pins the address_space in memory.
   * But zap_pte_range() does not lock the page, however in that case the
   * mapping is pinned by the vma's ->vm_file reference.
   *
   * We take care to handle the case where the page was truncated from the
   * mapping by re-checking page_mapping() inside tree_lock.
   */
  int __set_page_dirty_nobuffers(struct page *page)
  {
          if (!TestSetPageDirty(page)) {
                  struct address_space *mapping = page_mapping(page);
                  struct address_space *mapping2;
  
                  if (!mapping)
                          return 1;
  
                  spin_lock_irq(&mapping->tree_lock);
                  mapping2 = page_mapping(page);
                  if (mapping2) { /* Race with truncate? */
                          BUG_ON(mapping2 != mapping);
                          WARN_ON_ONCE(!PagePrivate(page) && !PageUptodate(page));
                          account_page_dirtied(page, mapping);
                          radix_tree_tag_set(&mapping->page_tree,
                                  page_index(page), PAGECACHE_TAG_DIRTY);
                  }
                  spin_unlock_irq(&mapping->tree_lock);
                  if (mapping->host) {
                          /* !PageAnon && !swapper_space */
                          __mark_inode_dirty(mapping->host, I_DIRTY_PAGES);
                  }
                  return 1;
          }
          return 0;
  }
  EXPORT_SYMBOL(__set_page_dirty_nobuffers);
  
  /*
   * Call this whenever redirtying a page, to de-account the dirty counters
   * (NR_DIRTIED, BDI_DIRTIED, tsk->nr_dirtied), so that they match the written
   * counters (NR_WRITTEN, BDI_WRITTEN) in long term. The mismatches will lead to
   * systematic errors in balanced_dirty_ratelimit and the dirty pages position
   * control.
   */
  void account_page_redirty(struct page *page)
  {
          struct address_space *mapping = page->mapping;
          if (mapping && mapping_cap_account_dirty(mapping)) {
                  current->nr_dirtied--;
                  dec_zone_page_state(page, NR_DIRTIED);
                  dec_bdi_stat(mapping->backing_dev_info, BDI_DIRTIED);
          }
  }
  EXPORT_SYMBOL(account_page_redirty);
  
  /*
   * When a writepage implementation decides that it doesn't want to write this
   * page for some reason, it should redirty the locked page via
   * redirty_page_for_writepage() and it should then unlock the page and return 0
   */
  int redirty_page_for_writepage(struct writeback_control *wbc, struct page *page)
  {
          wbc->pages_skipped++;
          account_page_redirty(page);
          return __set_page_dirty_nobuffers(page);
  }
  EXPORT_SYMBOL(redirty_page_for_writepage);
  
  /*
   * Dirty a page.
   *
   * For pages with a mapping this should be done under the page lock
   * for the benefit of asynchronous memory errors who prefer a consistent
   * dirty state. This rule can be broken in some special cases,
   * but should be better not to.
   *
   * If the mapping doesn't provide a set_page_dirty a_op, then
   * just fall through and assume that it wants buffer_heads.
   */
  int set_page_dirty(struct page *page)
  {
          struct address_space *mapping = page_mapping(page);
  
          if (likely(mapping)) {
                  int (*spd)(struct page *) = mapping->a_ops->set_page_dirty;
                  /*
                   * readahead/lru_deactivate_page could remain
                   * PG_readahead/PG_reclaim due to race with end_page_writeback
                   * About readahead, if the page is written, the flags would be
                   * reset. So no problem.
                   * About lru_deactivate_page, if the page is redirty, the flag
                   * will be reset. So no problem. but if the page is used by readahead
                   * it will confuse readahead and make it restart the size rampup
                   * process. But it's a trivial problem.
                   */
                  ClearPageReclaim(page);
  #ifdef CONFIG_BLOCK
                  if (!spd)
                          spd = __set_page_dirty_buffers;
  #endif
                  return (*spd)(page);
          }
          if (!PageDirty(page)) {
                  if (!TestSetPageDirty(page))
                          return 1;
          }
          return 0;
  }
  EXPORT_SYMBOL(set_page_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);
  
  /*
   * Clear a page's dirty flag, while caring for dirty memory accounting.
   * Returns true if the page was previously dirty.
   *
   * This is for preparing to put the page under writeout.  We leave the page
   * tagged as dirty in the radix tree so that a concurrent write-for-sync
   * can discover it via a PAGECACHE_TAG_DIRTY walk.  The ->writepage
   * implementation will run either set_page_writeback() or set_page_dirty(),
   * at which stage we bring the page's dirty flag and radix-tree dirty tag
   * back into sync.
   *
   * This incoherency between the page's dirty flag and radix-tree tag is
   * unfortunate, but it only exists while the page is locked.
   */
  int clear_page_dirty_for_io(struct page *page)
  {
          struct address_space *mapping = page_mapping(page);
  
          BUG_ON(!PageLocked(page));
  
          if (mapping && mapping_cap_account_dirty(mapping)) {
                  /*
                   * 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 page dirty if it was
                   *      dirty in a pagetable. Only to then
                   *  (c) clean the page 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 "set_page_dirty(page)"
                   * 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 page "master dirty bit"
                   * as a serialization point for all the different
                   * threads doing their things.
                   */
                  if (page_mkclean(page))
                          set_page_dirty(page);
                  /*
                   * We carefully synchronise fault handlers against
                   * installing a dirty pte and marking the page dirty
                   * at this point. We do this by having them hold the
                   * page lock at some point after installing their
                   * pte, but before marking the page dirty.
                   * Pages are always locked coming in here, so we get
                   * the desired exclusion. See mm/memory.c:do_wp_page()
                   * for more comments.
                   */
                  if (TestClearPageDirty(page)) {
                          dec_zone_page_state(page, NR_FILE_DIRTY);
                          dec_bdi_stat(mapping->backing_dev_info,
                                          BDI_RECLAIMABLE);
                          return 1;
                  }
                  return 0;
          }
          return TestClearPageDirty(page);
  }
  EXPORT_SYMBOL(clear_page_dirty_for_io);
  
  int test_clear_page_writeback(struct page *page)
  {
          struct address_space *mapping = page_mapping(page);
          int ret;
  
          if (mapping) {
                  struct backing_dev_info *bdi = mapping->backing_dev_info;
                  unsigned long flags;
  
                  spin_lock_irqsave(&mapping->tree_lock, flags);
                  ret = TestClearPageWriteback(page);
                  if (ret) {
                          radix_tree_tag_clear(&mapping->page_tree,
                                                  page_index(page),
                                                  PAGECACHE_TAG_WRITEBACK);
                          if (bdi_cap_account_writeback(bdi)) {
                                  __dec_bdi_stat(bdi, BDI_WRITEBACK);
                                  __bdi_writeout_inc(bdi);
                          }
                  }
                  spin_unlock_irqrestore(&mapping->tree_lock, flags);
          } else {
                  ret = TestClearPageWriteback(page);
          }
          if (ret) {
                  dec_zone_page_state(page, NR_WRITEBACK);
                  inc_zone_page_state(page, NR_WRITTEN);
          }
          return ret;
  }
  
  int test_set_page_writeback(struct page *page)
  {
          struct address_space *mapping = page_mapping(page);
          int ret;
  
          if (mapping) {
                  struct backing_dev_info *bdi = mapping->backing_dev_info;
                  unsigned long flags;
  
                  spin_lock_irqsave(&mapping->tree_lock, flags);
                  ret = TestSetPageWriteback(page);
                  if (!ret) {
                          radix_tree_tag_set(&mapping->page_tree,
                                                  page_index(page),
                                                  PAGECACHE_TAG_WRITEBACK);
                          if (bdi_cap_account_writeback(bdi))
                                  __inc_bdi_stat(bdi, BDI_WRITEBACK);
                  }
                  if (!PageDirty(page))
                          radix_tree_tag_clear(&mapping->page_tree,
                                                  page_index(page),
                                                  PAGECACHE_TAG_DIRTY);
                  radix_tree_tag_clear(&mapping->page_tree,
                                       page_index(page),
                                       PAGECACHE_TAG_TOWRITE);
                  spin_unlock_irqrestore(&mapping->tree_lock, flags);
          } else {
                  ret = TestSetPageWriteback(page);
          }
          if (!ret)
                  account_page_writeback(page);
          return ret;
  
  }
  EXPORT_SYMBOL(test_set_page_writeback);
  
  /*
   * Return true if any of the pages in the mapping are marked with the
   * passed tag.
   */
  int mapping_tagged(struct address_space *mapping, int tag)
  {
          return radix_tree_tagged(&mapping->page_tree, tag);
  }
  EXPORT_SYMBOL(mapping_tagged);

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