/*

   * 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 <linux/sched/rt.h>

  #include <linux/mm_inline.h>

  #include <trace/events/writeback.h>

  #include "internal.h"

  /*

   * Sleep at most 200ms at a time in balance_dirty_pages().
   */
  #define MAX_PAUSE		max(HZ/5, 1)
  
  /*

   * Try to keep balance_dirty_pages() call intervals higher than this many pages
   * by raising pause time to max_pause when falls below it.
   */
  #define DIRTY_POLL_THRESH	(128 >> (PAGE_SHIFT - 10))
  
  /*

   * Estimate write bandwidth at 200ms intervals.
   */
  #define BANDWIDTH_INTERVAL	max(HZ/5, 1)

  #define RATELIMIT_CALC_SHIFT	10

  /*

   * After a CPU has dirtied this many pages, balance_dirty_pages_ratelimited
   * will look to see if it needs to force writeback or throttling.
   */
  static long ratelimit_pages = 32;

  /* The following parameters are exported via /proc/sys/vm */
  
  /*

   * Start background writeback (via writeback threads) at this percentage

   */

  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)

  /*

   * 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.
   */

  /**
   * 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)
  {
  	unsigned long nr_pages;
  
  	nr_pages = zone_page_state(zone, NR_FREE_PAGES);
  	nr_pages -= min(nr_pages, zone->dirty_balance_reserve);

  	nr_pages += zone_page_state(zone, NR_INACTIVE_FILE);
  	nr_pages += zone_page_state(zone, NR_ACTIVE_FILE);

  
  	return nr_pages;
  }

  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_dirtyable_memory(z);

  	}
  	/*

  	 * 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);

  	x -= min(x, dirty_balance_reserve);

  	x += global_page_state(NR_INACTIVE_FILE);
  	x += global_page_state(NR_ACTIVE_FILE);

  	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)
  {

  	const unsigned long available_memory = global_dirtyable_memory();

  	unsigned long background;
  	unsigned long dirty;

  	struct task_struct *tsk;

  	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_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;

  }

  /*

   *                           setpoint - dirty 3
   *        f(dirty) := 1.0 + (----------------)
   *                           limit - setpoint
   *
   * it's a 3rd order polynomial that subjects to
   *
   * (1) f(freerun)  = 2.0 => rampup dirty_ratelimit reasonably fast
   * (2) f(setpoint) = 1.0 => the balance point
   * (3) f(limit)    = 0   => the hard limit
   * (4) df/dx      <= 0	 => negative feedback control
   * (5) the closer to setpoint, the smaller |df/dx| (and the reverse)
   *     => fast response on large errors; small oscillation near setpoint
   */

  static long long pos_ratio_polynom(unsigned long setpoint,

  					  unsigned long dirty,
  					  unsigned long limit)
  {
  	long long pos_ratio;
  	long x;

  	x = div64_s64(((s64)setpoint - (s64)dirty) << RATELIMIT_CALC_SHIFT,

  		    limit - setpoint + 1);
  	pos_ratio = x;
  	pos_ratio = pos_ratio * x >> RATELIMIT_CALC_SHIFT;
  	pos_ratio = pos_ratio * x >> RATELIMIT_CALC_SHIFT;
  	pos_ratio += 1 << RATELIMIT_CALC_SHIFT;
  
  	return clamp(pos_ratio, 0LL, 2LL << RATELIMIT_CALC_SHIFT);
  }
  
  /*

   * Dirty position control.
   *
   * (o) global/bdi setpoints
   *
   * We want the dirty pages be balanced around the global/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
  	 *

  	 * See comment for pos_ratio_polynom().
  	 */
  	setpoint = (freerun + limit) / 2;
  	pos_ratio = pos_ratio_polynom(setpoint, dirty, limit);
  
  	/*
  	 * The strictlimit feature is a tool preventing mistrusted filesystems
  	 * from growing a large number of dirty pages before throttling. For
  	 * such filesystems balance_dirty_pages always checks bdi counters
  	 * against bdi limits. Even if global "nr_dirty" is under "freerun".
  	 * This is especially important for fuse which sets bdi->max_ratio to
  	 * 1% by default. Without strictlimit feature, fuse writeback may
  	 * consume arbitrary amount of RAM because it is accounted in
  	 * NR_WRITEBACK_TEMP which is not involved in calculating "nr_dirty".

  	 *

  	 * Here, in bdi_position_ratio(), we calculate pos_ratio based on
  	 * two values: bdi_dirty and bdi_thresh. Let's consider an example:
  	 * total amount of RAM is 16GB, bdi->max_ratio is equal to 1%, global
  	 * limits are set by default to 10% and 20% (background and throttle).
  	 * Then bdi_thresh is 1% of 20% of 16GB. This amounts to ~8K pages.
  	 * bdi_dirty_limit(bdi, bg_thresh) is about ~4K pages. bdi_setpoint is
  	 * about ~6K pages (as the average of background and throttle bdi
  	 * limits). The 3rd order polynomial will provide positive feedback if
  	 * bdi_dirty is under bdi_setpoint and vice versa.

  	 *

  	 * Note, that we cannot use global counters in these calculations
  	 * because we want to throttle process writing to a strictlimit BDI
  	 * much earlier than global "freerun" is reached (~23MB vs. ~2.3GB
  	 * in the example above).

  	 */

  	if (unlikely(bdi->capabilities & BDI_CAP_STRICTLIMIT)) {
  		long long bdi_pos_ratio;
  		unsigned long bdi_bg_thresh;
  
  		if (bdi_dirty < 8)
  			return min_t(long long, pos_ratio * 2,
  				     2 << RATELIMIT_CALC_SHIFT);
  
  		if (bdi_dirty >= bdi_thresh)
  			return 0;
  
  		bdi_bg_thresh = div_u64((u64)bdi_thresh * bg_thresh, thresh);
  		bdi_setpoint = dirty_freerun_ceiling(bdi_thresh,
  						     bdi_bg_thresh);
  
  		if (bdi_setpoint == 0 || bdi_setpoint == bdi_thresh)
  			return 0;
  
  		bdi_pos_ratio = pos_ratio_polynom(bdi_setpoint, bdi_dirty,
  						  bdi_thresh);
  
  		/*
  		 * Typically, for strictlimit case, bdi_setpoint << setpoint
  		 * and pos_ratio >> bdi_pos_ratio. In the other words global
  		 * state ("dirty") is not limiting factor and we have to
  		 * make decision based on bdi counters. But there is an
  		 * important case when global pos_ratio should get precedence:
  		 * global limits are exceeded (e.g. due to activities on other
  		 * BDIs) while given strictlimit BDI is below limit.
  		 *
  		 * "pos_ratio * bdi_pos_ratio" would work for the case above,
  		 * but it would look too non-natural for the case of all
  		 * activity in the system coming from a single strictlimit BDI
  		 * with bdi->max_ratio == 100%.
  		 *
  		 * Note that min() below somewhat changes the dynamics of the
  		 * control system. Normally, pos_ratio value can be well over 3
  		 * (when globally we are at freerun and bdi is well below bdi
  		 * setpoint). Now the maximum pos_ratio in the same situation
  		 * is 2. We might want to tweak this if we observe the control
  		 * system is too slow to adapt.
  		 */
  		return min(pos_ratio, bdi_pos_ratio);
  	}

  
  	/*
  	 * 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 = div64_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;

  
  	/*
  	 * For strictlimit case, calculations above were based on bdi counters
  	 * and limits (starting from pos_ratio = bdi_position_ratio() and up to
  	 * balanced_dirty_ratelimit = task_ratelimit * write_bw / dirty_rate).
  	 * Hence, to calculate "step" properly, we have to use bdi_dirty as
  	 * "dirty" and bdi_setpoint as "setpoint".
  	 *
  	 * We rampup dirty_ratelimit forcibly if bdi_dirty is low because
  	 * it's possible that bdi_thresh is close to zero due to inactivity
  	 * of backing device (see the implementation of bdi_dirty_limit()).
  	 */
  	if (unlikely(bdi->capabilities & BDI_CAP_STRICTLIMIT)) {
  		dirty = bdi_dirty;
  		if (bdi_dirty < 8)
  			setpoint = bdi_dirty + 1;
  		else
  			setpoint = (bdi_thresh +
  				    bdi_dirty_limit(bdi, bg_thresh)) / 2;
  	}

  	if (dirty < setpoint) {

  		x = min3(bdi->balanced_dirty_ratelimit,
  			 balanced_dirty_ratelimit, task_ratelimit);

  		if (dirty_ratelimit < x)
  			step = x - dirty_ratelimit;
  	} else {

  		x = max3(bdi->balanced_dirty_ratelimit,
  			 balanced_dirty_ratelimit, task_ratelimit);

  		if (dirty_ratelimit > x)
  			step = dirty_ratelimit - x;
  	}
  
  	/*
  	 * Don't pursue 100% rate matching. It's impossible since the balanced
  	 * rate itself is constantly fluctuating. So decrease the track speed
  	 * when it gets close to the target. Helps eliminate pointless tremors.
  	 */
  	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 unsigned long bdi_max_pause(struct backing_dev_info *bdi,
  				   unsigned long bdi_dirty)

  {

  	unsigned long bw = bdi->avg_write_bandwidth;
  	unsigned long t;

  	/*
  	 * Limit pause time for small memory systems. If sleeping for too long
  	 * time, a small pool of dirty/writeback pages may go empty and disk go
  	 * idle.
  	 *
  	 * 8 serves as the safety ratio.
  	 */
  	t = bdi_dirty / (1 + bw / roundup_pow_of_two(1 + HZ / 8));
  	t++;

  	return min_t(unsigned 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;

  }

  static inline void bdi_dirty_limits(struct backing_dev_info *bdi,
  				    unsigned long dirty_thresh,
  				    unsigned long background_thresh,
  				    unsigned long *bdi_dirty,
  				    unsigned long *bdi_thresh,
  				    unsigned long *bdi_bg_thresh)
  {
  	unsigned long bdi_reclaimable;
  
  	/*
  	 * 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);
  
  	if (bdi_bg_thresh)

  		*bdi_bg_thresh = dirty_thresh ? div_u64((u64)*bdi_thresh *
  							background_thresh,
  							dirty_thresh) : 0;

  
  	/*
  	 * In order to avoid the stacked BDI deadlock we need
  	 * to ensure we accurately count the 'dirty' pages when
  	 * the threshold is low.
  	 *
  	 * Otherwise it would be possible to get thresh+n pages
  	 * reported dirty, even though there are thresh-m pages
  	 * actually dirty; with m+n sitting in the percpu
  	 * deltas.
  	 */
  	if (*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);
  	}
  }

  /*

   * 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 nr_dirty;  /* = file_dirty + writeback + unstable_nfs */

  	unsigned long background_thresh;
  	unsigned long dirty_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;

  	bool strictlimit = bdi->capabilities & BDI_CAP_STRICTLIMIT;

  	unsigned long start_time = jiffies;

  
  	for (;;) {

  		unsigned long now = jiffies;

  		unsigned long uninitialized_var(bdi_thresh);
  		unsigned long thresh;
  		unsigned long uninitialized_var(bdi_dirty);
  		unsigned long dirty;
  		unsigned long bg_thresh;

  		/*
  		 * 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);

  		if (unlikely(strictlimit)) {
  			bdi_dirty_limits(bdi, dirty_thresh, background_thresh,
  					 &bdi_dirty, &bdi_thresh, &bg_thresh);
  
  			dirty = bdi_dirty;
  			thresh = bdi_thresh;
  		} else {
  			dirty = nr_dirty;
  			thresh = dirty_thresh;
  			bg_thresh = background_thresh;
  		}

  		/*
  		 * Throttle it only when the background writeback cannot
  		 * catch-up. This avoids (excessively) small writeouts

  		 * when the bdi limits are ramping up in case of !strictlimit.
  		 *
  		 * In strictlimit case make decision based on the bdi counters
  		 * and limits. Small writeouts when the bdi limits are ramping
  		 * up are the price we consciously pay for strictlimit-ing.

  		 */

  		if (dirty <= dirty_freerun_ceiling(thresh, bg_thresh)) {

  			current->dirty_paused_when = now;
  			current->nr_dirtied = 0;

  			current->nr_dirtied_pause =

  				dirty_poll_interval(dirty, thresh);

  			break;

  		}

  		if (unlikely(!writeback_in_progress(bdi)))
  			bdi_start_background_writeback(bdi);

  		if (!strictlimit)
  			bdi_dirty_limits(bdi, dirty_thresh, background_thresh,
  					 &bdi_dirty, &bdi_thresh, NULL);

  		dirty_exceeded = (bdi_dirty > bdi_thresh) &&

  				 ((nr_dirty > dirty_thresh) || strictlimit);

  		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);

  }

  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 =  this_cpu_ptr(&bdp_ratelimits);

  	if (unlikely(current->nr_dirtied >= ratelimit))

  		*p = 0;

  	else if (unlikely(*p >= ratelimit_pages)) {
  		*p = 0;
  		ratelimit = 0;

  	}

  	/*
  	 * Pick up the dirtied pages by the exited tasks. This avoids lots of
  	 * short-lived tasks (eg. gcc invocations in a kernel build) escaping
  	 * the dirty throttling and livelock other long-run dirtiers.
  	 */

  	p = this_cpu_ptr(&dirty_throttle_leaks);

  	if (*p > 0 && current->nr_dirtied < ratelimit) {

  		unsigned long nr_pages_dirtied;

  		nr_pages_dirtied = min(*p, ratelimit - current->nr_dirtied);
  		*p -= nr_pages_dirtied;
  		current->nr_dirtied += nr_pages_dirtied;

  	}

  	preempt_enable();

  
  	if (unlikely(current->nr_dirtied >= ratelimit))
  		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(struct 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

  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 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, GFP_KERNEL);

  }

  /**

   * 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)
  {

  	trace_writeback_dirty_page(page, 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);

  
  /*

   * 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.
   *

   * The caller must ensure this doesn't race with truncation.  Most will simply
   * hold the page lock, but e.g. zap_pte_range() calls with the page mapped and
   * the pte lock held, which also locks out truncation.

   */
  int __set_page_dirty_nobuffers(struct page *page)
  {

  	if (!TestSetPageDirty(page)) {
  		struct address_space *mapping = page_mapping(page);

  		unsigned long flags;

  		if (!mapping)
  			return 1;

  		spin_lock_irqsave(&mapping->tree_lock, flags);

  		BUG_ON(page_mapping(page) != 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_irqrestore(&mapping->tree_lock, flags);

  		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 while dirtying the page, and pages are
  		 * always locked coming in here, so we get the desired
  		 * exclusion.

  		 */

  		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);

  	unsigned long memcg_flags;

  	struct mem_cgroup *memcg;
  	bool locked;
  	int ret;

  	memcg = mem_cgroup_begin_page_stat(page, &locked, &memcg_flags);

  	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) {

  		mem_cgroup_dec_page_stat(memcg, MEM_CGROUP_STAT_WRITEBACK);

  		dec_zone_page_state(page, NR_WRITEBACK);

  		inc_zone_page_state(page, NR_WRITTEN);
  	}

  	mem_cgroup_end_page_stat(memcg, locked, memcg_flags);

  	return ret;
  }

  int __test_set_page_writeback(struct page *page, bool keep_write)

  {
  	struct address_space *mapping = page_mapping(page);

  	unsigned long memcg_flags;

  	struct mem_cgroup *memcg;
  	bool locked;
  	int ret;

  	memcg = mem_cgroup_begin_page_stat(page, &locked, &memcg_flags);

  	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);

  		if (!keep_write)
  			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) {

  		mem_cgroup_inc_page_stat(memcg, MEM_CGROUP_STAT_WRITEBACK);

  		inc_zone_page_state(page, NR_WRITEBACK);
  	}

  	mem_cgroup_end_page_stat(memcg, locked, memcg_flags);

  	return ret;
  
  }