/*

   * 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/module.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>

  #include <linux/pagevec.h>

  #include <trace/events/writeback.h>

  
  /*

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

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

   * 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;

  /*
   * When balance_dirty_pages decides that the caller needs to perform some
   * non-background writeback, this is how many pages it will attempt to write.

   * It should be somewhat larger than dirtied pages to ensure that reasonably

   * large amounts of I/O are submitted.
   */

  static inline long sync_writeback_pages(unsigned long dirtied)

  {

  	if (dirtied < ratelimit_pages)
  		dirtied = ratelimit_pages;
  
  	return dirtied + dirtied / 2;

  }
  
  /* 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 */

  
  /*

   * 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 prop_descriptor vm_completions;

  static struct prop_descriptor vm_dirties;

  /*
   * couple the period to the dirty_ratio:
   *
   *   period/2 ~ roundup_pow_of_two(dirty limit)
   */
  static int calc_period_shift(void)
  {
  	unsigned long dirty_total;

  	if (vm_dirty_bytes)
  		dirty_total = vm_dirty_bytes / PAGE_SIZE;
  	else
  		dirty_total = (vm_dirty_ratio * determine_dirtyable_memory()) /
  				100;

  	return 2 + ilog2(dirty_total - 1);
  }
  
  /*

   * update the period when the dirty threshold changes.

   */

  static void update_completion_period(void)
  {
  	int shift = calc_period_shift();
  	prop_change_shift(&vm_completions, shift);
  	prop_change_shift(&vm_dirties, shift);
  }
  
  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) {

  		update_completion_period();
  		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) {
  		update_completion_period();
  		vm_dirty_ratio = 0;

  	}
  	return ret;
  }
  
  /*
   * 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);

  	__prop_inc_percpu_max(&vm_completions, &bdi->completions,
  			      bdi->max_prop_frac);

  }

  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);

  void task_dirty_inc(struct task_struct *tsk)

  {
  	prop_inc_single(&vm_dirties, &tsk->dirties);
  }

  /*
   * Obtain an accurate fraction of the BDI's portion.
   */
  static void bdi_writeout_fraction(struct backing_dev_info *bdi,
  		long *numerator, long *denominator)
  {

  	prop_fraction_percpu(&vm_completions, &bdi->completions,

  				numerator, denominator);

  }

  static inline void task_dirties_fraction(struct task_struct *tsk,
  		long *numerator, long *denominator)
  {
  	prop_fraction_single(&vm_dirties, &tsk->dirties,
  				numerator, denominator);
  }
  
  /*

   * task_dirty_limit - scale down dirty throttling threshold for one task

   *
   * task specific dirty limit:
   *
   *   dirty -= (dirty/8) * p_{t}

   *
   * To protect light/slow dirtying tasks from heavier/fast ones, we start
   * throttling individual tasks before reaching the bdi dirty limit.
   * Relatively low thresholds will be allocated to heavy dirtiers. So when
   * dirty pages grow large, heavy dirtiers will be throttled first, which will
   * effectively curb the growth of dirty pages. Light dirtiers with high enough
   * dirty threshold may never get throttled.

   */

  #define TASK_LIMIT_FRACTION 8

  static unsigned long task_dirty_limit(struct task_struct *tsk,
  				       unsigned long bdi_dirty)

  {
  	long numerator, denominator;

  	unsigned long dirty = bdi_dirty;

  	u64 inv = dirty / TASK_LIMIT_FRACTION;

  
  	task_dirties_fraction(tsk, &numerator, &denominator);
  	inv *= numerator;
  	do_div(inv, denominator);
  
  	dirty -= inv;

  	return max(dirty, bdi_dirty/2);

  }

  /* Minimum limit for any task */
  static unsigned long task_min_dirty_limit(unsigned long bdi_dirty)
  {
  	return bdi_dirty - bdi_dirty / TASK_LIMIT_FRACTION;
  }

  /*

   *
   */

  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 = (PROP_FRAC_BASE * max_ratio) / 100;
  	}

  	spin_unlock_bh(&bdi_lock);

  
  	return ret;
  }

  EXPORT_SYMBOL(bdi_set_max_ratio);

  
  /*

   * 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.
   */

  
  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);

  	}
  	/*
  	 * 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
  }

  /**
   * determine_dirtyable_memory - amount of memory that may be used
   *
   * Returns the numebr of pages that can currently be freed and used
   * by the kernel for direct mappings.
   */
  unsigned long determine_dirtyable_memory(void)

  {
  	unsigned long x;

  	x = global_page_state(NR_FREE_PAGES) + global_reclaimable_pages();

  
  	if (!vm_highmem_is_dirtyable)
  		x -= highmem_dirtyable_memory(x);

  	return x + 1;	/* Ensure that we never return 0 */
  }

  static unsigned long hard_dirty_limit(unsigned long thresh)
  {
  	return max(thresh, global_dirty_limit);
  }

  /*

   * 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 = determine_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);

  }

  /**

   * 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.
   * And the "limit" in the name is not seriously taken as hard limit in
   * balance_dirty_pages().

   *

   * 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;

  }

  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);
  }

  void __bdi_update_bandwidth(struct backing_dev_info *bdi,

  			    unsigned long 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 written;
  
  	/*
  	 * rate-limit, only update once every 200ms.
  	 */
  	if (elapsed < BANDWIDTH_INTERVAL)
  		return;
  
  	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_write_bandwidth(bdi, elapsed, written);
  
  snapshot:
  	bdi->written_stamp = written;
  	bdi->bw_time_stamp = now;
  }
  
  static void bdi_update_bandwidth(struct backing_dev_info *bdi,

  				 unsigned long 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, dirty, bdi_thresh, bdi_dirty,
  			       start_time);

  	spin_unlock(&bdi->wb.list_lock);
  }

  /*
   * 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 perform writeback if the system is over `vm_dirty_ratio'.

   * 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 write_chunk)

  {

  	unsigned long nr_reclaimable, bdi_nr_reclaimable;
  	unsigned long nr_dirty;  /* = file_dirty + writeback + unstable_nfs */
  	unsigned long bdi_dirty;

  	unsigned long background_thresh;
  	unsigned long dirty_thresh;
  	unsigned long bdi_thresh;

  	unsigned long task_bdi_thresh;
  	unsigned long min_task_bdi_thresh;

  	unsigned long pages_written = 0;

  	unsigned long pause = 1;

  	bool dirty_exceeded = false;

  	bool clear_dirty_exceeded = true;

  	struct backing_dev_info *bdi = mapping->backing_dev_info;

  	unsigned long start_time = jiffies;

  
  	for (;;) {

  		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.
  		 */

  		if (nr_dirty <= (background_thresh + dirty_thresh) / 2)

  			break;
  
  		bdi_thresh = bdi_dirty_limit(bdi, dirty_thresh);

  		min_task_bdi_thresh = task_min_dirty_limit(bdi_thresh);
  		task_bdi_thresh = task_dirty_limit(current, bdi_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 (task_bdi_thresh < 2 * bdi_stat_error(bdi)) {

  			bdi_nr_reclaimable = bdi_stat_sum(bdi, BDI_RECLAIMABLE);

  			bdi_dirty = bdi_nr_reclaimable +
  				    bdi_stat_sum(bdi, BDI_WRITEBACK);

  		} else {
  			bdi_nr_reclaimable = bdi_stat(bdi, BDI_RECLAIMABLE);

  			bdi_dirty = bdi_nr_reclaimable +
  				    bdi_stat(bdi, BDI_WRITEBACK);

  		}

  		/*
  		 * The bdi thresh is somehow "soft" limit derived from the
  		 * global "hard" limit. The former helps to prevent heavy IO
  		 * bdi or process from holding back light ones; The latter is
  		 * the last resort safeguard.
  		 */

  		dirty_exceeded = (bdi_dirty > task_bdi_thresh) ||

  				  (nr_dirty > dirty_thresh);

  		clear_dirty_exceeded = (bdi_dirty <= min_task_bdi_thresh) &&
  					(nr_dirty <= dirty_thresh);

  
  		if (!dirty_exceeded)

  			break;

  		if (!bdi->dirty_exceeded)
  			bdi->dirty_exceeded = 1;

  		bdi_update_bandwidth(bdi, dirty_thresh, nr_dirty,
  				     bdi_thresh, bdi_dirty, start_time);

  		/* Note: nr_reclaimable denotes nr_dirty + nr_unstable.
  		 * 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.

  		 * Only move pages to writeback if this bdi is over its
  		 * threshold otherwise wait until the disk writes catch
  		 * up.

  		 */

  		trace_balance_dirty_start(bdi);

  		if (bdi_nr_reclaimable > task_bdi_thresh) {

  			pages_written += writeback_inodes_wb(&bdi->wb,
  							     write_chunk);
  			trace_balance_dirty_written(bdi, pages_written);

  			if (pages_written >= write_chunk)
  				break;		/* We've done our duty */

  		}

  		__set_current_state(TASK_UNINTERRUPTIBLE);

  		io_schedule_timeout(pause);

  		trace_balance_dirty_wait(bdi);

  		dirty_thresh = hard_dirty_limit(dirty_thresh);
  		/*
  		 * max-pause area. If dirty exceeded but still within this
  		 * area, no need to sleep for more than 200ms: (a) 8 pages per
  		 * 200ms is typically more than enough to curb heavy dirtiers;
  		 * (b) the pause time limit makes the dirtiers more responsive.
  		 */

  		if (nr_dirty < dirty_thresh &&
  		    bdi_dirty < (task_bdi_thresh + bdi_thresh) / 2 &&

  		    time_after(jiffies, start_time + MAX_PAUSE))
  			break;

  
  		/*
  		 * Increase the delay for each loop, up to our previous
  		 * default of taking a 100ms nap.
  		 */
  		pause <<= 1;
  		if (pause > HZ / 10)
  			pause = HZ / 10;

  	}

  	/* Clear dirty_exceeded flag only when no task can exceed the limit */
  	if (clear_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 && pages_written) ||

  	    (!laptop_mode && (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(unsigned long, bdp_ratelimits) = 0;

  /**

   * balance_dirty_pages_ratelimited_nr - balance dirty memory state

   * @mapping: address_space which was dirtied

   * @nr_pages_dirtied: number of pages which the caller has just 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_nr(struct address_space *mapping,
  					unsigned long nr_pages_dirtied)

  {

  	struct backing_dev_info *bdi = mapping->backing_dev_info;

  	unsigned long ratelimit;
  	unsigned long *p;

  	if (!bdi_cap_account_dirty(bdi))
  		return;

  	ratelimit = ratelimit_pages;

  	if (mapping->backing_dev_info->dirty_exceeded)

  		ratelimit = 8;
  
  	/*
  	 * Check the rate limiting. Also, we do not want to throttle real-time
  	 * tasks in balance_dirty_pages(). Period.
  	 */

  	preempt_disable();

  	p =  &__get_cpu_var(bdp_ratelimits);

  	*p += nr_pages_dirtied;
  	if (unlikely(*p >= ratelimit)) {

  		ratelimit = sync_writeback_pages(*p);

  		*p = 0;
  		preempt_enable();

  		balance_dirty_pages(mapping, ratelimit);

  		return;
  	}

  	preempt_enable();

  }

  EXPORT_SYMBOL(balance_dirty_pages_ratelimited_nr);

  void throttle_vm_writeout(gfp_t gfp_mask)

  {

  	unsigned long background_thresh;
  	unsigned long dirty_thresh;

  
          for ( ; ; ) {

  		global_dirty_limits(&background_thresh, &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);

  	bdi_arm_supers_timer();

  	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);

  }
  
  /*
   * 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 before writeback cuts in.
   *
   * But the limit should not be set too high.  Because it also controls the
   * amount of memory which the balance_dirty_pages() caller has to write back.
   * If this is too large then the caller will block on the IO queue all the
   * time.  So limit it to four megabytes - the balance_dirty_pages() caller
   * will write six megabyte chunks, max.
   */

  void writeback_set_ratelimit(void)

  {

  	ratelimit_pages = vm_total_pages / (num_online_cpus() * 32);

  	if (ratelimit_pages < 16)
  		ratelimit_pages = 16;
  	if (ratelimit_pages * PAGE_CACHE_SIZE > 4096 * 1024)
  		ratelimit_pages = (4096 * 1024) / PAGE_CACHE_SIZE;
  }

  static int __cpuinit

  ratelimit_handler(struct notifier_block *self, unsigned long u, void *v)
  {

  	writeback_set_ratelimit();

  	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)
  {

  	int shift;

  	writeback_set_ratelimit();

  	register_cpu_notifier(&ratelimit_nb);

  
  	shift = calc_period_shift();
  	prop_descriptor_init(&vm_completions, shift);

  	prop_descriptor_init(&vm_dirties, shift);

  }

  /**

   * 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);
  		task_dirty_inc(current);
  		task_io_account_write(PAGE_CACHE_SIZE);
  	}
  }

  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);
  
  /*
   * 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++;
  	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);