writeback: s/bdi/wb/ in mm/page-writeback.c

 *

 * (o) global/bdi setpoints

 *

 * We want the dirty pages be balanced around the global/bdi setpoints.

 * We want the dirty pages be balanced around the global/wb setpoints.

 * When the number of dirty pages is higher/lower than the setpoint, the

 * dirty position control ratio (and hence task dirty ratelimit) will be

 * decreased/increased to bring the dirty pages back to the setpoint.

 *     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

 *     if (wb_dirty < wb_setpoint) scale up   pos_ratio

 *     if (wb_dirty > wb_setpoint) scale down pos_ratio

 *

 *     task_ratelimit = dirty_ratelimit * pos_ratio >> RATELIMIT_CALC_SHIFT

 *

 *   0 +------------.------------------.----------------------*------------->

 *           freerun^          setpoint^                 limit^   dirty pages

 *

 * (o) bdi control line

 * (o) wb control line

 *

 *     ^ pos_ratio

 *     |

 *     |                      .                           .

 *     |                      .                             .

 *   0 +----------------------.-------------------------------.------------->

 *                bdi_setpoint^                    x_intercept^

 *                wb_setpoint^                    x_intercept^

 *

 * The bdi control line won't drop below pos_ratio=1/4, so that bdi_dirty can

 * The wb control line won't drop below pos_ratio=1/4, so that wb_dirty can

 * be smoothly throttled down to normal if it starts high in situations like

 * - start writing to a slow SD card and a fast disk at the same time. The SD

 *   card's bdi_dirty may rush to many times higher than bdi_setpoint.

 * - the bdi dirty thresh drops quickly due to change of JBOD workload

 *   card's wb_dirty may rush to many times higher than wb_setpoint.

 * - the wb dirty thresh drops quickly due to change of JBOD workload

 */

static unsigned long wb_position_ratio(struct bdi_writeback *wb,

				       unsigned long thresh,

				       unsigned long bg_thresh,

				       unsigned long dirty,

				       unsigned long bdi_thresh,

				       unsigned long bdi_dirty)

				       unsigned long wb_thresh,

				       unsigned long wb_dirty)

{

	unsigned long write_bw = wb->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 wb_setpoint;

	unsigned long span;

	long long pos_ratio;		/* for scaling up/down the rate limit */

	long x;

	/*

	 * 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".

	 * such filesystems balance_dirty_pages always checks wb counters

	 * against wb limits. Even if global "nr_dirty" is under "freerun".

	 * This is especially important for fuse which sets bdi->max_ratio to

	 * 1% by default. Without strictlimit feature, fuse writeback may

	 * consume arbitrary amount of RAM because it is accounted in

	 * NR_WRITEBACK_TEMP which is not involved in calculating "nr_dirty".

	 *

	 * Here, in wb_position_ratio(), we calculate pos_ratio based on

	 * two values: bdi_dirty and bdi_thresh. Let's consider an example:

	 * two values: wb_dirty and wb_thresh. Let's consider an example:

	 * total amount of RAM is 16GB, bdi->max_ratio is equal to 1%, global

	 * limits are set by default to 10% and 20% (background and throttle).

	 * Then bdi_thresh is 1% of 20% of 16GB. This amounts to ~8K pages.

	 * wb_dirty_limit(wb, bg_thresh) is about ~4K pages. bdi_setpoint is

	 * about ~6K pages (as the average of background and throttle bdi

	 * Then wb_thresh is 1% of 20% of 16GB. This amounts to ~8K pages.

	 * wb_dirty_limit(wb, bg_thresh) is about ~4K pages. wb_setpoint is

	 * about ~6K pages (as the average of background and throttle wb

	 * limits). The 3rd order polynomial will provide positive feedback if

	 * bdi_dirty is under bdi_setpoint and vice versa.

	 * wb_dirty is under wb_setpoint and vice versa.

	 *

	 * Note, that we cannot use global counters in these calculations

	 * because we want to throttle process writing to a strictlimit BDI

	 * because we want to throttle process writing to a strictlimit wb

	 * much earlier than global "freerun" is reached (~23MB vs. ~2.3GB

	 * in the example above).

	 */

	if (unlikely(wb->bdi->capabilities & BDI_CAP_STRICTLIMIT)) {

		long long bdi_pos_ratio;

		unsigned long bdi_bg_thresh;

		long long wb_pos_ratio;

		unsigned long wb_bg_thresh;

		if (bdi_dirty < 8)

		if (wb_dirty < 8)

			return min_t(long long, pos_ratio * 2,

				     2 << RATELIMIT_CALC_SHIFT);

		if (bdi_dirty >= bdi_thresh)

		if (wb_dirty >= wb_thresh)

			return 0;

		bdi_bg_thresh = div_u64((u64)bdi_thresh * bg_thresh, thresh);

		bdi_setpoint = dirty_freerun_ceiling(bdi_thresh,

						     bdi_bg_thresh);

		wb_bg_thresh = div_u64((u64)wb_thresh * bg_thresh, thresh);

		wb_setpoint = dirty_freerun_ceiling(wb_thresh, wb_bg_thresh);

		if (bdi_setpoint == 0 || bdi_setpoint == bdi_thresh)

		if (wb_setpoint == 0 || wb_setpoint == wb_thresh)

			return 0;

		bdi_pos_ratio = pos_ratio_polynom(bdi_setpoint, bdi_dirty,

						  bdi_thresh);

		wb_pos_ratio = pos_ratio_polynom(wb_setpoint, wb_dirty,

						 wb_thresh);

		/*

		 * Typically, for strictlimit case, bdi_setpoint << setpoint

		 * and pos_ratio >> bdi_pos_ratio. In the other words global

		 * Typically, for strictlimit case, wb_setpoint << setpoint

		 * and pos_ratio >> wb_pos_ratio. In the other words global

		 * state ("dirty") is not limiting factor and we have to

		 * make decision based on bdi counters. But there is an

		 * make decision based on wb counters. But there is an

		 * important case when global pos_ratio should get precedence:

		 * global limits are exceeded (e.g. due to activities on other

		 * BDIs) while given strictlimit BDI is below limit.

		 * wb's) while given strictlimit wb is below limit.

		 *

		 * "pos_ratio * bdi_pos_ratio" would work for the case above,

		 * "pos_ratio * wb_pos_ratio" would work for the case above,

		 * but it would look too non-natural for the case of all

		 * activity in the system coming from a single strictlimit BDI

		 * activity in the system coming from a single strictlimit wb

		 * with bdi->max_ratio == 100%.

		 *

		 * Note that min() below somewhat changes the dynamics of the

		 * control system. Normally, pos_ratio value can be well over 3

		 * (when globally we are at freerun and bdi is well below bdi

		 * (when globally we are at freerun and wb is well below wb

		 * setpoint). Now the maximum pos_ratio in the same situation

		 * is 2. We might want to tweak this if we observe the control

		 * system is too slow to adapt.

		 */

		return min(pos_ratio, bdi_pos_ratio);

		return min(pos_ratio, wb_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

	 * the wb is over/under its share of dirty pages, we want to scale

	 * pos_ratio further down/up. That is done by the following mechanism.

	 */

	/*

	 * bdi setpoint

	 * wb setpoint

	 *

	 *        f(bdi_dirty) := 1.0 + k * (bdi_dirty - bdi_setpoint)

	 *        f(wb_dirty) := 1.0 + k * (wb_dirty - wb_setpoint)

	 *

	 *                        x_intercept - bdi_dirty

	 *                        x_intercept - wb_dirty

	 *                     := --------------------------

	 *                        x_intercept - bdi_setpoint

	 *                        x_intercept - wb_setpoint

	 *

	 * The main bdi control line is a linear function that subjects to

	 * The main wb 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

	 * (1) f(wb_setpoint) = 1.0

	 * (2) k = - 1 / (8 * write_bw)  (in single wb case)

	 *     or equally: x_intercept = wb_setpoint + 8 * write_bw

	 *

	 * For single bdi case, the dirty pages are observed to fluctuate

	 * For single wb case, the dirty pages are observed to fluctuate

	 * regularly within range

	 *        [bdi_setpoint - write_bw/2, bdi_setpoint + write_bw/2]

	 *        [wb_setpoint - write_bw/2, wb_setpoint + write_bw/2]

	 * for various filesystems, where (2) can yield in a reasonable 12.5%

	 * fluctuation range for pos_ratio.

	 *

	 * For JBOD case, bdi_thresh (not bdi_dirty!) could fluctuate up to its

	 * For JBOD case, wb_thresh (not wb_dirty!) could fluctuate up to its

	 * own size, so move the slope over accordingly and choose a slope that

	 * yields 100% pos_ratio fluctuation on suddenly doubled bdi_thresh.

	 * yields 100% pos_ratio fluctuation on suddenly doubled wb_thresh.

	 */

	if (unlikely(bdi_thresh > thresh))

		bdi_thresh = thresh;

	if (unlikely(wb_thresh > thresh))

		wb_thresh = thresh;

	/*

	 * It's very possible that bdi_thresh is close to 0 not because the

	 * It's very possible that wb_thresh is close to 0 not because the

	 * device is slow, but that it has remained inactive for long time.

	 * Honour such devices a reasonable good (hopefully IO efficient)

	 * threshold, so that the occasional writes won't be blocked and active

	 * writes can rampup the threshold quickly.

	 */

	bdi_thresh = max(bdi_thresh, (limit - dirty) / 8);

	wb_thresh = max(wb_thresh, (limit - dirty) / 8);

	/*

	 * scale global setpoint to bdi's:

	 *	bdi_setpoint = setpoint * bdi_thresh / thresh

	 * scale global setpoint to wb's:

	 *	wb_setpoint = setpoint * wb_thresh / thresh

	 */

	x = div_u64((u64)bdi_thresh << 16, thresh + 1);

	bdi_setpoint = setpoint * (u64)x >> 16;

	x = div_u64((u64)wb_thresh << 16, thresh + 1);

	wb_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.

	 * Use span=(8*write_bw) in single wb case as indicated by

	 * (thresh - wb_thresh ~= 0) and transit to wb_thresh in JBOD case.

	 *

	 *        bdi_thresh                    thresh - bdi_thresh

	 * span = ---------- * (8 * write_bw) + ------------------- * bdi_thresh

	 *        wb_thresh                    thresh - wb_thresh

	 * span = --------- * (8 * write_bw) + ------------------ * wb_thresh

	 *         thresh                           thresh

	 */

	span = (thresh - bdi_thresh + 8 * write_bw) * (u64)x >> 16;

	x_intercept = bdi_setpoint + span;

	span = (thresh - wb_thresh + 8 * write_bw) * (u64)x >> 16;

	x_intercept = wb_setpoint + span;

	if (bdi_dirty < x_intercept - span / 4) {

		pos_ratio = div64_u64(pos_ratio * (x_intercept - bdi_dirty),

				    x_intercept - bdi_setpoint + 1);

	if (wb_dirty < x_intercept - span / 4) {

		pos_ratio = div64_u64(pos_ratio * (x_intercept - wb_dirty),

				    x_intercept - wb_setpoint + 1);

	} else

		pos_ratio /= 4;

	/*

	 * bdi reserve area, safeguard against dirty pool underrun and disk idle

	 * wb reserve area, safeguard against dirty pool underrun and disk idle

	 * It may push the desired control point of global dirty pages higher

	 * than setpoint.

	 */

	x_intercept = bdi_thresh / 2;

	if (bdi_dirty < x_intercept) {

		if (bdi_dirty > x_intercept / 8)

			pos_ratio = div_u64(pos_ratio * x_intercept, bdi_dirty);

	x_intercept = wb_thresh / 2;

	if (wb_dirty < x_intercept) {

		if (wb_dirty > x_intercept / 8)

			pos_ratio = div_u64(pos_ratio * x_intercept, wb_dirty);

		else

			pos_ratio *= 8;

	}

}

/*

 * Maintain bdi->dirty_ratelimit, the base dirty throttle rate.

 * Maintain wb->dirty_ratelimit, the base dirty throttle rate.

 *

 * Normal bdi tasks will be curbed at or below it in long term.

 * Normal wb tasks will be curbed at or below it in long term.

 * Obviously it should be around (write_bw / N) when there are N dd tasks.

 */

static void wb_update_dirty_ratelimit(struct bdi_writeback *wb,

				      unsigned long thresh,

				      unsigned long bg_thresh,

				      unsigned long dirty,

				      unsigned long bdi_thresh,

				      unsigned long bdi_dirty,

				      unsigned long wb_thresh,

				      unsigned long wb_dirty,

				      unsigned long dirtied,

				      unsigned long elapsed)

{

	dirty_rate = (dirtied - wb->dirtied_stamp) * HZ / elapsed;

	pos_ratio = wb_position_ratio(wb, thresh, bg_thresh, dirty,

				      bdi_thresh, bdi_dirty);

				      wb_thresh, wb_dirty);

	/*

	 * task_ratelimit reflects each dd's dirty rate for the past 200ms.

	 */

	/*

	 * 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

	 * if there are N dd tasks, each throttled at task_ratelimit, the wb's

	 * dirty_rate will be measured to be (N * task_ratelimit). So the below

	 * formula will yield the balanced rate limit (write_bw / N).

	 *

	/*

	 * We could safely do this and return immediately:

	 *

	 *	bdi->dirty_ratelimit = balanced_dirty_ratelimit;

	 *	wb->dirty_ratelimit = balanced_dirty_ratelimit;

	 *

	 * However to get a more stable dirty_ratelimit, the below elaborated

	 * code makes use of task_ratelimit to filter out singular points and

	step = 0;

	/*

	 * For strictlimit case, calculations above were based on bdi counters

	 * For strictlimit case, calculations above were based on wb counters

	 * and limits (starting from pos_ratio = wb_position_ratio() and up to

	 * balanced_dirty_ratelimit = task_ratelimit * write_bw / dirty_rate).

	 * Hence, to calculate "step" properly, we have to use bdi_dirty as

	 * "dirty" and bdi_setpoint as "setpoint".

	 * Hence, to calculate "step" properly, we have to use wb_dirty as

	 * "dirty" and wb_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

	 * We rampup dirty_ratelimit forcibly if wb_dirty is low because

	 * it's possible that wb_thresh is close to zero due to inactivity

	 * of backing device (see the implementation of wb_dirty_limit()).

	 */

	if (unlikely(wb->bdi->capabilities & BDI_CAP_STRICTLIMIT)) {

		dirty = bdi_dirty;

		if (bdi_dirty < 8)

			setpoint = bdi_dirty + 1;

		dirty = wb_dirty;

		if (wb_dirty < 8)

			setpoint = wb_dirty + 1;

		else

			setpoint = (bdi_thresh +

			setpoint = (wb_thresh +

				    wb_dirty_limit(wb, bg_thresh)) / 2;

	}

			   unsigned long thresh,

			   unsigned long bg_thresh,

			   unsigned long dirty,

			   unsigned long bdi_thresh,

			   unsigned long bdi_dirty,

			   unsigned long wb_thresh,

			   unsigned long wb_dirty,

			   unsigned long start_time)

{

	unsigned long now = jiffies;

	if (thresh) {

		global_update_bandwidth(thresh, dirty, now);

		wb_update_dirty_ratelimit(wb, thresh, bg_thresh, dirty,

					  bdi_thresh, bdi_dirty,

					  wb_thresh, wb_dirty,

					  dirtied, elapsed);

	}

	wb_update_write_bandwidth(wb, elapsed, written);

				unsigned long thresh,

				unsigned long bg_thresh,

				unsigned long dirty,

				unsigned long bdi_thresh,

				unsigned long bdi_dirty,

				unsigned long wb_thresh,

				unsigned long wb_dirty,

				unsigned long start_time)

{

	if (time_is_after_eq_jiffies(wb->bw_time_stamp + BANDWIDTH_INTERVAL))

		return;

	spin_lock(&wb->list_lock);

	__wb_update_bandwidth(wb, thresh, bg_thresh, dirty,

			      bdi_thresh, bdi_dirty, start_time);

			      wb_thresh, wb_dirty, start_time);

	spin_unlock(&wb->list_lock);

}

}

static unsigned long wb_max_pause(struct bdi_writeback *wb,

				      unsigned long bdi_dirty)

				  unsigned long wb_dirty)

{

	unsigned long bw = wb->avg_write_bandwidth;

	unsigned long t;

	 *

	 * 8 serves as the safety ratio.

	 */

	t = bdi_dirty / (1 + bw / roundup_pow_of_two(1 + HZ / 8));

	t = wb_dirty / (1 + bw / roundup_pow_of_two(1 + HZ / 8));

	t++;

	return min_t(unsigned long, t, MAX_PAUSE);

static inline void wb_dirty_limits(struct bdi_writeback *wb,

				   unsigned long dirty_thresh,

				   unsigned long background_thresh,

				   unsigned long *bdi_dirty,

				   unsigned long *bdi_thresh,

				   unsigned long *bdi_bg_thresh)

				   unsigned long *wb_dirty,

				   unsigned long *wb_thresh,

				   unsigned long *wb_bg_thresh)

{

	unsigned long wb_reclaimable;

	/*

	 * bdi_thresh is not treated as some limiting factor as

	 * wb_thresh is not treated as some limiting factor as

	 * dirty_thresh, due to reasons

	 * - in JBOD setup, bdi_thresh can fluctuate a lot

	 * - in JBOD setup, wb_thresh can fluctuate a lot

	 * - in a system with HDD and USB key, the USB key may somehow

	 *   go into state (bdi_dirty >> bdi_thresh) either because

	 *   bdi_dirty starts high, or because bdi_thresh drops low.

	 *   go into state (wb_dirty >> wb_thresh) either because

	 *   wb_dirty starts high, or because wb_thresh drops low.

	 *   In this case we don't want to hard throttle the USB key

	 *   dirtiers for 100 seconds until bdi_dirty drops under

	 *   bdi_thresh. Instead the auxiliary bdi control line in

	 *   dirtiers for 100 seconds until wb_dirty drops under

	 *   wb_thresh. Instead the auxiliary wb control line in

	 *   wb_position_ratio() will let the dirtier task progress

	 *   at some rate <= (write_bw / 2) for bringing down bdi_dirty.

	 *   at some rate <= (write_bw / 2) for bringing down wb_dirty.

	 */

	*bdi_thresh = wb_dirty_limit(wb, dirty_thresh);

	*wb_thresh = wb_dirty_limit(wb, dirty_thresh);

	if (bdi_bg_thresh)

		*bdi_bg_thresh = dirty_thresh ? div_u64((u64)*bdi_thresh *

	if (wb_bg_thresh)

		*wb_bg_thresh = dirty_thresh ? div_u64((u64)*wb_thresh *

						       background_thresh,

						       dirty_thresh) : 0;

	 * actually dirty; with m+n sitting in the percpu

	 * deltas.

	 */

	if (*bdi_thresh < 2 * wb_stat_error(wb)) {

	if (*wb_thresh < 2 * wb_stat_error(wb)) {

		wb_reclaimable = wb_stat_sum(wb, WB_RECLAIMABLE);

		*bdi_dirty = wb_reclaimable + wb_stat_sum(wb, WB_WRITEBACK);

		*wb_dirty = wb_reclaimable + wb_stat_sum(wb, WB_WRITEBACK);

	} else {

		wb_reclaimable = wb_stat(wb, WB_RECLAIMABLE);

		*bdi_dirty = wb_reclaimable + wb_stat(wb, WB_WRITEBACK);

		*wb_dirty = wb_reclaimable + wb_stat(wb, WB_WRITEBACK);

	}

}

	for (;;) {

		unsigned long now = jiffies;

		unsigned long uninitialized_var(bdi_thresh);

		unsigned long uninitialized_var(wb_thresh);

		unsigned long thresh;

		unsigned long uninitialized_var(bdi_dirty);

		unsigned long uninitialized_var(wb_dirty);

		unsigned long dirty;

		unsigned long bg_thresh;

		if (unlikely(strictlimit)) {

			wb_dirty_limits(wb, dirty_thresh, background_thresh,

					&bdi_dirty, &bdi_thresh, &bg_thresh);

					&wb_dirty, &wb_thresh, &bg_thresh);

			dirty = bdi_dirty;

			thresh = bdi_thresh;

			dirty = wb_dirty;

			thresh = wb_thresh;

		} else {

			dirty = nr_dirty;

			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 in case of !strictlimit.

		 * when the wb 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

		 * In strictlimit case make decision based on the wb counters

		 * and limits. Small writeouts when the wb limits are ramping

		 * up are the price we consciously pay for strictlimit-ing.

		 */

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

		if (!strictlimit)

			wb_dirty_limits(wb, dirty_thresh, background_thresh,

					&bdi_dirty, &bdi_thresh, NULL);

					&wb_dirty, &wb_thresh, NULL);

		dirty_exceeded = (bdi_dirty > bdi_thresh) &&

		dirty_exceeded = (wb_dirty > wb_thresh) &&

				 ((nr_dirty > dirty_thresh) || strictlimit);

		if (dirty_exceeded && !wb->dirty_exceeded)

			wb->dirty_exceeded = 1;

		wb_update_bandwidth(wb, dirty_thresh, background_thresh,

				    nr_dirty, bdi_thresh, bdi_dirty,

				    start_time);

				    nr_dirty, wb_thresh, wb_dirty, start_time);

		dirty_ratelimit = wb->dirty_ratelimit;

		pos_ratio = wb_position_ratio(wb, dirty_thresh,

					      background_thresh, nr_dirty,

					      bdi_thresh, bdi_dirty);

					      wb_thresh, wb_dirty);

		task_ratelimit = ((u64)dirty_ratelimit * pos_ratio) >>

							RATELIMIT_CALC_SHIFT;

		max_pause = wb_max_pause(wb, bdi_dirty);

		max_pause = wb_max_pause(wb, wb_dirty);

		min_pause = wb_min_pause(wb, max_pause,

					 task_ratelimit, dirty_ratelimit,

					 &nr_dirtied_pause);

						  dirty_thresh,

						  background_thresh,

						  nr_dirty,

						  bdi_thresh,

						  bdi_dirty,

						  wb_thresh,

						  wb_dirty,

						  dirty_ratelimit,

						  task_ratelimit,

						  pages_dirtied,

					  dirty_thresh,

					  background_thresh,

					  nr_dirty,

					  bdi_thresh,

					  bdi_dirty,

					  wb_thresh,

					  wb_dirty,

					  dirty_ratelimit,

					  task_ratelimit,

					  pages_dirtied,

		/*

		 * In the case of an unresponding NFS server and the NFS dirty

		 * pages exceeds dirty_thresh, give the other good bdi's a pipe

		 * pages exceeds dirty_thresh, give the other good wb's a pipe

		 * to go through, so that tasks on them still remain responsive.

		 *

		 * In theory 1 page is enough to keep the comsumer-producer

		 * pipe going: the flusher cleans 1 page => the task dirties 1

		 * more page. However bdi_dirty has accounting errors.  So use

		 * more page. However wb_dirty has accounting errors.  So use

		 * the larger and more IO friendly wb_stat_error.

		 */

		if (bdi_dirty <= wb_stat_error(wb))

		if (wb_dirty <= wb_stat_error(wb))

			break;

		if (fatal_signal_pending(current))