diff --git a/src/kernel/linux/v4.14/mm/page-writeback.c b/src/kernel/linux/v4.14/mm/page-writeback.c
new file mode 100644
index 0000000..29f9980
--- /dev/null
+++ b/src/kernel/linux/v4.14/mm/page-writeback.c
@@ -0,0 +1,2857 @@
+/*
+ * mm/page-writeback.c
+ *
+ * Copyright (C) 2002, Linus Torvalds.
+ * Copyright (C) 2007 Red Hat, Inc., Peter Zijlstra
+ *
+ * Contains functions related to writing back dirty pages at the
+ * address_space level.
+ *
+ * 10Apr2002	Andrew Morton
+ *		Initial version
+ */
+
+#include <linux/kernel.h>
+#include <linux/export.h>
+#include <linux/spinlock.h>
+#include <linux/fs.h>
+#include <linux/mm.h>
+#include <linux/swap.h>
+#include <linux/slab.h>
+#include <linux/pagemap.h>
+#include <linux/writeback.h>
+#include <linux/init.h>
+#include <linux/backing-dev.h>
+#include <linux/task_io_accounting_ops.h>
+#include <linux/blkdev.h>
+#include <linux/mpage.h>
+#include <linux/rmap.h>
+#include <linux/percpu.h>
+#include <linux/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/sched/signal.h>
+#include <linux/mm_inline.h>
+#include <trace/events/writeback.h>
+
+#include "internal.h"
+
+/*
+ * Sleep at most 200ms at a time in balance_dirty_pages().
+ */
+#define MAX_PAUSE		max(HZ/5, 1)
+
+/*
+ * Try to keep balance_dirty_pages() call intervals higher than this many pages
+ * by raising pause time to max_pause when falls below it.
+ */
+#define DIRTY_POLL_THRESH	(128 >> (PAGE_SHIFT - 10))
+
+/*
+ * Estimate write bandwidth at 200ms intervals.
+ */
+#define BANDWIDTH_INTERVAL	max(HZ/5, 1)
+
+#define RATELIMIT_CALC_SHIFT	10
+
+/*
+ * After a CPU has dirtied this many pages, balance_dirty_pages_ratelimited
+ * will look to see if it needs to force writeback or throttling.
+ */
+static long ratelimit_pages = 32;
+
+/* The following parameters are exported via /proc/sys/vm */
+
+/*
+ * Start background writeback (via writeback threads) at this percentage
+ */
+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 */
+
+struct wb_domain global_wb_domain;
+
+/* consolidated parameters for balance_dirty_pages() and its subroutines */
+struct dirty_throttle_control {
+#ifdef CONFIG_CGROUP_WRITEBACK
+	struct wb_domain	*dom;
+	struct dirty_throttle_control *gdtc;	/* only set in memcg dtc's */
+#endif
+	struct bdi_writeback	*wb;
+	struct fprop_local_percpu *wb_completions;
+
+	unsigned long		avail;		/* dirtyable */
+	unsigned long		dirty;		/* file_dirty + write + nfs */
+	unsigned long		thresh;		/* dirty threshold */
+	unsigned long		bg_thresh;	/* dirty background threshold */
+
+	unsigned long		wb_dirty;	/* per-wb counterparts */
+	unsigned long		wb_thresh;
+	unsigned long		wb_bg_thresh;
+
+	unsigned long		pos_ratio;
+};
+
+/*
+ * Length of period for aging writeout fractions of bdis. This is an
+ * arbitrarily chosen number. The longer the period, the slower fractions will
+ * reflect changes in current writeout rate.
+ */
+#define VM_COMPLETIONS_PERIOD_LEN (3*HZ)
+
+#ifdef CONFIG_CGROUP_WRITEBACK
+
+#define GDTC_INIT(__wb)		.wb = (__wb),				\
+				.dom = &global_wb_domain,		\
+				.wb_completions = &(__wb)->completions
+
+#define GDTC_INIT_NO_WB		.dom = &global_wb_domain
+
+#define MDTC_INIT(__wb, __gdtc)	.wb = (__wb),				\
+				.dom = mem_cgroup_wb_domain(__wb),	\
+				.wb_completions = &(__wb)->memcg_completions, \
+				.gdtc = __gdtc
+
+static bool mdtc_valid(struct dirty_throttle_control *dtc)
+{
+	return dtc->dom;
+}
+
+static struct wb_domain *dtc_dom(struct dirty_throttle_control *dtc)
+{
+	return dtc->dom;
+}
+
+static struct dirty_throttle_control *mdtc_gdtc(struct dirty_throttle_control *mdtc)
+{
+	return mdtc->gdtc;
+}
+
+static struct fprop_local_percpu *wb_memcg_completions(struct bdi_writeback *wb)
+{
+	return &wb->memcg_completions;
+}
+
+static void wb_min_max_ratio(struct bdi_writeback *wb,
+			     unsigned long *minp, unsigned long *maxp)
+{
+	unsigned long this_bw = wb->avg_write_bandwidth;
+	unsigned long tot_bw = atomic_long_read(&wb->bdi->tot_write_bandwidth);
+	unsigned long long min = wb->bdi->min_ratio;
+	unsigned long long max = wb->bdi->max_ratio;
+
+	/*
+	 * @wb may already be clean by the time control reaches here and
+	 * the total may not include its bw.
+	 */
+	if (this_bw < tot_bw) {
+		if (min) {
+			min *= this_bw;
+			min = div64_ul(min, tot_bw);
+		}
+		if (max < 100) {
+			max *= this_bw;
+			max = div64_ul(max, tot_bw);
+		}
+	}
+
+	*minp = min;
+	*maxp = max;
+}
+
+#else	/* CONFIG_CGROUP_WRITEBACK */
+
+#define GDTC_INIT(__wb)		.wb = (__wb),                           \
+				.wb_completions = &(__wb)->completions
+#define GDTC_INIT_NO_WB
+#define MDTC_INIT(__wb, __gdtc)
+
+static bool mdtc_valid(struct dirty_throttle_control *dtc)
+{
+	return false;
+}
+
+static struct wb_domain *dtc_dom(struct dirty_throttle_control *dtc)
+{
+	return &global_wb_domain;
+}
+
+static struct dirty_throttle_control *mdtc_gdtc(struct dirty_throttle_control *mdtc)
+{
+	return NULL;
+}
+
+static struct fprop_local_percpu *wb_memcg_completions(struct bdi_writeback *wb)
+{
+	return NULL;
+}
+
+static void wb_min_max_ratio(struct bdi_writeback *wb,
+			     unsigned long *minp, unsigned long *maxp)
+{
+	*minp = wb->bdi->min_ratio;
+	*maxp = wb->bdi->max_ratio;
+}
+
+#endif	/* CONFIG_CGROUP_WRITEBACK */
+
+/*
+ * In a memory zone, there is a certain amount of pages we consider
+ * available for the page cache, which is essentially the number of
+ * free and reclaimable pages, minus some zone reserves to protect
+ * lowmem and the ability to uphold the zone's watermarks without
+ * requiring writeback.
+ *
+ * This number of dirtyable pages is the base value of which the
+ * user-configurable dirty ratio is the 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.
+ */
+
+/**
+ * node_dirtyable_memory - number of dirtyable pages in a node
+ * @pgdat: the node
+ *
+ * Returns the node's number of pages potentially available for dirty
+ * page cache.  This is the base value for the per-node dirty limits.
+ */
+static unsigned long node_dirtyable_memory(struct pglist_data *pgdat)
+{
+	unsigned long nr_pages = 0;
+	int z;
+
+	for (z = 0; z < MAX_NR_ZONES; z++) {
+		struct zone *zone = pgdat->node_zones + z;
+
+		if (!populated_zone(zone))
+			continue;
+
+		nr_pages += zone_page_state(zone, NR_FREE_PAGES);
+	}
+
+	/*
+	 * Pages reserved for the kernel should not be considered
+	 * dirtyable, to prevent a situation where reclaim has to
+	 * clean pages in order to balance the zones.
+	 */
+	nr_pages -= min(nr_pages, pgdat->totalreserve_pages);
+
+	nr_pages += node_page_state(pgdat, NR_INACTIVE_FILE);
+	nr_pages += node_page_state(pgdat, NR_ACTIVE_FILE);
+
+	return nr_pages;
+}
+
+static unsigned long highmem_dirtyable_memory(unsigned long total)
+{
+#ifdef CONFIG_HIGHMEM
+	int node;
+	unsigned long x = 0;
+	int i;
+
+	for_each_node_state(node, N_HIGH_MEMORY) {
+		for (i = ZONE_NORMAL + 1; i < MAX_NR_ZONES; i++) {
+			struct zone *z;
+			unsigned long nr_pages;
+
+			if (!is_highmem_idx(i))
+				continue;
+
+			z = &NODE_DATA(node)->node_zones[i];
+			if (!populated_zone(z))
+				continue;
+
+			nr_pages = zone_page_state(z, NR_FREE_PAGES);
+			/* watch for underflows */
+			nr_pages -= min(nr_pages, high_wmark_pages(z));
+			nr_pages += zone_page_state(z, NR_ZONE_INACTIVE_FILE);
+			nr_pages += zone_page_state(z, NR_ZONE_ACTIVE_FILE);
+			x += nr_pages;
+		}
+	}
+
+	/*
+	 * 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_zone_page_state(NR_FREE_PAGES);
+	/*
+	 * Pages reserved for the kernel should not be considered
+	 * dirtyable, to prevent a situation where reclaim has to
+	 * clean pages in order to balance the zones.
+	 */
+	x -= min(x, totalreserve_pages);
+
+	x += global_node_page_state(NR_INACTIVE_FILE);
+	x += global_node_page_state(NR_ACTIVE_FILE);
+
+	if (!vm_highmem_is_dirtyable)
+		x -= highmem_dirtyable_memory(x);
+
+	return x + 1;	/* Ensure that we never return 0 */
+}
+
+/**
+ * domain_dirty_limits - calculate thresh and bg_thresh for a wb_domain
+ * @dtc: dirty_throttle_control of interest
+ *
+ * Calculate @dtc->thresh and ->bg_thresh considering
+ * vm_dirty_{bytes|ratio} and dirty_background_{bytes|ratio}.  The caller
+ * must ensure that @dtc->avail is set before calling this function.  The
+ * dirty limits will be lifted by 1/4 for PF_LESS_THROTTLE (ie. nfsd) and
+ * real-time tasks.
+ */
+static void domain_dirty_limits(struct dirty_throttle_control *dtc)
+{
+	const unsigned long available_memory = dtc->avail;
+	struct dirty_throttle_control *gdtc = mdtc_gdtc(dtc);
+	unsigned long bytes = vm_dirty_bytes;
+	unsigned long bg_bytes = dirty_background_bytes;
+	/* convert ratios to per-PAGE_SIZE for higher precision */
+	unsigned long ratio = (vm_dirty_ratio * PAGE_SIZE) / 100;
+	unsigned long bg_ratio = (dirty_background_ratio * PAGE_SIZE) / 100;
+	unsigned long thresh;
+	unsigned long bg_thresh;
+	struct task_struct *tsk;
+
+	/* gdtc is !NULL iff @dtc is for memcg domain */
+	if (gdtc) {
+		unsigned long global_avail = gdtc->avail;
+
+		/*
+		 * The byte settings can't be applied directly to memcg
+		 * domains.  Convert them to ratios by scaling against
+		 * globally available memory.  As the ratios are in
+		 * per-PAGE_SIZE, they can be obtained by dividing bytes by
+		 * number of pages.
+		 */
+		if (bytes)
+			ratio = min(DIV_ROUND_UP(bytes, global_avail),
+				    PAGE_SIZE);
+		if (bg_bytes)
+			bg_ratio = min(DIV_ROUND_UP(bg_bytes, global_avail),
+				       PAGE_SIZE);
+		bytes = bg_bytes = 0;
+	}
+
+	if (bytes)
+		thresh = DIV_ROUND_UP(bytes, PAGE_SIZE);
+	else
+		thresh = (ratio * available_memory) / PAGE_SIZE;
+
+	if (bg_bytes)
+		bg_thresh = DIV_ROUND_UP(bg_bytes, PAGE_SIZE);
+	else
+		bg_thresh = (bg_ratio * available_memory) / PAGE_SIZE;
+
+	if (bg_thresh >= thresh)
+		bg_thresh = thresh / 2;
+	tsk = current;
+	if (tsk->flags & PF_LESS_THROTTLE || rt_task(tsk)) {
+		bg_thresh += bg_thresh / 4 + global_wb_domain.dirty_limit / 32;
+		thresh += thresh / 4 + global_wb_domain.dirty_limit / 32;
+	}
+	dtc->thresh = thresh;
+	dtc->bg_thresh = bg_thresh;
+
+	/* we should eventually report the domain in the TP */
+	if (!gdtc)
+		trace_global_dirty_state(bg_thresh, thresh);
+}
+
+/**
+ * global_dirty_limits - background-writeback and dirty-throttling thresholds
+ * @pbackground: out parameter for bg_thresh
+ * @pdirty: out parameter for thresh
+ *
+ * Calculate bg_thresh and thresh for global_wb_domain.  See
+ * domain_dirty_limits() for details.
+ */
+void global_dirty_limits(unsigned long *pbackground, unsigned long *pdirty)
+{
+	struct dirty_throttle_control gdtc = { GDTC_INIT_NO_WB };
+
+	gdtc.avail = global_dirtyable_memory();
+	domain_dirty_limits(&gdtc);
+
+	*pbackground = gdtc.bg_thresh;
+	*pdirty = gdtc.thresh;
+}
+
+/**
+ * node_dirty_limit - maximum number of dirty pages allowed in a node
+ * @pgdat: the node
+ *
+ * Returns the maximum number of dirty pages allowed in a node, based
+ * on the node's dirtyable memory.
+ */
+static unsigned long node_dirty_limit(struct pglist_data *pgdat)
+{
+	unsigned long node_memory = node_dirtyable_memory(pgdat);
+	struct task_struct *tsk = current;
+	unsigned long dirty;
+
+	if (vm_dirty_bytes)
+		dirty = DIV_ROUND_UP(vm_dirty_bytes, PAGE_SIZE) *
+			node_memory / global_dirtyable_memory();
+	else
+		dirty = vm_dirty_ratio * node_memory / 100;
+
+	if (tsk->flags & PF_LESS_THROTTLE || rt_task(tsk))
+		dirty += dirty / 4;
+
+	return dirty;
+}
+
+/**
+ * node_dirty_ok - tells whether a node is within its dirty limits
+ * @pgdat: the node to check
+ *
+ * Returns %true when the dirty pages in @pgdat are within the node's
+ * dirty limit, %false if the limit is exceeded.
+ */
+bool node_dirty_ok(struct pglist_data *pgdat)
+{
+	unsigned long limit = node_dirty_limit(pgdat);
+	unsigned long nr_pages = 0;
+
+	nr_pages += node_page_state(pgdat, NR_FILE_DIRTY);
+	nr_pages += node_page_state(pgdat, NR_UNSTABLE_NFS);
+	nr_pages += node_page_state(pgdat, NR_WRITEBACK);
+
+	return nr_pages <= 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;
+}
+
+static void wb_domain_writeout_inc(struct wb_domain *dom,
+				   struct fprop_local_percpu *completions,
+				   unsigned int max_prop_frac)
+{
+	__fprop_inc_percpu_max(&dom->completions, completions,
+			       max_prop_frac);
+	/* First event after period switching was turned off? */
+	if (unlikely(!dom->period_time)) {
+		/*
+		 * We can race with other __bdi_writeout_inc calls here but
+		 * it does not cause any harm since the resulting time when
+		 * timer will fire and what is in writeout_period_time will be
+		 * roughly the same.
+		 */
+		dom->period_time = wp_next_time(jiffies);
+		mod_timer(&dom->period_timer, dom->period_time);
+	}
+}
+
+/*
+ * Increment @wb's writeout completion count and the global writeout
+ * completion count. Called from test_clear_page_writeback().
+ */
+static inline void __wb_writeout_inc(struct bdi_writeback *wb)
+{
+	struct wb_domain *cgdom;
+
+	inc_wb_stat(wb, WB_WRITTEN);
+	wb_domain_writeout_inc(&global_wb_domain, &wb->completions,
+			       wb->bdi->max_prop_frac);
+
+	cgdom = mem_cgroup_wb_domain(wb);
+	if (cgdom)
+		wb_domain_writeout_inc(cgdom, wb_memcg_completions(wb),
+				       wb->bdi->max_prop_frac);
+}
+
+void wb_writeout_inc(struct bdi_writeback *wb)
+{
+	unsigned long flags;
+
+	local_irq_save(flags);
+	__wb_writeout_inc(wb);
+	local_irq_restore(flags);
+}
+EXPORT_SYMBOL_GPL(wb_writeout_inc);
+
+/*
+ * On idle system, we can be called long after we scheduled because we use
+ * deferred timers so count with missed periods.
+ */
+static void writeout_period(unsigned long t)
+{
+	struct wb_domain *dom = (void *)t;
+	int miss_periods = (jiffies - dom->period_time) /
+						 VM_COMPLETIONS_PERIOD_LEN;
+
+	if (fprop_new_period(&dom->completions, miss_periods + 1)) {
+		dom->period_time = wp_next_time(dom->period_time +
+				miss_periods * VM_COMPLETIONS_PERIOD_LEN);
+		mod_timer(&dom->period_timer, dom->period_time);
+	} else {
+		/*
+		 * Aging has zeroed all fractions. Stop wasting CPU on period
+		 * updates.
+		 */
+		dom->period_time = 0;
+	}
+}
+
+int wb_domain_init(struct wb_domain *dom, gfp_t gfp)
+{
+	memset(dom, 0, sizeof(*dom));
+
+	spin_lock_init(&dom->lock);
+
+	setup_deferrable_timer(&dom->period_timer, writeout_period,
+			       (unsigned long)dom);
+
+	dom->dirty_limit_tstamp = jiffies;
+
+	return fprop_global_init(&dom->completions, gfp);
+}
+
+#ifdef CONFIG_CGROUP_WRITEBACK
+void wb_domain_exit(struct wb_domain *dom)
+{
+	del_timer_sync(&dom->period_timer);
+	fprop_global_destroy(&dom->completions);
+}
+#endif
+
+/*
+ * bdi_min_ratio keeps the sum of the minimum dirty shares of all
+ * registered backing devices, which, for obvious reasons, can not
+ * exceed 100%.
+ */
+static unsigned int bdi_min_ratio;
+
+int bdi_set_min_ratio(struct backing_dev_info *bdi, unsigned int min_ratio)
+{
+	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(struct wb_domain *dom,
+				      unsigned long thresh)
+{
+	return max(thresh, dom->dirty_limit);
+}
+
+/*
+ * Memory which can be further allocated to a memcg domain is capped by
+ * system-wide clean memory excluding the amount being used in the domain.
+ */
+static void mdtc_calc_avail(struct dirty_throttle_control *mdtc,
+			    unsigned long filepages, unsigned long headroom)
+{
+	struct dirty_throttle_control *gdtc = mdtc_gdtc(mdtc);
+	unsigned long clean = filepages - min(filepages, mdtc->dirty);
+	unsigned long global_clean = gdtc->avail - min(gdtc->avail, gdtc->dirty);
+	unsigned long other_clean = global_clean - min(global_clean, clean);
+
+	mdtc->avail = filepages + min(headroom, other_clean);
+}
+
+/**
+ * __wb_calc_thresh - @wb's share of dirty throttling threshold
+ * @dtc: dirty_throttle_context of interest
+ *
+ * Returns @wb'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 wb dirty pages go high.
+ *
+ * It allocates high/low dirty limits to fast/slow devices, in order to prevent
+ * - starving fast devices
+ * - piling up dirty pages (that will take long time to sync) on slow devices
+ *
+ * The wb's share of dirty limit will be adapting to its throughput and
+ * bounded by the bdi->min_ratio and/or bdi->max_ratio parameters, if set.
+ */
+static unsigned long __wb_calc_thresh(struct dirty_throttle_control *dtc)
+{
+	struct wb_domain *dom = dtc_dom(dtc);
+	unsigned long thresh = dtc->thresh;
+	u64 wb_thresh;
+	long numerator, denominator;
+	unsigned long wb_min_ratio, wb_max_ratio;
+
+	/*
+	 * Calculate this BDI's share of the thresh ratio.
+	 */
+	fprop_fraction_percpu(&dom->completions, dtc->wb_completions,
+			      &numerator, &denominator);
+
+	wb_thresh = (thresh * (100 - bdi_min_ratio)) / 100;
+	wb_thresh *= numerator;
+	do_div(wb_thresh, denominator);
+
+	wb_min_max_ratio(dtc->wb, &wb_min_ratio, &wb_max_ratio);
+
+	wb_thresh += (thresh * wb_min_ratio) / 100;
+	if (wb_thresh > (thresh * wb_max_ratio) / 100)
+		wb_thresh = thresh * wb_max_ratio / 100;
+
+	return wb_thresh;
+}
+
+unsigned long wb_calc_thresh(struct bdi_writeback *wb, unsigned long thresh)
+{
+	struct dirty_throttle_control gdtc = { GDTC_INIT(wb),
+					       .thresh = thresh };
+	return __wb_calc_thresh(&gdtc);
+}
+
+/*
+ *                           setpoint - dirty 3
+ *        f(dirty) := 1.0 + (----------------)
+ *                           limit - setpoint
+ *
+ * it's a 3rd order polynomial that subjects to
+ *
+ * (1) f(freerun)  = 2.0 => rampup dirty_ratelimit reasonably fast
+ * (2) f(setpoint) = 1.0 => the balance point
+ * (3) f(limit)    = 0   => the hard limit
+ * (4) df/dx      <= 0	 => negative feedback control
+ * (5) the closer to setpoint, the smaller |df/dx| (and the reverse)
+ *     => fast response on large errors; small oscillation near setpoint
+ */
+static long long pos_ratio_polynom(unsigned long setpoint,
+					  unsigned long dirty,
+					  unsigned long limit)
+{
+	long long pos_ratio;
+	long x;
+
+	x = div64_s64(((s64)setpoint - (s64)dirty) << RATELIMIT_CALC_SHIFT,
+		      (limit - setpoint) | 1);
+	pos_ratio = x;
+	pos_ratio = pos_ratio * x >> RATELIMIT_CALC_SHIFT;
+	pos_ratio = pos_ratio * x >> RATELIMIT_CALC_SHIFT;
+	pos_ratio += 1 << RATELIMIT_CALC_SHIFT;
+
+	return clamp(pos_ratio, 0LL, 2LL << RATELIMIT_CALC_SHIFT);
+}
+
+/*
+ * Dirty position control.
+ *
+ * (o) global/bdi setpoints
+ *
+ * We want the dirty pages be balanced around the global/wb setpoints.
+ * When the number of dirty pages is higher/lower than the setpoint, the
+ * dirty position control ratio (and hence task dirty ratelimit) will be
+ * decreased/increased to bring the dirty pages back to the setpoint.
+ *
+ *     pos_ratio = 1 << RATELIMIT_CALC_SHIFT
+ *
+ *     if (dirty < setpoint) scale up   pos_ratio
+ *     if (dirty > setpoint) scale down pos_ratio
+ *
+ *     if (wb_dirty < wb_setpoint) scale up   pos_ratio
+ *     if (wb_dirty > wb_setpoint) scale down pos_ratio
+ *
+ *     task_ratelimit = dirty_ratelimit * pos_ratio >> RATELIMIT_CALC_SHIFT
+ *
+ * (o) global control line
+ *
+ *     ^ pos_ratio
+ *     |
+ *     |            |<===== global dirty control scope ======>|
+ * 2.0 .............*
+ *     |            .*
+ *     |            . *
+ *     |            .   *
+ *     |            .     *
+ *     |            .        *
+ *     |            .            *
+ * 1.0 ................................*
+ *     |            .                  .     *
+ *     |            .                  .          *
+ *     |            .                  .              *
+ *     |            .                  .                 *
+ *     |            .                  .                    *
+ *   0 +------------.------------------.----------------------*------------->
+ *           freerun^          setpoint^                 limit^   dirty pages
+ *
+ * (o) wb control line
+ *
+ *     ^ pos_ratio
+ *     |
+ *     |            *
+ *     |              *
+ *     |                *
+ *     |                  *
+ *     |                    * |<=========== span ============>|
+ * 1.0 .......................*
+ *     |                      . *
+ *     |                      .   *
+ *     |                      .     *
+ *     |                      .       *
+ *     |                      .         *
+ *     |                      .           *
+ *     |                      .             *
+ *     |                      .               *
+ *     |                      .                 *
+ *     |                      .                   *
+ *     |                      .                     *
+ * 1/4 ...............................................* * * * * * * * * * * *
+ *     |                      .                         .
+ *     |                      .                           .
+ *     |                      .                             .
+ *   0 +----------------------.-------------------------------.------------->
+ *                wb_setpoint^                    x_intercept^
+ *
+ * The wb control line won't drop below pos_ratio=1/4, so that wb_dirty can
+ * be smoothly throttled down to normal if it starts high in situations like
+ * - start writing to a slow SD card and a fast disk at the same time. The SD
+ *   card's wb_dirty may rush to many times higher than wb_setpoint.
+ * - the wb dirty thresh drops quickly due to change of JBOD workload
+ */
+static void wb_position_ratio(struct dirty_throttle_control *dtc)
+{
+	struct bdi_writeback *wb = dtc->wb;
+	unsigned long write_bw = wb->avg_write_bandwidth;
+	unsigned long freerun = dirty_freerun_ceiling(dtc->thresh, dtc->bg_thresh);
+	unsigned long limit = hard_dirty_limit(dtc_dom(dtc), dtc->thresh);
+	unsigned long wb_thresh = dtc->wb_thresh;
+	unsigned long x_intercept;
+	unsigned long setpoint;		/* dirty pages' target balance point */
+	unsigned long wb_setpoint;
+	unsigned long span;
+	long long pos_ratio;		/* for scaling up/down the rate limit */
+	long x;
+
+	dtc->pos_ratio = 0;
+
+	if (unlikely(dtc->dirty >= limit))
+		return;
+
+	/*
+	 * global setpoint
+	 *
+	 * See comment for pos_ratio_polynom().
+	 */
+	setpoint = (freerun + limit) / 2;
+	pos_ratio = pos_ratio_polynom(setpoint, dtc->dirty, limit);
+
+	/*
+	 * The strictlimit feature is a tool preventing mistrusted filesystems
+	 * from growing a large number of dirty pages before throttling. For
+	 * such filesystems balance_dirty_pages always checks wb counters
+	 * against wb limits. Even if global "nr_dirty" is under "freerun".
+	 * This is especially important for fuse which sets bdi->max_ratio to
+	 * 1% by default. Without strictlimit feature, fuse writeback may
+	 * consume arbitrary amount of RAM because it is accounted in
+	 * NR_WRITEBACK_TEMP which is not involved in calculating "nr_dirty".
+	 *
+	 * Here, in wb_position_ratio(), we calculate pos_ratio based on
+	 * two values: wb_dirty and wb_thresh. Let's consider an example:
+	 * total amount of RAM is 16GB, bdi->max_ratio is equal to 1%, global
+	 * limits are set by default to 10% and 20% (background and throttle).
+	 * Then wb_thresh is 1% of 20% of 16GB. This amounts to ~8K pages.
+	 * wb_calc_thresh(wb, bg_thresh) is about ~4K pages. wb_setpoint is
+	 * about ~6K pages (as the average of background and throttle wb
+	 * limits). The 3rd order polynomial will provide positive feedback if
+	 * wb_dirty is under wb_setpoint and vice versa.
+	 *
+	 * Note, that we cannot use global counters in these calculations
+	 * because we want to throttle process writing to a strictlimit wb
+	 * much earlier than global "freerun" is reached (~23MB vs. ~2.3GB
+	 * in the example above).
+	 */
+	if (unlikely(wb->bdi->capabilities & BDI_CAP_STRICTLIMIT)) {
+		long long wb_pos_ratio;
+
+		if (dtc->wb_dirty < 8) {
+			dtc->pos_ratio = min_t(long long, pos_ratio * 2,
+					   2 << RATELIMIT_CALC_SHIFT);
+			return;
+		}
+
+		if (dtc->wb_dirty >= wb_thresh)
+			return;
+
+		wb_setpoint = dirty_freerun_ceiling(wb_thresh,
+						    dtc->wb_bg_thresh);
+
+		if (wb_setpoint == 0 || wb_setpoint == wb_thresh)
+			return;
+
+		wb_pos_ratio = pos_ratio_polynom(wb_setpoint, dtc->wb_dirty,
+						 wb_thresh);
+
+		/*
+		 * Typically, for strictlimit case, wb_setpoint << setpoint
+		 * and pos_ratio >> wb_pos_ratio. In the other words global
+		 * state ("dirty") is not limiting factor and we have to
+		 * make decision based on wb counters. But there is an
+		 * important case when global pos_ratio should get precedence:
+		 * global limits are exceeded (e.g. due to activities on other
+		 * wb's) while given strictlimit wb is below limit.
+		 *
+		 * "pos_ratio * wb_pos_ratio" would work for the case above,
+		 * but it would look too non-natural for the case of all
+		 * activity in the system coming from a single strictlimit wb
+		 * with bdi->max_ratio == 100%.
+		 *
+		 * Note that min() below somewhat changes the dynamics of the
+		 * control system. Normally, pos_ratio value can be well over 3
+		 * (when globally we are at freerun and wb is well below wb
+		 * setpoint). Now the maximum pos_ratio in the same situation
+		 * is 2. We might want to tweak this if we observe the control
+		 * system is too slow to adapt.
+		 */
+		dtc->pos_ratio = min(pos_ratio, wb_pos_ratio);
+		return;
+	}
+
+	/*
+	 * We have computed basic pos_ratio above based on global situation. If
+	 * the wb is over/under its share of dirty pages, we want to scale
+	 * pos_ratio further down/up. That is done by the following mechanism.
+	 */
+
+	/*
+	 * wb setpoint
+	 *
+	 *        f(wb_dirty) := 1.0 + k * (wb_dirty - wb_setpoint)
+	 *
+	 *                        x_intercept - wb_dirty
+	 *                     := --------------------------
+	 *                        x_intercept - wb_setpoint
+	 *
+	 * The main wb control line is a linear function that subjects to
+	 *
+	 * (1) f(wb_setpoint) = 1.0
+	 * (2) k = - 1 / (8 * write_bw)  (in single wb case)
+	 *     or equally: x_intercept = wb_setpoint + 8 * write_bw
+	 *
+	 * For single wb case, the dirty pages are observed to fluctuate
+	 * regularly within range
+	 *        [wb_setpoint - write_bw/2, wb_setpoint + write_bw/2]
+	 * for various filesystems, where (2) can yield in a reasonable 12.5%
+	 * fluctuation range for pos_ratio.
+	 *
+	 * For JBOD case, wb_thresh (not wb_dirty!) could fluctuate up to its
+	 * own size, so move the slope over accordingly and choose a slope that
+	 * yields 100% pos_ratio fluctuation on suddenly doubled wb_thresh.
+	 */
+	if (unlikely(wb_thresh > dtc->thresh))
+		wb_thresh = dtc->thresh;
+	/*
+	 * It's very possible that wb_thresh is close to 0 not because the
+	 * device is slow, but that it has remained inactive for long time.
+	 * Honour such devices a reasonable good (hopefully IO efficient)
+	 * threshold, so that the occasional writes won't be blocked and active
+	 * writes can rampup the threshold quickly.
+	 */
+	wb_thresh = max(wb_thresh, (limit - dtc->dirty) / 8);
+	/*
+	 * scale global setpoint to wb's:
+	 *	wb_setpoint = setpoint * wb_thresh / thresh
+	 */
+	x = div_u64((u64)wb_thresh << 16, dtc->thresh | 1);
+	wb_setpoint = setpoint * (u64)x >> 16;
+	/*
+	 * Use span=(8*write_bw) in single wb case as indicated by
+	 * (thresh - wb_thresh ~= 0) and transit to wb_thresh in JBOD case.
+	 *
+	 *        wb_thresh                    thresh - wb_thresh
+	 * span = --------- * (8 * write_bw) + ------------------ * wb_thresh
+	 *         thresh                           thresh
+	 */
+	span = (dtc->thresh - wb_thresh + 8 * write_bw) * (u64)x >> 16;
+	x_intercept = wb_setpoint + span;
+
+	if (dtc->wb_dirty < x_intercept - span / 4) {
+		pos_ratio = div64_u64(pos_ratio * (x_intercept - dtc->wb_dirty),
+				      (x_intercept - wb_setpoint) | 1);
+	} else
+		pos_ratio /= 4;
+
+	/*
+	 * wb reserve area, safeguard against dirty pool underrun and disk idle
+	 * It may push the desired control point of global dirty pages higher
+	 * than setpoint.
+	 */
+	x_intercept = wb_thresh / 2;
+	if (dtc->wb_dirty < x_intercept) {
+		if (dtc->wb_dirty > x_intercept / 8)
+			pos_ratio = div_u64(pos_ratio * x_intercept,
+					    dtc->wb_dirty);
+		else
+			pos_ratio *= 8;
+	}
+
+	dtc->pos_ratio = pos_ratio;
+}
+
+static void wb_update_write_bandwidth(struct bdi_writeback *wb,
+				      unsigned long elapsed,
+				      unsigned long written)
+{
+	const unsigned long period = roundup_pow_of_two(3 * HZ);
+	unsigned long avg = wb->avg_write_bandwidth;
+	unsigned long old = wb->write_bandwidth;
+	u64 bw;
+
+	/*
+	 * bw = written * HZ / elapsed
+	 *
+	 *                   bw * elapsed + write_bandwidth * (period - elapsed)
+	 * write_bandwidth = ---------------------------------------------------
+	 *                                          period
+	 *
+	 * @written may have decreased due to account_page_redirty().
+	 * Avoid underflowing @bw calculation.
+	 */
+	bw = written - min(written, wb->written_stamp);
+	bw *= HZ;
+	if (unlikely(elapsed > period)) {
+		do_div(bw, elapsed);
+		avg = bw;
+		goto out;
+	}
+	bw += (u64)wb->write_bandwidth * (period - elapsed);
+	bw >>= ilog2(period);
+
+	/*
+	 * one more level of smoothing, for filtering out sudden spikes
+	 */
+	if (avg > old && old >= (unsigned long)bw)
+		avg -= (avg - old) >> 3;
+
+	if (avg < old && old <= (unsigned long)bw)
+		avg += (old - avg) >> 3;
+
+out:
+	/* keep avg > 0 to guarantee that tot > 0 if there are dirty wbs */
+	avg = max(avg, 1LU);
+	if (wb_has_dirty_io(wb)) {
+		long delta = avg - wb->avg_write_bandwidth;
+		WARN_ON_ONCE(atomic_long_add_return(delta,
+					&wb->bdi->tot_write_bandwidth) <= 0);
+	}
+	wb->write_bandwidth = bw;
+	wb->avg_write_bandwidth = avg;
+}
+
+static void update_dirty_limit(struct dirty_throttle_control *dtc)
+{
+	struct wb_domain *dom = dtc_dom(dtc);
+	unsigned long thresh = dtc->thresh;
+	unsigned long limit = dom->dirty_limit;
+
+	/*
+	 * Follow up in one step.
+	 */
+	if (limit < thresh) {
+		limit = thresh;
+		goto update;
+	}
+
+	/*
+	 * Follow down slowly. Use the higher one as the target, because thresh
+	 * may drop below dirty. This is exactly the reason to introduce
+	 * dom->dirty_limit which is guaranteed to lie above the dirty pages.
+	 */
+	thresh = max(thresh, dtc->dirty);
+	if (limit > thresh) {
+		limit -= (limit - thresh) >> 5;
+		goto update;
+	}
+	return;
+update:
+	dom->dirty_limit = limit;
+}
+
+static void domain_update_bandwidth(struct dirty_throttle_control *dtc,
+				    unsigned long now)
+{
+	struct wb_domain *dom = dtc_dom(dtc);
+
+	/*
+	 * check locklessly first to optimize away locking for the most time
+	 */
+	if (time_before(now, dom->dirty_limit_tstamp + BANDWIDTH_INTERVAL))
+		return;
+
+	spin_lock(&dom->lock);
+	if (time_after_eq(now, dom->dirty_limit_tstamp + BANDWIDTH_INTERVAL)) {
+		update_dirty_limit(dtc);
+		dom->dirty_limit_tstamp = now;
+	}
+	spin_unlock(&dom->lock);
+}
+
+/*
+ * Maintain wb->dirty_ratelimit, the base dirty throttle rate.
+ *
+ * Normal wb tasks will be curbed at or below it in long term.
+ * Obviously it should be around (write_bw / N) when there are N dd tasks.
+ */
+static void wb_update_dirty_ratelimit(struct dirty_throttle_control *dtc,
+				      unsigned long dirtied,
+				      unsigned long elapsed)
+{
+	struct bdi_writeback *wb = dtc->wb;
+	unsigned long dirty = dtc->dirty;
+	unsigned long freerun = dirty_freerun_ceiling(dtc->thresh, dtc->bg_thresh);
+	unsigned long limit = hard_dirty_limit(dtc_dom(dtc), dtc->thresh);
+	unsigned long setpoint = (freerun + limit) / 2;
+	unsigned long write_bw = wb->avg_write_bandwidth;
+	unsigned long dirty_ratelimit = wb->dirty_ratelimit;
+	unsigned long dirty_rate;
+	unsigned long task_ratelimit;
+	unsigned long balanced_dirty_ratelimit;
+	unsigned long step;
+	unsigned long x;
+	unsigned long shift;
+
+	/*
+	 * The dirty rate will match the writeout rate in long term, except
+	 * when dirty pages are truncated by userspace or re-dirtied by FS.
+	 */
+	dirty_rate = (dirtied - wb->dirtied_stamp) * HZ / elapsed;
+
+	/*
+	 * task_ratelimit reflects each dd's dirty rate for the past 200ms.
+	 */
+	task_ratelimit = (u64)dirty_ratelimit *
+					dtc->pos_ratio >> RATELIMIT_CALC_SHIFT;
+	task_ratelimit++; /* it helps rampup dirty_ratelimit from tiny values */
+
+	/*
+	 * A linear estimation of the "balanced" throttle rate. The theory is,
+	 * if there are N dd tasks, each throttled at task_ratelimit, the wb's
+	 * dirty_rate will be measured to be (N * task_ratelimit). So the below
+	 * formula will yield the balanced rate limit (write_bw / N).
+	 *
+	 * Note that the expanded form is not a pure rate feedback:
+	 *	rate_(i+1) = rate_(i) * (write_bw / dirty_rate)		     (1)
+	 * but also takes pos_ratio into account:
+	 *	rate_(i+1) = rate_(i) * (write_bw / dirty_rate) * pos_ratio  (2)
+	 *
+	 * (1) is not realistic because pos_ratio also takes part in balancing
+	 * the dirty rate.  Consider the state
+	 *	pos_ratio = 0.5						     (3)
+	 *	rate = 2 * (write_bw / N)				     (4)
+	 * If (1) is used, it will stuck in that state! Because each dd will
+	 * be throttled at
+	 *	task_ratelimit = pos_ratio * rate = (write_bw / N)	     (5)
+	 * yielding
+	 *	dirty_rate = N * task_ratelimit = write_bw		     (6)
+	 * put (6) into (1) we get
+	 *	rate_(i+1) = rate_(i)					     (7)
+	 *
+	 * So we end up using (2) to always keep
+	 *	rate_(i+1) ~= (write_bw / N)				     (8)
+	 * regardless of the value of pos_ratio. As long as (8) is satisfied,
+	 * pos_ratio is able to drive itself to 1.0, which is not only where
+	 * the dirty count meet the setpoint, but also where the slope of
+	 * pos_ratio is most flat and hence task_ratelimit is least fluctuated.
+	 */
+	balanced_dirty_ratelimit = div_u64((u64)task_ratelimit * write_bw,
+					   dirty_rate | 1);
+	/*
+	 * balanced_dirty_ratelimit ~= (write_bw / N) <= write_bw
+	 */
+	if (unlikely(balanced_dirty_ratelimit > write_bw))
+		balanced_dirty_ratelimit = write_bw;
+
+	/*
+	 * We could safely do this and return immediately:
+	 *
+	 *	wb->dirty_ratelimit = balanced_dirty_ratelimit;
+	 *
+	 * However to get a more stable dirty_ratelimit, the below elaborated
+	 * code makes use of task_ratelimit to filter out singular points and
+	 * limit the step size.
+	 *
+	 * The below code essentially only uses the relative value of
+	 *
+	 *	task_ratelimit - dirty_ratelimit
+	 *	= (pos_ratio - 1) * dirty_ratelimit
+	 *
+	 * which reflects the direction and size of dirty position error.
+	 */
+
+	/*
+	 * dirty_ratelimit will follow balanced_dirty_ratelimit iff
+	 * task_ratelimit is on the same side of dirty_ratelimit, too.
+	 * For example, when
+	 * - dirty_ratelimit > balanced_dirty_ratelimit
+	 * - dirty_ratelimit > task_ratelimit (dirty pages are above setpoint)
+	 * lowering dirty_ratelimit will help meet both the position and rate
+	 * control targets. Otherwise, don't update dirty_ratelimit if it will
+	 * only help meet the rate target. After all, what the users ultimately
+	 * feel and care are stable dirty rate and small position error.
+	 *
+	 * |task_ratelimit - dirty_ratelimit| is used to limit the step size
+	 * and filter out the singular points of balanced_dirty_ratelimit. Which
+	 * keeps jumping around randomly and can even leap far away at times
+	 * due to the small 200ms estimation period of dirty_rate (we want to
+	 * keep that period small to reduce time lags).
+	 */
+	step = 0;
+
+	/*
+	 * For strictlimit case, calculations above were based on wb counters
+	 * and limits (starting from pos_ratio = wb_position_ratio() and up to
+	 * balanced_dirty_ratelimit = task_ratelimit * write_bw / dirty_rate).
+	 * Hence, to calculate "step" properly, we have to use wb_dirty as
+	 * "dirty" and wb_setpoint as "setpoint".
+	 *
+	 * We rampup dirty_ratelimit forcibly if wb_dirty is low because
+	 * it's possible that wb_thresh is close to zero due to inactivity
+	 * of backing device.
+	 */
+	if (unlikely(wb->bdi->capabilities & BDI_CAP_STRICTLIMIT)) {
+		dirty = dtc->wb_dirty;
+		if (dtc->wb_dirty < 8)
+			setpoint = dtc->wb_dirty + 1;
+		else
+			setpoint = (dtc->wb_thresh + dtc->wb_bg_thresh) / 2;
+	}
+
+	if (dirty < setpoint) {
+		x = min3(wb->balanced_dirty_ratelimit,
+			 balanced_dirty_ratelimit, task_ratelimit);
+		if (dirty_ratelimit < x)
+			step = x - dirty_ratelimit;
+	} else {
+		x = max3(wb->balanced_dirty_ratelimit,
+			 balanced_dirty_ratelimit, task_ratelimit);
+		if (dirty_ratelimit > x)
+			step = dirty_ratelimit - x;
+	}
+
+	/*
+	 * Don't pursue 100% rate matching. It's impossible since the balanced
+	 * rate itself is constantly fluctuating. So decrease the track speed
+	 * when it gets close to the target. Helps eliminate pointless tremors.
+	 */
+	shift = dirty_ratelimit / (2 * step + 1);
+	if (shift < BITS_PER_LONG)
+		step = DIV_ROUND_UP(step >> shift, 8);
+	else
+		step = 0;
+
+	if (dirty_ratelimit < balanced_dirty_ratelimit)
+		dirty_ratelimit += step;
+	else
+		dirty_ratelimit -= step;
+
+	wb->dirty_ratelimit = max(dirty_ratelimit, 1UL);
+	wb->balanced_dirty_ratelimit = balanced_dirty_ratelimit;
+
+	trace_bdi_dirty_ratelimit(wb, dirty_rate, task_ratelimit);
+}
+
+static void __wb_update_bandwidth(struct dirty_throttle_control *gdtc,
+				  struct dirty_throttle_control *mdtc,
+				  unsigned long start_time,
+				  bool update_ratelimit)
+{
+	struct bdi_writeback *wb = gdtc->wb;
+	unsigned long now = jiffies;
+	unsigned long elapsed = now - wb->bw_time_stamp;
+	unsigned long dirtied;
+	unsigned long written;
+
+	lockdep_assert_held(&wb->list_lock);
+
+	/*
+	 * rate-limit, only update once every 200ms.
+	 */
+	if (elapsed < BANDWIDTH_INTERVAL)
+		return;
+
+	dirtied = percpu_counter_read(&wb->stat[WB_DIRTIED]);
+	written = percpu_counter_read(&wb->stat[WB_WRITTEN]);
+
+	/*
+	 * Skip quiet periods when disk bandwidth is under-utilized.
+	 * (at least 1s idle time between two flusher runs)
+	 */
+	if (elapsed > HZ && time_before(wb->bw_time_stamp, start_time))
+		goto snapshot;
+
+	if (update_ratelimit) {
+		domain_update_bandwidth(gdtc, now);
+		wb_update_dirty_ratelimit(gdtc, dirtied, elapsed);
+
+		/*
+		 * @mdtc is always NULL if !CGROUP_WRITEBACK but the
+		 * compiler has no way to figure that out.  Help it.
+		 */
+		if (IS_ENABLED(CONFIG_CGROUP_WRITEBACK) && mdtc) {
+			domain_update_bandwidth(mdtc, now);
+			wb_update_dirty_ratelimit(mdtc, dirtied, elapsed);
+		}
+	}
+	wb_update_write_bandwidth(wb, elapsed, written);
+
+snapshot:
+	wb->dirtied_stamp = dirtied;
+	wb->written_stamp = written;
+	wb->bw_time_stamp = now;
+}
+
+void wb_update_bandwidth(struct bdi_writeback *wb, unsigned long start_time)
+{
+	struct dirty_throttle_control gdtc = { GDTC_INIT(wb) };
+
+	__wb_update_bandwidth(&gdtc, NULL, start_time, false);
+}
+
+/*
+ * After a task dirtied this many pages, balance_dirty_pages_ratelimited()
+ * will look to see if it needs to start dirty throttling.
+ *
+ * If dirty_poll_interval is too low, big NUMA machines will call the expensive
+ * global_zone_page_state() too often. So scale it near-sqrt to the safety margin
+ * (the number of pages we may dirty without exceeding the dirty limits).
+ */
+static unsigned long dirty_poll_interval(unsigned long dirty,
+					 unsigned long thresh)
+{
+	if (thresh > dirty)
+		return 1UL << (ilog2(thresh - dirty) >> 1);
+
+	return 1;
+}
+
+static unsigned long wb_max_pause(struct bdi_writeback *wb,
+				  unsigned long wb_dirty)
+{
+	unsigned long bw = wb->avg_write_bandwidth;
+	unsigned long t;
+
+	/*
+	 * Limit pause time for small memory systems. If sleeping for too long
+	 * time, a small pool of dirty/writeback pages may go empty and disk go
+	 * idle.
+	 *
+	 * 8 serves as the safety ratio.
+	 */
+	t = wb_dirty / (1 + bw / roundup_pow_of_two(1 + HZ / 8));
+	t++;
+
+	return min_t(unsigned long, t, MAX_PAUSE);
+}
+
+static long wb_min_pause(struct bdi_writeback *wb,
+			 long max_pause,
+			 unsigned long task_ratelimit,
+			 unsigned long dirty_ratelimit,
+			 int *nr_dirtied_pause)
+{
+	long hi = ilog2(wb->avg_write_bandwidth);
+	long lo = ilog2(wb->dirty_ratelimit);
+	long t;		/* target pause */
+	long pause;	/* estimated next pause */
+	int pages;	/* target nr_dirtied_pause */
+
+	/* target for 10ms pause on 1-dd case */
+	t = max(1, HZ / 100);
+
+	/*
+	 * Scale up pause time for concurrent dirtiers in order to reduce CPU
+	 * overheads.
+	 *
+	 * (N * 10ms) on 2^N concurrent tasks.
+	 */
+	if (hi > lo)
+		t += (hi - lo) * (10 * HZ) / 1024;
+
+	/*
+	 * This is a bit convoluted. We try to base the next nr_dirtied_pause
+	 * on the much more stable dirty_ratelimit. However the next pause time
+	 * will be computed based on task_ratelimit and the two rate limits may
+	 * depart considerably at some time. Especially if task_ratelimit goes
+	 * below dirty_ratelimit/2 and the target pause is max_pause, the next
+	 * pause time will be max_pause*2 _trimmed down_ to max_pause.  As a
+	 * result task_ratelimit won't be executed faithfully, which could
+	 * eventually bring down dirty_ratelimit.
+	 *
+	 * We apply two rules to fix it up:
+	 * 1) try to estimate the next pause time and if necessary, use a lower
+	 *    nr_dirtied_pause so as not to exceed max_pause. When this happens,
+	 *    nr_dirtied_pause will be "dancing" with task_ratelimit.
+	 * 2) limit the target pause time to max_pause/2, so that the normal
+	 *    small fluctuations of task_ratelimit won't trigger rule (1) and
+	 *    nr_dirtied_pause will remain as stable as dirty_ratelimit.
+	 */
+	t = min(t, 1 + max_pause / 2);
+	pages = dirty_ratelimit * t / roundup_pow_of_two(HZ);
+
+	/*
+	 * Tiny nr_dirtied_pause is found to hurt I/O performance in the test
+	 * case fio-mmap-randwrite-64k, which does 16*{sync read, async write}.
+	 * When the 16 consecutive reads are often interrupted by some dirty
+	 * throttling pause during the async writes, cfq will go into idles
+	 * (deadline is fine). So push nr_dirtied_pause as high as possible
+	 * until reaches DIRTY_POLL_THRESH=32 pages.
+	 */
+	if (pages < DIRTY_POLL_THRESH) {
+		t = max_pause;
+		pages = dirty_ratelimit * t / roundup_pow_of_two(HZ);
+		if (pages > DIRTY_POLL_THRESH) {
+			pages = DIRTY_POLL_THRESH;
+			t = HZ * DIRTY_POLL_THRESH / dirty_ratelimit;
+		}
+	}
+
+	pause = HZ * pages / (task_ratelimit + 1);
+	if (pause > max_pause) {
+		t = max_pause;
+		pages = task_ratelimit * t / roundup_pow_of_two(HZ);
+	}
+
+	*nr_dirtied_pause = pages;
+	/*
+	 * The minimal pause time will normally be half the target pause time.
+	 */
+	return pages >= DIRTY_POLL_THRESH ? 1 + t / 2 : t;
+}
+
+static inline void wb_dirty_limits(struct dirty_throttle_control *dtc)
+{
+	struct bdi_writeback *wb = dtc->wb;
+	unsigned long wb_reclaimable;
+
+	/*
+	 * wb_thresh is not treated as some limiting factor as
+	 * dirty_thresh, due to reasons
+	 * - in JBOD setup, wb_thresh can fluctuate a lot
+	 * - in a system with HDD and USB key, the USB key may somehow
+	 *   go into state (wb_dirty >> wb_thresh) either because
+	 *   wb_dirty starts high, or because wb_thresh drops low.
+	 *   In this case we don't want to hard throttle the USB key
+	 *   dirtiers for 100 seconds until wb_dirty drops under
+	 *   wb_thresh. Instead the auxiliary wb control line in
+	 *   wb_position_ratio() will let the dirtier task progress
+	 *   at some rate <= (write_bw / 2) for bringing down wb_dirty.
+	 */
+	dtc->wb_thresh = __wb_calc_thresh(dtc);
+	dtc->wb_bg_thresh = dtc->thresh ?
+		div_u64((u64)dtc->wb_thresh * dtc->bg_thresh, dtc->thresh) : 0;
+
+	/*
+	 * In order to avoid the stacked BDI deadlock we need
+	 * to ensure we accurately count the 'dirty' pages when
+	 * the threshold is low.
+	 *
+	 * Otherwise it would be possible to get thresh+n pages
+	 * reported dirty, even though there are thresh-m pages
+	 * actually dirty; with m+n sitting in the percpu
+	 * deltas.
+	 */
+	if (dtc->wb_thresh < 2 * wb_stat_error(wb)) {
+		wb_reclaimable = wb_stat_sum(wb, WB_RECLAIMABLE);
+		dtc->wb_dirty = wb_reclaimable + wb_stat_sum(wb, WB_WRITEBACK);
+	} else {
+		wb_reclaimable = wb_stat(wb, WB_RECLAIMABLE);
+		dtc->wb_dirty = wb_reclaimable + wb_stat(wb, WB_WRITEBACK);
+	}
+}
+
+/*
+ * balance_dirty_pages() must be called by processes which are generating dirty
+ * data.  It looks at the number of dirty pages in the machine and will force
+ * the caller to wait once crossing the (background_thresh + dirty_thresh) / 2.
+ * If we're over `background_thresh' then the writeback threads are woken to
+ * perform some writeout.
+ */
+static void balance_dirty_pages(struct address_space *mapping,
+				struct bdi_writeback *wb,
+				unsigned long pages_dirtied)
+{
+	struct dirty_throttle_control gdtc_stor = { GDTC_INIT(wb) };
+	struct dirty_throttle_control mdtc_stor = { MDTC_INIT(wb, &gdtc_stor) };
+	struct dirty_throttle_control * const gdtc = &gdtc_stor;
+	struct dirty_throttle_control * const mdtc = mdtc_valid(&mdtc_stor) ?
+						     &mdtc_stor : NULL;
+	struct dirty_throttle_control *sdtc;
+	unsigned long nr_reclaimable;	/* = file_dirty + unstable_nfs */
+	long period;
+	long pause;
+	long max_pause;
+	long min_pause;
+	int nr_dirtied_pause;
+	bool dirty_exceeded = false;
+	unsigned long task_ratelimit;
+	unsigned long dirty_ratelimit;
+	struct backing_dev_info *bdi = wb->bdi;
+	bool strictlimit = bdi->capabilities & BDI_CAP_STRICTLIMIT;
+	unsigned long start_time = jiffies;
+
+	for (;;) {
+		unsigned long now = jiffies;
+		unsigned long dirty, thresh, bg_thresh;
+		unsigned long m_dirty = 0;	/* stop bogus uninit warnings */
+		unsigned long m_thresh = 0;
+		unsigned long m_bg_thresh = 0;
+
+		/*
+		 * 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_node_page_state(NR_FILE_DIRTY) +
+					global_node_page_state(NR_UNSTABLE_NFS);
+		gdtc->avail = global_dirtyable_memory();
+		gdtc->dirty = nr_reclaimable + global_node_page_state(NR_WRITEBACK);
+
+		domain_dirty_limits(gdtc);
+
+		if (unlikely(strictlimit)) {
+			wb_dirty_limits(gdtc);
+
+			dirty = gdtc->wb_dirty;
+			thresh = gdtc->wb_thresh;
+			bg_thresh = gdtc->wb_bg_thresh;
+		} else {
+			dirty = gdtc->dirty;
+			thresh = gdtc->thresh;
+			bg_thresh = gdtc->bg_thresh;
+		}
+
+		if (mdtc) {
+			unsigned long filepages, headroom, writeback;
+
+			/*
+			 * If @wb belongs to !root memcg, repeat the same
+			 * basic calculations for the memcg domain.
+			 */
+			mem_cgroup_wb_stats(wb, &filepages, &headroom,
+					    &mdtc->dirty, &writeback);
+			mdtc->dirty += writeback;
+			mdtc_calc_avail(mdtc, filepages, headroom);
+
+			domain_dirty_limits(mdtc);
+
+			if (unlikely(strictlimit)) {
+				wb_dirty_limits(mdtc);
+				m_dirty = mdtc->wb_dirty;
+				m_thresh = mdtc->wb_thresh;
+				m_bg_thresh = mdtc->wb_bg_thresh;
+			} else {
+				m_dirty = mdtc->dirty;
+				m_thresh = mdtc->thresh;
+				m_bg_thresh = mdtc->bg_thresh;
+			}
+		}
+
+		/*
+		 * Throttle it only when the background writeback cannot
+		 * catch-up. This avoids (excessively) small writeouts
+		 * when the wb limits are ramping up in case of !strictlimit.
+		 *
+		 * In strictlimit case make decision based on the wb counters
+		 * and limits. Small writeouts when the wb limits are ramping
+		 * up are the price we consciously pay for strictlimit-ing.
+		 *
+		 * If memcg domain is in effect, @dirty should be under
+		 * both global and memcg freerun ceilings.
+		 */
+		if (dirty <= dirty_freerun_ceiling(thresh, bg_thresh) &&
+		    (!mdtc ||
+		     m_dirty <= dirty_freerun_ceiling(m_thresh, m_bg_thresh))) {
+			unsigned long intv = dirty_poll_interval(dirty, thresh);
+			unsigned long m_intv = ULONG_MAX;
+
+			current->dirty_paused_when = now;
+			current->nr_dirtied = 0;
+			if (mdtc)
+				m_intv = dirty_poll_interval(m_dirty, m_thresh);
+			current->nr_dirtied_pause = min(intv, m_intv);
+			break;
+		}
+
+		if (unlikely(!writeback_in_progress(wb)))
+			wb_start_background_writeback(wb);
+
+		/*
+		 * Calculate global domain's pos_ratio and select the
+		 * global dtc by default.
+		 */
+		if (!strictlimit)
+			wb_dirty_limits(gdtc);
+
+		dirty_exceeded = (gdtc->wb_dirty > gdtc->wb_thresh) &&
+			((gdtc->dirty > gdtc->thresh) || strictlimit);
+
+		wb_position_ratio(gdtc);
+		sdtc = gdtc;
+
+		if (mdtc) {
+			/*
+			 * If memcg domain is in effect, calculate its
+			 * pos_ratio.  @wb should satisfy constraints from
+			 * both global and memcg domains.  Choose the one
+			 * w/ lower pos_ratio.
+			 */
+			if (!strictlimit)
+				wb_dirty_limits(mdtc);
+
+			dirty_exceeded |= (mdtc->wb_dirty > mdtc->wb_thresh) &&
+				((mdtc->dirty > mdtc->thresh) || strictlimit);
+
+			wb_position_ratio(mdtc);
+			if (mdtc->pos_ratio < gdtc->pos_ratio)
+				sdtc = mdtc;
+		}
+
+		if (dirty_exceeded && !wb->dirty_exceeded)
+			wb->dirty_exceeded = 1;
+
+		if (time_is_before_jiffies(wb->bw_time_stamp +
+					   BANDWIDTH_INTERVAL)) {
+			spin_lock(&wb->list_lock);
+			__wb_update_bandwidth(gdtc, mdtc, start_time, true);
+			spin_unlock(&wb->list_lock);
+		}
+
+		/* throttle according to the chosen dtc */
+		dirty_ratelimit = wb->dirty_ratelimit;
+		task_ratelimit = ((u64)dirty_ratelimit * sdtc->pos_ratio) >>
+							RATELIMIT_CALC_SHIFT;
+		max_pause = wb_max_pause(wb, sdtc->wb_dirty);
+		min_pause = wb_min_pause(wb, max_pause,
+					 task_ratelimit, dirty_ratelimit,
+					 &nr_dirtied_pause);
+
+		if (unlikely(task_ratelimit == 0)) {
+			period = max_pause;
+			pause = max_pause;
+			goto pause;
+		}
+		period = HZ * pages_dirtied / task_ratelimit;
+		pause = period;
+		if (current->dirty_paused_when)
+			pause -= now - current->dirty_paused_when;
+		/*
+		 * For less than 1s think time (ext3/4 may block the dirtier
+		 * for up to 800ms from time to time on 1-HDD; so does xfs,
+		 * however at much less frequency), try to compensate it in
+		 * future periods by updating the virtual time; otherwise just
+		 * do a reset, as it may be a light dirtier.
+		 */
+		if (pause < min_pause) {
+			trace_balance_dirty_pages(wb,
+						  sdtc->thresh,
+						  sdtc->bg_thresh,
+						  sdtc->dirty,
+						  sdtc->wb_thresh,
+						  sdtc->wb_dirty,
+						  dirty_ratelimit,
+						  task_ratelimit,
+						  pages_dirtied,
+						  period,
+						  min(pause, 0L),
+						  start_time);
+			if (pause < -HZ) {
+				current->dirty_paused_when = now;
+				current->nr_dirtied = 0;
+			} else if (period) {
+				current->dirty_paused_when += period;
+				current->nr_dirtied = 0;
+			} else if (current->nr_dirtied_pause <= pages_dirtied)
+				current->nr_dirtied_pause += pages_dirtied;
+			break;
+		}
+		if (unlikely(pause > max_pause)) {
+			/* for occasional dropped task_ratelimit */
+			now += min(pause - max_pause, max_pause);
+			pause = max_pause;
+		}
+
+pause:
+		trace_balance_dirty_pages(wb,
+					  sdtc->thresh,
+					  sdtc->bg_thresh,
+					  sdtc->dirty,
+					  sdtc->wb_thresh,
+					  sdtc->wb_dirty,
+					  dirty_ratelimit,
+					  task_ratelimit,
+					  pages_dirtied,
+					  period,
+					  pause,
+					  start_time);
+		__set_current_state(TASK_KILLABLE);
+		wb->dirty_sleep = now;
+		io_schedule_timeout(pause);
+
+		current->dirty_paused_when = now + pause;
+		current->nr_dirtied = 0;
+		current->nr_dirtied_pause = nr_dirtied_pause;
+
+		/*
+		 * This is typically equal to (dirty < thresh) and can also
+		 * keep "1000+ dd on a slow USB stick" under control.
+		 */
+		if (task_ratelimit)
+			break;
+
+		/*
+		 * In the case of an unresponding NFS server and the NFS dirty
+		 * pages exceeds dirty_thresh, give the other good wb's a pipe
+		 * to go through, so that tasks on them still remain responsive.
+		 *
+		 * In theory 1 page is enough to keep the consumer-producer
+		 * pipe going: the flusher cleans 1 page => the task dirties 1
+		 * more page. However wb_dirty has accounting errors.  So use
+		 * the larger and more IO friendly wb_stat_error.
+		 */
+		if (sdtc->wb_dirty <= wb_stat_error(wb))
+			break;
+
+		if (fatal_signal_pending(current))
+			break;
+	}
+
+	if (!dirty_exceeded && wb->dirty_exceeded)
+		wb->dirty_exceeded = 0;
+
+	if (writeback_in_progress(wb))
+		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 > gdtc->bg_thresh)
+		wb_start_background_writeback(wb);
+}
+
+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 inode *inode = mapping->host;
+	struct backing_dev_info *bdi = inode_to_bdi(inode);
+	struct bdi_writeback *wb = NULL;
+	int ratelimit;
+	int *p;
+
+	if (!bdi_cap_account_dirty(bdi))
+		return;
+
+	if (inode_cgwb_enabled(inode))
+		wb = wb_get_create_current(bdi, GFP_KERNEL);
+	if (!wb)
+		wb = &bdi->wb;
+
+	ratelimit = current->nr_dirtied_pause;
+	if (wb->dirty_exceeded)
+		ratelimit = min(ratelimit, 32 >> (PAGE_SHIFT - 10));
+
+	preempt_disable();
+	/*
+	 * This prevents one CPU to accumulate too many dirtied pages without
+	 * calling into balance_dirty_pages(), which can happen when there are
+	 * 1000+ tasks, all of them start dirtying pages at exactly the same
+	 * time, hence all honoured too large initial task->nr_dirtied_pause.
+	 */
+	p =  this_cpu_ptr(&bdp_ratelimits);
+	if (unlikely(current->nr_dirtied >= ratelimit))
+		*p = 0;
+	else if (unlikely(*p >= ratelimit_pages)) {
+		*p = 0;
+		ratelimit = 0;
+	}
+	/*
+	 * Pick up the dirtied pages by the exited tasks. This avoids lots of
+	 * short-lived tasks (eg. gcc invocations in a kernel build) escaping
+	 * the dirty throttling and livelock other long-run dirtiers.
+	 */
+	p = this_cpu_ptr(&dirty_throttle_leaks);
+	if (*p > 0 && current->nr_dirtied < ratelimit) {
+		unsigned long nr_pages_dirtied;
+		nr_pages_dirtied = min(*p, ratelimit - current->nr_dirtied);
+		*p -= nr_pages_dirtied;
+		current->nr_dirtied += nr_pages_dirtied;
+	}
+	preempt_enable();
+
+	if (unlikely(current->nr_dirtied >= ratelimit))
+		balance_dirty_pages(mapping, wb, current->nr_dirtied);
+
+	wb_put(wb);
+}
+EXPORT_SYMBOL(balance_dirty_pages_ratelimited);
+
+/**
+ * wb_over_bg_thresh - does @wb need to be written back?
+ * @wb: bdi_writeback of interest
+ *
+ * Determines whether background writeback should keep writing @wb or it's
+ * clean enough.  Returns %true if writeback should continue.
+ */
+bool wb_over_bg_thresh(struct bdi_writeback *wb)
+{
+	struct dirty_throttle_control gdtc_stor = { GDTC_INIT(wb) };
+	struct dirty_throttle_control mdtc_stor = { MDTC_INIT(wb, &gdtc_stor) };
+	struct dirty_throttle_control * const gdtc = &gdtc_stor;
+	struct dirty_throttle_control * const mdtc = mdtc_valid(&mdtc_stor) ?
+						     &mdtc_stor : NULL;
+
+	/*
+	 * Similar to balance_dirty_pages() but ignores pages being written
+	 * as we're trying to decide whether to put more under writeback.
+	 */
+	gdtc->avail = global_dirtyable_memory();
+	gdtc->dirty = global_node_page_state(NR_FILE_DIRTY) +
+		      global_node_page_state(NR_UNSTABLE_NFS);
+	domain_dirty_limits(gdtc);
+
+	if (gdtc->dirty > gdtc->bg_thresh)
+		return true;
+
+	if (wb_stat(wb, WB_RECLAIMABLE) >
+	    wb_calc_thresh(gdtc->wb, gdtc->bg_thresh))
+		return true;
+
+	if (mdtc) {
+		unsigned long filepages, headroom, writeback;
+
+		mem_cgroup_wb_stats(wb, &filepages, &headroom, &mdtc->dirty,
+				    &writeback);
+		mdtc_calc_avail(mdtc, filepages, headroom);
+		domain_dirty_limits(mdtc);	/* ditto, ignore writeback */
+
+		if (mdtc->dirty > mdtc->bg_thresh)
+			return true;
+
+		if (wb_stat(wb, WB_RECLAIMABLE) >
+		    wb_calc_thresh(mdtc->wb, mdtc->bg_thresh))
+			return true;
+	}
+
+	return false;
+}
+
+/*
+ * 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_node_page_state(NR_FILE_DIRTY) +
+		global_node_page_state(NR_UNSTABLE_NFS);
+	struct bdi_writeback *wb;
+
+	/*
+	 * We want to write everything out, not just down to the dirty
+	 * threshold
+	 */
+	if (!bdi_has_dirty_io(q->backing_dev_info))
+		return;
+
+	rcu_read_lock();
+	list_for_each_entry_rcu(wb, &q->backing_dev_info->wb_list, bdi_node)
+		if (wb_has_dirty_io(wb))
+			wb_start_writeback(wb, nr_pages, true,
+					   WB_REASON_LAPTOP_TIMER);
+	rcu_read_unlock();
+}
+
+/*
+ * 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)
+{
+	struct wb_domain *dom = &global_wb_domain;
+	unsigned long background_thresh;
+	unsigned long dirty_thresh;
+
+	global_dirty_limits(&background_thresh, &dirty_thresh);
+	dom->dirty_limit = dirty_thresh;
+	ratelimit_pages = dirty_thresh / (num_online_cpus() * 32);
+	if (ratelimit_pages < 16)
+		ratelimit_pages = 16;
+}
+
+static int page_writeback_cpu_online(unsigned int cpu)
+{
+	writeback_set_ratelimit();
+	return 0;
+}
+
+/*
+ * 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)
+{
+	BUG_ON(wb_domain_init(&global_wb_domain, GFP_KERNEL));
+
+	cpuhp_setup_state(CPUHP_AP_ONLINE_DYN, "mm/writeback:online",
+			  page_writeback_cpu_online, NULL);
+	cpuhp_setup_state(CPUHP_MM_WRITEBACK_DEAD, "mm/writeback:dead", NULL,
+			  page_writeback_cpu_online);
+}
+
+/**
+ * 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 = 0;
+	struct radix_tree_iter iter;
+	void **slot;
+
+	spin_lock_irq(&mapping->tree_lock);
+	radix_tree_for_each_tagged(slot, &mapping->page_tree, &iter, start,
+							PAGECACHE_TAG_DIRTY) {
+		if (iter.index > end)
+			break;
+		radix_tree_iter_tag_set(&mapping->page_tree, &iter,
+							PAGECACHE_TAG_TOWRITE);
+		tagged++;
+		if ((tagged % WRITEBACK_TAG_BATCH) != 0)
+			continue;
+		slot = radix_tree_iter_resume(slot, &iter);
+		spin_unlock_irq(&mapping->tree_lock);
+		cond_resched();
+		spin_lock_irq(&mapping->tree_lock);
+	}
+	spin_unlock_irq(&mapping->tree_lock);
+}
+EXPORT_SYMBOL(tag_pages_for_writeback);
+
+/**
+ * write_cache_pages - walk the list of dirty pages of the given address space and write all of them.
+ * @mapping: address space structure to write
+ * @wbc: subtract the number of written pages from *@wbc->nr_to_write
+ * @writepage: function called for each page
+ * @data: data passed to writepage function
+ *
+ * If a page is already under I/O, write_cache_pages() skips it, even
+ * if it's dirty.  This is desirable behaviour for memory-cleaning writeback,
+ * but it is INCORRECT for data-integrity system calls such as fsync().  fsync()
+ * and msync() need to guarantee that all the data which was dirty at the time
+ * the call was made get new I/O started against them.  If wbc->sync_mode is
+ * WB_SYNC_ALL then we were called for data integrity and we must wait for
+ * existing IO to complete.
+ *
+ * To avoid livelocks (when other process dirties new pages), we first tag
+ * pages which should be written back with TOWRITE tag and only then start
+ * writing them. For data-integrity sync we have to be careful so that we do
+ * not miss some pages (e.g., because some other process has cleared TOWRITE
+ * tag we set). The rule we follow is that TOWRITE tag can be cleared only
+ * by the process clearing the DIRTY tag (and submitting the page for IO).
+ *
+ * To avoid deadlocks between range_cyclic writeback and callers that hold
+ * pages in PageWriteback to aggregate IO until write_cache_pages() returns,
+ * we do not loop back to the start of the file. Doing so causes a page
+ * lock/page writeback access order inversion - we should only ever lock
+ * multiple pages in ascending page->index order, and looping back to the start
+ * of the file violates that rule and causes deadlocks.
+ */
+int write_cache_pages(struct address_space *mapping,
+		      struct writeback_control *wbc, writepage_t writepage,
+		      void *data)
+{
+	int ret = 0;
+	int done = 0;
+	int error;
+	struct pagevec pvec;
+	int nr_pages;
+	pgoff_t uninitialized_var(writeback_index);
+	pgoff_t index;
+	pgoff_t end;		/* Inclusive */
+	pgoff_t done_index;
+	int range_whole = 0;
+	int tag;
+
+	pagevec_init(&pvec, 0);
+	if (wbc->range_cyclic) {
+		writeback_index = mapping->writeback_index; /* prev offset */
+		index = writeback_index;
+		end = -1;
+	} else {
+		index = wbc->range_start >> PAGE_SHIFT;
+		end = wbc->range_end >> PAGE_SHIFT;
+		if (wbc->range_start == 0 && wbc->range_end == LLONG_MAX)
+			range_whole = 1;
+	}
+	if (wbc->sync_mode == WB_SYNC_ALL || wbc->tagged_writepages)
+		tag = PAGECACHE_TAG_TOWRITE;
+	else
+		tag = PAGECACHE_TAG_DIRTY;
+	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, inode_to_bdi(mapping->host));
+			error = (*writepage)(page, wbc, data);
+			if (unlikely(error)) {
+				/*
+				 * Handle errors according to the type of
+				 * writeback. There's no need to continue for
+				 * background writeback. Just push done_index
+				 * past this page so media errors won't choke
+				 * writeout for the entire file. For integrity
+				 * writeback, we must process the entire dirty
+				 * set regardless of errors because the fs may
+				 * still have state to clear for each page. In
+				 * that case we continue processing and return
+				 * the first error.
+				 */
+				if (error == AOP_WRITEPAGE_ACTIVATE) {
+					unlock_page(page);
+					error = 0;
+				} else if (wbc->sync_mode != WB_SYNC_ALL) {
+					ret = error;
+					done_index = page->index + 1;
+					done = 1;
+					break;
+				}
+				if (!ret)
+					ret = error;
+			}
+
+			/*
+			 * We stop writing back only if we are not doing
+			 * integrity sync. In case of integrity sync we have to
+			 * keep going until we have written all the pages
+			 * we tagged for writeback prior to entering this loop.
+			 */
+			if (--wbc->nr_to_write <= 0 &&
+			    wbc->sync_mode == WB_SYNC_NONE) {
+				done = 1;
+				break;
+			}
+		}
+		pagevec_release(&pvec);
+		cond_resched();
+	}
+
+	/*
+	 * If we hit the last page and there is more work to be done: wrap
+	 * back the index back to the start of the file for the next
+	 * time we are called.
+	 */
+	if (wbc->range_cyclic && !done)
+		done_index = 0;
+	if (wbc->range_cyclic || (range_whole && wbc->nr_to_write > 0))
+		mapping->writeback_index = done_index;
+
+	return ret;
+}
+EXPORT_SYMBOL(write_cache_pages);
+
+/*
+ * Function used by generic_writepages to call the real writepage
+ * function and set the mapping flags on error
+ */
+static int __writepage(struct page *page, struct writeback_control *wbc,
+		       void *data)
+{
+	struct address_space *mapping = data;
+	int ret = mapping->a_ops->writepage(page, wbc);
+	mapping_set_error(mapping, ret);
+	return ret;
+}
+
+/**
+ * generic_writepages - walk the list of dirty pages of the given address space and writepage() all of them.
+ * @mapping: address space structure to write
+ * @wbc: subtract the number of written pages from *@wbc->nr_to_write
+ *
+ * This is a library function, which implements the writepages()
+ * address_space_operation.
+ */
+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;
+	while (1) {
+		if (mapping->a_ops->writepages)
+			ret = mapping->a_ops->writepages(mapping, wbc);
+		else
+			ret = generic_writepages(mapping, wbc);
+		if ((ret != -ENOMEM) || (wbc->sync_mode != WB_SYNC_ALL))
+			break;
+		cond_resched();
+		congestion_wait(BLK_RW_ASYNC, HZ/50);
+	}
+	return ret;
+}
+
+/**
+ * write_one_page - write out a single page and wait on I/O
+ * @page: the page to write
+ *
+ * The page must be locked by the caller and will be unlocked upon return.
+ *
+ * Note that the mapping's AS_EIO/AS_ENOSPC flags will be cleared when this
+ * function returns.
+ */
+int write_one_page(struct page *page)
+{
+	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));
+
+	wait_on_page_writeback(page);
+
+	if (clear_page_dirty_for_io(page)) {
+		get_page(page);
+		ret = mapping->a_ops->writepage(page, &wbc);
+		if (ret == 0)
+			wait_on_page_writeback(page);
+		put_page(page);
+	} else {
+		unlock_page(page);
+	}
+
+	if (!ret)
+		ret = filemap_check_errors(mapping);
+	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.
+ *
+ * Caller must hold lock_page_memcg().
+ *
+ * NOTE: This relies on being atomic wrt interrupts.
+ */
+void account_page_dirtied(struct page *page, struct address_space *mapping)
+{
+	struct inode *inode = mapping->host;
+
+	trace_writeback_dirty_page(page, mapping);
+
+	if (mapping_cap_account_dirty(mapping)) {
+		struct bdi_writeback *wb;
+
+		inode_attach_wb(inode, page);
+		wb = inode_to_wb(inode);
+
+		__inc_lruvec_page_state(page, NR_FILE_DIRTY);
+		__inc_zone_page_state(page, NR_ZONE_WRITE_PENDING);
+		__inc_node_page_state(page, NR_DIRTIED);
+		inc_wb_stat(wb, WB_RECLAIMABLE);
+		inc_wb_stat(wb, WB_DIRTIED);
+		task_io_account_write(PAGE_SIZE);
+		current->nr_dirtied++;
+		this_cpu_inc(bdp_ratelimits);
+	}
+}
+EXPORT_SYMBOL(account_page_dirtied);
+
+/*
+ * Helper function for deaccounting dirty page without writeback.
+ *
+ * Caller must hold lock_page_memcg().
+ */
+void account_page_cleaned(struct page *page, struct address_space *mapping,
+			  struct bdi_writeback *wb)
+{
+	if (mapping_cap_account_dirty(mapping)) {
+		dec_lruvec_page_state(page, NR_FILE_DIRTY);
+		dec_zone_page_state(page, NR_ZONE_WRITE_PENDING);
+		dec_wb_stat(wb, WB_RECLAIMABLE);
+		task_io_account_cancelled_write(PAGE_SIZE);
+	}
+}
+
+/*
+ * 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)
+{
+	lock_page_memcg(page);
+	if (!TestSetPageDirty(page)) {
+		struct address_space *mapping = page_mapping(page);
+		unsigned long flags;
+
+		if (!mapping) {
+			unlock_page_memcg(page);
+			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);
+		unlock_page_memcg(page);
+
+		if (mapping->host) {
+			/* !PageAnon && !swapper_space */
+			__mark_inode_dirty(mapping->host, I_DIRTY_PAGES);
+		}
+		return 1;
+	}
+	unlock_page_memcg(page);
+	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)) {
+		struct inode *inode = mapping->host;
+		struct bdi_writeback *wb;
+		struct wb_lock_cookie cookie = {};
+
+		wb = unlocked_inode_to_wb_begin(inode, &cookie);
+		current->nr_dirtied--;
+		dec_node_page_state(page, NR_DIRTIED);
+		dec_wb_stat(wb, WB_DIRTIED);
+		unlocked_inode_to_wb_end(inode, &cookie);
+	}
+}
+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)
+{
+	int ret;
+
+	wbc->pages_skipped++;
+	ret = __set_page_dirty_nobuffers(page);
+	account_page_redirty(page);
+	return ret;
+}
+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);
+
+	page = compound_head(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.
+		 */
+		if (PageReclaim(page))
+			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);
+
+/*
+ * This cancels just the dirty bit on the kernel page itself, it does NOT
+ * actually remove dirty bits on any mmap's that may be around. It also
+ * leaves the page tagged dirty, so any sync activity will still find it on
+ * the dirty lists, and in particular, clear_page_dirty_for_io() will still
+ * look at the dirty bits in the VM.
+ *
+ * Doing this should *normally* only ever be done when a page is truncated,
+ * and is not actually mapped anywhere at all. However, fs/buffer.c does
+ * this when it notices that somebody has cleaned out all the buffers on a
+ * page without actually doing it through the VM. Can you say "ext3 is
+ * horribly ugly"? Thought you could.
+ */
+void cancel_dirty_page(struct page *page)
+{
+	struct address_space *mapping = page_mapping(page);
+
+	if (mapping_cap_account_dirty(mapping)) {
+		struct inode *inode = mapping->host;
+		struct bdi_writeback *wb;
+		struct wb_lock_cookie cookie = {};
+
+		lock_page_memcg(page);
+		wb = unlocked_inode_to_wb_begin(inode, &cookie);
+
+		if (TestClearPageDirty(page))
+			account_page_cleaned(page, mapping, wb);
+
+		unlocked_inode_to_wb_end(inode, &cookie);
+		unlock_page_memcg(page);
+	} else {
+		ClearPageDirty(page);
+	}
+}
+EXPORT_SYMBOL(cancel_dirty_page);
+
+/*
+ * 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);
+	int ret = 0;
+
+	BUG_ON(!PageLocked(page));
+
+	if (mapping && mapping_cap_account_dirty(mapping)) {
+		struct inode *inode = mapping->host;
+		struct bdi_writeback *wb;
+		struct wb_lock_cookie cookie = {};
+
+		/*
+		 * Yes, Virginia, this is indeed insane.
+		 *
+		 * We use this sequence to make sure that
+		 *  (a) we account for dirty stats properly
+		 *  (b) we tell the low-level filesystem to
+		 *      mark the whole 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.
+		 */
+		wb = unlocked_inode_to_wb_begin(inode, &cookie);
+		if (TestClearPageDirty(page)) {
+			dec_lruvec_page_state(page, NR_FILE_DIRTY);
+			dec_zone_page_state(page, NR_ZONE_WRITE_PENDING);
+			dec_wb_stat(wb, WB_RECLAIMABLE);
+			ret = 1;
+		}
+		unlocked_inode_to_wb_end(inode, &cookie);
+		return ret;
+	}
+	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);
+	struct mem_cgroup *memcg;
+	struct lruvec *lruvec;
+	int ret;
+
+	memcg = lock_page_memcg(page);
+	lruvec = mem_cgroup_page_lruvec(page, page_pgdat(page));
+	if (mapping && mapping_use_writeback_tags(mapping)) {
+		struct inode *inode = mapping->host;
+		struct backing_dev_info *bdi = inode_to_bdi(inode);
+		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)) {
+				struct bdi_writeback *wb = inode_to_wb(inode);
+
+				dec_wb_stat(wb, WB_WRITEBACK);
+				__wb_writeout_inc(wb);
+			}
+		}
+
+		if (mapping->host && !mapping_tagged(mapping,
+						     PAGECACHE_TAG_WRITEBACK))
+			sb_clear_inode_writeback(mapping->host);
+
+		spin_unlock_irqrestore(&mapping->tree_lock, flags);
+	} else {
+		ret = TestClearPageWriteback(page);
+	}
+	/*
+	 * NOTE: Page might be free now! Writeback doesn't hold a page
+	 * reference on its own, it relies on truncation to wait for
+	 * the clearing of PG_writeback. The below can only access
+	 * page state that is static across allocation cycles.
+	 */
+	if (ret) {
+		dec_lruvec_state(lruvec, NR_WRITEBACK);
+		dec_zone_page_state(page, NR_ZONE_WRITE_PENDING);
+		inc_node_page_state(page, NR_WRITTEN);
+	}
+	__unlock_page_memcg(memcg);
+	return ret;
+}
+
+int __test_set_page_writeback(struct page *page, bool keep_write)
+{
+	struct address_space *mapping = page_mapping(page);
+	int ret;
+
+	lock_page_memcg(page);
+	if (mapping && mapping_use_writeback_tags(mapping)) {
+		struct inode *inode = mapping->host;
+		struct backing_dev_info *bdi = inode_to_bdi(inode);
+		unsigned long flags;
+
+		spin_lock_irqsave(&mapping->tree_lock, flags);
+		ret = TestSetPageWriteback(page);
+		if (!ret) {
+			bool on_wblist;
+
+			on_wblist = mapping_tagged(mapping,
+						   PAGECACHE_TAG_WRITEBACK);
+
+			radix_tree_tag_set(&mapping->page_tree,
+						page_index(page),
+						PAGECACHE_TAG_WRITEBACK);
+			if (bdi_cap_account_writeback(bdi))
+				inc_wb_stat(inode_to_wb(inode), WB_WRITEBACK);
+
+			/*
+			 * We can come through here when swapping anonymous
+			 * pages, so we don't necessarily have an inode to track
+			 * for sync.
+			 */
+			if (mapping->host && !on_wblist)
+				sb_mark_inode_writeback(mapping->host);
+		}
+		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) {
+		inc_lruvec_page_state(page, NR_WRITEBACK);
+		inc_zone_page_state(page, NR_ZONE_WRITE_PENDING);
+	}
+	unlock_page_memcg(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);
+
+/**
+ * wait_for_stable_page() - wait for writeback to finish, if necessary.
+ * @page:	The page to wait on.
+ *
+ * This function determines if the given page is related to a backing device
+ * that requires page contents to be held stable during writeback.  If so, then
+ * it will wait for any pending writeback to complete.
+ */
+void wait_for_stable_page(struct page *page)
+{
+	if (bdi_cap_stable_pages_required(inode_to_bdi(page->mapping->host)))
+		wait_on_page_writeback(page);
+}
+EXPORT_SYMBOL_GPL(wait_for_stable_page);