1	// SPDX-License-Identifier: GPL-2.0-only
2	/*
3	* mm/page-writeback.c
4	*
5	* Copyright (C) 2002, Linus Torvalds.
6	* Copyright (C) 2007 Red Hat, Inc., Peter Zijlstra
7	*
8	* Contains functions related to writing back dirty pages at the
9	* address_space level.
10	*
11	* 10Apr2002 Andrew Morton
12	* Initial version
13	*/
14
15	#include <linux/kernel.h>
16	#include <linux/math64.h>
17	#include <linux/export.h>
18	#include <linux/spinlock.h>
19	#include <linux/fs.h>
20	#include <linux/mm.h>
21	#include <linux/swap.h>
22	#include <linux/slab.h>
23	#include <linux/pagemap.h>
24	#include <linux/writeback.h>
25	#include <linux/init.h>
26	#include <linux/backing-dev.h>
27	#include <linux/task_io_accounting_ops.h>
28	#include <linux/blkdev.h>
29	#include <linux/mpage.h>
30	#include <linux/rmap.h>
31	#include <linux/percpu.h>
32	#include <linux/smp.h>
33	#include <linux/sysctl.h>
34	#include <linux/cpu.h>
35	#include <linux/syscalls.h>
36	#include <linux/pagevec.h>
37	#include <linux/timer.h>
38	#include <linux/sched/rt.h>
39	#include <linux/sched/signal.h>
40	#include <linux/mm_inline.h>
41	#include <trace/events/writeback.h>
42
43	#include "internal.h"
44
45	/*
46	* Sleep at most 200ms at a time in balance_dirty_pages().
47	*/
48	#define MAX_PAUSE max(HZ/5, 1)
49
50	/*
51	* Try to keep balance_dirty_pages() call intervals higher than this many pages
52	* by raising pause time to max_pause when falls below it.
53	*/
54	#define DIRTY_POLL_THRESH (128 >> (PAGE_SHIFT - 10))
55
56	/*
57	* Estimate write bandwidth at 200ms intervals.
58	*/
59	#define BANDWIDTH_INTERVAL max(HZ/5, 1)
60
61	#define RATELIMIT_CALC_SHIFT 10
62
63	/*
64	* After a CPU has dirtied this many pages, balance_dirty_pages_ratelimited
65	* will look to see if it needs to force writeback or throttling.
66	*/
67	static long ratelimit_pages = 32;
68
69	/* The following parameters are exported via /proc/sys/vm */
70
71	/*
72	* Start background writeback (via writeback threads) at this percentage
73	*/
74	static int dirty_background_ratio = 10;
75
76	/*
77	* dirty_background_bytes starts at 0 (disabled) so that it is a function of
78	* dirty_background_ratio * the amount of dirtyable memory
79	*/
80	static unsigned long dirty_background_bytes;
81
82	/*
83	* free highmem will not be subtracted from the total free memory
84	* for calculating free ratios if vm_highmem_is_dirtyable is true
85	*/
86	static int vm_highmem_is_dirtyable;
87
88	/*
89	* The generator of dirty data starts writeback at this percentage
90	*/
91	static int vm_dirty_ratio = 20;
92
93	/*
94	* vm_dirty_bytes starts at 0 (disabled) so that it is a function of
95	* vm_dirty_ratio * the amount of dirtyable memory
96	*/
97	static unsigned long vm_dirty_bytes;
98
99	/*
100	* The interval between `kupdate'-style writebacks
101	*/
102	unsigned int dirty_writeback_interval = 5 * 100; /* centiseconds */
103
104	EXPORT_SYMBOL_GPL(dirty_writeback_interval);
105
106	/*
107	* The longest time for which data is allowed to remain dirty
108	*/
109	unsigned int dirty_expire_interval = 30 * 100; /* centiseconds */
110
111	/*
112	* Flag that puts the machine in "laptop mode". Doubles as a timeout in jiffies:
113	* a full sync is triggered after this time elapses without any disk activity.
114	*/
115	int laptop_mode;
116
117	EXPORT_SYMBOL(laptop_mode);
118
119	/* End of sysctl-exported parameters */
120
121	struct wb_domain global_wb_domain;
122
123	/* consolidated parameters for balance_dirty_pages() and its subroutines */
124	struct dirty_throttle_control {
125	#ifdef CONFIG_CGROUP_WRITEBACK
126	struct wb_domain *dom;
127	struct dirty_throttle_control *gdtc; /* only set in memcg dtc's */
128	#endif
129	struct bdi_writeback *wb;
130	struct fprop_local_percpu *wb_completions;
131
132	unsigned long avail; /* dirtyable */
133	unsigned long dirty; /* file_dirty + write + nfs */
134	unsigned long thresh; /* dirty threshold */
135	unsigned long bg_thresh; /* dirty background threshold */
136
137	unsigned long wb_dirty; /* per-wb counterparts */
138	unsigned long wb_thresh;
139	unsigned long wb_bg_thresh;
140
141	unsigned long pos_ratio;
142	};
143
144	/*
145	* Length of period for aging writeout fractions of bdis. This is an
146	* arbitrarily chosen number. The longer the period, the slower fractions will
147	* reflect changes in current writeout rate.
148	*/
149	#define VM_COMPLETIONS_PERIOD_LEN (3*HZ)
150
151	#ifdef CONFIG_CGROUP_WRITEBACK
152
153	#define GDTC_INIT(__wb) .wb = (__wb), \
154	.dom = &global_wb_domain, \
155	.wb_completions = &(__wb)->completions
156
157	#define GDTC_INIT_NO_WB .dom = &global_wb_domain
158
159	#define MDTC_INIT(__wb, __gdtc) .wb = (__wb), \
160	.dom = mem_cgroup_wb_domain(__wb), \
161	.wb_completions = &(__wb)->memcg_completions, \
162	.gdtc = __gdtc
163
164	static bool mdtc_valid(struct dirty_throttle_control *dtc)
165	{
166	return dtc->dom;
167	}
168
169	static struct wb_domain *dtc_dom(struct dirty_throttle_control *dtc)
170	{
171	return dtc->dom;
172	}
173
174	static struct dirty_throttle_control *mdtc_gdtc(struct dirty_throttle_control *mdtc)
175	{
176	return mdtc->gdtc;
177	}
178
179	static struct fprop_local_percpu *wb_memcg_completions(struct bdi_writeback *wb)
180	{
181	return &wb->memcg_completions;
182	}
183
184	static void wb_min_max_ratio(struct bdi_writeback *wb,
185	unsigned long *minp, unsigned long *maxp)
186	{
187	unsigned long this_bw = READ_ONCE(wb->avg_write_bandwidth);
188	unsigned long tot_bw = atomic_long_read(v: &wb->bdi->tot_write_bandwidth);
189	unsigned long long min = wb->bdi->min_ratio;
190	unsigned long long max = wb->bdi->max_ratio;
191
192	/*
193	* @wb may already be clean by the time control reaches here and
194	* the total may not include its bw.
195	*/
196	if (this_bw < tot_bw) {
197	if (min) {
198	min *= this_bw;
199	min = div64_ul(min, tot_bw);
200	}
201	if (max < 100 * BDI_RATIO_SCALE) {
202	max *= this_bw;
203	max = div64_ul(max, tot_bw);
204	}
205	}
206
207	*minp = min;
208	*maxp = max;
209	}
210
211	#else /* CONFIG_CGROUP_WRITEBACK */
212
213	#define GDTC_INIT(__wb) .wb = (__wb), \
214	.wb_completions = &(__wb)->completions
215	#define GDTC_INIT_NO_WB
216	#define MDTC_INIT(__wb, __gdtc)
217
218	static bool mdtc_valid(struct dirty_throttle_control *dtc)
219	{
220	return false;
221	}
222
223	static struct wb_domain *dtc_dom(struct dirty_throttle_control *dtc)
224	{
225	return &global_wb_domain;
226	}
227
228	static struct dirty_throttle_control *mdtc_gdtc(struct dirty_throttle_control *mdtc)
229	{
230	return NULL;
231	}
232
233	static struct fprop_local_percpu *wb_memcg_completions(struct bdi_writeback *wb)
234	{
235	return NULL;
236	}
237
238	static void wb_min_max_ratio(struct bdi_writeback *wb,
239	unsigned long *minp, unsigned long *maxp)
240	{
241	*minp = wb->bdi->min_ratio;
242	*maxp = wb->bdi->max_ratio;
243	}
244
245	#endif /* CONFIG_CGROUP_WRITEBACK */
246
247	/*
248	* In a memory zone, there is a certain amount of pages we consider
249	* available for the page cache, which is essentially the number of
250	* free and reclaimable pages, minus some zone reserves to protect
251	* lowmem and the ability to uphold the zone's watermarks without
252	* requiring writeback.
253	*
254	* This number of dirtyable pages is the base value of which the
255	* user-configurable dirty ratio is the effective number of pages that
256	* are allowed to be actually dirtied. Per individual zone, or
257	* globally by using the sum of dirtyable pages over all zones.
258	*
259	* Because the user is allowed to specify the dirty limit globally as
260	* absolute number of bytes, calculating the per-zone dirty limit can
261	* require translating the configured limit into a percentage of
262	* global dirtyable memory first.
263	*/
264
265	/**
266	* node_dirtyable_memory - number of dirtyable pages in a node
267	* @pgdat: the node
268	*
269	* Return: the node's number of pages potentially available for dirty
270	* page cache. This is the base value for the per-node dirty limits.
271	*/
272	static unsigned long node_dirtyable_memory(struct pglist_data *pgdat)
273	{
274	unsigned long nr_pages = 0;
275	int z;
276
277	for (z = 0; z < MAX_NR_ZONES; z++) {
278	struct zone *zone = pgdat->node_zones + z;
279
280	if (!populated_zone(zone))
281	continue;
282
283	nr_pages += zone_page_state(zone, item: NR_FREE_PAGES);
284	}
285
286	/*
287	* Pages reserved for the kernel should not be considered
288	* dirtyable, to prevent a situation where reclaim has to
289	* clean pages in order to balance the zones.
290	*/
291	nr_pages -= min(nr_pages, pgdat->totalreserve_pages);
292
293	nr_pages += node_page_state(pgdat, item: NR_INACTIVE_FILE);
294	nr_pages += node_page_state(pgdat, item: NR_ACTIVE_FILE);
295
296	return nr_pages;
297	}
298
299	static unsigned long highmem_dirtyable_memory(unsigned long total)
300	{
301	#ifdef CONFIG_HIGHMEM
302	int node;
303	unsigned long x = 0;
304	int i;
305
306	for_each_node_state(node, N_HIGH_MEMORY) {
307	for (i = ZONE_NORMAL + 1; i < MAX_NR_ZONES; i++) {
308	struct zone *z;
309	unsigned long nr_pages;
310
311	if (!is_highmem_idx(i))
312	continue;
313
314	z = &NODE_DATA(node)->node_zones[i];
315	if (!populated_zone(z))
316	continue;
317
318	nr_pages = zone_page_state(z, NR_FREE_PAGES);
319	/* watch for underflows */
320	nr_pages -= min(nr_pages, high_wmark_pages(z));
321	nr_pages += zone_page_state(z, NR_ZONE_INACTIVE_FILE);
322	nr_pages += zone_page_state(z, NR_ZONE_ACTIVE_FILE);
323	x += nr_pages;
324	}
325	}
326
327	/*
328	* Make sure that the number of highmem pages is never larger
329	* than the number of the total dirtyable memory. This can only
330	* occur in very strange VM situations but we want to make sure
331	* that this does not occur.
332	*/
333	return min(x, total);
334	#else
335	return 0;
336	#endif
337	}
338
339	/**
340	* global_dirtyable_memory - number of globally dirtyable pages
341	*
342	* Return: the global number of pages potentially available for dirty
343	* page cache. This is the base value for the global dirty limits.
344	*/
345	static unsigned long global_dirtyable_memory(void)
346	{
347	unsigned long x;
348
349	x = global_zone_page_state(item: NR_FREE_PAGES);
350	/*
351	* Pages reserved for the kernel should not be considered
352	* dirtyable, to prevent a situation where reclaim has to
353	* clean pages in order to balance the zones.
354	*/
355	x -= min(x, totalreserve_pages);
356
357	x += global_node_page_state(item: NR_INACTIVE_FILE);
358	x += global_node_page_state(item: NR_ACTIVE_FILE);
359
360	if (!vm_highmem_is_dirtyable)
361	x -= highmem_dirtyable_memory(total: x);
362
363	return x + 1; /* Ensure that we never return 0 */
364	}
365
366	/**
367	* domain_dirty_limits - calculate thresh and bg_thresh for a wb_domain
368	* @dtc: dirty_throttle_control of interest
369	*
370	* Calculate @dtc->thresh and ->bg_thresh considering
371	* vm_dirty_{bytes|ratio} and dirty_background_{bytes|ratio}. The caller
372	* must ensure that @dtc->avail is set before calling this function. The
373	* dirty limits will be lifted by 1/4 for real-time tasks.
374	*/
375	static void domain_dirty_limits(struct dirty_throttle_control *dtc)
376	{
377	const unsigned long available_memory = dtc->avail;
378	struct dirty_throttle_control *gdtc = mdtc_gdtc(mdtc: dtc);
379	unsigned long bytes = vm_dirty_bytes;
380	unsigned long bg_bytes = dirty_background_bytes;
381	/* convert ratios to per-PAGE_SIZE for higher precision */
382	unsigned long ratio = (vm_dirty_ratio * PAGE_SIZE) / 100;
383	unsigned long bg_ratio = (dirty_background_ratio * PAGE_SIZE) / 100;
384	unsigned long thresh;
385	unsigned long bg_thresh;
386	struct task_struct *tsk;
387
388	/* gdtc is !NULL iff @dtc is for memcg domain */
389	if (gdtc) {
390	unsigned long global_avail = gdtc->avail;
391
392	/*
393	* The byte settings can't be applied directly to memcg
394	* domains. Convert them to ratios by scaling against
395	* globally available memory. As the ratios are in
396	* per-PAGE_SIZE, they can be obtained by dividing bytes by
397	* number of pages.
398	*/
399	if (bytes)
400	ratio = min(DIV_ROUND_UP(bytes, global_avail),
401	PAGE_SIZE);
402	if (bg_bytes)
403	bg_ratio = min(DIV_ROUND_UP(bg_bytes, global_avail),
404	PAGE_SIZE);
405	bytes = bg_bytes = 0;
406	}
407
408	if (bytes)
409	thresh = DIV_ROUND_UP(bytes, PAGE_SIZE);
410	else
411	thresh = (ratio * available_memory) / PAGE_SIZE;
412
413	if (bg_bytes)
414	bg_thresh = DIV_ROUND_UP(bg_bytes, PAGE_SIZE);
415	else
416	bg_thresh = (bg_ratio * available_memory) / PAGE_SIZE;
417
418	if (bg_thresh >= thresh)
419	bg_thresh = thresh / 2;
420	tsk = current;
421	if (rt_task(p: tsk)) {
422	bg_thresh += bg_thresh / 4 + global_wb_domain.dirty_limit / 32;
423	thresh += thresh / 4 + global_wb_domain.dirty_limit / 32;
424	}
425	dtc->thresh = thresh;
426	dtc->bg_thresh = bg_thresh;
427
428	/* we should eventually report the domain in the TP */
429	if (!gdtc)
430	trace_global_dirty_state(background_thresh: bg_thresh, dirty_thresh: thresh);
431	}
432
433	/**
434	* global_dirty_limits - background-writeback and dirty-throttling thresholds
435	* @pbackground: out parameter for bg_thresh
436	* @pdirty: out parameter for thresh
437	*
438	* Calculate bg_thresh and thresh for global_wb_domain. See
439	* domain_dirty_limits() for details.
440	*/
441	void global_dirty_limits(unsigned long *pbackground, unsigned long *pdirty)
442	{
443	struct dirty_throttle_control gdtc = { GDTC_INIT_NO_WB };
444
445	gdtc.avail = global_dirtyable_memory();
446	domain_dirty_limits(dtc: &gdtc);
447
448	*pbackground = gdtc.bg_thresh;
449	*pdirty = gdtc.thresh;
450	}
451
452	/**
453	* node_dirty_limit - maximum number of dirty pages allowed in a node
454	* @pgdat: the node
455	*
456	* Return: the maximum number of dirty pages allowed in a node, based
457	* on the node's dirtyable memory.
458	*/
459	static unsigned long node_dirty_limit(struct pglist_data *pgdat)
460	{
461	unsigned long node_memory = node_dirtyable_memory(pgdat);
462	struct task_struct *tsk = current;
463	unsigned long dirty;
464
465	if (vm_dirty_bytes)
466	dirty = DIV_ROUND_UP(vm_dirty_bytes, PAGE_SIZE) *
467	node_memory / global_dirtyable_memory();
468	else
469	dirty = vm_dirty_ratio * node_memory / 100;
470
471	if (rt_task(p: tsk))
472	dirty += dirty / 4;
473
474	return dirty;
475	}
476
477	/**
478	* node_dirty_ok - tells whether a node is within its dirty limits
479	* @pgdat: the node to check
480	*
481	* Return: %true when the dirty pages in @pgdat are within the node's
482	* dirty limit, %false if the limit is exceeded.
483	*/
484	bool node_dirty_ok(struct pglist_data *pgdat)
485	{
486	unsigned long limit = node_dirty_limit(pgdat);
487	unsigned long nr_pages = 0;
488
489	nr_pages += node_page_state(pgdat, item: NR_FILE_DIRTY);
490	nr_pages += node_page_state(pgdat, item: NR_WRITEBACK);
491
492	return nr_pages <= limit;
493	}
494
495	#ifdef CONFIG_SYSCTL
496	static int dirty_background_ratio_handler(struct ctl_table *table, int write,
497	void *buffer, size_t *lenp, loff_t *ppos)
498	{
499	int ret;
500
501	ret = proc_dointvec_minmax(table, write, buffer, lenp, ppos);
502	if (ret == 0 && write)
503	dirty_background_bytes = 0;
504	return ret;
505	}
506
507	static int dirty_background_bytes_handler(struct ctl_table *table, int write,
508	void *buffer, size_t *lenp, loff_t *ppos)
509	{
510	int ret;
511
512	ret = proc_doulongvec_minmax(table, write, buffer, lenp, ppos);
513	if (ret == 0 && write)
514	dirty_background_ratio = 0;
515	return ret;
516	}
517
518	static int dirty_ratio_handler(struct ctl_table *table, int write, void *buffer,
519	size_t *lenp, loff_t *ppos)
520	{
521	int old_ratio = vm_dirty_ratio;
522	int ret;
523
524	ret = proc_dointvec_minmax(table, write, buffer, lenp, ppos);
525	if (ret == 0 && write && vm_dirty_ratio != old_ratio) {
526	writeback_set_ratelimit();
527	vm_dirty_bytes = 0;
528	}
529	return ret;
530	}
531
532	static int dirty_bytes_handler(struct ctl_table *table, int write,
533	void *buffer, size_t *lenp, loff_t *ppos)
534	{
535	unsigned long old_bytes = vm_dirty_bytes;
536	int ret;
537
538	ret = proc_doulongvec_minmax(table, write, buffer, lenp, ppos);
539	if (ret == 0 && write && vm_dirty_bytes != old_bytes) {
540	writeback_set_ratelimit();
541	vm_dirty_ratio = 0;
542	}
543	return ret;
544	}
545	#endif
546
547	static unsigned long wp_next_time(unsigned long cur_time)
548	{
549	cur_time += VM_COMPLETIONS_PERIOD_LEN;
550	/* 0 has a special meaning... */
551	if (!cur_time)
552	return 1;
553	return cur_time;
554	}
555
556	static void wb_domain_writeout_add(struct wb_domain *dom,
557	struct fprop_local_percpu *completions,
558	unsigned int max_prop_frac, long nr)
559	{
560	__fprop_add_percpu_max(p: &dom->completions, pl: completions,
561	max_frac: max_prop_frac, nr);
562	/* First event after period switching was turned off? */
563	if (unlikely(!dom->period_time)) {
564	/*
565	* We can race with other __bdi_writeout_inc calls here but
566	* it does not cause any harm since the resulting time when
567	* timer will fire and what is in writeout_period_time will be
568	* roughly the same.
569	*/
570	dom->period_time = wp_next_time(cur_time: jiffies);
571	mod_timer(timer: &dom->period_timer, expires: dom->period_time);
572	}
573	}
574
575	/*
576	* Increment @wb's writeout completion count and the global writeout
577	* completion count. Called from __folio_end_writeback().
578	*/
579	static inline void __wb_writeout_add(struct bdi_writeback *wb, long nr)
580	{
581	struct wb_domain *cgdom;
582
583	wb_stat_mod(wb, item: WB_WRITTEN, amount: nr);
584	wb_domain_writeout_add(dom: &global_wb_domain, completions: &wb->completions,
585	max_prop_frac: wb->bdi->max_prop_frac, nr);
586
587	cgdom = mem_cgroup_wb_domain(wb);
588	if (cgdom)
589	wb_domain_writeout_add(dom: cgdom, completions: wb_memcg_completions(wb),
590	max_prop_frac: wb->bdi->max_prop_frac, nr);
591	}
592
593	void wb_writeout_inc(struct bdi_writeback *wb)
594	{
595	unsigned long flags;
596
597	local_irq_save(flags);
598	__wb_writeout_add(wb, nr: 1);
599	local_irq_restore(flags);
600	}
601	EXPORT_SYMBOL_GPL(wb_writeout_inc);
602
603	/*
604	* On idle system, we can be called long after we scheduled because we use
605	* deferred timers so count with missed periods.
606	*/
607	static void writeout_period(struct timer_list *t)
608	{
609	struct wb_domain *dom = from_timer(dom, t, period_timer);
610	int miss_periods = (jiffies - dom->period_time) /
611	VM_COMPLETIONS_PERIOD_LEN;
612
613	if (fprop_new_period(p: &dom->completions, periods: miss_periods + 1)) {
614	dom->period_time = wp_next_time(cur_time: dom->period_time +
615	miss_periods * VM_COMPLETIONS_PERIOD_LEN);
616	mod_timer(timer: &dom->period_timer, expires: dom->period_time);
617	} else {
618	/*
619	* Aging has zeroed all fractions. Stop wasting CPU on period
620	* updates.
621	*/
622	dom->period_time = 0;
623	}
624	}
625
626	int wb_domain_init(struct wb_domain *dom, gfp_t gfp)
627	{
628	memset(dom, 0, sizeof(*dom));
629
630	spin_lock_init(&dom->lock);
631
632	timer_setup(&dom->period_timer, writeout_period, TIMER_DEFERRABLE);
633
634	dom->dirty_limit_tstamp = jiffies;
635
636	return fprop_global_init(p: &dom->completions, gfp);
637	}
638
639	#ifdef CONFIG_CGROUP_WRITEBACK
640	void wb_domain_exit(struct wb_domain *dom)
641	{
642	del_timer_sync(timer: &dom->period_timer);
643	fprop_global_destroy(p: &dom->completions);
644	}
645	#endif
646
647	/*
648	* bdi_min_ratio keeps the sum of the minimum dirty shares of all
649	* registered backing devices, which, for obvious reasons, can not
650	* exceed 100%.
651	*/
652	static unsigned int bdi_min_ratio;
653
654	static int bdi_check_pages_limit(unsigned long pages)
655	{
656	unsigned long max_dirty_pages = global_dirtyable_memory();
657
658	if (pages > max_dirty_pages)
659	return -EINVAL;
660
661	return 0;
662	}
663
664	static unsigned long bdi_ratio_from_pages(unsigned long pages)
665	{
666	unsigned long background_thresh;
667	unsigned long dirty_thresh;
668	unsigned long ratio;
669
670	global_dirty_limits(pbackground: &background_thresh, pdirty: &dirty_thresh);
671	ratio = div64_u64(dividend: pages * 100ULL * BDI_RATIO_SCALE, divisor: dirty_thresh);
672
673	return ratio;
674	}
675
676	static u64 bdi_get_bytes(unsigned int ratio)
677	{
678	unsigned long background_thresh;
679	unsigned long dirty_thresh;
680	u64 bytes;
681
682	global_dirty_limits(pbackground: &background_thresh, pdirty: &dirty_thresh);
683	bytes = (dirty_thresh * PAGE_SIZE * ratio) / BDI_RATIO_SCALE / 100;
684
685	return bytes;
686	}
687
688	static int __bdi_set_min_ratio(struct backing_dev_info *bdi, unsigned int min_ratio)
689	{
690	unsigned int delta;
691	int ret = 0;
692
693	if (min_ratio > 100 * BDI_RATIO_SCALE)
694	return -EINVAL;
695
696	spin_lock_bh(lock: &bdi_lock);
697	if (min_ratio > bdi->max_ratio) {
698	ret = -EINVAL;
699	} else {
700	if (min_ratio < bdi->min_ratio) {
701	delta = bdi->min_ratio - min_ratio;
702	bdi_min_ratio -= delta;
703	bdi->min_ratio = min_ratio;
704	} else {
705	delta = min_ratio - bdi->min_ratio;
706	if (bdi_min_ratio + delta < 100 * BDI_RATIO_SCALE) {
707	bdi_min_ratio += delta;
708	bdi->min_ratio = min_ratio;
709	} else {
710	ret = -EINVAL;
711	}
712	}
713	}
714	spin_unlock_bh(lock: &bdi_lock);
715
716	return ret;
717	}
718
719	static int __bdi_set_max_ratio(struct backing_dev_info *bdi, unsigned int max_ratio)
720	{
721	int ret = 0;
722
723	if (max_ratio > 100 * BDI_RATIO_SCALE)
724	return -EINVAL;
725
726	spin_lock_bh(lock: &bdi_lock);
727	if (bdi->min_ratio > max_ratio) {
728	ret = -EINVAL;
729	} else {
730	bdi->max_ratio = max_ratio;
731	bdi->max_prop_frac = (FPROP_FRAC_BASE * max_ratio) /
732	(100 * BDI_RATIO_SCALE);
733	}
734	spin_unlock_bh(lock: &bdi_lock);
735
736	return ret;
737	}
738
739	int bdi_set_min_ratio_no_scale(struct backing_dev_info *bdi, unsigned int min_ratio)
740	{
741	return __bdi_set_min_ratio(bdi, min_ratio);
742	}
743
744	int bdi_set_max_ratio_no_scale(struct backing_dev_info *bdi, unsigned int max_ratio)
745	{
746	return __bdi_set_max_ratio(bdi, max_ratio);
747	}
748
749	int bdi_set_min_ratio(struct backing_dev_info *bdi, unsigned int min_ratio)
750	{
751	return __bdi_set_min_ratio(bdi, min_ratio: min_ratio * BDI_RATIO_SCALE);
752	}
753
754	int bdi_set_max_ratio(struct backing_dev_info *bdi, unsigned int max_ratio)
755	{
756	return __bdi_set_max_ratio(bdi, max_ratio: max_ratio * BDI_RATIO_SCALE);
757	}
758	EXPORT_SYMBOL(bdi_set_max_ratio);
759
760	u64 bdi_get_min_bytes(struct backing_dev_info *bdi)
761	{
762	return bdi_get_bytes(ratio: bdi->min_ratio);
763	}
764
765	int bdi_set_min_bytes(struct backing_dev_info *bdi, u64 min_bytes)
766	{
767	int ret;
768	unsigned long pages = min_bytes >> PAGE_SHIFT;
769	unsigned long min_ratio;
770
771	ret = bdi_check_pages_limit(pages);
772	if (ret)
773	return ret;
774
775	min_ratio = bdi_ratio_from_pages(pages);
776	return __bdi_set_min_ratio(bdi, min_ratio);
777	}
778
779	u64 bdi_get_max_bytes(struct backing_dev_info *bdi)
780	{
781	return bdi_get_bytes(ratio: bdi->max_ratio);
782	}
783
784	int bdi_set_max_bytes(struct backing_dev_info *bdi, u64 max_bytes)
785	{
786	int ret;
787	unsigned long pages = max_bytes >> PAGE_SHIFT;
788	unsigned long max_ratio;
789
790	ret = bdi_check_pages_limit(pages);
791	if (ret)
792	return ret;
793
794	max_ratio = bdi_ratio_from_pages(pages);
795	return __bdi_set_max_ratio(bdi, max_ratio);
796	}
797
798	int bdi_set_strict_limit(struct backing_dev_info *bdi, unsigned int strict_limit)
799	{
800	if (strict_limit > 1)
801	return -EINVAL;
802
803	spin_lock_bh(lock: &bdi_lock);
804	if (strict_limit)
805	bdi->capabilities |= BDI_CAP_STRICTLIMIT;
806	else
807	bdi->capabilities &= ~BDI_CAP_STRICTLIMIT;
808	spin_unlock_bh(lock: &bdi_lock);
809
810	return 0;
811	}
812
813	static unsigned long dirty_freerun_ceiling(unsigned long thresh,
814	unsigned long bg_thresh)
815	{
816	return (thresh + bg_thresh) / 2;
817	}
818
819	static unsigned long hard_dirty_limit(struct wb_domain *dom,
820	unsigned long thresh)
821	{
822	return max(thresh, dom->dirty_limit);
823	}
824
825	/*
826	* Memory which can be further allocated to a memcg domain is capped by
827	* system-wide clean memory excluding the amount being used in the domain.
828	*/
829	static void mdtc_calc_avail(struct dirty_throttle_control *mdtc,
830	unsigned long filepages, unsigned long headroom)
831	{
832	struct dirty_throttle_control *gdtc = mdtc_gdtc(mdtc);
833	unsigned long clean = filepages - min(filepages, mdtc->dirty);
834	unsigned long global_clean = gdtc->avail - min(gdtc->avail, gdtc->dirty);
835	unsigned long other_clean = global_clean - min(global_clean, clean);
836
837	mdtc->avail = filepages + min(headroom, other_clean);
838	}
839
840	/**
841	* __wb_calc_thresh - @wb's share of dirty throttling threshold
842	* @dtc: dirty_throttle_context of interest
843	*
844	* Note that balance_dirty_pages() will only seriously take it as a hard limit
845	* when sleeping max_pause per page is not enough to keep the dirty pages under
846	* control. For example, when the device is completely stalled due to some error
847	* conditions, or when there are 1000 dd tasks writing to a slow 10MB/s USB key.
848	* In the other normal situations, it acts more gently by throttling the tasks
849	* more (rather than completely block them) when the wb dirty pages go high.
850	*
851	* It allocates high/low dirty limits to fast/slow devices, in order to prevent
852	* - starving fast devices
853	* - piling up dirty pages (that will take long time to sync) on slow devices
854	*
855	* The wb's share of dirty limit will be adapting to its throughput and
856	* bounded by the bdi->min_ratio and/or bdi->max_ratio parameters, if set.
857	*
858	* Return: @wb's dirty limit in pages. The term "dirty" in the context of
859	* dirty balancing includes all PG_dirty and PG_writeback pages.
860	*/
861	static unsigned long __wb_calc_thresh(struct dirty_throttle_control *dtc)
862	{
863	struct wb_domain *dom = dtc_dom(dtc);
864	unsigned long thresh = dtc->thresh;
865	u64 wb_thresh;
866	unsigned long numerator, denominator;
867	unsigned long wb_min_ratio, wb_max_ratio;
868
869	/*
870	* Calculate this BDI's share of the thresh ratio.
871	*/
872	fprop_fraction_percpu(p: &dom->completions, pl: dtc->wb_completions,
873	numerator: &numerator, denominator: &denominator);
874
875	wb_thresh = (thresh * (100 * BDI_RATIO_SCALE - bdi_min_ratio)) / (100 * BDI_RATIO_SCALE);
876	wb_thresh *= numerator;
877	wb_thresh = div64_ul(wb_thresh, denominator);
878
879	wb_min_max_ratio(wb: dtc->wb, minp: &wb_min_ratio, maxp: &wb_max_ratio);
880
881	wb_thresh += (thresh * wb_min_ratio) / (100 * BDI_RATIO_SCALE);
882	if (wb_thresh > (thresh * wb_max_ratio) / (100 * BDI_RATIO_SCALE))
883	wb_thresh = thresh * wb_max_ratio / (100 * BDI_RATIO_SCALE);
884
885	return wb_thresh;
886	}
887
888	unsigned long wb_calc_thresh(struct bdi_writeback *wb, unsigned long thresh)
889	{
890	struct dirty_throttle_control gdtc = { GDTC_INIT(wb),
891	.thresh = thresh };
892	return __wb_calc_thresh(dtc: &gdtc);
893	}
894
895	/*
896	* setpoint - dirty 3
897	* f(dirty) := 1.0 + (----------------)
898	* limit - setpoint
899	*
900	* it's a 3rd order polynomial that subjects to
901	*
902	* (1) f(freerun) = 2.0 => rampup dirty_ratelimit reasonably fast
903	* (2) f(setpoint) = 1.0 => the balance point
904	* (3) f(limit) = 0 => the hard limit
905	* (4) df/dx <= 0 => negative feedback control
906	* (5) the closer to setpoint, the smaller |df/dx| (and the reverse)
907	* => fast response on large errors; small oscillation near setpoint
908	*/
909	static long long pos_ratio_polynom(unsigned long setpoint,
910	unsigned long dirty,
911	unsigned long limit)
912	{
913	long long pos_ratio;
914	long x;
915
916	x = div64_s64(dividend: ((s64)setpoint - (s64)dirty) << RATELIMIT_CALC_SHIFT,
917	divisor: (limit - setpoint) | 1);
918	pos_ratio = x;
919	pos_ratio = pos_ratio * x >> RATELIMIT_CALC_SHIFT;
920	pos_ratio = pos_ratio * x >> RATELIMIT_CALC_SHIFT;
921	pos_ratio += 1 << RATELIMIT_CALC_SHIFT;
922
923	return clamp(pos_ratio, 0LL, 2LL << RATELIMIT_CALC_SHIFT);
924	}
925
926	/*
927	* Dirty position control.
928	*
929	* (o) global/bdi setpoints
930	*
931	* We want the dirty pages be balanced around the global/wb setpoints.
932	* When the number of dirty pages is higher/lower than the setpoint, the
933	* dirty position control ratio (and hence task dirty ratelimit) will be
934	* decreased/increased to bring the dirty pages back to the setpoint.
935	*
936	* pos_ratio = 1 << RATELIMIT_CALC_SHIFT
937	*
938	* if (dirty < setpoint) scale up pos_ratio
939	* if (dirty > setpoint) scale down pos_ratio
940	*
941	* if (wb_dirty < wb_setpoint) scale up pos_ratio
942	* if (wb_dirty > wb_setpoint) scale down pos_ratio
943	*
944	* task_ratelimit = dirty_ratelimit * pos_ratio >> RATELIMIT_CALC_SHIFT
945	*
946	* (o) global control line
947	*
948	* ^ pos_ratio
949	* |
950	* | |<===== global dirty control scope ======>|
951	* 2.0 * * * * * * *
952	* | .*
953	* | . *
954	* | . *
955	* | . *
956	* | . *
957	* | . *
958	* 1.0 ................................*
959	* | . . *
960	* | . . *
961	* | . . *
962	* | . . *
963	* | . . *
964	* 0 +------------.------------------.----------------------*------------->
965	* freerun^ setpoint^ limit^ dirty pages
966	*
967	* (o) wb control line
968	*
969	* ^ pos_ratio
970	* |
971	* | *
972	* | *
973	* | *
974	* | *
975	* | * |<=========== span ============>|
976	* 1.0 .......................*
977	* | . *
978	* | . *
979	* | . *
980	* | . *
981	* | . *
982	* | . *
983	* | . *
984	* | . *
985	* | . *
986	* | . *
987	* | . *
988	* 1/4 ...............................................* * * * * * * * * * * *
989	* | . .
990	* | . .
991	* | . .
992	* 0 +----------------------.-------------------------------.------------->
993	* wb_setpoint^ x_intercept^
994	*
995	* The wb control line won't drop below pos_ratio=1/4, so that wb_dirty can
996	* be smoothly throttled down to normal if it starts high in situations like
997	* - start writing to a slow SD card and a fast disk at the same time. The SD
998	* card's wb_dirty may rush to many times higher than wb_setpoint.
999	* - the wb dirty thresh drops quickly due to change of JBOD workload
1000	*/
1001	static void wb_position_ratio(struct dirty_throttle_control *dtc)
1002	{
1003	struct bdi_writeback *wb = dtc->wb;
1004	unsigned long write_bw = READ_ONCE(wb->avg_write_bandwidth);
1005	unsigned long freerun = dirty_freerun_ceiling(thresh: dtc->thresh, bg_thresh: dtc->bg_thresh);
1006	unsigned long limit = hard_dirty_limit(dom: dtc_dom(dtc), thresh: dtc->thresh);
1007	unsigned long wb_thresh = dtc->wb_thresh;
1008	unsigned long x_intercept;
1009	unsigned long setpoint; /* dirty pages' target balance point */
1010	unsigned long wb_setpoint;
1011	unsigned long span;
1012	long long pos_ratio; /* for scaling up/down the rate limit */
1013	long x;
1014
1015	dtc->pos_ratio = 0;
1016
1017	if (unlikely(dtc->dirty >= limit))
1018	return;
1019
1020	/*
1021	* global setpoint
1022	*
1023	* See comment for pos_ratio_polynom().
1024	*/
1025	setpoint = (freerun + limit) / 2;
1026	pos_ratio = pos_ratio_polynom(setpoint, dirty: dtc->dirty, limit);
1027
1028	/*
1029	* The strictlimit feature is a tool preventing mistrusted filesystems
1030	* from growing a large number of dirty pages before throttling. For
1031	* such filesystems balance_dirty_pages always checks wb counters
1032	* against wb limits. Even if global "nr_dirty" is under "freerun".
1033	* This is especially important for fuse which sets bdi->max_ratio to
1034	* 1% by default. Without strictlimit feature, fuse writeback may
1035	* consume arbitrary amount of RAM because it is accounted in
1036	* NR_WRITEBACK_TEMP which is not involved in calculating "nr_dirty".
1037	*
1038	* Here, in wb_position_ratio(), we calculate pos_ratio based on
1039	* two values: wb_dirty and wb_thresh. Let's consider an example:
1040	* total amount of RAM is 16GB, bdi->max_ratio is equal to 1%, global
1041	* limits are set by default to 10% and 20% (background and throttle).
1042	* Then wb_thresh is 1% of 20% of 16GB. This amounts to ~8K pages.
1043	* wb_calc_thresh(wb, bg_thresh) is about ~4K pages. wb_setpoint is
1044	* about ~6K pages (as the average of background and throttle wb
1045	* limits). The 3rd order polynomial will provide positive feedback if
1046	* wb_dirty is under wb_setpoint and vice versa.
1047	*
1048	* Note, that we cannot use global counters in these calculations
1049	* because we want to throttle process writing to a strictlimit wb
1050	* much earlier than global "freerun" is reached (~23MB vs. ~2.3GB
1051	* in the example above).
1052	*/
1053	if (unlikely(wb->bdi->capabilities & BDI_CAP_STRICTLIMIT)) {
1054	long long wb_pos_ratio;
1055
1056	if (dtc->wb_dirty < 8) {
1057	dtc->pos_ratio = min_t(long long, pos_ratio * 2,
1058	2 << RATELIMIT_CALC_SHIFT);
1059	return;
1060	}
1061
1062	if (dtc->wb_dirty >= wb_thresh)
1063	return;
1064
1065	wb_setpoint = dirty_freerun_ceiling(thresh: wb_thresh,
1066	bg_thresh: dtc->wb_bg_thresh);
1067
1068	if (wb_setpoint == 0 || wb_setpoint == wb_thresh)
1069	return;
1070
1071	wb_pos_ratio = pos_ratio_polynom(setpoint: wb_setpoint, dirty: dtc->wb_dirty,
1072	limit: wb_thresh);
1073
1074	/*
1075	* Typically, for strictlimit case, wb_setpoint << setpoint
1076	* and pos_ratio >> wb_pos_ratio. In the other words global
1077	* state ("dirty") is not limiting factor and we have to
1078	* make decision based on wb counters. But there is an
1079	* important case when global pos_ratio should get precedence:
1080	* global limits are exceeded (e.g. due to activities on other
1081	* wb's) while given strictlimit wb is below limit.
1082	*
1083	* "pos_ratio * wb_pos_ratio" would work for the case above,
1084	* but it would look too non-natural for the case of all
1085	* activity in the system coming from a single strictlimit wb
1086	* with bdi->max_ratio == 100%.
1087	*
1088	* Note that min() below somewhat changes the dynamics of the
1089	* control system. Normally, pos_ratio value can be well over 3
1090	* (when globally we are at freerun and wb is well below wb
1091	* setpoint). Now the maximum pos_ratio in the same situation
1092	* is 2. We might want to tweak this if we observe the control
1093	* system is too slow to adapt.
1094	*/
1095	dtc->pos_ratio = min(pos_ratio, wb_pos_ratio);
1096	return;
1097	}
1098
1099	/*
1100	* We have computed basic pos_ratio above based on global situation. If
1101	* the wb is over/under its share of dirty pages, we want to scale
1102	* pos_ratio further down/up. That is done by the following mechanism.
1103	*/
1104
1105	/*
1106	* wb setpoint
1107	*
1108	* f(wb_dirty) := 1.0 + k * (wb_dirty - wb_setpoint)
1109	*
1110	* x_intercept - wb_dirty
1111	* := --------------------------
1112	* x_intercept - wb_setpoint
1113	*
1114	* The main wb control line is a linear function that subjects to
1115	*
1116	* (1) f(wb_setpoint) = 1.0
1117	* (2) k = - 1 / (8 * write_bw) (in single wb case)
1118	* or equally: x_intercept = wb_setpoint + 8 * write_bw
1119	*
1120	* For single wb case, the dirty pages are observed to fluctuate
1121	* regularly within range
1122	* [wb_setpoint - write_bw/2, wb_setpoint + write_bw/2]
1123	* for various filesystems, where (2) can yield in a reasonable 12.5%
1124	* fluctuation range for pos_ratio.
1125	*
1126	* For JBOD case, wb_thresh (not wb_dirty!) could fluctuate up to its
1127	* own size, so move the slope over accordingly and choose a slope that
1128	* yields 100% pos_ratio fluctuation on suddenly doubled wb_thresh.
1129	*/
1130	if (unlikely(wb_thresh > dtc->thresh))
1131	wb_thresh = dtc->thresh;
1132	/*
1133	* It's very possible that wb_thresh is close to 0 not because the
1134	* device is slow, but that it has remained inactive for long time.
1135	* Honour such devices a reasonable good (hopefully IO efficient)
1136	* threshold, so that the occasional writes won't be blocked and active
1137	* writes can rampup the threshold quickly.
1138	*/
1139	wb_thresh = max(wb_thresh, (limit - dtc->dirty) / 8);
1140	/*
1141	* scale global setpoint to wb's:
1142	* wb_setpoint = setpoint * wb_thresh / thresh
1143	*/
1144	x = div_u64(dividend: (u64)wb_thresh << 16, divisor: dtc->thresh | 1);
1145	wb_setpoint = setpoint * (u64)x >> 16;
1146	/*
1147	* Use span=(8*write_bw) in single wb case as indicated by
1148	* (thresh - wb_thresh ~= 0) and transit to wb_thresh in JBOD case.
1149	*
1150	* wb_thresh thresh - wb_thresh
1151	* span = --------- * (8 * write_bw) + ------------------ * wb_thresh
1152	* thresh thresh
1153	*/
1154	span = (dtc->thresh - wb_thresh + 8 * write_bw) * (u64)x >> 16;
1155	x_intercept = wb_setpoint + span;
1156
1157	if (dtc->wb_dirty < x_intercept - span / 4) {
1158	pos_ratio = div64_u64(dividend: pos_ratio * (x_intercept - dtc->wb_dirty),
1159	divisor: (x_intercept - wb_setpoint) | 1);
1160	} else
1161	pos_ratio /= 4;
1162
1163	/*
1164	* wb reserve area, safeguard against dirty pool underrun and disk idle
1165	* It may push the desired control point of global dirty pages higher
1166	* than setpoint.
1167	*/
1168	x_intercept = wb_thresh / 2;
1169	if (dtc->wb_dirty < x_intercept) {
1170	if (dtc->wb_dirty > x_intercept / 8)
1171	pos_ratio = div_u64(dividend: pos_ratio * x_intercept,
1172	divisor: dtc->wb_dirty);
1173	else
1174	pos_ratio *= 8;
1175	}
1176
1177	dtc->pos_ratio = pos_ratio;
1178	}
1179
1180	static void wb_update_write_bandwidth(struct bdi_writeback *wb,
1181	unsigned long elapsed,
1182	unsigned long written)
1183	{
1184	const unsigned long period = roundup_pow_of_two(3 * HZ);
1185	unsigned long avg = wb->avg_write_bandwidth;
1186	unsigned long old = wb->write_bandwidth;
1187	u64 bw;
1188
1189	/*
1190	* bw = written * HZ / elapsed
1191	*
1192	* bw * elapsed + write_bandwidth * (period - elapsed)
1193	* write_bandwidth = ---------------------------------------------------
1194	* period
1195	*
1196	* @written may have decreased due to folio_redirty_for_writepage().
1197	* Avoid underflowing @bw calculation.
1198	*/
1199	bw = written - min(written, wb->written_stamp);
1200	bw *= HZ;
1201	if (unlikely(elapsed > period)) {
1202	bw = div64_ul(bw, elapsed);
1203	avg = bw;
1204	goto out;
1205	}
1206	bw += (u64)wb->write_bandwidth * (period - elapsed);
1207	bw >>= ilog2(period);
1208
1209	/*
1210	* one more level of smoothing, for filtering out sudden spikes
1211	*/
1212	if (avg > old && old >= (unsigned long)bw)
1213	avg -= (avg - old) >> 3;
1214
1215	if (avg < old && old <= (unsigned long)bw)
1216	avg += (old - avg) >> 3;
1217
1218	out:
1219	/* keep avg > 0 to guarantee that tot > 0 if there are dirty wbs */
1220	avg = max(avg, 1LU);
1221	if (wb_has_dirty_io(wb)) {
1222	long delta = avg - wb->avg_write_bandwidth;
1223	WARN_ON_ONCE(atomic_long_add_return(delta,
1224	&wb->bdi->tot_write_bandwidth) <= 0);
1225	}
1226	wb->write_bandwidth = bw;
1227	WRITE_ONCE(wb->avg_write_bandwidth, avg);
1228	}
1229
1230	static void update_dirty_limit(struct dirty_throttle_control *dtc)
1231	{
1232	struct wb_domain *dom = dtc_dom(dtc);
1233	unsigned long thresh = dtc->thresh;
1234	unsigned long limit = dom->dirty_limit;
1235
1236	/*
1237	* Follow up in one step.
1238	*/
1239	if (limit < thresh) {
1240	limit = thresh;
1241	goto update;
1242	}
1243
1244	/*
1245	* Follow down slowly. Use the higher one as the target, because thresh
1246	* may drop below dirty. This is exactly the reason to introduce
1247	* dom->dirty_limit which is guaranteed to lie above the dirty pages.
1248	*/
1249	thresh = max(thresh, dtc->dirty);
1250	if (limit > thresh) {
1251	limit -= (limit - thresh) >> 5;
1252	goto update;
1253	}
1254	return;
1255	update:
1256	dom->dirty_limit = limit;
1257	}
1258
1259	static void domain_update_dirty_limit(struct dirty_throttle_control *dtc,
1260	unsigned long now)
1261	{
1262	struct wb_domain *dom = dtc_dom(dtc);
1263
1264	/*
1265	* check locklessly first to optimize away locking for the most time
1266	*/
1267	if (time_before(now, dom->dirty_limit_tstamp + BANDWIDTH_INTERVAL))
1268	return;
1269
1270	spin_lock(lock: &dom->lock);
1271	if (time_after_eq(now, dom->dirty_limit_tstamp + BANDWIDTH_INTERVAL)) {
1272	update_dirty_limit(dtc);
1273	dom->dirty_limit_tstamp = now;
1274	}
1275	spin_unlock(lock: &dom->lock);
1276	}
1277
1278	/*
1279	* Maintain wb->dirty_ratelimit, the base dirty throttle rate.
1280	*
1281	* Normal wb tasks will be curbed at or below it in long term.
1282	* Obviously it should be around (write_bw / N) when there are N dd tasks.
1283	*/
1284	static void wb_update_dirty_ratelimit(struct dirty_throttle_control *dtc,
1285	unsigned long dirtied,
1286	unsigned long elapsed)
1287	{
1288	struct bdi_writeback *wb = dtc->wb;
1289	unsigned long dirty = dtc->dirty;
1290	unsigned long freerun = dirty_freerun_ceiling(thresh: dtc->thresh, bg_thresh: dtc->bg_thresh);
1291	unsigned long limit = hard_dirty_limit(dom: dtc_dom(dtc), thresh: dtc->thresh);
1292	unsigned long setpoint = (freerun + limit) / 2;
1293	unsigned long write_bw = wb->avg_write_bandwidth;
1294	unsigned long dirty_ratelimit = wb->dirty_ratelimit;
1295	unsigned long dirty_rate;
1296	unsigned long task_ratelimit;
1297	unsigned long balanced_dirty_ratelimit;
1298	unsigned long step;
1299	unsigned long x;
1300	unsigned long shift;
1301
1302	/*
1303	* The dirty rate will match the writeout rate in long term, except
1304	* when dirty pages are truncated by userspace or re-dirtied by FS.
1305	*/
1306	dirty_rate = (dirtied - wb->dirtied_stamp) * HZ / elapsed;
1307
1308	/*
1309	* task_ratelimit reflects each dd's dirty rate for the past 200ms.
1310	*/
1311	task_ratelimit = (u64)dirty_ratelimit *
1312	dtc->pos_ratio >> RATELIMIT_CALC_SHIFT;
1313	task_ratelimit++; /* it helps rampup dirty_ratelimit from tiny values */
1314
1315	/*
1316	* A linear estimation of the "balanced" throttle rate. The theory is,
1317	* if there are N dd tasks, each throttled at task_ratelimit, the wb's
1318	* dirty_rate will be measured to be (N * task_ratelimit). So the below
1319	* formula will yield the balanced rate limit (write_bw / N).
1320	*
1321	* Note that the expanded form is not a pure rate feedback:
1322	* rate_(i+1) = rate_(i) * (write_bw / dirty_rate) (1)
1323	* but also takes pos_ratio into account:
1324	* rate_(i+1) = rate_(i) * (write_bw / dirty_rate) * pos_ratio (2)
1325	*
1326	* (1) is not realistic because pos_ratio also takes part in balancing
1327	* the dirty rate. Consider the state
1328	* pos_ratio = 0.5 (3)
1329	* rate = 2 * (write_bw / N) (4)
1330	* If (1) is used, it will stuck in that state! Because each dd will
1331	* be throttled at
1332	* task_ratelimit = pos_ratio * rate = (write_bw / N) (5)
1333	* yielding
1334	* dirty_rate = N * task_ratelimit = write_bw (6)
1335	* put (6) into (1) we get
1336	* rate_(i+1) = rate_(i) (7)
1337	*
1338	* So we end up using (2) to always keep
1339	* rate_(i+1) ~= (write_bw / N) (8)
1340	* regardless of the value of pos_ratio. As long as (8) is satisfied,
1341	* pos_ratio is able to drive itself to 1.0, which is not only where
1342	* the dirty count meet the setpoint, but also where the slope of
1343	* pos_ratio is most flat and hence task_ratelimit is least fluctuated.
1344	*/
1345	balanced_dirty_ratelimit = div_u64(dividend: (u64)task_ratelimit * write_bw,
1346	divisor: dirty_rate | 1);
1347	/*
1348	* balanced_dirty_ratelimit ~= (write_bw / N) <= write_bw
1349	*/
1350	if (unlikely(balanced_dirty_ratelimit > write_bw))
1351	balanced_dirty_ratelimit = write_bw;
1352
1353	/*
1354	* We could safely do this and return immediately:
1355	*
1356	* wb->dirty_ratelimit = balanced_dirty_ratelimit;
1357	*
1358	* However to get a more stable dirty_ratelimit, the below elaborated
1359	* code makes use of task_ratelimit to filter out singular points and
1360	* limit the step size.
1361	*
1362	* The below code essentially only uses the relative value of
1363	*
1364	* task_ratelimit - dirty_ratelimit
1365	* = (pos_ratio - 1) * dirty_ratelimit
1366	*
1367	* which reflects the direction and size of dirty position error.
1368	*/
1369
1370	/*
1371	* dirty_ratelimit will follow balanced_dirty_ratelimit iff
1372	* task_ratelimit is on the same side of dirty_ratelimit, too.
1373	* For example, when
1374	* - dirty_ratelimit > balanced_dirty_ratelimit
1375	* - dirty_ratelimit > task_ratelimit (dirty pages are above setpoint)
1376	* lowering dirty_ratelimit will help meet both the position and rate
1377	* control targets. Otherwise, don't update dirty_ratelimit if it will
1378	* only help meet the rate target. After all, what the users ultimately
1379	* feel and care are stable dirty rate and small position error.
1380	*
1381	* |task_ratelimit - dirty_ratelimit| is used to limit the step size
1382	* and filter out the singular points of balanced_dirty_ratelimit. Which
1383	* keeps jumping around randomly and can even leap far away at times
1384	* due to the small 200ms estimation period of dirty_rate (we want to
1385	* keep that period small to reduce time lags).
1386	*/
1387	step = 0;
1388
1389	/*
1390	* For strictlimit case, calculations above were based on wb counters
1391	* and limits (starting from pos_ratio = wb_position_ratio() and up to
1392	* balanced_dirty_ratelimit = task_ratelimit * write_bw / dirty_rate).
1393	* Hence, to calculate "step" properly, we have to use wb_dirty as
1394	* "dirty" and wb_setpoint as "setpoint".
1395	*
1396	* We rampup dirty_ratelimit forcibly if wb_dirty is low because
1397	* it's possible that wb_thresh is close to zero due to inactivity
1398	* of backing device.
1399	*/
1400	if (unlikely(wb->bdi->capabilities & BDI_CAP_STRICTLIMIT)) {
1401	dirty = dtc->wb_dirty;
1402	if (dtc->wb_dirty < 8)
1403	setpoint = dtc->wb_dirty + 1;
1404	else
1405	setpoint = (dtc->wb_thresh + dtc->wb_bg_thresh) / 2;
1406	}
1407
1408	if (dirty < setpoint) {
1409	x = min3(wb->balanced_dirty_ratelimit,
1410	balanced_dirty_ratelimit, task_ratelimit);
1411	if (dirty_ratelimit < x)
1412	step = x - dirty_ratelimit;
1413	} else {
1414	x = max3(wb->balanced_dirty_ratelimit,
1415	balanced_dirty_ratelimit, task_ratelimit);
1416	if (dirty_ratelimit > x)
1417	step = dirty_ratelimit - x;
1418	}
1419
1420	/*
1421	* Don't pursue 100% rate matching. It's impossible since the balanced
1422	* rate itself is constantly fluctuating. So decrease the track speed
1423	* when it gets close to the target. Helps eliminate pointless tremors.
1424	*/
1425	shift = dirty_ratelimit / (2 * step + 1);
1426	if (shift < BITS_PER_LONG)
1427	step = DIV_ROUND_UP(step >> shift, 8);
1428	else
1429	step = 0;
1430
1431	if (dirty_ratelimit < balanced_dirty_ratelimit)
1432	dirty_ratelimit += step;
1433	else
1434	dirty_ratelimit -= step;
1435
1436	WRITE_ONCE(wb->dirty_ratelimit, max(dirty_ratelimit, 1UL));
1437	wb->balanced_dirty_ratelimit = balanced_dirty_ratelimit;
1438
1439	trace_bdi_dirty_ratelimit(wb, dirty_rate, task_ratelimit);
1440	}
1441
1442	static void __wb_update_bandwidth(struct dirty_throttle_control *gdtc,
1443	struct dirty_throttle_control *mdtc,
1444	bool update_ratelimit)
1445	{
1446	struct bdi_writeback *wb = gdtc->wb;
1447	unsigned long now = jiffies;
1448	unsigned long elapsed;
1449	unsigned long dirtied;
1450	unsigned long written;
1451
1452	spin_lock(lock: &wb->list_lock);
1453
1454	/*
1455	* Lockless checks for elapsed time are racy and delayed update after
1456	* IO completion doesn't do it at all (to make sure written pages are
1457	* accounted reasonably quickly). Make sure elapsed >= 1 to avoid
1458	* division errors.
1459	*/
1460	elapsed = max(now - wb->bw_time_stamp, 1UL);
1461	dirtied = percpu_counter_read(fbc: &wb->stat[WB_DIRTIED]);
1462	written = percpu_counter_read(fbc: &wb->stat[WB_WRITTEN]);
1463
1464	if (update_ratelimit) {
1465	domain_update_dirty_limit(dtc: gdtc, now);
1466	wb_update_dirty_ratelimit(dtc: gdtc, dirtied, elapsed);
1467
1468	/*
1469	* @mdtc is always NULL if !CGROUP_WRITEBACK but the
1470	* compiler has no way to figure that out. Help it.
1471	*/
1472	if (IS_ENABLED(CONFIG_CGROUP_WRITEBACK) && mdtc) {
1473	domain_update_dirty_limit(dtc: mdtc, now);
1474	wb_update_dirty_ratelimit(dtc: mdtc, dirtied, elapsed);
1475	}
1476	}
1477	wb_update_write_bandwidth(wb, elapsed, written);
1478
1479	wb->dirtied_stamp = dirtied;
1480	wb->written_stamp = written;
1481	WRITE_ONCE(wb->bw_time_stamp, now);
1482	spin_unlock(lock: &wb->list_lock);
1483	}
1484
1485	void wb_update_bandwidth(struct bdi_writeback *wb)
1486	{
1487	struct dirty_throttle_control gdtc = { GDTC_INIT(wb) };
1488
1489	__wb_update_bandwidth(gdtc: &gdtc, NULL, update_ratelimit: false);
1490	}
1491
1492	/* Interval after which we consider wb idle and don't estimate bandwidth */
1493	#define WB_BANDWIDTH_IDLE_JIF (HZ)
1494
1495	static void wb_bandwidth_estimate_start(struct bdi_writeback *wb)
1496	{
1497	unsigned long now = jiffies;
1498	unsigned long elapsed = now - READ_ONCE(wb->bw_time_stamp);
1499
1500	if (elapsed > WB_BANDWIDTH_IDLE_JIF &&
1501	!atomic_read(v: &wb->writeback_inodes)) {
1502	spin_lock(lock: &wb->list_lock);
1503	wb->dirtied_stamp = wb_stat(wb, item: WB_DIRTIED);
1504	wb->written_stamp = wb_stat(wb, item: WB_WRITTEN);
1505	WRITE_ONCE(wb->bw_time_stamp, now);
1506	spin_unlock(lock: &wb->list_lock);
1507	}
1508	}
1509
1510	/*
1511	* After a task dirtied this many pages, balance_dirty_pages_ratelimited()
1512	* will look to see if it needs to start dirty throttling.
1513	*
1514	* If dirty_poll_interval is too low, big NUMA machines will call the expensive
1515	* global_zone_page_state() too often. So scale it near-sqrt to the safety margin
1516	* (the number of pages we may dirty without exceeding the dirty limits).
1517	*/
1518	static unsigned long dirty_poll_interval(unsigned long dirty,
1519	unsigned long thresh)
1520	{
1521	if (thresh > dirty)
1522	return 1UL << (ilog2(thresh - dirty) >> 1);
1523
1524	return 1;
1525	}
1526
1527	static unsigned long wb_max_pause(struct bdi_writeback *wb,
1528	unsigned long wb_dirty)
1529	{
1530	unsigned long bw = READ_ONCE(wb->avg_write_bandwidth);
1531	unsigned long t;
1532
1533	/*
1534	* Limit pause time for small memory systems. If sleeping for too long
1535	* time, a small pool of dirty/writeback pages may go empty and disk go
1536	* idle.
1537	*
1538	* 8 serves as the safety ratio.
1539	*/
1540	t = wb_dirty / (1 + bw / roundup_pow_of_two(1 + HZ / 8));
1541	t++;
1542
1543	return min_t(unsigned long, t, MAX_PAUSE);
1544	}
1545
1546	static long wb_min_pause(struct bdi_writeback *wb,
1547	long max_pause,
1548	unsigned long task_ratelimit,
1549	unsigned long dirty_ratelimit,
1550	int *nr_dirtied_pause)
1551	{
1552	long hi = ilog2(READ_ONCE(wb->avg_write_bandwidth));
1553	long lo = ilog2(READ_ONCE(wb->dirty_ratelimit));
1554	long t; /* target pause */
1555	long pause; /* estimated next pause */
1556	int pages; /* target nr_dirtied_pause */
1557
1558	/* target for 10ms pause on 1-dd case */
1559	t = max(1, HZ / 100);
1560
1561	/*
1562	* Scale up pause time for concurrent dirtiers in order to reduce CPU
1563	* overheads.
1564	*
1565	* (N * 10ms) on 2^N concurrent tasks.
1566	*/
1567	if (hi > lo)
1568	t += (hi - lo) * (10 * HZ) / 1024;
1569
1570	/*
1571	* This is a bit convoluted. We try to base the next nr_dirtied_pause
1572	* on the much more stable dirty_ratelimit. However the next pause time
1573	* will be computed based on task_ratelimit and the two rate limits may
1574	* depart considerably at some time. Especially if task_ratelimit goes
1575	* below dirty_ratelimit/2 and the target pause is max_pause, the next
1576	* pause time will be max_pause*2 _trimmed down_ to max_pause. As a
1577	* result task_ratelimit won't be executed faithfully, which could
1578	* eventually bring down dirty_ratelimit.
1579	*
1580	* We apply two rules to fix it up:
1581	* 1) try to estimate the next pause time and if necessary, use a lower
1582	* nr_dirtied_pause so as not to exceed max_pause. When this happens,
1583	* nr_dirtied_pause will be "dancing" with task_ratelimit.
1584	* 2) limit the target pause time to max_pause/2, so that the normal
1585	* small fluctuations of task_ratelimit won't trigger rule (1) and
1586	* nr_dirtied_pause will remain as stable as dirty_ratelimit.
1587	*/
1588	t = min(t, 1 + max_pause / 2);
1589	pages = dirty_ratelimit * t / roundup_pow_of_two(HZ);
1590
1591	/*
1592	* Tiny nr_dirtied_pause is found to hurt I/O performance in the test
1593	* case fio-mmap-randwrite-64k, which does 16*{sync read, async write}.
1594	* When the 16 consecutive reads are often interrupted by some dirty
1595	* throttling pause during the async writes, cfq will go into idles
1596	* (deadline is fine). So push nr_dirtied_pause as high as possible
1597	* until reaches DIRTY_POLL_THRESH=32 pages.
1598	*/
1599	if (pages < DIRTY_POLL_THRESH) {
1600	t = max_pause;
1601	pages = dirty_ratelimit * t / roundup_pow_of_two(HZ);
1602	if (pages > DIRTY_POLL_THRESH) {
1603	pages = DIRTY_POLL_THRESH;
1604	t = HZ * DIRTY_POLL_THRESH / dirty_ratelimit;
1605	}
1606	}
1607
1608	pause = HZ * pages / (task_ratelimit + 1);
1609	if (pause > max_pause) {
1610	t = max_pause;
1611	pages = task_ratelimit * t / roundup_pow_of_two(HZ);
1612	}
1613
1614	*nr_dirtied_pause = pages;
1615	/*
1616	* The minimal pause time will normally be half the target pause time.
1617	*/
1618	return pages >= DIRTY_POLL_THRESH ? 1 + t / 2 : t;
1619	}
1620
1621	static inline void wb_dirty_limits(struct dirty_throttle_control *dtc)
1622	{
1623	struct bdi_writeback *wb = dtc->wb;
1624	unsigned long wb_reclaimable;
1625
1626	/*
1627	* wb_thresh is not treated as some limiting factor as
1628	* dirty_thresh, due to reasons
1629	* - in JBOD setup, wb_thresh can fluctuate a lot
1630	* - in a system with HDD and USB key, the USB key may somehow
1631	* go into state (wb_dirty >> wb_thresh) either because
1632	* wb_dirty starts high, or because wb_thresh drops low.
1633	* In this case we don't want to hard throttle the USB key
1634	* dirtiers for 100 seconds until wb_dirty drops under
1635	* wb_thresh. Instead the auxiliary wb control line in
1636	* wb_position_ratio() will let the dirtier task progress
1637	* at some rate <= (write_bw / 2) for bringing down wb_dirty.
1638	*/
1639	dtc->wb_thresh = __wb_calc_thresh(dtc);
1640	dtc->wb_bg_thresh = dtc->thresh ?
1641	div64_u64(dividend: dtc->wb_thresh * dtc->bg_thresh, divisor: dtc->thresh) : 0;
1642
1643	/*
1644	* In order to avoid the stacked BDI deadlock we need
1645	* to ensure we accurately count the 'dirty' pages when
1646	* the threshold is low.
1647	*
1648	* Otherwise it would be possible to get thresh+n pages
1649	* reported dirty, even though there are thresh-m pages
1650	* actually dirty; with m+n sitting in the percpu
1651	* deltas.
1652	*/
1653	if (dtc->wb_thresh < 2 * wb_stat_error()) {
1654	wb_reclaimable = wb_stat_sum(wb, item: WB_RECLAIMABLE);
1655	dtc->wb_dirty = wb_reclaimable + wb_stat_sum(wb, item: WB_WRITEBACK);
1656	} else {
1657	wb_reclaimable = wb_stat(wb, item: WB_RECLAIMABLE);
1658	dtc->wb_dirty = wb_reclaimable + wb_stat(wb, item: WB_WRITEBACK);
1659	}
1660	}
1661
1662	/*
1663	* balance_dirty_pages() must be called by processes which are generating dirty
1664	* data. It looks at the number of dirty pages in the machine and will force
1665	* the caller to wait once crossing the (background_thresh + dirty_thresh) / 2.
1666	* If we're over `background_thresh' then the writeback threads are woken to
1667	* perform some writeout.
1668	*/
1669	static int balance_dirty_pages(struct bdi_writeback *wb,
1670	unsigned long pages_dirtied, unsigned int flags)
1671	{
1672	struct dirty_throttle_control gdtc_stor = { GDTC_INIT(wb) };
1673	struct dirty_throttle_control mdtc_stor = { MDTC_INIT(wb, &gdtc_stor) };
1674	struct dirty_throttle_control * const gdtc = &gdtc_stor;
1675	struct dirty_throttle_control * const mdtc = mdtc_valid(dtc: &mdtc_stor) ?
1676	&mdtc_stor : NULL;
1677	struct dirty_throttle_control *sdtc;
1678	unsigned long nr_reclaimable; /* = file_dirty */
1679	long period;
1680	long pause;
1681	long max_pause;
1682	long min_pause;
1683	int nr_dirtied_pause;
1684	bool dirty_exceeded = false;
1685	unsigned long task_ratelimit;
1686	unsigned long dirty_ratelimit;
1687	struct backing_dev_info *bdi = wb->bdi;
1688	bool strictlimit = bdi->capabilities & BDI_CAP_STRICTLIMIT;
1689	unsigned long start_time = jiffies;
1690	int ret = 0;
1691
1692	for (;;) {
1693	unsigned long now = jiffies;
1694	unsigned long dirty, thresh, bg_thresh;
1695	unsigned long m_dirty = 0; /* stop bogus uninit warnings */
1696	unsigned long m_thresh = 0;
1697	unsigned long m_bg_thresh = 0;
1698
1699	nr_reclaimable = global_node_page_state(item: NR_FILE_DIRTY);
1700	gdtc->avail = global_dirtyable_memory();
1701	gdtc->dirty = nr_reclaimable + global_node_page_state(item: NR_WRITEBACK);
1702
1703	domain_dirty_limits(dtc: gdtc);
1704
1705	if (unlikely(strictlimit)) {
1706	wb_dirty_limits(dtc: gdtc);
1707
1708	dirty = gdtc->wb_dirty;
1709	thresh = gdtc->wb_thresh;
1710	bg_thresh = gdtc->wb_bg_thresh;
1711	} else {
1712	dirty = gdtc->dirty;
1713	thresh = gdtc->thresh;
1714	bg_thresh = gdtc->bg_thresh;
1715	}
1716
1717	if (mdtc) {
1718	unsigned long filepages, headroom, writeback;
1719
1720	/*
1721	* If @wb belongs to !root memcg, repeat the same
1722	* basic calculations for the memcg domain.
1723	*/
1724	mem_cgroup_wb_stats(wb, pfilepages: &filepages, pheadroom: &headroom,
1725	pdirty: &mdtc->dirty, pwriteback: &writeback);
1726	mdtc->dirty += writeback;
1727	mdtc_calc_avail(mdtc, filepages, headroom);
1728
1729	domain_dirty_limits(dtc: mdtc);
1730
1731	if (unlikely(strictlimit)) {
1732	wb_dirty_limits(dtc: mdtc);
1733	m_dirty = mdtc->wb_dirty;
1734	m_thresh = mdtc->wb_thresh;
1735	m_bg_thresh = mdtc->wb_bg_thresh;
1736	} else {
1737	m_dirty = mdtc->dirty;
1738	m_thresh = mdtc->thresh;
1739	m_bg_thresh = mdtc->bg_thresh;
1740	}
1741	}
1742
1743	/*
1744	* In laptop mode, we wait until hitting the higher threshold
1745	* before starting background writeout, and then write out all
1746	* the way down to the lower threshold. So slow writers cause
1747	* minimal disk activity.
1748	*
1749	* In normal mode, we start background writeout at the lower
1750	* background_thresh, to keep the amount of dirty memory low.
1751	*/
1752	if (!laptop_mode && nr_reclaimable > gdtc->bg_thresh &&
1753	!writeback_in_progress(wb))
1754	wb_start_background_writeback(wb);
1755
1756	/*
1757	* Throttle it only when the background writeback cannot
1758	* catch-up. This avoids (excessively) small writeouts
1759	* when the wb limits are ramping up in case of !strictlimit.
1760	*
1761	* In strictlimit case make decision based on the wb counters
1762	* and limits. Small writeouts when the wb limits are ramping
1763	* up are the price we consciously pay for strictlimit-ing.
1764	*
1765	* If memcg domain is in effect, @dirty should be under
1766	* both global and memcg freerun ceilings.
1767	*/
1768	if (dirty <= dirty_freerun_ceiling(thresh, bg_thresh) &&
1769	(!mdtc ||
1770	m_dirty <= dirty_freerun_ceiling(thresh: m_thresh, bg_thresh: m_bg_thresh))) {
1771	unsigned long intv;
1772	unsigned long m_intv;
1773
1774	free_running:
1775	intv = dirty_poll_interval(dirty, thresh);
1776	m_intv = ULONG_MAX;
1777
1778	current->dirty_paused_when = now;
1779	current->nr_dirtied = 0;
1780	if (mdtc)
1781	m_intv = dirty_poll_interval(dirty: m_dirty, thresh: m_thresh);
1782	current->nr_dirtied_pause = min(intv, m_intv);
1783	break;
1784	}
1785
1786	/* Start writeback even when in laptop mode */
1787	if (unlikely(!writeback_in_progress(wb)))
1788	wb_start_background_writeback(wb);
1789
1790	mem_cgroup_flush_foreign(wb);
1791
1792	/*
1793	* Calculate global domain's pos_ratio and select the
1794	* global dtc by default.
1795	*/
1796	if (!strictlimit) {
1797	wb_dirty_limits(dtc: gdtc);
1798
1799	if ((current->flags & PF_LOCAL_THROTTLE) &&
1800	gdtc->wb_dirty <
1801	dirty_freerun_ceiling(thresh: gdtc->wb_thresh,
1802	bg_thresh: gdtc->wb_bg_thresh))
1803	/*
1804	* LOCAL_THROTTLE tasks must not be throttled
1805	* when below the per-wb freerun ceiling.
1806	*/
1807	goto free_running;
1808	}
1809
1810	dirty_exceeded = (gdtc->wb_dirty > gdtc->wb_thresh) &&
1811	((gdtc->dirty > gdtc->thresh) || strictlimit);
1812
1813	wb_position_ratio(dtc: gdtc);
1814	sdtc = gdtc;
1815
1816	if (mdtc) {
1817	/*
1818	* If memcg domain is in effect, calculate its
1819	* pos_ratio. @wb should satisfy constraints from
1820	* both global and memcg domains. Choose the one
1821	* w/ lower pos_ratio.
1822	*/
1823	if (!strictlimit) {
1824	wb_dirty_limits(dtc: mdtc);
1825
1826	if ((current->flags & PF_LOCAL_THROTTLE) &&
1827	mdtc->wb_dirty <
1828	dirty_freerun_ceiling(thresh: mdtc->wb_thresh,
1829	bg_thresh: mdtc->wb_bg_thresh))
1830	/*
1831	* LOCAL_THROTTLE tasks must not be
1832	* throttled when below the per-wb
1833	* freerun ceiling.
1834	*/
1835	goto free_running;
1836	}
1837	dirty_exceeded |= (mdtc->wb_dirty > mdtc->wb_thresh) &&
1838	((mdtc->dirty > mdtc->thresh) || strictlimit);
1839
1840	wb_position_ratio(dtc: mdtc);
1841	if (mdtc->pos_ratio < gdtc->pos_ratio)
1842	sdtc = mdtc;
1843	}
1844
1845	if (dirty_exceeded != wb->dirty_exceeded)
1846	wb->dirty_exceeded = dirty_exceeded;
1847
1848	if (time_is_before_jiffies(READ_ONCE(wb->bw_time_stamp) +
1849	BANDWIDTH_INTERVAL))
1850	__wb_update_bandwidth(gdtc, mdtc, update_ratelimit: true);
1851
1852	/* throttle according to the chosen dtc */
1853	dirty_ratelimit = READ_ONCE(wb->dirty_ratelimit);
1854	task_ratelimit = ((u64)dirty_ratelimit * sdtc->pos_ratio) >>
1855	RATELIMIT_CALC_SHIFT;
1856	max_pause = wb_max_pause(wb, wb_dirty: sdtc->wb_dirty);
1857	min_pause = wb_min_pause(wb, max_pause,
1858	task_ratelimit, dirty_ratelimit,
1859	nr_dirtied_pause: &nr_dirtied_pause);
1860
1861	if (unlikely(task_ratelimit == 0)) {
1862	period = max_pause;
1863	pause = max_pause;
1864	goto pause;
1865	}
1866	period = HZ * pages_dirtied / task_ratelimit;
1867	pause = period;
1868	if (current->dirty_paused_when)
1869	pause -= now - current->dirty_paused_when;
1870	/*
1871	* For less than 1s think time (ext3/4 may block the dirtier
1872	* for up to 800ms from time to time on 1-HDD; so does xfs,
1873	* however at much less frequency), try to compensate it in
1874	* future periods by updating the virtual time; otherwise just
1875	* do a reset, as it may be a light dirtier.
1876	*/
1877	if (pause < min_pause) {
1878	trace_balance_dirty_pages(wb,
1879	thresh: sdtc->thresh,
1880	bg_thresh: sdtc->bg_thresh,
1881	dirty: sdtc->dirty,
1882	bdi_thresh: sdtc->wb_thresh,
1883	bdi_dirty: sdtc->wb_dirty,
1884	dirty_ratelimit,
1885	task_ratelimit,
1886	dirtied: pages_dirtied,
1887	period,
1888	min(pause, 0L),
1889	start_time);
1890	if (pause < -HZ) {
1891	current->dirty_paused_when = now;
1892	current->nr_dirtied = 0;
1893	} else if (period) {
1894	current->dirty_paused_when += period;
1895	current->nr_dirtied = 0;
1896	} else if (current->nr_dirtied_pause <= pages_dirtied)
1897	current->nr_dirtied_pause += pages_dirtied;
1898	break;
1899	}
1900	if (unlikely(pause > max_pause)) {
1901	/* for occasional dropped task_ratelimit */
1902	now += min(pause - max_pause, max_pause);
1903	pause = max_pause;
1904	}
1905
1906	pause:
1907	trace_balance_dirty_pages(wb,
1908	thresh: sdtc->thresh,
1909	bg_thresh: sdtc->bg_thresh,
1910	dirty: sdtc->dirty,
1911	bdi_thresh: sdtc->wb_thresh,
1912	bdi_dirty: sdtc->wb_dirty,
1913	dirty_ratelimit,
1914	task_ratelimit,
1915	dirtied: pages_dirtied,
1916	period,
1917	pause,
1918	start_time);
1919	if (flags & BDP_ASYNC) {
1920	ret = -EAGAIN;
1921	break;
1922	}
1923	__set_current_state(TASK_KILLABLE);
1924	bdi->last_bdp_sleep = jiffies;
1925	io_schedule_timeout(timeout: pause);
1926
1927	current->dirty_paused_when = now + pause;
1928	current->nr_dirtied = 0;
1929	current->nr_dirtied_pause = nr_dirtied_pause;
1930
1931	/*
1932	* This is typically equal to (dirty < thresh) and can also
1933	* keep "1000+ dd on a slow USB stick" under control.
1934	*/
1935	if (task_ratelimit)
1936	break;
1937
1938	/*
1939	* In the case of an unresponsive NFS server and the NFS dirty
1940	* pages exceeds dirty_thresh, give the other good wb's a pipe
1941	* to go through, so that tasks on them still remain responsive.
1942	*
1943	* In theory 1 page is enough to keep the consumer-producer
1944	* pipe going: the flusher cleans 1 page => the task dirties 1
1945	* more page. However wb_dirty has accounting errors. So use
1946	* the larger and more IO friendly wb_stat_error.
1947	*/
1948	if (sdtc->wb_dirty <= wb_stat_error())
1949	break;
1950
1951	if (fatal_signal_pending(current))
1952	break;
1953	}
1954	return ret;
1955	}
1956
1957	static DEFINE_PER_CPU(int, bdp_ratelimits);
1958
1959	/*
1960	* Normal tasks are throttled by
1961	* loop {
1962	* dirty tsk->nr_dirtied_pause pages;
1963	* take a snap in balance_dirty_pages();
1964	* }
1965	* However there is a worst case. If every task exit immediately when dirtied
1966	* (tsk->nr_dirtied_pause - 1) pages, balance_dirty_pages() will never be
1967	* called to throttle the page dirties. The solution is to save the not yet
1968	* throttled page dirties in dirty_throttle_leaks on task exit and charge them
1969	* randomly into the running tasks. This works well for the above worst case,
1970	* as the new task will pick up and accumulate the old task's leaked dirty
1971	* count and eventually get throttled.
1972	*/
1973	DEFINE_PER_CPU(int, dirty_throttle_leaks) = 0;
1974
1975	/**
1976	* balance_dirty_pages_ratelimited_flags - Balance dirty memory state.
1977	* @mapping: address_space which was dirtied.
1978	* @flags: BDP flags.
1979	*
1980	* Processes which are dirtying memory should call in here once for each page
1981	* which was newly dirtied. The function will periodically check the system's
1982	* dirty state and will initiate writeback if needed.
1983	*
1984	* See balance_dirty_pages_ratelimited() for details.
1985	*
1986	* Return: If @flags contains BDP_ASYNC, it may return -EAGAIN to
1987	* indicate that memory is out of balance and the caller must wait
1988	* for I/O to complete. Otherwise, it will return 0 to indicate
1989	* that either memory was already in balance, or it was able to sleep
1990	* until the amount of dirty memory returned to balance.
1991	*/
1992	int balance_dirty_pages_ratelimited_flags(struct address_space *mapping,
1993	unsigned int flags)
1994	{
1995	struct inode *inode = mapping->host;
1996	struct backing_dev_info *bdi = inode_to_bdi(inode);
1997	struct bdi_writeback *wb = NULL;
1998	int ratelimit;
1999	int ret = 0;
2000	int *p;
2001
2002	if (!(bdi->capabilities & BDI_CAP_WRITEBACK))
2003	return ret;
2004
2005	if (inode_cgwb_enabled(inode))
2006	wb = wb_get_create_current(bdi, GFP_KERNEL);
2007	if (!wb)
2008	wb = &bdi->wb;
2009
2010	ratelimit = current->nr_dirtied_pause;
2011	if (wb->dirty_exceeded)
2012	ratelimit = min(ratelimit, 32 >> (PAGE_SHIFT - 10));
2013
2014	preempt_disable();
2015	/*
2016	* This prevents one CPU to accumulate too many dirtied pages without
2017	* calling into balance_dirty_pages(), which can happen when there are
2018	* 1000+ tasks, all of them start dirtying pages at exactly the same
2019	* time, hence all honoured too large initial task->nr_dirtied_pause.
2020	*/
2021	p = this_cpu_ptr(&bdp_ratelimits);
2022	if (unlikely(current->nr_dirtied >= ratelimit))
2023	*p = 0;
2024	else if (unlikely(*p >= ratelimit_pages)) {
2025	*p = 0;
2026	ratelimit = 0;
2027	}
2028	/*
2029	* Pick up the dirtied pages by the exited tasks. This avoids lots of
2030	* short-lived tasks (eg. gcc invocations in a kernel build) escaping
2031	* the dirty throttling and livelock other long-run dirtiers.
2032	*/
2033	p = this_cpu_ptr(&dirty_throttle_leaks);
2034	if (*p > 0 && current->nr_dirtied < ratelimit) {
2035	unsigned long nr_pages_dirtied;
2036	nr_pages_dirtied = min(*p, ratelimit - current->nr_dirtied);
2037	*p -= nr_pages_dirtied;
2038	current->nr_dirtied += nr_pages_dirtied;
2039	}
2040	preempt_enable();
2041
2042	if (unlikely(current->nr_dirtied >= ratelimit))
2043	ret = balance_dirty_pages(wb, current->nr_dirtied, flags);
2044
2045	wb_put(wb);
2046	return ret;
2047	}
2048	EXPORT_SYMBOL_GPL(balance_dirty_pages_ratelimited_flags);
2049
2050	/**
2051	* balance_dirty_pages_ratelimited - balance dirty memory state.
2052	* @mapping: address_space which was dirtied.
2053	*
2054	* Processes which are dirtying memory should call in here once for each page
2055	* which was newly dirtied. The function will periodically check the system's
2056	* dirty state and will initiate writeback if needed.
2057	*
2058	* Once we're over the dirty memory limit we decrease the ratelimiting
2059	* by a lot, to prevent individual processes from overshooting the limit
2060	* by (ratelimit_pages) each.
2061	*/
2062	void balance_dirty_pages_ratelimited(struct address_space *mapping)
2063	{
2064	balance_dirty_pages_ratelimited_flags(mapping, 0);
2065	}
2066	EXPORT_SYMBOL(balance_dirty_pages_ratelimited);
2067
2068	/**
2069	* wb_over_bg_thresh - does @wb need to be written back?
2070	* @wb: bdi_writeback of interest
2071	*
2072	* Determines whether background writeback should keep writing @wb or it's
2073	* clean enough.
2074	*
2075	* Return: %true if writeback should continue.
2076	*/
2077	bool wb_over_bg_thresh(struct bdi_writeback *wb)
2078	{
2079	struct dirty_throttle_control gdtc_stor = { GDTC_INIT(wb) };
2080	struct dirty_throttle_control mdtc_stor = { MDTC_INIT(wb, &gdtc_stor) };
2081	struct dirty_throttle_control * const gdtc = &gdtc_stor;
2082	struct dirty_throttle_control * const mdtc = mdtc_valid(dtc: &mdtc_stor) ?
2083	&mdtc_stor : NULL;
2084	unsigned long reclaimable;
2085	unsigned long thresh;
2086
2087	/*
2088	* Similar to balance_dirty_pages() but ignores pages being written
2089	* as we're trying to decide whether to put more under writeback.
2090	*/
2091	gdtc->avail = global_dirtyable_memory();
2092	gdtc->dirty = global_node_page_state(item: NR_FILE_DIRTY);
2093	domain_dirty_limits(dtc: gdtc);
2094
2095	if (gdtc->dirty > gdtc->bg_thresh)
2096	return true;
2097
2098	thresh = wb_calc_thresh(wb: gdtc->wb, thresh: gdtc->bg_thresh);
2099	if (thresh < 2 * wb_stat_error())
2100	reclaimable = wb_stat_sum(wb, item: WB_RECLAIMABLE);
2101	else
2102	reclaimable = wb_stat(wb, item: WB_RECLAIMABLE);
2103
2104	if (reclaimable > thresh)
2105	return true;
2106
2107	if (mdtc) {
2108	unsigned long filepages, headroom, writeback;
2109
2110	mem_cgroup_wb_stats(wb, pfilepages: &filepages, pheadroom: &headroom, pdirty: &mdtc->dirty,
2111	pwriteback: &writeback);
2112	mdtc_calc_avail(mdtc, filepages, headroom);
2113	domain_dirty_limits(dtc: mdtc); /* ditto, ignore writeback */
2114
2115	if (mdtc->dirty > mdtc->bg_thresh)
2116	return true;
2117
2118	thresh = wb_calc_thresh(wb: mdtc->wb, thresh: mdtc->bg_thresh);
2119	if (thresh < 2 * wb_stat_error())
2120	reclaimable = wb_stat_sum(wb, item: WB_RECLAIMABLE);
2121	else
2122	reclaimable = wb_stat(wb, item: WB_RECLAIMABLE);
2123
2124	if (reclaimable > thresh)
2125	return true;
2126	}
2127
2128	return false;
2129	}
2130
2131	#ifdef CONFIG_SYSCTL
2132	/*
2133	* sysctl handler for /proc/sys/vm/dirty_writeback_centisecs
2134	*/
2135	static int dirty_writeback_centisecs_handler(struct ctl_table *table, int write,
2136	void *buffer, size_t *length, loff_t *ppos)
2137	{
2138	unsigned int old_interval = dirty_writeback_interval;
2139	int ret;
2140
2141	ret = proc_dointvec(table, write, buffer, length, ppos);
2142
2143	/*
2144	* Writing 0 to dirty_writeback_interval will disable periodic writeback
2145	* and a different non-zero value will wakeup the writeback threads.
2146	* wb_wakeup_delayed() would be more appropriate, but it's a pain to
2147	* iterate over all bdis and wbs.
2148	* The reason we do this is to make the change take effect immediately.
2149	*/
2150	if (!ret && write && dirty_writeback_interval &&
2151	dirty_writeback_interval != old_interval)
2152	wakeup_flusher_threads(reason: WB_REASON_PERIODIC);
2153
2154	return ret;
2155	}
2156	#endif
2157
2158	void laptop_mode_timer_fn(struct timer_list *t)
2159	{
2160	struct backing_dev_info *backing_dev_info =
2161	from_timer(backing_dev_info, t, laptop_mode_wb_timer);
2162
2163	wakeup_flusher_threads_bdi(bdi: backing_dev_info, reason: WB_REASON_LAPTOP_TIMER);
2164	}
2165
2166	/*
2167	* We've spun up the disk and we're in laptop mode: schedule writeback
2168	* of all dirty data a few seconds from now. If the flush is already scheduled
2169	* then push it back - the user is still using the disk.
2170	*/
2171	void laptop_io_completion(struct backing_dev_info *info)
2172	{
2173	mod_timer(timer: &info->laptop_mode_wb_timer, expires: jiffies + laptop_mode);
2174	}
2175
2176	/*
2177	* We're in laptop mode and we've just synced. The sync's writes will have
2178	* caused another writeback to be scheduled by laptop_io_completion.
2179	* Nothing needs to be written back anymore, so we unschedule the writeback.
2180	*/
2181	void laptop_sync_completion(void)
2182	{
2183	struct backing_dev_info *bdi;
2184
2185	rcu_read_lock();
2186
2187	list_for_each_entry_rcu(bdi, &bdi_list, bdi_list)
2188	del_timer(timer: &bdi->laptop_mode_wb_timer);
2189
2190	rcu_read_unlock();
2191	}
2192
2193	/*
2194	* If ratelimit_pages is too high then we can get into dirty-data overload
2195	* if a large number of processes all perform writes at the same time.
2196	*
2197	* Here we set ratelimit_pages to a level which ensures that when all CPUs are
2198	* dirtying in parallel, we cannot go more than 3% (1/32) over the dirty memory
2199	* thresholds.
2200	*/
2201
2202	void writeback_set_ratelimit(void)
2203	{
2204	struct wb_domain *dom = &global_wb_domain;
2205	unsigned long background_thresh;
2206	unsigned long dirty_thresh;
2207
2208	global_dirty_limits(pbackground: &background_thresh, pdirty: &dirty_thresh);
2209	dom->dirty_limit = dirty_thresh;
2210	ratelimit_pages = dirty_thresh / (num_online_cpus() * 32);
2211	if (ratelimit_pages < 16)
2212	ratelimit_pages = 16;
2213	}
2214
2215	static int page_writeback_cpu_online(unsigned int cpu)
2216	{
2217	writeback_set_ratelimit();
2218	return 0;
2219	}
2220
2221	#ifdef CONFIG_SYSCTL
2222
2223	/* this is needed for the proc_doulongvec_minmax of vm_dirty_bytes */
2224	static const unsigned long dirty_bytes_min = 2 * PAGE_SIZE;
2225
2226	static struct ctl_table vm_page_writeback_sysctls[] = {
2227	{
2228	.procname = "dirty_background_ratio",
2229	.data = &dirty_background_ratio,
2230	.maxlen = sizeof(dirty_background_ratio),
2231	.mode = 0644,
2232	.proc_handler = dirty_background_ratio_handler,
2233	.extra1 = SYSCTL_ZERO,
2234	.extra2 = SYSCTL_ONE_HUNDRED,
2235	},
2236	{
2237	.procname = "dirty_background_bytes",
2238	.data = &dirty_background_bytes,
2239	.maxlen = sizeof(dirty_background_bytes),
2240	.mode = 0644,
2241	.proc_handler = dirty_background_bytes_handler,
2242	.extra1 = SYSCTL_LONG_ONE,
2243	},
2244	{
2245	.procname = "dirty_ratio",
2246	.data = &vm_dirty_ratio,
2247	.maxlen = sizeof(vm_dirty_ratio),
2248	.mode = 0644,
2249	.proc_handler = dirty_ratio_handler,
2250	.extra1 = SYSCTL_ZERO,
2251	.extra2 = SYSCTL_ONE_HUNDRED,
2252	},
2253	{
2254	.procname = "dirty_bytes",
2255	.data = &vm_dirty_bytes,
2256	.maxlen = sizeof(vm_dirty_bytes),
2257	.mode = 0644,
2258	.proc_handler = dirty_bytes_handler,
2259	.extra1 = (void *)&dirty_bytes_min,
2260	},
2261	{
2262	.procname = "dirty_writeback_centisecs",
2263	.data = &dirty_writeback_interval,
2264	.maxlen = sizeof(dirty_writeback_interval),
2265	.mode = 0644,
2266	.proc_handler = dirty_writeback_centisecs_handler,
2267	},
2268	{
2269	.procname = "dirty_expire_centisecs",
2270	.data = &dirty_expire_interval,
2271	.maxlen = sizeof(dirty_expire_interval),
2272	.mode = 0644,
2273	.proc_handler = proc_dointvec_minmax,
2274	.extra1 = SYSCTL_ZERO,
2275	},
2276	#ifdef CONFIG_HIGHMEM
2277	{
2278	.procname = "highmem_is_dirtyable",
2279	.data = &vm_highmem_is_dirtyable,
2280	.maxlen = sizeof(vm_highmem_is_dirtyable),
2281	.mode = 0644,
2282	.proc_handler = proc_dointvec_minmax,
2283	.extra1 = SYSCTL_ZERO,
2284	.extra2 = SYSCTL_ONE,
2285	},
2286	#endif
2287	{
2288	.procname = "laptop_mode",
2289	.data = &laptop_mode,
2290	.maxlen = sizeof(laptop_mode),
2291	.mode = 0644,
2292	.proc_handler = proc_dointvec_jiffies,
2293	},
2294	{}
2295	};
2296	#endif
2297
2298	/*
2299	* Called early on to tune the page writeback dirty limits.
2300	*
2301	* We used to scale dirty pages according to how total memory
2302	* related to pages that could be allocated for buffers.
2303	*
2304	* However, that was when we used "dirty_ratio" to scale with
2305	* all memory, and we don't do that any more. "dirty_ratio"
2306	* is now applied to total non-HIGHPAGE memory, and as such we can't
2307	* get into the old insane situation any more where we had
2308	* large amounts of dirty pages compared to a small amount of
2309	* non-HIGHMEM memory.
2310	*
2311	* But we might still want to scale the dirty_ratio by how
2312	* much memory the box has..
2313	*/
2314	void __init page_writeback_init(void)
2315	{
2316	BUG_ON(wb_domain_init(&global_wb_domain, GFP_KERNEL));
2317
2318	cpuhp_setup_state(state: CPUHP_AP_ONLINE_DYN, name: "mm/writeback:online",
2319	startup: page_writeback_cpu_online, NULL);
2320	cpuhp_setup_state(state: CPUHP_MM_WRITEBACK_DEAD, name: "mm/writeback:dead", NULL,
2321	teardown: page_writeback_cpu_online);
2322	#ifdef CONFIG_SYSCTL
2323	register_sysctl_init("vm", vm_page_writeback_sysctls);
2324	#endif
2325	}
2326
2327	/**
2328	* tag_pages_for_writeback - tag pages to be written by writeback
2329	* @mapping: address space structure to write
2330	* @start: starting page index
2331	* @end: ending page index (inclusive)
2332	*
2333	* This function scans the page range from @start to @end (inclusive) and tags
2334	* all pages that have DIRTY tag set with a special TOWRITE tag. The caller
2335	* can then use the TOWRITE tag to identify pages eligible for writeback.
2336	* This mechanism is used to avoid livelocking of writeback by a process
2337	* steadily creating new dirty pages in the file (thus it is important for this
2338	* function to be quick so that it can tag pages faster than a dirtying process
2339	* can create them).
2340	*/
2341	void tag_pages_for_writeback(struct address_space *mapping,
2342	pgoff_t start, pgoff_t end)
2343	{
2344	XA_STATE(xas, &mapping->i_pages, start);
2345	unsigned int tagged = 0;
2346	void *page;
2347
2348	xas_lock_irq(&xas);
2349	xas_for_each_marked(&xas, page, end, PAGECACHE_TAG_DIRTY) {
2350	xas_set_mark(&xas, PAGECACHE_TAG_TOWRITE);
2351	if (++tagged % XA_CHECK_SCHED)
2352	continue;
2353
2354	xas_pause(&xas);
2355	xas_unlock_irq(&xas);
2356	cond_resched();
2357	xas_lock_irq(&xas);
2358	}
2359	xas_unlock_irq(&xas);
2360	}
2361	EXPORT_SYMBOL(tag_pages_for_writeback);
2362
2363	static bool folio_prepare_writeback(struct address_space *mapping,
2364	struct writeback_control *wbc, struct folio *folio)
2365	{
2366	/*
2367	* Folio truncated or invalidated. We can freely skip it then,
2368	* even for data integrity operations: the folio has disappeared
2369	* concurrently, so there could be no real expectation of this
2370	* data integrity operation even if there is now a new, dirty
2371	* folio at the same pagecache index.
2372	*/
2373	if (unlikely(folio->mapping != mapping))
2374	return false;
2375
2376	/*
2377	* Did somebody else write it for us?
2378	*/
2379	if (!folio_test_dirty(folio))
2380	return false;
2381
2382	if (folio_test_writeback(folio)) {
2383	if (wbc->sync_mode == WB_SYNC_NONE)
2384	return false;
2385	folio_wait_writeback(folio);
2386	}
2387	BUG_ON(folio_test_writeback(folio));
2388
2389	if (!folio_clear_dirty_for_io(folio))
2390	return false;
2391
2392	return true;
2393	}
2394
2395	static xa_mark_t wbc_to_tag(struct writeback_control *wbc)
2396	{
2397	if (wbc->sync_mode == WB_SYNC_ALL || wbc->tagged_writepages)
2398	return PAGECACHE_TAG_TOWRITE;
2399	return PAGECACHE_TAG_DIRTY;
2400	}
2401
2402	static pgoff_t wbc_end(struct writeback_control *wbc)
2403	{
2404	if (wbc->range_cyclic)
2405	return -1;
2406	return wbc->range_end >> PAGE_SHIFT;
2407	}
2408
2409	static struct folio *writeback_get_folio(struct address_space *mapping,
2410	struct writeback_control *wbc)
2411	{
2412	struct folio *folio;
2413
2414	retry:
2415	folio = folio_batch_next(fbatch: &wbc->fbatch);
2416	if (!folio) {
2417	folio_batch_release(fbatch: &wbc->fbatch);
2418	cond_resched();
2419	filemap_get_folios_tag(mapping, start: &wbc->index, end: wbc_end(wbc),
2420	tag: wbc_to_tag(wbc), fbatch: &wbc->fbatch);
2421	folio = folio_batch_next(fbatch: &wbc->fbatch);
2422	if (!folio)
2423	return NULL;
2424	}
2425
2426	folio_lock(folio);
2427	if (unlikely(!folio_prepare_writeback(mapping, wbc, folio))) {
2428	folio_unlock(folio);
2429	goto retry;
2430	}
2431
2432	trace_wbc_writepage(wbc, bdi: inode_to_bdi(inode: mapping->host));
2433	return folio;
2434	}
2435
2436	/**
2437	* writeback_iter - iterate folio of a mapping for writeback
2438	* @mapping: address space structure to write
2439	* @wbc: writeback context
2440	* @folio: previously iterated folio (%NULL to start)
2441	* @error: in-out pointer for writeback errors (see below)
2442	*
2443	* This function returns the next folio for the writeback operation described by
2444	* @wbc on @mapping and should be called in a while loop in the ->writepages
2445	* implementation.
2446	*
2447	* To start the writeback operation, %NULL is passed in the @folio argument, and
2448	* for every subsequent iteration the folio returned previously should be passed
2449	* back in.
2450	*
2451	* If there was an error in the per-folio writeback inside the writeback_iter()
2452	* loop, @error should be set to the error value.
2453	*
2454	* Once the writeback described in @wbc has finished, this function will return
2455	* %NULL and if there was an error in any iteration restore it to @error.
2456	*
2457	* Note: callers should not manually break out of the loop using break or goto
2458	* but must keep calling writeback_iter() until it returns %NULL.
2459	*
2460	* Return: the folio to write or %NULL if the loop is done.
2461	*/
2462	struct folio *writeback_iter(struct address_space *mapping,
2463	struct writeback_control *wbc, struct folio *folio, int *error)
2464	{
2465	if (!folio) {
2466	folio_batch_init(fbatch: &wbc->fbatch);
2467	wbc->saved_err = *error = 0;
2468
2469	/*
2470	* For range cyclic writeback we remember where we stopped so
2471	* that we can continue where we stopped.
2472	*
2473	* For non-cyclic writeback we always start at the beginning of
2474	* the passed in range.
2475	*/
2476	if (wbc->range_cyclic)
2477	wbc->index = mapping->writeback_index;
2478	else
2479	wbc->index = wbc->range_start >> PAGE_SHIFT;
2480
2481	/*
2482	* To avoid livelocks when other processes dirty new pages, we
2483	* first tag pages which should be written back and only then
2484	* start writing them.
2485	*
2486	* For data-integrity writeback we have to be careful so that we
2487	* do not miss some pages (e.g., because some other process has
2488	* cleared the TOWRITE tag we set). The rule we follow is that
2489	* TOWRITE tag can be cleared only by the process clearing the
2490	* DIRTY tag (and submitting the page for I/O).
2491	*/
2492	if (wbc->sync_mode == WB_SYNC_ALL || wbc->tagged_writepages)
2493	tag_pages_for_writeback(mapping, wbc->index,
2494	wbc_end(wbc));
2495	} else {
2496	wbc->nr_to_write -= folio_nr_pages(folio);
2497
2498	WARN_ON_ONCE(*error > 0);
2499
2500	/*
2501	* For integrity writeback we have to keep going until we have
2502	* written all the folios we tagged for writeback above, even if
2503	* we run past wbc->nr_to_write or encounter errors.
2504	* We stash away the first error we encounter in wbc->saved_err
2505	* so that it can be retrieved when we're done. This is because
2506	* the file system may still have state to clear for each folio.
2507	*
2508	* For background writeback we exit as soon as we run past
2509	* wbc->nr_to_write or encounter the first error.
2510	*/
2511	if (wbc->sync_mode == WB_SYNC_ALL) {
2512	if (*error && !wbc->saved_err)
2513	wbc->saved_err = *error;
2514	} else {
2515	if (*error || wbc->nr_to_write <= 0)
2516	goto done;
2517	}
2518	}
2519
2520	folio = writeback_get_folio(mapping, wbc);
2521	if (!folio) {
2522	/*
2523	* To avoid deadlocks between range_cyclic writeback and callers
2524	* that hold pages in PageWriteback to aggregate I/O until
2525	* the writeback iteration finishes, we do not loop back to the
2526	* start of the file. Doing so causes a page lock/page
2527	* writeback access order inversion - we should only ever lock
2528	* multiple pages in ascending page->index order, and looping
2529	* back to the start of the file violates that rule and causes
2530	* deadlocks.
2531	*/
2532	if (wbc->range_cyclic)
2533	mapping->writeback_index = 0;
2534
2535	/*
2536	* Return the first error we encountered (if there was any) to
2537	* the caller.
2538	*/
2539	*error = wbc->saved_err;
2540	}
2541	return folio;
2542
2543	done:
2544	if (wbc->range_cyclic)
2545	mapping->writeback_index = folio->index + folio_nr_pages(folio);
2546	folio_batch_release(fbatch: &wbc->fbatch);
2547	return NULL;
2548	}
2549
2550	/**
2551	* write_cache_pages - walk the list of dirty pages of the given address space and write all of them.
2552	* @mapping: address space structure to write
2553	* @wbc: subtract the number of written pages from *@wbc->nr_to_write
2554	* @writepage: function called for each page
2555	* @data: data passed to writepage function
2556	*
2557	* Return: %0 on success, negative error code otherwise
2558	*
2559	* Note: please use writeback_iter() instead.
2560	*/
2561	int write_cache_pages(struct address_space *mapping,
2562	struct writeback_control *wbc, writepage_t writepage,
2563	void *data)
2564	{
2565	struct folio *folio = NULL;
2566	int error;
2567
2568	while ((folio = writeback_iter(mapping, wbc, folio, error: &error))) {
2569	error = writepage(folio, wbc, data);
2570	if (error == AOP_WRITEPAGE_ACTIVATE) {
2571	folio_unlock(folio);
2572	error = 0;
2573	}
2574	}
2575
2576	return error;
2577	}
2578	EXPORT_SYMBOL(write_cache_pages);
2579
2580	static int writeback_use_writepage(struct address_space *mapping,
2581	struct writeback_control *wbc)
2582	{
2583	struct folio *folio = NULL;
2584	struct blk_plug plug;
2585	int err;
2586
2587	blk_start_plug(&plug);
2588	while ((folio = writeback_iter(mapping, wbc, folio, error: &err))) {
2589	err = mapping->a_ops->writepage(&folio->page, wbc);
2590	if (err == AOP_WRITEPAGE_ACTIVATE) {
2591	folio_unlock(folio);
2592	err = 0;
2593	}
2594	mapping_set_error(mapping, error: err);
2595	}
2596	blk_finish_plug(&plug);
2597
2598	return err;
2599	}
2600
2601	int do_writepages(struct address_space *mapping, struct writeback_control *wbc)
2602	{
2603	int ret;
2604	struct bdi_writeback *wb;
2605
2606	if (wbc->nr_to_write <= 0)
2607	return 0;
2608	wb = inode_to_wb_wbc(inode: mapping->host, wbc);
2609	wb_bandwidth_estimate_start(wb);
2610	while (1) {
2611	if (mapping->a_ops->writepages) {
2612	ret = mapping->a_ops->writepages(mapping, wbc);
2613	} else if (mapping->a_ops->writepage) {
2614	ret = writeback_use_writepage(mapping, wbc);
2615	} else {
2616	/* deal with chardevs and other special files */
2617	ret = 0;
2618	}
2619	if (ret != -ENOMEM || wbc->sync_mode != WB_SYNC_ALL)
2620	break;
2621
2622	/*
2623	* Lacking an allocation context or the locality or writeback
2624	* state of any of the inode's pages, throttle based on
2625	* writeback activity on the local node. It's as good a
2626	* guess as any.
2627	*/
2628	reclaim_throttle(NODE_DATA(numa_node_id()),
2629	reason: VMSCAN_THROTTLE_WRITEBACK);
2630	}
2631	/*
2632	* Usually few pages are written by now from those we've just submitted
2633	* but if there's constant writeback being submitted, this makes sure
2634	* writeback bandwidth is updated once in a while.
2635	*/
2636	if (time_is_before_jiffies(READ_ONCE(wb->bw_time_stamp) +
2637	BANDWIDTH_INTERVAL))
2638	wb_update_bandwidth(wb);
2639	return ret;
2640	}
2641
2642	/*
2643	* For address_spaces which do not use buffers nor write back.
2644	*/
2645	bool noop_dirty_folio(struct address_space *mapping, struct folio *folio)
2646	{
2647	if (!folio_test_dirty(folio))
2648	return !folio_test_set_dirty(folio);
2649	return false;
2650	}
2651	EXPORT_SYMBOL(noop_dirty_folio);
2652
2653	/*
2654	* Helper function for set_page_dirty family.
2655	*
2656	* Caller must hold folio_memcg_lock().
2657	*
2658	* NOTE: This relies on being atomic wrt interrupts.
2659	*/
2660	static void folio_account_dirtied(struct folio *folio,
2661	struct address_space *mapping)
2662	{
2663	struct inode *inode = mapping->host;
2664
2665	trace_writeback_dirty_folio(folio, mapping);
2666
2667	if (mapping_can_writeback(mapping)) {
2668	struct bdi_writeback *wb;
2669	long nr = folio_nr_pages(folio);
2670
2671	inode_attach_wb(inode, folio);
2672	wb = inode_to_wb(inode);
2673
2674	__lruvec_stat_mod_folio(folio, idx: NR_FILE_DIRTY, val: nr);
2675	__zone_stat_mod_folio(folio, item: NR_ZONE_WRITE_PENDING, nr);
2676	__node_stat_mod_folio(folio, item: NR_DIRTIED, nr);
2677	wb_stat_mod(wb, item: WB_RECLAIMABLE, amount: nr);
2678	wb_stat_mod(wb, item: WB_DIRTIED, amount: nr);
2679	task_io_account_write(bytes: nr * PAGE_SIZE);
2680	current->nr_dirtied += nr;
2681	__this_cpu_add(bdp_ratelimits, nr);
2682
2683	mem_cgroup_track_foreign_dirty(folio, wb);
2684	}
2685	}
2686
2687	/*
2688	* Helper function for deaccounting dirty page without writeback.
2689	*
2690	* Caller must hold folio_memcg_lock().
2691	*/
2692	void folio_account_cleaned(struct folio *folio, struct bdi_writeback *wb)
2693	{
2694	long nr = folio_nr_pages(folio);
2695
2696	lruvec_stat_mod_folio(folio, idx: NR_FILE_DIRTY, val: -nr);
2697	zone_stat_mod_folio(folio, item: NR_ZONE_WRITE_PENDING, nr: -nr);
2698	wb_stat_mod(wb, item: WB_RECLAIMABLE, amount: -nr);
2699	task_io_account_cancelled_write(bytes: nr * PAGE_SIZE);
2700	}
2701
2702	/*
2703	* Mark the folio dirty, and set it dirty in the page cache, and mark
2704	* the inode dirty.
2705	*
2706	* If warn is true, then emit a warning if the folio is not uptodate and has
2707	* not been truncated.
2708	*
2709	* The caller must hold folio_memcg_lock(). Most callers have the folio
2710	* locked. A few have the folio blocked from truncation through other
2711	* means (eg zap_vma_pages() has it mapped and is holding the page table
2712	* lock). This can also be called from mark_buffer_dirty(), which I
2713	* cannot prove is always protected against truncate.
2714	*/
2715	void __folio_mark_dirty(struct folio *folio, struct address_space *mapping,
2716	int warn)
2717	{
2718	unsigned long flags;
2719
2720	xa_lock_irqsave(&mapping->i_pages, flags);
2721	if (folio->mapping) { /* Race with truncate? */
2722	WARN_ON_ONCE(warn && !folio_test_uptodate(folio));
2723	folio_account_dirtied(folio, mapping);
2724	__xa_set_mark(&mapping->i_pages, index: folio_index(folio),
2725	PAGECACHE_TAG_DIRTY);
2726	}
2727	xa_unlock_irqrestore(&mapping->i_pages, flags);
2728	}
2729
2730	/**
2731	* filemap_dirty_folio - Mark a folio dirty for filesystems which do not use buffer_heads.
2732	* @mapping: Address space this folio belongs to.
2733	* @folio: Folio to be marked as dirty.
2734	*
2735	* Filesystems which do not use buffer heads should call this function
2736	* from their dirty_folio address space operation. It ignores the
2737	* contents of folio_get_private(), so if the filesystem marks individual
2738	* blocks as dirty, the filesystem should handle that itself.
2739	*
2740	* This is also sometimes used by filesystems which use buffer_heads when
2741	* a single buffer is being dirtied: we want to set the folio dirty in
2742	* that case, but not all the buffers. This is a "bottom-up" dirtying,
2743	* whereas block_dirty_folio() is a "top-down" dirtying.
2744	*
2745	* The caller must ensure this doesn't race with truncation. Most will
2746	* simply hold the folio lock, but e.g. zap_pte_range() calls with the
2747	* folio mapped and the pte lock held, which also locks out truncation.
2748	*/
2749	bool filemap_dirty_folio(struct address_space *mapping, struct folio *folio)
2750	{
2751	folio_memcg_lock(folio);
2752	if (folio_test_set_dirty(folio)) {
2753	folio_memcg_unlock(folio);
2754	return false;
2755	}
2756
2757	__folio_mark_dirty(folio, mapping, warn: !folio_test_private(folio));
2758	folio_memcg_unlock(folio);
2759
2760	if (mapping->host) {
2761	/* !PageAnon && !swapper_space */
2762	__mark_inode_dirty(mapping->host, I_DIRTY_PAGES);
2763	}
2764	return true;
2765	}
2766	EXPORT_SYMBOL(filemap_dirty_folio);
2767
2768	/**
2769	* folio_redirty_for_writepage - Decline to write a dirty folio.
2770	* @wbc: The writeback control.
2771	* @folio: The folio.
2772	*
2773	* When a writepage implementation decides that it doesn't want to write
2774	* @folio for some reason, it should call this function, unlock @folio and
2775	* return 0.
2776	*
2777	* Return: True if we redirtied the folio. False if someone else dirtied
2778	* it first.
2779	*/
2780	bool folio_redirty_for_writepage(struct writeback_control *wbc,
2781	struct folio *folio)
2782	{
2783	struct address_space *mapping = folio->mapping;
2784	long nr = folio_nr_pages(folio);
2785	bool ret;
2786
2787	wbc->pages_skipped += nr;
2788	ret = filemap_dirty_folio(mapping, folio);
2789	if (mapping && mapping_can_writeback(mapping)) {
2790	struct inode *inode = mapping->host;
2791	struct bdi_writeback *wb;
2792	struct wb_lock_cookie cookie = {};
2793
2794	wb = unlocked_inode_to_wb_begin(inode, cookie: &cookie);
2795	current->nr_dirtied -= nr;
2796	node_stat_mod_folio(folio, item: NR_DIRTIED, nr: -nr);
2797	wb_stat_mod(wb, item: WB_DIRTIED, amount: -nr);
2798	unlocked_inode_to_wb_end(inode, cookie: &cookie);
2799	}
2800	return ret;
2801	}
2802	EXPORT_SYMBOL(folio_redirty_for_writepage);
2803
2804	/**
2805	* folio_mark_dirty - Mark a folio as being modified.
2806	* @folio: The folio.
2807	*
2808	* The folio may not be truncated while this function is running.
2809	* Holding the folio lock is sufficient to prevent truncation, but some
2810	* callers cannot acquire a sleeping lock. These callers instead hold
2811	* the page table lock for a page table which contains at least one page
2812	* in this folio. Truncation will block on the page table lock as it
2813	* unmaps pages before removing the folio from its mapping.
2814	*
2815	* Return: True if the folio was newly dirtied, false if it was already dirty.
2816	*/
2817	bool folio_mark_dirty(struct folio *folio)
2818	{
2819	struct address_space *mapping = folio_mapping(folio);
2820
2821	if (likely(mapping)) {
2822	/*
2823	* readahead/folio_deactivate could remain
2824	* PG_readahead/PG_reclaim due to race with folio_end_writeback
2825	* About readahead, if the folio is written, the flags would be
2826	* reset. So no problem.
2827	* About folio_deactivate, if the folio is redirtied,
2828	* the flag will be reset. So no problem. but if the
2829	* folio is used by readahead it will confuse readahead
2830	* and make it restart the size rampup process. But it's
2831	* a trivial problem.
2832	*/
2833	if (folio_test_reclaim(folio))
2834	folio_clear_reclaim(folio);
2835	return mapping->a_ops->dirty_folio(mapping, folio);
2836	}
2837
2838	return noop_dirty_folio(mapping, folio);
2839	}
2840	EXPORT_SYMBOL(folio_mark_dirty);
2841
2842	/*
2843	* set_page_dirty() is racy if the caller has no reference against
2844	* page->mapping->host, and if the page is unlocked. This is because another
2845	* CPU could truncate the page off the mapping and then free the mapping.
2846	*
2847	* Usually, the page _is_ locked, or the caller is a user-space process which
2848	* holds a reference on the inode by having an open file.
2849	*
2850	* In other cases, the page should be locked before running set_page_dirty().
2851	*/
2852	int set_page_dirty_lock(struct page *page)
2853	{
2854	int ret;
2855
2856	lock_page(page);
2857	ret = set_page_dirty(page);
2858	unlock_page(page);
2859	return ret;
2860	}
2861	EXPORT_SYMBOL(set_page_dirty_lock);
2862
2863	/*
2864	* This cancels just the dirty bit on the kernel page itself, it does NOT
2865	* actually remove dirty bits on any mmap's that may be around. It also
2866	* leaves the page tagged dirty, so any sync activity will still find it on
2867	* the dirty lists, and in particular, clear_page_dirty_for_io() will still
2868	* look at the dirty bits in the VM.
2869	*
2870	* Doing this should *normally* only ever be done when a page is truncated,
2871	* and is not actually mapped anywhere at all. However, fs/buffer.c does
2872	* this when it notices that somebody has cleaned out all the buffers on a
2873	* page without actually doing it through the VM. Can you say "ext3 is
2874	* horribly ugly"? Thought you could.
2875	*/
2876	void __folio_cancel_dirty(struct folio *folio)
2877	{
2878	struct address_space *mapping = folio_mapping(folio);
2879
2880	if (mapping_can_writeback(mapping)) {
2881	struct inode *inode = mapping->host;
2882	struct bdi_writeback *wb;
2883	struct wb_lock_cookie cookie = {};
2884
2885	folio_memcg_lock(folio);
2886	wb = unlocked_inode_to_wb_begin(inode, cookie: &cookie);
2887
2888	if (folio_test_clear_dirty(folio))
2889	folio_account_cleaned(folio, wb);
2890
2891	unlocked_inode_to_wb_end(inode, cookie: &cookie);
2892	folio_memcg_unlock(folio);
2893	} else {
2894	folio_clear_dirty(folio);
2895	}
2896	}
2897	EXPORT_SYMBOL(__folio_cancel_dirty);
2898
2899	/*
2900	* Clear a folio's dirty flag, while caring for dirty memory accounting.
2901	* Returns true if the folio was previously dirty.
2902	*
2903	* This is for preparing to put the folio under writeout. We leave
2904	* the folio tagged as dirty in the xarray so that a concurrent
2905	* write-for-sync can discover it via a PAGECACHE_TAG_DIRTY walk.
2906	* The ->writepage implementation will run either folio_start_writeback()
2907	* or folio_mark_dirty(), at which stage we bring the folio's dirty flag
2908	* and xarray dirty tag back into sync.
2909	*
2910	* This incoherency between the folio's dirty flag and xarray tag is
2911	* unfortunate, but it only exists while the folio is locked.
2912	*/
2913	bool folio_clear_dirty_for_io(struct folio *folio)
2914	{
2915	struct address_space *mapping = folio_mapping(folio);
2916	bool ret = false;
2917
2918	VM_BUG_ON_FOLIO(!folio_test_locked(folio), folio);
2919
2920	if (mapping && mapping_can_writeback(mapping)) {
2921	struct inode *inode = mapping->host;
2922	struct bdi_writeback *wb;
2923	struct wb_lock_cookie cookie = {};
2924
2925	/*
2926	* Yes, Virginia, this is indeed insane.
2927	*
2928	* We use this sequence to make sure that
2929	* (a) we account for dirty stats properly
2930	* (b) we tell the low-level filesystem to
2931	* mark the whole folio dirty if it was
2932	* dirty in a pagetable. Only to then
2933	* (c) clean the folio again and return 1 to
2934	* cause the writeback.
2935	*
2936	* This way we avoid all nasty races with the
2937	* dirty bit in multiple places and clearing
2938	* them concurrently from different threads.
2939	*
2940	* Note! Normally the "folio_mark_dirty(folio)"
2941	* has no effect on the actual dirty bit - since
2942	* that will already usually be set. But we
2943	* need the side effects, and it can help us
2944	* avoid races.
2945	*
2946	* We basically use the folio "master dirty bit"
2947	* as a serialization point for all the different
2948	* threads doing their things.
2949	*/
2950	if (folio_mkclean(folio))
2951	folio_mark_dirty(folio);
2952	/*
2953	* We carefully synchronise fault handlers against
2954	* installing a dirty pte and marking the folio dirty
2955	* at this point. We do this by having them hold the
2956	* page lock while dirtying the folio, and folios are
2957	* always locked coming in here, so we get the desired
2958	* exclusion.
2959	*/
2960	wb = unlocked_inode_to_wb_begin(inode, cookie: &cookie);
2961	if (folio_test_clear_dirty(folio)) {
2962	long nr = folio_nr_pages(folio);
2963	lruvec_stat_mod_folio(folio, idx: NR_FILE_DIRTY, val: -nr);
2964	zone_stat_mod_folio(folio, item: NR_ZONE_WRITE_PENDING, nr: -nr);
2965	wb_stat_mod(wb, item: WB_RECLAIMABLE, amount: -nr);
2966	ret = true;
2967	}
2968	unlocked_inode_to_wb_end(inode, cookie: &cookie);
2969	return ret;
2970	}
2971	return folio_test_clear_dirty(folio);
2972	}
2973	EXPORT_SYMBOL(folio_clear_dirty_for_io);
2974
2975	static void wb_inode_writeback_start(struct bdi_writeback *wb)
2976	{
2977	atomic_inc(v: &wb->writeback_inodes);
2978	}
2979
2980	static void wb_inode_writeback_end(struct bdi_writeback *wb)
2981	{
2982	unsigned long flags;
2983	atomic_dec(v: &wb->writeback_inodes);
2984	/*
2985	* Make sure estimate of writeback throughput gets updated after
2986	* writeback completed. We delay the update by BANDWIDTH_INTERVAL
2987	* (which is the interval other bandwidth updates use for batching) so
2988	* that if multiple inodes end writeback at a similar time, they get
2989	* batched into one bandwidth update.
2990	*/
2991	spin_lock_irqsave(&wb->work_lock, flags);
2992	if (test_bit(WB_registered, &wb->state))
2993	queue_delayed_work(wq: bdi_wq, dwork: &wb->bw_dwork, BANDWIDTH_INTERVAL);
2994	spin_unlock_irqrestore(lock: &wb->work_lock, flags);
2995	}
2996
2997	bool __folio_end_writeback(struct folio *folio)
2998	{
2999	long nr = folio_nr_pages(folio);
3000	struct address_space *mapping = folio_mapping(folio);
3001	bool ret;
3002
3003	folio_memcg_lock(folio);
3004	if (mapping && mapping_use_writeback_tags(mapping)) {
3005	struct inode *inode = mapping->host;
3006	struct backing_dev_info *bdi = inode_to_bdi(inode);
3007	unsigned long flags;
3008
3009	xa_lock_irqsave(&mapping->i_pages, flags);
3010	ret = folio_xor_flags_has_waiters(folio, mask: 1 << PG_writeback);
3011	__xa_clear_mark(&mapping->i_pages, index: folio_index(folio),
3012	PAGECACHE_TAG_WRITEBACK);
3013	if (bdi->capabilities & BDI_CAP_WRITEBACK_ACCT) {
3014	struct bdi_writeback *wb = inode_to_wb(inode);
3015
3016	wb_stat_mod(wb, item: WB_WRITEBACK, amount: -nr);
3017	__wb_writeout_add(wb, nr);
3018	if (!mapping_tagged(mapping, PAGECACHE_TAG_WRITEBACK))
3019	wb_inode_writeback_end(wb);
3020	}
3021
3022	if (mapping->host && !mapping_tagged(mapping,
3023	PAGECACHE_TAG_WRITEBACK))
3024	sb_clear_inode_writeback(inode: mapping->host);
3025
3026	xa_unlock_irqrestore(&mapping->i_pages, flags);
3027	} else {
3028	ret = folio_xor_flags_has_waiters(folio, mask: 1 << PG_writeback);
3029	}
3030
3031	lruvec_stat_mod_folio(folio, idx: NR_WRITEBACK, val: -nr);
3032	zone_stat_mod_folio(folio, item: NR_ZONE_WRITE_PENDING, nr: -nr);
3033	node_stat_mod_folio(folio, item: NR_WRITTEN, nr);
3034	folio_memcg_unlock(folio);
3035
3036	return ret;
3037	}
3038
3039	void __folio_start_writeback(struct folio *folio, bool keep_write)
3040	{
3041	long nr = folio_nr_pages(folio);
3042	struct address_space *mapping = folio_mapping(folio);
3043	int access_ret;
3044
3045	VM_BUG_ON_FOLIO(folio_test_writeback(folio), folio);
3046
3047	folio_memcg_lock(folio);
3048	if (mapping && mapping_use_writeback_tags(mapping)) {
3049	XA_STATE(xas, &mapping->i_pages, folio_index(folio));
3050	struct inode *inode = mapping->host;
3051	struct backing_dev_info *bdi = inode_to_bdi(inode);
3052	unsigned long flags;
3053	bool on_wblist;
3054
3055	xas_lock_irqsave(&xas, flags);
3056	xas_load(&xas);
3057	folio_test_set_writeback(folio);
3058
3059	on_wblist = mapping_tagged(mapping, PAGECACHE_TAG_WRITEBACK);
3060
3061	xas_set_mark(&xas, PAGECACHE_TAG_WRITEBACK);
3062	if (bdi->capabilities & BDI_CAP_WRITEBACK_ACCT) {
3063	struct bdi_writeback *wb = inode_to_wb(inode);
3064
3065	wb_stat_mod(wb, item: WB_WRITEBACK, amount: nr);
3066	if (!on_wblist)
3067	wb_inode_writeback_start(wb);
3068	}
3069
3070	/*
3071	* We can come through here when swapping anonymous
3072	* folios, so we don't necessarily have an inode to
3073	* track for sync.
3074	*/
3075	if (mapping->host && !on_wblist)
3076	sb_mark_inode_writeback(inode: mapping->host);
3077	if (!folio_test_dirty(folio))
3078	xas_clear_mark(&xas, PAGECACHE_TAG_DIRTY);
3079	if (!keep_write)
3080	xas_clear_mark(&xas, PAGECACHE_TAG_TOWRITE);
3081	xas_unlock_irqrestore(&xas, flags);
3082	} else {
3083	folio_test_set_writeback(folio);
3084	}
3085
3086	lruvec_stat_mod_folio(folio, idx: NR_WRITEBACK, val: nr);
3087	zone_stat_mod_folio(folio, item: NR_ZONE_WRITE_PENDING, nr);
3088	folio_memcg_unlock(folio);
3089
3090	access_ret = arch_make_folio_accessible(folio);
3091	/*
3092	* If writeback has been triggered on a page that cannot be made
3093	* accessible, it is too late to recover here.
3094	*/
3095	VM_BUG_ON_FOLIO(access_ret != 0, folio);
3096	}
3097	EXPORT_SYMBOL(__folio_start_writeback);
3098
3099	/**
3100	* folio_wait_writeback - Wait for a folio to finish writeback.
3101	* @folio: The folio to wait for.
3102	*
3103	* If the folio is currently being written back to storage, wait for the
3104	* I/O to complete.
3105	*
3106	* Context: Sleeps. Must be called in process context and with
3107	* no spinlocks held. Caller should hold a reference on the folio.
3108	* If the folio is not locked, writeback may start again after writeback
3109	* has finished.
3110	*/
3111	void folio_wait_writeback(struct folio *folio)
3112	{
3113	while (folio_test_writeback(folio)) {
3114	trace_folio_wait_writeback(folio, mapping: folio_mapping(folio));
3115	folio_wait_bit(folio, bit_nr: PG_writeback);
3116	}
3117	}
3118	EXPORT_SYMBOL_GPL(folio_wait_writeback);
3119
3120	/**
3121	* folio_wait_writeback_killable - Wait for a folio to finish writeback.
3122	* @folio: The folio to wait for.
3123	*
3124	* If the folio is currently being written back to storage, wait for the
3125	* I/O to complete or a fatal signal to arrive.
3126	*
3127	* Context: Sleeps. Must be called in process context and with
3128	* no spinlocks held. Caller should hold a reference on the folio.
3129	* If the folio is not locked, writeback may start again after writeback
3130	* has finished.
3131	* Return: 0 on success, -EINTR if we get a fatal signal while waiting.
3132	*/
3133	int folio_wait_writeback_killable(struct folio *folio)
3134	{
3135	while (folio_test_writeback(folio)) {
3136	trace_folio_wait_writeback(folio, mapping: folio_mapping(folio));
3137	if (folio_wait_bit_killable(folio, bit_nr: PG_writeback))
3138	return -EINTR;
3139	}
3140
3141	return 0;
3142	}
3143	EXPORT_SYMBOL_GPL(folio_wait_writeback_killable);
3144
3145	/**
3146	* folio_wait_stable() - wait for writeback to finish, if necessary.
3147	* @folio: The folio to wait on.
3148	*
3149	* This function determines if the given folio is related to a backing
3150	* device that requires folio contents to be held stable during writeback.
3151	* If so, then it will wait for any pending writeback to complete.
3152	*
3153	* Context: Sleeps. Must be called in process context and with
3154	* no spinlocks held. Caller should hold a reference on the folio.
3155	* If the folio is not locked, writeback may start again after writeback
3156	* has finished.
3157	*/
3158	void folio_wait_stable(struct folio *folio)
3159	{
3160	if (mapping_stable_writes(mapping: folio_mapping(folio)))
3161	folio_wait_writeback(folio);
3162	}
3163	EXPORT_SYMBOL_GPL(folio_wait_stable);
3164