xref: /openbmc/linux/mm/page-writeback.c (revision 151f4e2b)
1 /*
2  * mm/page-writeback.c
3  *
4  * Copyright (C) 2002, Linus Torvalds.
5  * Copyright (C) 2007 Red Hat, Inc., Peter Zijlstra
6  *
7  * Contains functions related to writing back dirty pages at the
8  * address_space level.
9  *
10  * 10Apr2002	Andrew Morton
11  *		Initial version
12  */
13 
14 #include <linux/kernel.h>
15 #include <linux/export.h>
16 #include <linux/spinlock.h>
17 #include <linux/fs.h>
18 #include <linux/mm.h>
19 #include <linux/swap.h>
20 #include <linux/slab.h>
21 #include <linux/pagemap.h>
22 #include <linux/writeback.h>
23 #include <linux/init.h>
24 #include <linux/backing-dev.h>
25 #include <linux/task_io_accounting_ops.h>
26 #include <linux/blkdev.h>
27 #include <linux/mpage.h>
28 #include <linux/rmap.h>
29 #include <linux/percpu.h>
30 #include <linux/smp.h>
31 #include <linux/sysctl.h>
32 #include <linux/cpu.h>
33 #include <linux/syscalls.h>
34 #include <linux/buffer_head.h> /* __set_page_dirty_buffers */
35 #include <linux/pagevec.h>
36 #include <linux/timer.h>
37 #include <linux/sched/rt.h>
38 #include <linux/sched/signal.h>
39 #include <linux/mm_inline.h>
40 #include <trace/events/writeback.h>
41 
42 #include "internal.h"
43 
44 /*
45  * Sleep at most 200ms at a time in balance_dirty_pages().
46  */
47 #define MAX_PAUSE		max(HZ/5, 1)
48 
49 /*
50  * Try to keep balance_dirty_pages() call intervals higher than this many pages
51  * by raising pause time to max_pause when falls below it.
52  */
53 #define DIRTY_POLL_THRESH	(128 >> (PAGE_SHIFT - 10))
54 
55 /*
56  * Estimate write bandwidth at 200ms intervals.
57  */
58 #define BANDWIDTH_INTERVAL	max(HZ/5, 1)
59 
60 #define RATELIMIT_CALC_SHIFT	10
61 
62 /*
63  * After a CPU has dirtied this many pages, balance_dirty_pages_ratelimited
64  * will look to see if it needs to force writeback or throttling.
65  */
66 static long ratelimit_pages = 32;
67 
68 /* The following parameters are exported via /proc/sys/vm */
69 
70 /*
71  * Start background writeback (via writeback threads) at this percentage
72  */
73 int dirty_background_ratio = 10;
74 
75 /*
76  * dirty_background_bytes starts at 0 (disabled) so that it is a function of
77  * dirty_background_ratio * the amount of dirtyable memory
78  */
79 unsigned long dirty_background_bytes;
80 
81 /*
82  * free highmem will not be subtracted from the total free memory
83  * for calculating free ratios if vm_highmem_is_dirtyable is true
84  */
85 int vm_highmem_is_dirtyable;
86 
87 /*
88  * The generator of dirty data starts writeback at this percentage
89  */
90 int vm_dirty_ratio = 20;
91 
92 /*
93  * vm_dirty_bytes starts at 0 (disabled) so that it is a function of
94  * vm_dirty_ratio * the amount of dirtyable memory
95  */
96 unsigned long vm_dirty_bytes;
97 
98 /*
99  * The interval between `kupdate'-style writebacks
100  */
101 unsigned int dirty_writeback_interval = 5 * 100; /* centiseconds */
102 
103 EXPORT_SYMBOL_GPL(dirty_writeback_interval);
104 
105 /*
106  * The longest time for which data is allowed to remain dirty
107  */
108 unsigned int dirty_expire_interval = 30 * 100; /* centiseconds */
109 
110 /*
111  * Flag that makes the machine dump writes/reads and block dirtyings.
112  */
113 int block_dump;
114 
115 /*
116  * Flag that puts the machine in "laptop mode". Doubles as a timeout in jiffies:
117  * a full sync is triggered after this time elapses without any disk activity.
118  */
119 int laptop_mode;
120 
121 EXPORT_SYMBOL(laptop_mode);
122 
123 /* End of sysctl-exported parameters */
124 
125 struct wb_domain global_wb_domain;
126 
127 /* consolidated parameters for balance_dirty_pages() and its subroutines */
128 struct dirty_throttle_control {
129 #ifdef CONFIG_CGROUP_WRITEBACK
130 	struct wb_domain	*dom;
131 	struct dirty_throttle_control *gdtc;	/* only set in memcg dtc's */
132 #endif
133 	struct bdi_writeback	*wb;
134 	struct fprop_local_percpu *wb_completions;
135 
136 	unsigned long		avail;		/* dirtyable */
137 	unsigned long		dirty;		/* file_dirty + write + nfs */
138 	unsigned long		thresh;		/* dirty threshold */
139 	unsigned long		bg_thresh;	/* dirty background threshold */
140 
141 	unsigned long		wb_dirty;	/* per-wb counterparts */
142 	unsigned long		wb_thresh;
143 	unsigned long		wb_bg_thresh;
144 
145 	unsigned long		pos_ratio;
146 };
147 
148 /*
149  * Length of period for aging writeout fractions of bdis. This is an
150  * arbitrarily chosen number. The longer the period, the slower fractions will
151  * reflect changes in current writeout rate.
152  */
153 #define VM_COMPLETIONS_PERIOD_LEN (3*HZ)
154 
155 #ifdef CONFIG_CGROUP_WRITEBACK
156 
157 #define GDTC_INIT(__wb)		.wb = (__wb),				\
158 				.dom = &global_wb_domain,		\
159 				.wb_completions = &(__wb)->completions
160 
161 #define GDTC_INIT_NO_WB		.dom = &global_wb_domain
162 
163 #define MDTC_INIT(__wb, __gdtc)	.wb = (__wb),				\
164 				.dom = mem_cgroup_wb_domain(__wb),	\
165 				.wb_completions = &(__wb)->memcg_completions, \
166 				.gdtc = __gdtc
167 
168 static bool mdtc_valid(struct dirty_throttle_control *dtc)
169 {
170 	return dtc->dom;
171 }
172 
173 static struct wb_domain *dtc_dom(struct dirty_throttle_control *dtc)
174 {
175 	return dtc->dom;
176 }
177 
178 static struct dirty_throttle_control *mdtc_gdtc(struct dirty_throttle_control *mdtc)
179 {
180 	return mdtc->gdtc;
181 }
182 
183 static struct fprop_local_percpu *wb_memcg_completions(struct bdi_writeback *wb)
184 {
185 	return &wb->memcg_completions;
186 }
187 
188 static void wb_min_max_ratio(struct bdi_writeback *wb,
189 			     unsigned long *minp, unsigned long *maxp)
190 {
191 	unsigned long this_bw = wb->avg_write_bandwidth;
192 	unsigned long tot_bw = atomic_long_read(&wb->bdi->tot_write_bandwidth);
193 	unsigned long long min = wb->bdi->min_ratio;
194 	unsigned long long max = wb->bdi->max_ratio;
195 
196 	/*
197 	 * @wb may already be clean by the time control reaches here and
198 	 * the total may not include its bw.
199 	 */
200 	if (this_bw < tot_bw) {
201 		if (min) {
202 			min *= this_bw;
203 			do_div(min, tot_bw);
204 		}
205 		if (max < 100) {
206 			max *= this_bw;
207 			do_div(max, tot_bw);
208 		}
209 	}
210 
211 	*minp = min;
212 	*maxp = max;
213 }
214 
215 #else	/* CONFIG_CGROUP_WRITEBACK */
216 
217 #define GDTC_INIT(__wb)		.wb = (__wb),                           \
218 				.wb_completions = &(__wb)->completions
219 #define GDTC_INIT_NO_WB
220 #define MDTC_INIT(__wb, __gdtc)
221 
222 static bool mdtc_valid(struct dirty_throttle_control *dtc)
223 {
224 	return false;
225 }
226 
227 static struct wb_domain *dtc_dom(struct dirty_throttle_control *dtc)
228 {
229 	return &global_wb_domain;
230 }
231 
232 static struct dirty_throttle_control *mdtc_gdtc(struct dirty_throttle_control *mdtc)
233 {
234 	return NULL;
235 }
236 
237 static struct fprop_local_percpu *wb_memcg_completions(struct bdi_writeback *wb)
238 {
239 	return NULL;
240 }
241 
242 static void wb_min_max_ratio(struct bdi_writeback *wb,
243 			     unsigned long *minp, unsigned long *maxp)
244 {
245 	*minp = wb->bdi->min_ratio;
246 	*maxp = wb->bdi->max_ratio;
247 }
248 
249 #endif	/* CONFIG_CGROUP_WRITEBACK */
250 
251 /*
252  * In a memory zone, there is a certain amount of pages we consider
253  * available for the page cache, which is essentially the number of
254  * free and reclaimable pages, minus some zone reserves to protect
255  * lowmem and the ability to uphold the zone's watermarks without
256  * requiring writeback.
257  *
258  * This number of dirtyable pages is the base value of which the
259  * user-configurable dirty ratio is the effictive number of pages that
260  * are allowed to be actually dirtied.  Per individual zone, or
261  * globally by using the sum of dirtyable pages over all zones.
262  *
263  * Because the user is allowed to specify the dirty limit globally as
264  * absolute number of bytes, calculating the per-zone dirty limit can
265  * require translating the configured limit into a percentage of
266  * global dirtyable memory first.
267  */
268 
269 /**
270  * node_dirtyable_memory - number of dirtyable pages in a node
271  * @pgdat: the node
272  *
273  * Return: the node's number of pages potentially available for dirty
274  * page cache.  This is the base value for the per-node dirty limits.
275  */
276 static unsigned long node_dirtyable_memory(struct pglist_data *pgdat)
277 {
278 	unsigned long nr_pages = 0;
279 	int z;
280 
281 	for (z = 0; z < MAX_NR_ZONES; z++) {
282 		struct zone *zone = pgdat->node_zones + z;
283 
284 		if (!populated_zone(zone))
285 			continue;
286 
287 		nr_pages += zone_page_state(zone, NR_FREE_PAGES);
288 	}
289 
290 	/*
291 	 * Pages reserved for the kernel should not be considered
292 	 * dirtyable, to prevent a situation where reclaim has to
293 	 * clean pages in order to balance the zones.
294 	 */
295 	nr_pages -= min(nr_pages, pgdat->totalreserve_pages);
296 
297 	nr_pages += node_page_state(pgdat, NR_INACTIVE_FILE);
298 	nr_pages += node_page_state(pgdat, NR_ACTIVE_FILE);
299 
300 	return nr_pages;
301 }
302 
303 static unsigned long highmem_dirtyable_memory(unsigned long total)
304 {
305 #ifdef CONFIG_HIGHMEM
306 	int node;
307 	unsigned long x = 0;
308 	int i;
309 
310 	for_each_node_state(node, N_HIGH_MEMORY) {
311 		for (i = ZONE_NORMAL + 1; i < MAX_NR_ZONES; i++) {
312 			struct zone *z;
313 			unsigned long nr_pages;
314 
315 			if (!is_highmem_idx(i))
316 				continue;
317 
318 			z = &NODE_DATA(node)->node_zones[i];
319 			if (!populated_zone(z))
320 				continue;
321 
322 			nr_pages = zone_page_state(z, NR_FREE_PAGES);
323 			/* watch for underflows */
324 			nr_pages -= min(nr_pages, high_wmark_pages(z));
325 			nr_pages += zone_page_state(z, NR_ZONE_INACTIVE_FILE);
326 			nr_pages += zone_page_state(z, NR_ZONE_ACTIVE_FILE);
327 			x += nr_pages;
328 		}
329 	}
330 
331 	/*
332 	 * Unreclaimable memory (kernel memory or anonymous memory
333 	 * without swap) can bring down the dirtyable pages below
334 	 * the zone's dirty balance reserve and the above calculation
335 	 * will underflow.  However we still want to add in nodes
336 	 * which are below threshold (negative values) to get a more
337 	 * accurate calculation but make sure that the total never
338 	 * underflows.
339 	 */
340 	if ((long)x < 0)
341 		x = 0;
342 
343 	/*
344 	 * Make sure that the number of highmem pages is never larger
345 	 * than the number of the total dirtyable memory. This can only
346 	 * occur in very strange VM situations but we want to make sure
347 	 * that this does not occur.
348 	 */
349 	return min(x, total);
350 #else
351 	return 0;
352 #endif
353 }
354 
355 /**
356  * global_dirtyable_memory - number of globally dirtyable pages
357  *
358  * Return: the global number of pages potentially available for dirty
359  * page cache.  This is the base value for the global dirty limits.
360  */
361 static unsigned long global_dirtyable_memory(void)
362 {
363 	unsigned long x;
364 
365 	x = global_zone_page_state(NR_FREE_PAGES);
366 	/*
367 	 * Pages reserved for the kernel should not be considered
368 	 * dirtyable, to prevent a situation where reclaim has to
369 	 * clean pages in order to balance the zones.
370 	 */
371 	x -= min(x, totalreserve_pages);
372 
373 	x += global_node_page_state(NR_INACTIVE_FILE);
374 	x += global_node_page_state(NR_ACTIVE_FILE);
375 
376 	if (!vm_highmem_is_dirtyable)
377 		x -= highmem_dirtyable_memory(x);
378 
379 	return x + 1;	/* Ensure that we never return 0 */
380 }
381 
382 /**
383  * domain_dirty_limits - calculate thresh and bg_thresh for a wb_domain
384  * @dtc: dirty_throttle_control of interest
385  *
386  * Calculate @dtc->thresh and ->bg_thresh considering
387  * vm_dirty_{bytes|ratio} and dirty_background_{bytes|ratio}.  The caller
388  * must ensure that @dtc->avail is set before calling this function.  The
389  * dirty limits will be lifted by 1/4 for PF_LESS_THROTTLE (ie. nfsd) and
390  * real-time tasks.
391  */
392 static void domain_dirty_limits(struct dirty_throttle_control *dtc)
393 {
394 	const unsigned long available_memory = dtc->avail;
395 	struct dirty_throttle_control *gdtc = mdtc_gdtc(dtc);
396 	unsigned long bytes = vm_dirty_bytes;
397 	unsigned long bg_bytes = dirty_background_bytes;
398 	/* convert ratios to per-PAGE_SIZE for higher precision */
399 	unsigned long ratio = (vm_dirty_ratio * PAGE_SIZE) / 100;
400 	unsigned long bg_ratio = (dirty_background_ratio * PAGE_SIZE) / 100;
401 	unsigned long thresh;
402 	unsigned long bg_thresh;
403 	struct task_struct *tsk;
404 
405 	/* gdtc is !NULL iff @dtc is for memcg domain */
406 	if (gdtc) {
407 		unsigned long global_avail = gdtc->avail;
408 
409 		/*
410 		 * The byte settings can't be applied directly to memcg
411 		 * domains.  Convert them to ratios by scaling against
412 		 * globally available memory.  As the ratios are in
413 		 * per-PAGE_SIZE, they can be obtained by dividing bytes by
414 		 * number of pages.
415 		 */
416 		if (bytes)
417 			ratio = min(DIV_ROUND_UP(bytes, global_avail),
418 				    PAGE_SIZE);
419 		if (bg_bytes)
420 			bg_ratio = min(DIV_ROUND_UP(bg_bytes, global_avail),
421 				       PAGE_SIZE);
422 		bytes = bg_bytes = 0;
423 	}
424 
425 	if (bytes)
426 		thresh = DIV_ROUND_UP(bytes, PAGE_SIZE);
427 	else
428 		thresh = (ratio * available_memory) / PAGE_SIZE;
429 
430 	if (bg_bytes)
431 		bg_thresh = DIV_ROUND_UP(bg_bytes, PAGE_SIZE);
432 	else
433 		bg_thresh = (bg_ratio * available_memory) / PAGE_SIZE;
434 
435 	if (bg_thresh >= thresh)
436 		bg_thresh = thresh / 2;
437 	tsk = current;
438 	if (tsk->flags & PF_LESS_THROTTLE || rt_task(tsk)) {
439 		bg_thresh += bg_thresh / 4 + global_wb_domain.dirty_limit / 32;
440 		thresh += thresh / 4 + global_wb_domain.dirty_limit / 32;
441 	}
442 	dtc->thresh = thresh;
443 	dtc->bg_thresh = bg_thresh;
444 
445 	/* we should eventually report the domain in the TP */
446 	if (!gdtc)
447 		trace_global_dirty_state(bg_thresh, thresh);
448 }
449 
450 /**
451  * global_dirty_limits - background-writeback and dirty-throttling thresholds
452  * @pbackground: out parameter for bg_thresh
453  * @pdirty: out parameter for thresh
454  *
455  * Calculate bg_thresh and thresh for global_wb_domain.  See
456  * domain_dirty_limits() for details.
457  */
458 void global_dirty_limits(unsigned long *pbackground, unsigned long *pdirty)
459 {
460 	struct dirty_throttle_control gdtc = { GDTC_INIT_NO_WB };
461 
462 	gdtc.avail = global_dirtyable_memory();
463 	domain_dirty_limits(&gdtc);
464 
465 	*pbackground = gdtc.bg_thresh;
466 	*pdirty = gdtc.thresh;
467 }
468 
469 /**
470  * node_dirty_limit - maximum number of dirty pages allowed in a node
471  * @pgdat: the node
472  *
473  * Return: the maximum number of dirty pages allowed in a node, based
474  * on the node's dirtyable memory.
475  */
476 static unsigned long node_dirty_limit(struct pglist_data *pgdat)
477 {
478 	unsigned long node_memory = node_dirtyable_memory(pgdat);
479 	struct task_struct *tsk = current;
480 	unsigned long dirty;
481 
482 	if (vm_dirty_bytes)
483 		dirty = DIV_ROUND_UP(vm_dirty_bytes, PAGE_SIZE) *
484 			node_memory / global_dirtyable_memory();
485 	else
486 		dirty = vm_dirty_ratio * node_memory / 100;
487 
488 	if (tsk->flags & PF_LESS_THROTTLE || rt_task(tsk))
489 		dirty += dirty / 4;
490 
491 	return dirty;
492 }
493 
494 /**
495  * node_dirty_ok - tells whether a node is within its dirty limits
496  * @pgdat: the node to check
497  *
498  * Return: %true when the dirty pages in @pgdat are within the node's
499  * dirty limit, %false if the limit is exceeded.
500  */
501 bool node_dirty_ok(struct pglist_data *pgdat)
502 {
503 	unsigned long limit = node_dirty_limit(pgdat);
504 	unsigned long nr_pages = 0;
505 
506 	nr_pages += node_page_state(pgdat, NR_FILE_DIRTY);
507 	nr_pages += node_page_state(pgdat, NR_UNSTABLE_NFS);
508 	nr_pages += node_page_state(pgdat, NR_WRITEBACK);
509 
510 	return nr_pages <= limit;
511 }
512 
513 int dirty_background_ratio_handler(struct ctl_table *table, int write,
514 		void __user *buffer, size_t *lenp,
515 		loff_t *ppos)
516 {
517 	int ret;
518 
519 	ret = proc_dointvec_minmax(table, write, buffer, lenp, ppos);
520 	if (ret == 0 && write)
521 		dirty_background_bytes = 0;
522 	return ret;
523 }
524 
525 int dirty_background_bytes_handler(struct ctl_table *table, int write,
526 		void __user *buffer, size_t *lenp,
527 		loff_t *ppos)
528 {
529 	int ret;
530 
531 	ret = proc_doulongvec_minmax(table, write, buffer, lenp, ppos);
532 	if (ret == 0 && write)
533 		dirty_background_ratio = 0;
534 	return ret;
535 }
536 
537 int dirty_ratio_handler(struct ctl_table *table, int write,
538 		void __user *buffer, size_t *lenp,
539 		loff_t *ppos)
540 {
541 	int old_ratio = vm_dirty_ratio;
542 	int ret;
543 
544 	ret = proc_dointvec_minmax(table, write, buffer, lenp, ppos);
545 	if (ret == 0 && write && vm_dirty_ratio != old_ratio) {
546 		writeback_set_ratelimit();
547 		vm_dirty_bytes = 0;
548 	}
549 	return ret;
550 }
551 
552 int dirty_bytes_handler(struct ctl_table *table, int write,
553 		void __user *buffer, size_t *lenp,
554 		loff_t *ppos)
555 {
556 	unsigned long old_bytes = vm_dirty_bytes;
557 	int ret;
558 
559 	ret = proc_doulongvec_minmax(table, write, buffer, lenp, ppos);
560 	if (ret == 0 && write && vm_dirty_bytes != old_bytes) {
561 		writeback_set_ratelimit();
562 		vm_dirty_ratio = 0;
563 	}
564 	return ret;
565 }
566 
567 static unsigned long wp_next_time(unsigned long cur_time)
568 {
569 	cur_time += VM_COMPLETIONS_PERIOD_LEN;
570 	/* 0 has a special meaning... */
571 	if (!cur_time)
572 		return 1;
573 	return cur_time;
574 }
575 
576 static void wb_domain_writeout_inc(struct wb_domain *dom,
577 				   struct fprop_local_percpu *completions,
578 				   unsigned int max_prop_frac)
579 {
580 	__fprop_inc_percpu_max(&dom->completions, completions,
581 			       max_prop_frac);
582 	/* First event after period switching was turned off? */
583 	if (unlikely(!dom->period_time)) {
584 		/*
585 		 * We can race with other __bdi_writeout_inc calls here but
586 		 * it does not cause any harm since the resulting time when
587 		 * timer will fire and what is in writeout_period_time will be
588 		 * roughly the same.
589 		 */
590 		dom->period_time = wp_next_time(jiffies);
591 		mod_timer(&dom->period_timer, dom->period_time);
592 	}
593 }
594 
595 /*
596  * Increment @wb's writeout completion count and the global writeout
597  * completion count. Called from test_clear_page_writeback().
598  */
599 static inline void __wb_writeout_inc(struct bdi_writeback *wb)
600 {
601 	struct wb_domain *cgdom;
602 
603 	inc_wb_stat(wb, WB_WRITTEN);
604 	wb_domain_writeout_inc(&global_wb_domain, &wb->completions,
605 			       wb->bdi->max_prop_frac);
606 
607 	cgdom = mem_cgroup_wb_domain(wb);
608 	if (cgdom)
609 		wb_domain_writeout_inc(cgdom, wb_memcg_completions(wb),
610 				       wb->bdi->max_prop_frac);
611 }
612 
613 void wb_writeout_inc(struct bdi_writeback *wb)
614 {
615 	unsigned long flags;
616 
617 	local_irq_save(flags);
618 	__wb_writeout_inc(wb);
619 	local_irq_restore(flags);
620 }
621 EXPORT_SYMBOL_GPL(wb_writeout_inc);
622 
623 /*
624  * On idle system, we can be called long after we scheduled because we use
625  * deferred timers so count with missed periods.
626  */
627 static void writeout_period(struct timer_list *t)
628 {
629 	struct wb_domain *dom = from_timer(dom, t, period_timer);
630 	int miss_periods = (jiffies - dom->period_time) /
631 						 VM_COMPLETIONS_PERIOD_LEN;
632 
633 	if (fprop_new_period(&dom->completions, miss_periods + 1)) {
634 		dom->period_time = wp_next_time(dom->period_time +
635 				miss_periods * VM_COMPLETIONS_PERIOD_LEN);
636 		mod_timer(&dom->period_timer, dom->period_time);
637 	} else {
638 		/*
639 		 * Aging has zeroed all fractions. Stop wasting CPU on period
640 		 * updates.
641 		 */
642 		dom->period_time = 0;
643 	}
644 }
645 
646 int wb_domain_init(struct wb_domain *dom, gfp_t gfp)
647 {
648 	memset(dom, 0, sizeof(*dom));
649 
650 	spin_lock_init(&dom->lock);
651 
652 	timer_setup(&dom->period_timer, writeout_period, TIMER_DEFERRABLE);
653 
654 	dom->dirty_limit_tstamp = jiffies;
655 
656 	return fprop_global_init(&dom->completions, gfp);
657 }
658 
659 #ifdef CONFIG_CGROUP_WRITEBACK
660 void wb_domain_exit(struct wb_domain *dom)
661 {
662 	del_timer_sync(&dom->period_timer);
663 	fprop_global_destroy(&dom->completions);
664 }
665 #endif
666 
667 /*
668  * bdi_min_ratio keeps the sum of the minimum dirty shares of all
669  * registered backing devices, which, for obvious reasons, can not
670  * exceed 100%.
671  */
672 static unsigned int bdi_min_ratio;
673 
674 int bdi_set_min_ratio(struct backing_dev_info *bdi, unsigned int min_ratio)
675 {
676 	int ret = 0;
677 
678 	spin_lock_bh(&bdi_lock);
679 	if (min_ratio > bdi->max_ratio) {
680 		ret = -EINVAL;
681 	} else {
682 		min_ratio -= bdi->min_ratio;
683 		if (bdi_min_ratio + min_ratio < 100) {
684 			bdi_min_ratio += min_ratio;
685 			bdi->min_ratio += min_ratio;
686 		} else {
687 			ret = -EINVAL;
688 		}
689 	}
690 	spin_unlock_bh(&bdi_lock);
691 
692 	return ret;
693 }
694 
695 int bdi_set_max_ratio(struct backing_dev_info *bdi, unsigned max_ratio)
696 {
697 	int ret = 0;
698 
699 	if (max_ratio > 100)
700 		return -EINVAL;
701 
702 	spin_lock_bh(&bdi_lock);
703 	if (bdi->min_ratio > max_ratio) {
704 		ret = -EINVAL;
705 	} else {
706 		bdi->max_ratio = max_ratio;
707 		bdi->max_prop_frac = (FPROP_FRAC_BASE * max_ratio) / 100;
708 	}
709 	spin_unlock_bh(&bdi_lock);
710 
711 	return ret;
712 }
713 EXPORT_SYMBOL(bdi_set_max_ratio);
714 
715 static unsigned long dirty_freerun_ceiling(unsigned long thresh,
716 					   unsigned long bg_thresh)
717 {
718 	return (thresh + bg_thresh) / 2;
719 }
720 
721 static unsigned long hard_dirty_limit(struct wb_domain *dom,
722 				      unsigned long thresh)
723 {
724 	return max(thresh, dom->dirty_limit);
725 }
726 
727 /*
728  * Memory which can be further allocated to a memcg domain is capped by
729  * system-wide clean memory excluding the amount being used in the domain.
730  */
731 static void mdtc_calc_avail(struct dirty_throttle_control *mdtc,
732 			    unsigned long filepages, unsigned long headroom)
733 {
734 	struct dirty_throttle_control *gdtc = mdtc_gdtc(mdtc);
735 	unsigned long clean = filepages - min(filepages, mdtc->dirty);
736 	unsigned long global_clean = gdtc->avail - min(gdtc->avail, gdtc->dirty);
737 	unsigned long other_clean = global_clean - min(global_clean, clean);
738 
739 	mdtc->avail = filepages + min(headroom, other_clean);
740 }
741 
742 /**
743  * __wb_calc_thresh - @wb's share of dirty throttling threshold
744  * @dtc: dirty_throttle_context of interest
745  *
746  * Note that balance_dirty_pages() will only seriously take it as a hard limit
747  * when sleeping max_pause per page is not enough to keep the dirty pages under
748  * control. For example, when the device is completely stalled due to some error
749  * conditions, or when there are 1000 dd tasks writing to a slow 10MB/s USB key.
750  * In the other normal situations, it acts more gently by throttling the tasks
751  * more (rather than completely block them) when the wb dirty pages go high.
752  *
753  * It allocates high/low dirty limits to fast/slow devices, in order to prevent
754  * - starving fast devices
755  * - piling up dirty pages (that will take long time to sync) on slow devices
756  *
757  * The wb's share of dirty limit will be adapting to its throughput and
758  * bounded by the bdi->min_ratio and/or bdi->max_ratio parameters, if set.
759  *
760  * Return: @wb's dirty limit in pages. The term "dirty" in the context of
761  * dirty balancing includes all PG_dirty, PG_writeback and NFS unstable pages.
762  */
763 static unsigned long __wb_calc_thresh(struct dirty_throttle_control *dtc)
764 {
765 	struct wb_domain *dom = dtc_dom(dtc);
766 	unsigned long thresh = dtc->thresh;
767 	u64 wb_thresh;
768 	long numerator, denominator;
769 	unsigned long wb_min_ratio, wb_max_ratio;
770 
771 	/*
772 	 * Calculate this BDI's share of the thresh ratio.
773 	 */
774 	fprop_fraction_percpu(&dom->completions, dtc->wb_completions,
775 			      &numerator, &denominator);
776 
777 	wb_thresh = (thresh * (100 - bdi_min_ratio)) / 100;
778 	wb_thresh *= numerator;
779 	do_div(wb_thresh, denominator);
780 
781 	wb_min_max_ratio(dtc->wb, &wb_min_ratio, &wb_max_ratio);
782 
783 	wb_thresh += (thresh * wb_min_ratio) / 100;
784 	if (wb_thresh > (thresh * wb_max_ratio) / 100)
785 		wb_thresh = thresh * wb_max_ratio / 100;
786 
787 	return wb_thresh;
788 }
789 
790 unsigned long wb_calc_thresh(struct bdi_writeback *wb, unsigned long thresh)
791 {
792 	struct dirty_throttle_control gdtc = { GDTC_INIT(wb),
793 					       .thresh = thresh };
794 	return __wb_calc_thresh(&gdtc);
795 }
796 
797 /*
798  *                           setpoint - dirty 3
799  *        f(dirty) := 1.0 + (----------------)
800  *                           limit - setpoint
801  *
802  * it's a 3rd order polynomial that subjects to
803  *
804  * (1) f(freerun)  = 2.0 => rampup dirty_ratelimit reasonably fast
805  * (2) f(setpoint) = 1.0 => the balance point
806  * (3) f(limit)    = 0   => the hard limit
807  * (4) df/dx      <= 0	 => negative feedback control
808  * (5) the closer to setpoint, the smaller |df/dx| (and the reverse)
809  *     => fast response on large errors; small oscillation near setpoint
810  */
811 static long long pos_ratio_polynom(unsigned long setpoint,
812 					  unsigned long dirty,
813 					  unsigned long limit)
814 {
815 	long long pos_ratio;
816 	long x;
817 
818 	x = div64_s64(((s64)setpoint - (s64)dirty) << RATELIMIT_CALC_SHIFT,
819 		      (limit - setpoint) | 1);
820 	pos_ratio = x;
821 	pos_ratio = pos_ratio * x >> RATELIMIT_CALC_SHIFT;
822 	pos_ratio = pos_ratio * x >> RATELIMIT_CALC_SHIFT;
823 	pos_ratio += 1 << RATELIMIT_CALC_SHIFT;
824 
825 	return clamp(pos_ratio, 0LL, 2LL << RATELIMIT_CALC_SHIFT);
826 }
827 
828 /*
829  * Dirty position control.
830  *
831  * (o) global/bdi setpoints
832  *
833  * We want the dirty pages be balanced around the global/wb setpoints.
834  * When the number of dirty pages is higher/lower than the setpoint, the
835  * dirty position control ratio (and hence task dirty ratelimit) will be
836  * decreased/increased to bring the dirty pages back to the setpoint.
837  *
838  *     pos_ratio = 1 << RATELIMIT_CALC_SHIFT
839  *
840  *     if (dirty < setpoint) scale up   pos_ratio
841  *     if (dirty > setpoint) scale down pos_ratio
842  *
843  *     if (wb_dirty < wb_setpoint) scale up   pos_ratio
844  *     if (wb_dirty > wb_setpoint) scale down pos_ratio
845  *
846  *     task_ratelimit = dirty_ratelimit * pos_ratio >> RATELIMIT_CALC_SHIFT
847  *
848  * (o) global control line
849  *
850  *     ^ pos_ratio
851  *     |
852  *     |            |<===== global dirty control scope ======>|
853  * 2.0 .............*
854  *     |            .*
855  *     |            . *
856  *     |            .   *
857  *     |            .     *
858  *     |            .        *
859  *     |            .            *
860  * 1.0 ................................*
861  *     |            .                  .     *
862  *     |            .                  .          *
863  *     |            .                  .              *
864  *     |            .                  .                 *
865  *     |            .                  .                    *
866  *   0 +------------.------------------.----------------------*------------->
867  *           freerun^          setpoint^                 limit^   dirty pages
868  *
869  * (o) wb control line
870  *
871  *     ^ pos_ratio
872  *     |
873  *     |            *
874  *     |              *
875  *     |                *
876  *     |                  *
877  *     |                    * |<=========== span ============>|
878  * 1.0 .......................*
879  *     |                      . *
880  *     |                      .   *
881  *     |                      .     *
882  *     |                      .       *
883  *     |                      .         *
884  *     |                      .           *
885  *     |                      .             *
886  *     |                      .               *
887  *     |                      .                 *
888  *     |                      .                   *
889  *     |                      .                     *
890  * 1/4 ...............................................* * * * * * * * * * * *
891  *     |                      .                         .
892  *     |                      .                           .
893  *     |                      .                             .
894  *   0 +----------------------.-------------------------------.------------->
895  *                wb_setpoint^                    x_intercept^
896  *
897  * The wb control line won't drop below pos_ratio=1/4, so that wb_dirty can
898  * be smoothly throttled down to normal if it starts high in situations like
899  * - start writing to a slow SD card and a fast disk at the same time. The SD
900  *   card's wb_dirty may rush to many times higher than wb_setpoint.
901  * - the wb dirty thresh drops quickly due to change of JBOD workload
902  */
903 static void wb_position_ratio(struct dirty_throttle_control *dtc)
904 {
905 	struct bdi_writeback *wb = dtc->wb;
906 	unsigned long write_bw = wb->avg_write_bandwidth;
907 	unsigned long freerun = dirty_freerun_ceiling(dtc->thresh, dtc->bg_thresh);
908 	unsigned long limit = hard_dirty_limit(dtc_dom(dtc), dtc->thresh);
909 	unsigned long wb_thresh = dtc->wb_thresh;
910 	unsigned long x_intercept;
911 	unsigned long setpoint;		/* dirty pages' target balance point */
912 	unsigned long wb_setpoint;
913 	unsigned long span;
914 	long long pos_ratio;		/* for scaling up/down the rate limit */
915 	long x;
916 
917 	dtc->pos_ratio = 0;
918 
919 	if (unlikely(dtc->dirty >= limit))
920 		return;
921 
922 	/*
923 	 * global setpoint
924 	 *
925 	 * See comment for pos_ratio_polynom().
926 	 */
927 	setpoint = (freerun + limit) / 2;
928 	pos_ratio = pos_ratio_polynom(setpoint, dtc->dirty, limit);
929 
930 	/*
931 	 * The strictlimit feature is a tool preventing mistrusted filesystems
932 	 * from growing a large number of dirty pages before throttling. For
933 	 * such filesystems balance_dirty_pages always checks wb counters
934 	 * against wb limits. Even if global "nr_dirty" is under "freerun".
935 	 * This is especially important for fuse which sets bdi->max_ratio to
936 	 * 1% by default. Without strictlimit feature, fuse writeback may
937 	 * consume arbitrary amount of RAM because it is accounted in
938 	 * NR_WRITEBACK_TEMP which is not involved in calculating "nr_dirty".
939 	 *
940 	 * Here, in wb_position_ratio(), we calculate pos_ratio based on
941 	 * two values: wb_dirty and wb_thresh. Let's consider an example:
942 	 * total amount of RAM is 16GB, bdi->max_ratio is equal to 1%, global
943 	 * limits are set by default to 10% and 20% (background and throttle).
944 	 * Then wb_thresh is 1% of 20% of 16GB. This amounts to ~8K pages.
945 	 * wb_calc_thresh(wb, bg_thresh) is about ~4K pages. wb_setpoint is
946 	 * about ~6K pages (as the average of background and throttle wb
947 	 * limits). The 3rd order polynomial will provide positive feedback if
948 	 * wb_dirty is under wb_setpoint and vice versa.
949 	 *
950 	 * Note, that we cannot use global counters in these calculations
951 	 * because we want to throttle process writing to a strictlimit wb
952 	 * much earlier than global "freerun" is reached (~23MB vs. ~2.3GB
953 	 * in the example above).
954 	 */
955 	if (unlikely(wb->bdi->capabilities & BDI_CAP_STRICTLIMIT)) {
956 		long long wb_pos_ratio;
957 
958 		if (dtc->wb_dirty < 8) {
959 			dtc->pos_ratio = min_t(long long, pos_ratio * 2,
960 					   2 << RATELIMIT_CALC_SHIFT);
961 			return;
962 		}
963 
964 		if (dtc->wb_dirty >= wb_thresh)
965 			return;
966 
967 		wb_setpoint = dirty_freerun_ceiling(wb_thresh,
968 						    dtc->wb_bg_thresh);
969 
970 		if (wb_setpoint == 0 || wb_setpoint == wb_thresh)
971 			return;
972 
973 		wb_pos_ratio = pos_ratio_polynom(wb_setpoint, dtc->wb_dirty,
974 						 wb_thresh);
975 
976 		/*
977 		 * Typically, for strictlimit case, wb_setpoint << setpoint
978 		 * and pos_ratio >> wb_pos_ratio. In the other words global
979 		 * state ("dirty") is not limiting factor and we have to
980 		 * make decision based on wb counters. But there is an
981 		 * important case when global pos_ratio should get precedence:
982 		 * global limits are exceeded (e.g. due to activities on other
983 		 * wb's) while given strictlimit wb is below limit.
984 		 *
985 		 * "pos_ratio * wb_pos_ratio" would work for the case above,
986 		 * but it would look too non-natural for the case of all
987 		 * activity in the system coming from a single strictlimit wb
988 		 * with bdi->max_ratio == 100%.
989 		 *
990 		 * Note that min() below somewhat changes the dynamics of the
991 		 * control system. Normally, pos_ratio value can be well over 3
992 		 * (when globally we are at freerun and wb is well below wb
993 		 * setpoint). Now the maximum pos_ratio in the same situation
994 		 * is 2. We might want to tweak this if we observe the control
995 		 * system is too slow to adapt.
996 		 */
997 		dtc->pos_ratio = min(pos_ratio, wb_pos_ratio);
998 		return;
999 	}
1000 
1001 	/*
1002 	 * We have computed basic pos_ratio above based on global situation. If
1003 	 * the wb is over/under its share of dirty pages, we want to scale
1004 	 * pos_ratio further down/up. That is done by the following mechanism.
1005 	 */
1006 
1007 	/*
1008 	 * wb setpoint
1009 	 *
1010 	 *        f(wb_dirty) := 1.0 + k * (wb_dirty - wb_setpoint)
1011 	 *
1012 	 *                        x_intercept - wb_dirty
1013 	 *                     := --------------------------
1014 	 *                        x_intercept - wb_setpoint
1015 	 *
1016 	 * The main wb control line is a linear function that subjects to
1017 	 *
1018 	 * (1) f(wb_setpoint) = 1.0
1019 	 * (2) k = - 1 / (8 * write_bw)  (in single wb case)
1020 	 *     or equally: x_intercept = wb_setpoint + 8 * write_bw
1021 	 *
1022 	 * For single wb case, the dirty pages are observed to fluctuate
1023 	 * regularly within range
1024 	 *        [wb_setpoint - write_bw/2, wb_setpoint + write_bw/2]
1025 	 * for various filesystems, where (2) can yield in a reasonable 12.5%
1026 	 * fluctuation range for pos_ratio.
1027 	 *
1028 	 * For JBOD case, wb_thresh (not wb_dirty!) could fluctuate up to its
1029 	 * own size, so move the slope over accordingly and choose a slope that
1030 	 * yields 100% pos_ratio fluctuation on suddenly doubled wb_thresh.
1031 	 */
1032 	if (unlikely(wb_thresh > dtc->thresh))
1033 		wb_thresh = dtc->thresh;
1034 	/*
1035 	 * It's very possible that wb_thresh is close to 0 not because the
1036 	 * device is slow, but that it has remained inactive for long time.
1037 	 * Honour such devices a reasonable good (hopefully IO efficient)
1038 	 * threshold, so that the occasional writes won't be blocked and active
1039 	 * writes can rampup the threshold quickly.
1040 	 */
1041 	wb_thresh = max(wb_thresh, (limit - dtc->dirty) / 8);
1042 	/*
1043 	 * scale global setpoint to wb's:
1044 	 *	wb_setpoint = setpoint * wb_thresh / thresh
1045 	 */
1046 	x = div_u64((u64)wb_thresh << 16, dtc->thresh | 1);
1047 	wb_setpoint = setpoint * (u64)x >> 16;
1048 	/*
1049 	 * Use span=(8*write_bw) in single wb case as indicated by
1050 	 * (thresh - wb_thresh ~= 0) and transit to wb_thresh in JBOD case.
1051 	 *
1052 	 *        wb_thresh                    thresh - wb_thresh
1053 	 * span = --------- * (8 * write_bw) + ------------------ * wb_thresh
1054 	 *         thresh                           thresh
1055 	 */
1056 	span = (dtc->thresh - wb_thresh + 8 * write_bw) * (u64)x >> 16;
1057 	x_intercept = wb_setpoint + span;
1058 
1059 	if (dtc->wb_dirty < x_intercept - span / 4) {
1060 		pos_ratio = div64_u64(pos_ratio * (x_intercept - dtc->wb_dirty),
1061 				      (x_intercept - wb_setpoint) | 1);
1062 	} else
1063 		pos_ratio /= 4;
1064 
1065 	/*
1066 	 * wb reserve area, safeguard against dirty pool underrun and disk idle
1067 	 * It may push the desired control point of global dirty pages higher
1068 	 * than setpoint.
1069 	 */
1070 	x_intercept = wb_thresh / 2;
1071 	if (dtc->wb_dirty < x_intercept) {
1072 		if (dtc->wb_dirty > x_intercept / 8)
1073 			pos_ratio = div_u64(pos_ratio * x_intercept,
1074 					    dtc->wb_dirty);
1075 		else
1076 			pos_ratio *= 8;
1077 	}
1078 
1079 	dtc->pos_ratio = pos_ratio;
1080 }
1081 
1082 static void wb_update_write_bandwidth(struct bdi_writeback *wb,
1083 				      unsigned long elapsed,
1084 				      unsigned long written)
1085 {
1086 	const unsigned long period = roundup_pow_of_two(3 * HZ);
1087 	unsigned long avg = wb->avg_write_bandwidth;
1088 	unsigned long old = wb->write_bandwidth;
1089 	u64 bw;
1090 
1091 	/*
1092 	 * bw = written * HZ / elapsed
1093 	 *
1094 	 *                   bw * elapsed + write_bandwidth * (period - elapsed)
1095 	 * write_bandwidth = ---------------------------------------------------
1096 	 *                                          period
1097 	 *
1098 	 * @written may have decreased due to account_page_redirty().
1099 	 * Avoid underflowing @bw calculation.
1100 	 */
1101 	bw = written - min(written, wb->written_stamp);
1102 	bw *= HZ;
1103 	if (unlikely(elapsed > period)) {
1104 		do_div(bw, elapsed);
1105 		avg = bw;
1106 		goto out;
1107 	}
1108 	bw += (u64)wb->write_bandwidth * (period - elapsed);
1109 	bw >>= ilog2(period);
1110 
1111 	/*
1112 	 * one more level of smoothing, for filtering out sudden spikes
1113 	 */
1114 	if (avg > old && old >= (unsigned long)bw)
1115 		avg -= (avg - old) >> 3;
1116 
1117 	if (avg < old && old <= (unsigned long)bw)
1118 		avg += (old - avg) >> 3;
1119 
1120 out:
1121 	/* keep avg > 0 to guarantee that tot > 0 if there are dirty wbs */
1122 	avg = max(avg, 1LU);
1123 	if (wb_has_dirty_io(wb)) {
1124 		long delta = avg - wb->avg_write_bandwidth;
1125 		WARN_ON_ONCE(atomic_long_add_return(delta,
1126 					&wb->bdi->tot_write_bandwidth) <= 0);
1127 	}
1128 	wb->write_bandwidth = bw;
1129 	wb->avg_write_bandwidth = avg;
1130 }
1131 
1132 static void update_dirty_limit(struct dirty_throttle_control *dtc)
1133 {
1134 	struct wb_domain *dom = dtc_dom(dtc);
1135 	unsigned long thresh = dtc->thresh;
1136 	unsigned long limit = dom->dirty_limit;
1137 
1138 	/*
1139 	 * Follow up in one step.
1140 	 */
1141 	if (limit < thresh) {
1142 		limit = thresh;
1143 		goto update;
1144 	}
1145 
1146 	/*
1147 	 * Follow down slowly. Use the higher one as the target, because thresh
1148 	 * may drop below dirty. This is exactly the reason to introduce
1149 	 * dom->dirty_limit which is guaranteed to lie above the dirty pages.
1150 	 */
1151 	thresh = max(thresh, dtc->dirty);
1152 	if (limit > thresh) {
1153 		limit -= (limit - thresh) >> 5;
1154 		goto update;
1155 	}
1156 	return;
1157 update:
1158 	dom->dirty_limit = limit;
1159 }
1160 
1161 static void domain_update_bandwidth(struct dirty_throttle_control *dtc,
1162 				    unsigned long now)
1163 {
1164 	struct wb_domain *dom = dtc_dom(dtc);
1165 
1166 	/*
1167 	 * check locklessly first to optimize away locking for the most time
1168 	 */
1169 	if (time_before(now, dom->dirty_limit_tstamp + BANDWIDTH_INTERVAL))
1170 		return;
1171 
1172 	spin_lock(&dom->lock);
1173 	if (time_after_eq(now, dom->dirty_limit_tstamp + BANDWIDTH_INTERVAL)) {
1174 		update_dirty_limit(dtc);
1175 		dom->dirty_limit_tstamp = now;
1176 	}
1177 	spin_unlock(&dom->lock);
1178 }
1179 
1180 /*
1181  * Maintain wb->dirty_ratelimit, the base dirty throttle rate.
1182  *
1183  * Normal wb tasks will be curbed at or below it in long term.
1184  * Obviously it should be around (write_bw / N) when there are N dd tasks.
1185  */
1186 static void wb_update_dirty_ratelimit(struct dirty_throttle_control *dtc,
1187 				      unsigned long dirtied,
1188 				      unsigned long elapsed)
1189 {
1190 	struct bdi_writeback *wb = dtc->wb;
1191 	unsigned long dirty = dtc->dirty;
1192 	unsigned long freerun = dirty_freerun_ceiling(dtc->thresh, dtc->bg_thresh);
1193 	unsigned long limit = hard_dirty_limit(dtc_dom(dtc), dtc->thresh);
1194 	unsigned long setpoint = (freerun + limit) / 2;
1195 	unsigned long write_bw = wb->avg_write_bandwidth;
1196 	unsigned long dirty_ratelimit = wb->dirty_ratelimit;
1197 	unsigned long dirty_rate;
1198 	unsigned long task_ratelimit;
1199 	unsigned long balanced_dirty_ratelimit;
1200 	unsigned long step;
1201 	unsigned long x;
1202 	unsigned long shift;
1203 
1204 	/*
1205 	 * The dirty rate will match the writeout rate in long term, except
1206 	 * when dirty pages are truncated by userspace or re-dirtied by FS.
1207 	 */
1208 	dirty_rate = (dirtied - wb->dirtied_stamp) * HZ / elapsed;
1209 
1210 	/*
1211 	 * task_ratelimit reflects each dd's dirty rate for the past 200ms.
1212 	 */
1213 	task_ratelimit = (u64)dirty_ratelimit *
1214 					dtc->pos_ratio >> RATELIMIT_CALC_SHIFT;
1215 	task_ratelimit++; /* it helps rampup dirty_ratelimit from tiny values */
1216 
1217 	/*
1218 	 * A linear estimation of the "balanced" throttle rate. The theory is,
1219 	 * if there are N dd tasks, each throttled at task_ratelimit, the wb's
1220 	 * dirty_rate will be measured to be (N * task_ratelimit). So the below
1221 	 * formula will yield the balanced rate limit (write_bw / N).
1222 	 *
1223 	 * Note that the expanded form is not a pure rate feedback:
1224 	 *	rate_(i+1) = rate_(i) * (write_bw / dirty_rate)		     (1)
1225 	 * but also takes pos_ratio into account:
1226 	 *	rate_(i+1) = rate_(i) * (write_bw / dirty_rate) * pos_ratio  (2)
1227 	 *
1228 	 * (1) is not realistic because pos_ratio also takes part in balancing
1229 	 * the dirty rate.  Consider the state
1230 	 *	pos_ratio = 0.5						     (3)
1231 	 *	rate = 2 * (write_bw / N)				     (4)
1232 	 * If (1) is used, it will stuck in that state! Because each dd will
1233 	 * be throttled at
1234 	 *	task_ratelimit = pos_ratio * rate = (write_bw / N)	     (5)
1235 	 * yielding
1236 	 *	dirty_rate = N * task_ratelimit = write_bw		     (6)
1237 	 * put (6) into (1) we get
1238 	 *	rate_(i+1) = rate_(i)					     (7)
1239 	 *
1240 	 * So we end up using (2) to always keep
1241 	 *	rate_(i+1) ~= (write_bw / N)				     (8)
1242 	 * regardless of the value of pos_ratio. As long as (8) is satisfied,
1243 	 * pos_ratio is able to drive itself to 1.0, which is not only where
1244 	 * the dirty count meet the setpoint, but also where the slope of
1245 	 * pos_ratio is most flat and hence task_ratelimit is least fluctuated.
1246 	 */
1247 	balanced_dirty_ratelimit = div_u64((u64)task_ratelimit * write_bw,
1248 					   dirty_rate | 1);
1249 	/*
1250 	 * balanced_dirty_ratelimit ~= (write_bw / N) <= write_bw
1251 	 */
1252 	if (unlikely(balanced_dirty_ratelimit > write_bw))
1253 		balanced_dirty_ratelimit = write_bw;
1254 
1255 	/*
1256 	 * We could safely do this and return immediately:
1257 	 *
1258 	 *	wb->dirty_ratelimit = balanced_dirty_ratelimit;
1259 	 *
1260 	 * However to get a more stable dirty_ratelimit, the below elaborated
1261 	 * code makes use of task_ratelimit to filter out singular points and
1262 	 * limit the step size.
1263 	 *
1264 	 * The below code essentially only uses the relative value of
1265 	 *
1266 	 *	task_ratelimit - dirty_ratelimit
1267 	 *	= (pos_ratio - 1) * dirty_ratelimit
1268 	 *
1269 	 * which reflects the direction and size of dirty position error.
1270 	 */
1271 
1272 	/*
1273 	 * dirty_ratelimit will follow balanced_dirty_ratelimit iff
1274 	 * task_ratelimit is on the same side of dirty_ratelimit, too.
1275 	 * For example, when
1276 	 * - dirty_ratelimit > balanced_dirty_ratelimit
1277 	 * - dirty_ratelimit > task_ratelimit (dirty pages are above setpoint)
1278 	 * lowering dirty_ratelimit will help meet both the position and rate
1279 	 * control targets. Otherwise, don't update dirty_ratelimit if it will
1280 	 * only help meet the rate target. After all, what the users ultimately
1281 	 * feel and care are stable dirty rate and small position error.
1282 	 *
1283 	 * |task_ratelimit - dirty_ratelimit| is used to limit the step size
1284 	 * and filter out the singular points of balanced_dirty_ratelimit. Which
1285 	 * keeps jumping around randomly and can even leap far away at times
1286 	 * due to the small 200ms estimation period of dirty_rate (we want to
1287 	 * keep that period small to reduce time lags).
1288 	 */
1289 	step = 0;
1290 
1291 	/*
1292 	 * For strictlimit case, calculations above were based on wb counters
1293 	 * and limits (starting from pos_ratio = wb_position_ratio() and up to
1294 	 * balanced_dirty_ratelimit = task_ratelimit * write_bw / dirty_rate).
1295 	 * Hence, to calculate "step" properly, we have to use wb_dirty as
1296 	 * "dirty" and wb_setpoint as "setpoint".
1297 	 *
1298 	 * We rampup dirty_ratelimit forcibly if wb_dirty is low because
1299 	 * it's possible that wb_thresh is close to zero due to inactivity
1300 	 * of backing device.
1301 	 */
1302 	if (unlikely(wb->bdi->capabilities & BDI_CAP_STRICTLIMIT)) {
1303 		dirty = dtc->wb_dirty;
1304 		if (dtc->wb_dirty < 8)
1305 			setpoint = dtc->wb_dirty + 1;
1306 		else
1307 			setpoint = (dtc->wb_thresh + dtc->wb_bg_thresh) / 2;
1308 	}
1309 
1310 	if (dirty < setpoint) {
1311 		x = min3(wb->balanced_dirty_ratelimit,
1312 			 balanced_dirty_ratelimit, task_ratelimit);
1313 		if (dirty_ratelimit < x)
1314 			step = x - dirty_ratelimit;
1315 	} else {
1316 		x = max3(wb->balanced_dirty_ratelimit,
1317 			 balanced_dirty_ratelimit, task_ratelimit);
1318 		if (dirty_ratelimit > x)
1319 			step = dirty_ratelimit - x;
1320 	}
1321 
1322 	/*
1323 	 * Don't pursue 100% rate matching. It's impossible since the balanced
1324 	 * rate itself is constantly fluctuating. So decrease the track speed
1325 	 * when it gets close to the target. Helps eliminate pointless tremors.
1326 	 */
1327 	shift = dirty_ratelimit / (2 * step + 1);
1328 	if (shift < BITS_PER_LONG)
1329 		step = DIV_ROUND_UP(step >> shift, 8);
1330 	else
1331 		step = 0;
1332 
1333 	if (dirty_ratelimit < balanced_dirty_ratelimit)
1334 		dirty_ratelimit += step;
1335 	else
1336 		dirty_ratelimit -= step;
1337 
1338 	wb->dirty_ratelimit = max(dirty_ratelimit, 1UL);
1339 	wb->balanced_dirty_ratelimit = balanced_dirty_ratelimit;
1340 
1341 	trace_bdi_dirty_ratelimit(wb, dirty_rate, task_ratelimit);
1342 }
1343 
1344 static void __wb_update_bandwidth(struct dirty_throttle_control *gdtc,
1345 				  struct dirty_throttle_control *mdtc,
1346 				  unsigned long start_time,
1347 				  bool update_ratelimit)
1348 {
1349 	struct bdi_writeback *wb = gdtc->wb;
1350 	unsigned long now = jiffies;
1351 	unsigned long elapsed = now - wb->bw_time_stamp;
1352 	unsigned long dirtied;
1353 	unsigned long written;
1354 
1355 	lockdep_assert_held(&wb->list_lock);
1356 
1357 	/*
1358 	 * rate-limit, only update once every 200ms.
1359 	 */
1360 	if (elapsed < BANDWIDTH_INTERVAL)
1361 		return;
1362 
1363 	dirtied = percpu_counter_read(&wb->stat[WB_DIRTIED]);
1364 	written = percpu_counter_read(&wb->stat[WB_WRITTEN]);
1365 
1366 	/*
1367 	 * Skip quiet periods when disk bandwidth is under-utilized.
1368 	 * (at least 1s idle time between two flusher runs)
1369 	 */
1370 	if (elapsed > HZ && time_before(wb->bw_time_stamp, start_time))
1371 		goto snapshot;
1372 
1373 	if (update_ratelimit) {
1374 		domain_update_bandwidth(gdtc, now);
1375 		wb_update_dirty_ratelimit(gdtc, dirtied, elapsed);
1376 
1377 		/*
1378 		 * @mdtc is always NULL if !CGROUP_WRITEBACK but the
1379 		 * compiler has no way to figure that out.  Help it.
1380 		 */
1381 		if (IS_ENABLED(CONFIG_CGROUP_WRITEBACK) && mdtc) {
1382 			domain_update_bandwidth(mdtc, now);
1383 			wb_update_dirty_ratelimit(mdtc, dirtied, elapsed);
1384 		}
1385 	}
1386 	wb_update_write_bandwidth(wb, elapsed, written);
1387 
1388 snapshot:
1389 	wb->dirtied_stamp = dirtied;
1390 	wb->written_stamp = written;
1391 	wb->bw_time_stamp = now;
1392 }
1393 
1394 void wb_update_bandwidth(struct bdi_writeback *wb, unsigned long start_time)
1395 {
1396 	struct dirty_throttle_control gdtc = { GDTC_INIT(wb) };
1397 
1398 	__wb_update_bandwidth(&gdtc, NULL, start_time, false);
1399 }
1400 
1401 /*
1402  * After a task dirtied this many pages, balance_dirty_pages_ratelimited()
1403  * will look to see if it needs to start dirty throttling.
1404  *
1405  * If dirty_poll_interval is too low, big NUMA machines will call the expensive
1406  * global_zone_page_state() too often. So scale it near-sqrt to the safety margin
1407  * (the number of pages we may dirty without exceeding the dirty limits).
1408  */
1409 static unsigned long dirty_poll_interval(unsigned long dirty,
1410 					 unsigned long thresh)
1411 {
1412 	if (thresh > dirty)
1413 		return 1UL << (ilog2(thresh - dirty) >> 1);
1414 
1415 	return 1;
1416 }
1417 
1418 static unsigned long wb_max_pause(struct bdi_writeback *wb,
1419 				  unsigned long wb_dirty)
1420 {
1421 	unsigned long bw = wb->avg_write_bandwidth;
1422 	unsigned long t;
1423 
1424 	/*
1425 	 * Limit pause time for small memory systems. If sleeping for too long
1426 	 * time, a small pool of dirty/writeback pages may go empty and disk go
1427 	 * idle.
1428 	 *
1429 	 * 8 serves as the safety ratio.
1430 	 */
1431 	t = wb_dirty / (1 + bw / roundup_pow_of_two(1 + HZ / 8));
1432 	t++;
1433 
1434 	return min_t(unsigned long, t, MAX_PAUSE);
1435 }
1436 
1437 static long wb_min_pause(struct bdi_writeback *wb,
1438 			 long max_pause,
1439 			 unsigned long task_ratelimit,
1440 			 unsigned long dirty_ratelimit,
1441 			 int *nr_dirtied_pause)
1442 {
1443 	long hi = ilog2(wb->avg_write_bandwidth);
1444 	long lo = ilog2(wb->dirty_ratelimit);
1445 	long t;		/* target pause */
1446 	long pause;	/* estimated next pause */
1447 	int pages;	/* target nr_dirtied_pause */
1448 
1449 	/* target for 10ms pause on 1-dd case */
1450 	t = max(1, HZ / 100);
1451 
1452 	/*
1453 	 * Scale up pause time for concurrent dirtiers in order to reduce CPU
1454 	 * overheads.
1455 	 *
1456 	 * (N * 10ms) on 2^N concurrent tasks.
1457 	 */
1458 	if (hi > lo)
1459 		t += (hi - lo) * (10 * HZ) / 1024;
1460 
1461 	/*
1462 	 * This is a bit convoluted. We try to base the next nr_dirtied_pause
1463 	 * on the much more stable dirty_ratelimit. However the next pause time
1464 	 * will be computed based on task_ratelimit and the two rate limits may
1465 	 * depart considerably at some time. Especially if task_ratelimit goes
1466 	 * below dirty_ratelimit/2 and the target pause is max_pause, the next
1467 	 * pause time will be max_pause*2 _trimmed down_ to max_pause.  As a
1468 	 * result task_ratelimit won't be executed faithfully, which could
1469 	 * eventually bring down dirty_ratelimit.
1470 	 *
1471 	 * We apply two rules to fix it up:
1472 	 * 1) try to estimate the next pause time and if necessary, use a lower
1473 	 *    nr_dirtied_pause so as not to exceed max_pause. When this happens,
1474 	 *    nr_dirtied_pause will be "dancing" with task_ratelimit.
1475 	 * 2) limit the target pause time to max_pause/2, so that the normal
1476 	 *    small fluctuations of task_ratelimit won't trigger rule (1) and
1477 	 *    nr_dirtied_pause will remain as stable as dirty_ratelimit.
1478 	 */
1479 	t = min(t, 1 + max_pause / 2);
1480 	pages = dirty_ratelimit * t / roundup_pow_of_two(HZ);
1481 
1482 	/*
1483 	 * Tiny nr_dirtied_pause is found to hurt I/O performance in the test
1484 	 * case fio-mmap-randwrite-64k, which does 16*{sync read, async write}.
1485 	 * When the 16 consecutive reads are often interrupted by some dirty
1486 	 * throttling pause during the async writes, cfq will go into idles
1487 	 * (deadline is fine). So push nr_dirtied_pause as high as possible
1488 	 * until reaches DIRTY_POLL_THRESH=32 pages.
1489 	 */
1490 	if (pages < DIRTY_POLL_THRESH) {
1491 		t = max_pause;
1492 		pages = dirty_ratelimit * t / roundup_pow_of_two(HZ);
1493 		if (pages > DIRTY_POLL_THRESH) {
1494 			pages = DIRTY_POLL_THRESH;
1495 			t = HZ * DIRTY_POLL_THRESH / dirty_ratelimit;
1496 		}
1497 	}
1498 
1499 	pause = HZ * pages / (task_ratelimit + 1);
1500 	if (pause > max_pause) {
1501 		t = max_pause;
1502 		pages = task_ratelimit * t / roundup_pow_of_two(HZ);
1503 	}
1504 
1505 	*nr_dirtied_pause = pages;
1506 	/*
1507 	 * The minimal pause time will normally be half the target pause time.
1508 	 */
1509 	return pages >= DIRTY_POLL_THRESH ? 1 + t / 2 : t;
1510 }
1511 
1512 static inline void wb_dirty_limits(struct dirty_throttle_control *dtc)
1513 {
1514 	struct bdi_writeback *wb = dtc->wb;
1515 	unsigned long wb_reclaimable;
1516 
1517 	/*
1518 	 * wb_thresh is not treated as some limiting factor as
1519 	 * dirty_thresh, due to reasons
1520 	 * - in JBOD setup, wb_thresh can fluctuate a lot
1521 	 * - in a system with HDD and USB key, the USB key may somehow
1522 	 *   go into state (wb_dirty >> wb_thresh) either because
1523 	 *   wb_dirty starts high, or because wb_thresh drops low.
1524 	 *   In this case we don't want to hard throttle the USB key
1525 	 *   dirtiers for 100 seconds until wb_dirty drops under
1526 	 *   wb_thresh. Instead the auxiliary wb control line in
1527 	 *   wb_position_ratio() will let the dirtier task progress
1528 	 *   at some rate <= (write_bw / 2) for bringing down wb_dirty.
1529 	 */
1530 	dtc->wb_thresh = __wb_calc_thresh(dtc);
1531 	dtc->wb_bg_thresh = dtc->thresh ?
1532 		div_u64((u64)dtc->wb_thresh * dtc->bg_thresh, dtc->thresh) : 0;
1533 
1534 	/*
1535 	 * In order to avoid the stacked BDI deadlock we need
1536 	 * to ensure we accurately count the 'dirty' pages when
1537 	 * the threshold is low.
1538 	 *
1539 	 * Otherwise it would be possible to get thresh+n pages
1540 	 * reported dirty, even though there are thresh-m pages
1541 	 * actually dirty; with m+n sitting in the percpu
1542 	 * deltas.
1543 	 */
1544 	if (dtc->wb_thresh < 2 * wb_stat_error()) {
1545 		wb_reclaimable = wb_stat_sum(wb, WB_RECLAIMABLE);
1546 		dtc->wb_dirty = wb_reclaimable + wb_stat_sum(wb, WB_WRITEBACK);
1547 	} else {
1548 		wb_reclaimable = wb_stat(wb, WB_RECLAIMABLE);
1549 		dtc->wb_dirty = wb_reclaimable + wb_stat(wb, WB_WRITEBACK);
1550 	}
1551 }
1552 
1553 /*
1554  * balance_dirty_pages() must be called by processes which are generating dirty
1555  * data.  It looks at the number of dirty pages in the machine and will force
1556  * the caller to wait once crossing the (background_thresh + dirty_thresh) / 2.
1557  * If we're over `background_thresh' then the writeback threads are woken to
1558  * perform some writeout.
1559  */
1560 static void balance_dirty_pages(struct bdi_writeback *wb,
1561 				unsigned long pages_dirtied)
1562 {
1563 	struct dirty_throttle_control gdtc_stor = { GDTC_INIT(wb) };
1564 	struct dirty_throttle_control mdtc_stor = { MDTC_INIT(wb, &gdtc_stor) };
1565 	struct dirty_throttle_control * const gdtc = &gdtc_stor;
1566 	struct dirty_throttle_control * const mdtc = mdtc_valid(&mdtc_stor) ?
1567 						     &mdtc_stor : NULL;
1568 	struct dirty_throttle_control *sdtc;
1569 	unsigned long nr_reclaimable;	/* = file_dirty + unstable_nfs */
1570 	long period;
1571 	long pause;
1572 	long max_pause;
1573 	long min_pause;
1574 	int nr_dirtied_pause;
1575 	bool dirty_exceeded = false;
1576 	unsigned long task_ratelimit;
1577 	unsigned long dirty_ratelimit;
1578 	struct backing_dev_info *bdi = wb->bdi;
1579 	bool strictlimit = bdi->capabilities & BDI_CAP_STRICTLIMIT;
1580 	unsigned long start_time = jiffies;
1581 
1582 	for (;;) {
1583 		unsigned long now = jiffies;
1584 		unsigned long dirty, thresh, bg_thresh;
1585 		unsigned long m_dirty = 0;	/* stop bogus uninit warnings */
1586 		unsigned long m_thresh = 0;
1587 		unsigned long m_bg_thresh = 0;
1588 
1589 		/*
1590 		 * Unstable writes are a feature of certain networked
1591 		 * filesystems (i.e. NFS) in which data may have been
1592 		 * written to the server's write cache, but has not yet
1593 		 * been flushed to permanent storage.
1594 		 */
1595 		nr_reclaimable = global_node_page_state(NR_FILE_DIRTY) +
1596 					global_node_page_state(NR_UNSTABLE_NFS);
1597 		gdtc->avail = global_dirtyable_memory();
1598 		gdtc->dirty = nr_reclaimable + global_node_page_state(NR_WRITEBACK);
1599 
1600 		domain_dirty_limits(gdtc);
1601 
1602 		if (unlikely(strictlimit)) {
1603 			wb_dirty_limits(gdtc);
1604 
1605 			dirty = gdtc->wb_dirty;
1606 			thresh = gdtc->wb_thresh;
1607 			bg_thresh = gdtc->wb_bg_thresh;
1608 		} else {
1609 			dirty = gdtc->dirty;
1610 			thresh = gdtc->thresh;
1611 			bg_thresh = gdtc->bg_thresh;
1612 		}
1613 
1614 		if (mdtc) {
1615 			unsigned long filepages, headroom, writeback;
1616 
1617 			/*
1618 			 * If @wb belongs to !root memcg, repeat the same
1619 			 * basic calculations for the memcg domain.
1620 			 */
1621 			mem_cgroup_wb_stats(wb, &filepages, &headroom,
1622 					    &mdtc->dirty, &writeback);
1623 			mdtc->dirty += writeback;
1624 			mdtc_calc_avail(mdtc, filepages, headroom);
1625 
1626 			domain_dirty_limits(mdtc);
1627 
1628 			if (unlikely(strictlimit)) {
1629 				wb_dirty_limits(mdtc);
1630 				m_dirty = mdtc->wb_dirty;
1631 				m_thresh = mdtc->wb_thresh;
1632 				m_bg_thresh = mdtc->wb_bg_thresh;
1633 			} else {
1634 				m_dirty = mdtc->dirty;
1635 				m_thresh = mdtc->thresh;
1636 				m_bg_thresh = mdtc->bg_thresh;
1637 			}
1638 		}
1639 
1640 		/*
1641 		 * Throttle it only when the background writeback cannot
1642 		 * catch-up. This avoids (excessively) small writeouts
1643 		 * when the wb limits are ramping up in case of !strictlimit.
1644 		 *
1645 		 * In strictlimit case make decision based on the wb counters
1646 		 * and limits. Small writeouts when the wb limits are ramping
1647 		 * up are the price we consciously pay for strictlimit-ing.
1648 		 *
1649 		 * If memcg domain is in effect, @dirty should be under
1650 		 * both global and memcg freerun ceilings.
1651 		 */
1652 		if (dirty <= dirty_freerun_ceiling(thresh, bg_thresh) &&
1653 		    (!mdtc ||
1654 		     m_dirty <= dirty_freerun_ceiling(m_thresh, m_bg_thresh))) {
1655 			unsigned long intv = dirty_poll_interval(dirty, thresh);
1656 			unsigned long m_intv = ULONG_MAX;
1657 
1658 			current->dirty_paused_when = now;
1659 			current->nr_dirtied = 0;
1660 			if (mdtc)
1661 				m_intv = dirty_poll_interval(m_dirty, m_thresh);
1662 			current->nr_dirtied_pause = min(intv, m_intv);
1663 			break;
1664 		}
1665 
1666 		if (unlikely(!writeback_in_progress(wb)))
1667 			wb_start_background_writeback(wb);
1668 
1669 		/*
1670 		 * Calculate global domain's pos_ratio and select the
1671 		 * global dtc by default.
1672 		 */
1673 		if (!strictlimit)
1674 			wb_dirty_limits(gdtc);
1675 
1676 		dirty_exceeded = (gdtc->wb_dirty > gdtc->wb_thresh) &&
1677 			((gdtc->dirty > gdtc->thresh) || strictlimit);
1678 
1679 		wb_position_ratio(gdtc);
1680 		sdtc = gdtc;
1681 
1682 		if (mdtc) {
1683 			/*
1684 			 * If memcg domain is in effect, calculate its
1685 			 * pos_ratio.  @wb should satisfy constraints from
1686 			 * both global and memcg domains.  Choose the one
1687 			 * w/ lower pos_ratio.
1688 			 */
1689 			if (!strictlimit)
1690 				wb_dirty_limits(mdtc);
1691 
1692 			dirty_exceeded |= (mdtc->wb_dirty > mdtc->wb_thresh) &&
1693 				((mdtc->dirty > mdtc->thresh) || strictlimit);
1694 
1695 			wb_position_ratio(mdtc);
1696 			if (mdtc->pos_ratio < gdtc->pos_ratio)
1697 				sdtc = mdtc;
1698 		}
1699 
1700 		if (dirty_exceeded && !wb->dirty_exceeded)
1701 			wb->dirty_exceeded = 1;
1702 
1703 		if (time_is_before_jiffies(wb->bw_time_stamp +
1704 					   BANDWIDTH_INTERVAL)) {
1705 			spin_lock(&wb->list_lock);
1706 			__wb_update_bandwidth(gdtc, mdtc, start_time, true);
1707 			spin_unlock(&wb->list_lock);
1708 		}
1709 
1710 		/* throttle according to the chosen dtc */
1711 		dirty_ratelimit = wb->dirty_ratelimit;
1712 		task_ratelimit = ((u64)dirty_ratelimit * sdtc->pos_ratio) >>
1713 							RATELIMIT_CALC_SHIFT;
1714 		max_pause = wb_max_pause(wb, sdtc->wb_dirty);
1715 		min_pause = wb_min_pause(wb, max_pause,
1716 					 task_ratelimit, dirty_ratelimit,
1717 					 &nr_dirtied_pause);
1718 
1719 		if (unlikely(task_ratelimit == 0)) {
1720 			period = max_pause;
1721 			pause = max_pause;
1722 			goto pause;
1723 		}
1724 		period = HZ * pages_dirtied / task_ratelimit;
1725 		pause = period;
1726 		if (current->dirty_paused_when)
1727 			pause -= now - current->dirty_paused_when;
1728 		/*
1729 		 * For less than 1s think time (ext3/4 may block the dirtier
1730 		 * for up to 800ms from time to time on 1-HDD; so does xfs,
1731 		 * however at much less frequency), try to compensate it in
1732 		 * future periods by updating the virtual time; otherwise just
1733 		 * do a reset, as it may be a light dirtier.
1734 		 */
1735 		if (pause < min_pause) {
1736 			trace_balance_dirty_pages(wb,
1737 						  sdtc->thresh,
1738 						  sdtc->bg_thresh,
1739 						  sdtc->dirty,
1740 						  sdtc->wb_thresh,
1741 						  sdtc->wb_dirty,
1742 						  dirty_ratelimit,
1743 						  task_ratelimit,
1744 						  pages_dirtied,
1745 						  period,
1746 						  min(pause, 0L),
1747 						  start_time);
1748 			if (pause < -HZ) {
1749 				current->dirty_paused_when = now;
1750 				current->nr_dirtied = 0;
1751 			} else if (period) {
1752 				current->dirty_paused_when += period;
1753 				current->nr_dirtied = 0;
1754 			} else if (current->nr_dirtied_pause <= pages_dirtied)
1755 				current->nr_dirtied_pause += pages_dirtied;
1756 			break;
1757 		}
1758 		if (unlikely(pause > max_pause)) {
1759 			/* for occasional dropped task_ratelimit */
1760 			now += min(pause - max_pause, max_pause);
1761 			pause = max_pause;
1762 		}
1763 
1764 pause:
1765 		trace_balance_dirty_pages(wb,
1766 					  sdtc->thresh,
1767 					  sdtc->bg_thresh,
1768 					  sdtc->dirty,
1769 					  sdtc->wb_thresh,
1770 					  sdtc->wb_dirty,
1771 					  dirty_ratelimit,
1772 					  task_ratelimit,
1773 					  pages_dirtied,
1774 					  period,
1775 					  pause,
1776 					  start_time);
1777 		__set_current_state(TASK_KILLABLE);
1778 		wb->dirty_sleep = now;
1779 		io_schedule_timeout(pause);
1780 
1781 		current->dirty_paused_when = now + pause;
1782 		current->nr_dirtied = 0;
1783 		current->nr_dirtied_pause = nr_dirtied_pause;
1784 
1785 		/*
1786 		 * This is typically equal to (dirty < thresh) and can also
1787 		 * keep "1000+ dd on a slow USB stick" under control.
1788 		 */
1789 		if (task_ratelimit)
1790 			break;
1791 
1792 		/*
1793 		 * In the case of an unresponding NFS server and the NFS dirty
1794 		 * pages exceeds dirty_thresh, give the other good wb's a pipe
1795 		 * to go through, so that tasks on them still remain responsive.
1796 		 *
1797 		 * In theory 1 page is enough to keep the consumer-producer
1798 		 * pipe going: the flusher cleans 1 page => the task dirties 1
1799 		 * more page. However wb_dirty has accounting errors.  So use
1800 		 * the larger and more IO friendly wb_stat_error.
1801 		 */
1802 		if (sdtc->wb_dirty <= wb_stat_error())
1803 			break;
1804 
1805 		if (fatal_signal_pending(current))
1806 			break;
1807 	}
1808 
1809 	if (!dirty_exceeded && wb->dirty_exceeded)
1810 		wb->dirty_exceeded = 0;
1811 
1812 	if (writeback_in_progress(wb))
1813 		return;
1814 
1815 	/*
1816 	 * In laptop mode, we wait until hitting the higher threshold before
1817 	 * starting background writeout, and then write out all the way down
1818 	 * to the lower threshold.  So slow writers cause minimal disk activity.
1819 	 *
1820 	 * In normal mode, we start background writeout at the lower
1821 	 * background_thresh, to keep the amount of dirty memory low.
1822 	 */
1823 	if (laptop_mode)
1824 		return;
1825 
1826 	if (nr_reclaimable > gdtc->bg_thresh)
1827 		wb_start_background_writeback(wb);
1828 }
1829 
1830 static DEFINE_PER_CPU(int, bdp_ratelimits);
1831 
1832 /*
1833  * Normal tasks are throttled by
1834  *	loop {
1835  *		dirty tsk->nr_dirtied_pause pages;
1836  *		take a snap in balance_dirty_pages();
1837  *	}
1838  * However there is a worst case. If every task exit immediately when dirtied
1839  * (tsk->nr_dirtied_pause - 1) pages, balance_dirty_pages() will never be
1840  * called to throttle the page dirties. The solution is to save the not yet
1841  * throttled page dirties in dirty_throttle_leaks on task exit and charge them
1842  * randomly into the running tasks. This works well for the above worst case,
1843  * as the new task will pick up and accumulate the old task's leaked dirty
1844  * count and eventually get throttled.
1845  */
1846 DEFINE_PER_CPU(int, dirty_throttle_leaks) = 0;
1847 
1848 /**
1849  * balance_dirty_pages_ratelimited - balance dirty memory state
1850  * @mapping: address_space which was dirtied
1851  *
1852  * Processes which are dirtying memory should call in here once for each page
1853  * which was newly dirtied.  The function will periodically check the system's
1854  * dirty state and will initiate writeback if needed.
1855  *
1856  * On really big machines, get_writeback_state is expensive, so try to avoid
1857  * calling it too often (ratelimiting).  But once we're over the dirty memory
1858  * limit we decrease the ratelimiting by a lot, to prevent individual processes
1859  * from overshooting the limit by (ratelimit_pages) each.
1860  */
1861 void balance_dirty_pages_ratelimited(struct address_space *mapping)
1862 {
1863 	struct inode *inode = mapping->host;
1864 	struct backing_dev_info *bdi = inode_to_bdi(inode);
1865 	struct bdi_writeback *wb = NULL;
1866 	int ratelimit;
1867 	int *p;
1868 
1869 	if (!bdi_cap_account_dirty(bdi))
1870 		return;
1871 
1872 	if (inode_cgwb_enabled(inode))
1873 		wb = wb_get_create_current(bdi, GFP_KERNEL);
1874 	if (!wb)
1875 		wb = &bdi->wb;
1876 
1877 	ratelimit = current->nr_dirtied_pause;
1878 	if (wb->dirty_exceeded)
1879 		ratelimit = min(ratelimit, 32 >> (PAGE_SHIFT - 10));
1880 
1881 	preempt_disable();
1882 	/*
1883 	 * This prevents one CPU to accumulate too many dirtied pages without
1884 	 * calling into balance_dirty_pages(), which can happen when there are
1885 	 * 1000+ tasks, all of them start dirtying pages at exactly the same
1886 	 * time, hence all honoured too large initial task->nr_dirtied_pause.
1887 	 */
1888 	p =  this_cpu_ptr(&bdp_ratelimits);
1889 	if (unlikely(current->nr_dirtied >= ratelimit))
1890 		*p = 0;
1891 	else if (unlikely(*p >= ratelimit_pages)) {
1892 		*p = 0;
1893 		ratelimit = 0;
1894 	}
1895 	/*
1896 	 * Pick up the dirtied pages by the exited tasks. This avoids lots of
1897 	 * short-lived tasks (eg. gcc invocations in a kernel build) escaping
1898 	 * the dirty throttling and livelock other long-run dirtiers.
1899 	 */
1900 	p = this_cpu_ptr(&dirty_throttle_leaks);
1901 	if (*p > 0 && current->nr_dirtied < ratelimit) {
1902 		unsigned long nr_pages_dirtied;
1903 		nr_pages_dirtied = min(*p, ratelimit - current->nr_dirtied);
1904 		*p -= nr_pages_dirtied;
1905 		current->nr_dirtied += nr_pages_dirtied;
1906 	}
1907 	preempt_enable();
1908 
1909 	if (unlikely(current->nr_dirtied >= ratelimit))
1910 		balance_dirty_pages(wb, current->nr_dirtied);
1911 
1912 	wb_put(wb);
1913 }
1914 EXPORT_SYMBOL(balance_dirty_pages_ratelimited);
1915 
1916 /**
1917  * wb_over_bg_thresh - does @wb need to be written back?
1918  * @wb: bdi_writeback of interest
1919  *
1920  * Determines whether background writeback should keep writing @wb or it's
1921  * clean enough.
1922  *
1923  * Return: %true if writeback should continue.
1924  */
1925 bool wb_over_bg_thresh(struct bdi_writeback *wb)
1926 {
1927 	struct dirty_throttle_control gdtc_stor = { GDTC_INIT(wb) };
1928 	struct dirty_throttle_control mdtc_stor = { MDTC_INIT(wb, &gdtc_stor) };
1929 	struct dirty_throttle_control * const gdtc = &gdtc_stor;
1930 	struct dirty_throttle_control * const mdtc = mdtc_valid(&mdtc_stor) ?
1931 						     &mdtc_stor : NULL;
1932 
1933 	/*
1934 	 * Similar to balance_dirty_pages() but ignores pages being written
1935 	 * as we're trying to decide whether to put more under writeback.
1936 	 */
1937 	gdtc->avail = global_dirtyable_memory();
1938 	gdtc->dirty = global_node_page_state(NR_FILE_DIRTY) +
1939 		      global_node_page_state(NR_UNSTABLE_NFS);
1940 	domain_dirty_limits(gdtc);
1941 
1942 	if (gdtc->dirty > gdtc->bg_thresh)
1943 		return true;
1944 
1945 	if (wb_stat(wb, WB_RECLAIMABLE) >
1946 	    wb_calc_thresh(gdtc->wb, gdtc->bg_thresh))
1947 		return true;
1948 
1949 	if (mdtc) {
1950 		unsigned long filepages, headroom, writeback;
1951 
1952 		mem_cgroup_wb_stats(wb, &filepages, &headroom, &mdtc->dirty,
1953 				    &writeback);
1954 		mdtc_calc_avail(mdtc, filepages, headroom);
1955 		domain_dirty_limits(mdtc);	/* ditto, ignore writeback */
1956 
1957 		if (mdtc->dirty > mdtc->bg_thresh)
1958 			return true;
1959 
1960 		if (wb_stat(wb, WB_RECLAIMABLE) >
1961 		    wb_calc_thresh(mdtc->wb, mdtc->bg_thresh))
1962 			return true;
1963 	}
1964 
1965 	return false;
1966 }
1967 
1968 /*
1969  * sysctl handler for /proc/sys/vm/dirty_writeback_centisecs
1970  */
1971 int dirty_writeback_centisecs_handler(struct ctl_table *table, int write,
1972 	void __user *buffer, size_t *length, loff_t *ppos)
1973 {
1974 	unsigned int old_interval = dirty_writeback_interval;
1975 	int ret;
1976 
1977 	ret = proc_dointvec(table, write, buffer, length, ppos);
1978 
1979 	/*
1980 	 * Writing 0 to dirty_writeback_interval will disable periodic writeback
1981 	 * and a different non-zero value will wakeup the writeback threads.
1982 	 * wb_wakeup_delayed() would be more appropriate, but it's a pain to
1983 	 * iterate over all bdis and wbs.
1984 	 * The reason we do this is to make the change take effect immediately.
1985 	 */
1986 	if (!ret && write && dirty_writeback_interval &&
1987 		dirty_writeback_interval != old_interval)
1988 		wakeup_flusher_threads(WB_REASON_PERIODIC);
1989 
1990 	return ret;
1991 }
1992 
1993 #ifdef CONFIG_BLOCK
1994 void laptop_mode_timer_fn(struct timer_list *t)
1995 {
1996 	struct backing_dev_info *backing_dev_info =
1997 		from_timer(backing_dev_info, t, laptop_mode_wb_timer);
1998 
1999 	wakeup_flusher_threads_bdi(backing_dev_info, WB_REASON_LAPTOP_TIMER);
2000 }
2001 
2002 /*
2003  * We've spun up the disk and we're in laptop mode: schedule writeback
2004  * of all dirty data a few seconds from now.  If the flush is already scheduled
2005  * then push it back - the user is still using the disk.
2006  */
2007 void laptop_io_completion(struct backing_dev_info *info)
2008 {
2009 	mod_timer(&info->laptop_mode_wb_timer, jiffies + laptop_mode);
2010 }
2011 
2012 /*
2013  * We're in laptop mode and we've just synced. The sync's writes will have
2014  * caused another writeback to be scheduled by laptop_io_completion.
2015  * Nothing needs to be written back anymore, so we unschedule the writeback.
2016  */
2017 void laptop_sync_completion(void)
2018 {
2019 	struct backing_dev_info *bdi;
2020 
2021 	rcu_read_lock();
2022 
2023 	list_for_each_entry_rcu(bdi, &bdi_list, bdi_list)
2024 		del_timer(&bdi->laptop_mode_wb_timer);
2025 
2026 	rcu_read_unlock();
2027 }
2028 #endif
2029 
2030 /*
2031  * If ratelimit_pages is too high then we can get into dirty-data overload
2032  * if a large number of processes all perform writes at the same time.
2033  * If it is too low then SMP machines will call the (expensive)
2034  * get_writeback_state too often.
2035  *
2036  * Here we set ratelimit_pages to a level which ensures that when all CPUs are
2037  * dirtying in parallel, we cannot go more than 3% (1/32) over the dirty memory
2038  * thresholds.
2039  */
2040 
2041 void writeback_set_ratelimit(void)
2042 {
2043 	struct wb_domain *dom = &global_wb_domain;
2044 	unsigned long background_thresh;
2045 	unsigned long dirty_thresh;
2046 
2047 	global_dirty_limits(&background_thresh, &dirty_thresh);
2048 	dom->dirty_limit = dirty_thresh;
2049 	ratelimit_pages = dirty_thresh / (num_online_cpus() * 32);
2050 	if (ratelimit_pages < 16)
2051 		ratelimit_pages = 16;
2052 }
2053 
2054 static int page_writeback_cpu_online(unsigned int cpu)
2055 {
2056 	writeback_set_ratelimit();
2057 	return 0;
2058 }
2059 
2060 /*
2061  * Called early on to tune the page writeback dirty limits.
2062  *
2063  * We used to scale dirty pages according to how total memory
2064  * related to pages that could be allocated for buffers (by
2065  * comparing nr_free_buffer_pages() to vm_total_pages.
2066  *
2067  * However, that was when we used "dirty_ratio" to scale with
2068  * all memory, and we don't do that any more. "dirty_ratio"
2069  * is now applied to total non-HIGHPAGE memory (by subtracting
2070  * totalhigh_pages from vm_total_pages), and as such we can't
2071  * get into the old insane situation any more where we had
2072  * large amounts of dirty pages compared to a small amount of
2073  * non-HIGHMEM memory.
2074  *
2075  * But we might still want to scale the dirty_ratio by how
2076  * much memory the box has..
2077  */
2078 void __init page_writeback_init(void)
2079 {
2080 	BUG_ON(wb_domain_init(&global_wb_domain, GFP_KERNEL));
2081 
2082 	cpuhp_setup_state(CPUHP_AP_ONLINE_DYN, "mm/writeback:online",
2083 			  page_writeback_cpu_online, NULL);
2084 	cpuhp_setup_state(CPUHP_MM_WRITEBACK_DEAD, "mm/writeback:dead", NULL,
2085 			  page_writeback_cpu_online);
2086 }
2087 
2088 /**
2089  * tag_pages_for_writeback - tag pages to be written by write_cache_pages
2090  * @mapping: address space structure to write
2091  * @start: starting page index
2092  * @end: ending page index (inclusive)
2093  *
2094  * This function scans the page range from @start to @end (inclusive) and tags
2095  * all pages that have DIRTY tag set with a special TOWRITE tag. The idea is
2096  * that write_cache_pages (or whoever calls this function) will then use
2097  * TOWRITE tag to identify pages eligible for writeback.  This mechanism is
2098  * used to avoid livelocking of writeback by a process steadily creating new
2099  * dirty pages in the file (thus it is important for this function to be quick
2100  * so that it can tag pages faster than a dirtying process can create them).
2101  */
2102 void tag_pages_for_writeback(struct address_space *mapping,
2103 			     pgoff_t start, pgoff_t end)
2104 {
2105 	XA_STATE(xas, &mapping->i_pages, start);
2106 	unsigned int tagged = 0;
2107 	void *page;
2108 
2109 	xas_lock_irq(&xas);
2110 	xas_for_each_marked(&xas, page, end, PAGECACHE_TAG_DIRTY) {
2111 		xas_set_mark(&xas, PAGECACHE_TAG_TOWRITE);
2112 		if (++tagged % XA_CHECK_SCHED)
2113 			continue;
2114 
2115 		xas_pause(&xas);
2116 		xas_unlock_irq(&xas);
2117 		cond_resched();
2118 		xas_lock_irq(&xas);
2119 	}
2120 	xas_unlock_irq(&xas);
2121 }
2122 EXPORT_SYMBOL(tag_pages_for_writeback);
2123 
2124 /**
2125  * write_cache_pages - walk the list of dirty pages of the given address space and write all of them.
2126  * @mapping: address space structure to write
2127  * @wbc: subtract the number of written pages from *@wbc->nr_to_write
2128  * @writepage: function called for each page
2129  * @data: data passed to writepage function
2130  *
2131  * If a page is already under I/O, write_cache_pages() skips it, even
2132  * if it's dirty.  This is desirable behaviour for memory-cleaning writeback,
2133  * but it is INCORRECT for data-integrity system calls such as fsync().  fsync()
2134  * and msync() need to guarantee that all the data which was dirty at the time
2135  * the call was made get new I/O started against them.  If wbc->sync_mode is
2136  * WB_SYNC_ALL then we were called for data integrity and we must wait for
2137  * existing IO to complete.
2138  *
2139  * To avoid livelocks (when other process dirties new pages), we first tag
2140  * pages which should be written back with TOWRITE tag and only then start
2141  * writing them. For data-integrity sync we have to be careful so that we do
2142  * not miss some pages (e.g., because some other process has cleared TOWRITE
2143  * tag we set). The rule we follow is that TOWRITE tag can be cleared only
2144  * by the process clearing the DIRTY tag (and submitting the page for IO).
2145  *
2146  * To avoid deadlocks between range_cyclic writeback and callers that hold
2147  * pages in PageWriteback to aggregate IO until write_cache_pages() returns,
2148  * we do not loop back to the start of the file. Doing so causes a page
2149  * lock/page writeback access order inversion - we should only ever lock
2150  * multiple pages in ascending page->index order, and looping back to the start
2151  * of the file violates that rule and causes deadlocks.
2152  *
2153  * Return: %0 on success, negative error code otherwise
2154  */
2155 int write_cache_pages(struct address_space *mapping,
2156 		      struct writeback_control *wbc, writepage_t writepage,
2157 		      void *data)
2158 {
2159 	int ret = 0;
2160 	int done = 0;
2161 	int error;
2162 	struct pagevec pvec;
2163 	int nr_pages;
2164 	pgoff_t uninitialized_var(writeback_index);
2165 	pgoff_t index;
2166 	pgoff_t end;		/* Inclusive */
2167 	pgoff_t done_index;
2168 	int range_whole = 0;
2169 	xa_mark_t tag;
2170 
2171 	pagevec_init(&pvec);
2172 	if (wbc->range_cyclic) {
2173 		writeback_index = mapping->writeback_index; /* prev offset */
2174 		index = writeback_index;
2175 		end = -1;
2176 	} else {
2177 		index = wbc->range_start >> PAGE_SHIFT;
2178 		end = wbc->range_end >> PAGE_SHIFT;
2179 		if (wbc->range_start == 0 && wbc->range_end == LLONG_MAX)
2180 			range_whole = 1;
2181 	}
2182 	if (wbc->sync_mode == WB_SYNC_ALL || wbc->tagged_writepages)
2183 		tag = PAGECACHE_TAG_TOWRITE;
2184 	else
2185 		tag = PAGECACHE_TAG_DIRTY;
2186 	if (wbc->sync_mode == WB_SYNC_ALL || wbc->tagged_writepages)
2187 		tag_pages_for_writeback(mapping, index, end);
2188 	done_index = index;
2189 	while (!done && (index <= end)) {
2190 		int i;
2191 
2192 		nr_pages = pagevec_lookup_range_tag(&pvec, mapping, &index, end,
2193 				tag);
2194 		if (nr_pages == 0)
2195 			break;
2196 
2197 		for (i = 0; i < nr_pages; i++) {
2198 			struct page *page = pvec.pages[i];
2199 
2200 			done_index = page->index;
2201 
2202 			lock_page(page);
2203 
2204 			/*
2205 			 * Page truncated or invalidated. We can freely skip it
2206 			 * then, even for data integrity operations: the page
2207 			 * has disappeared concurrently, so there could be no
2208 			 * real expectation of this data interity operation
2209 			 * even if there is now a new, dirty page at the same
2210 			 * pagecache address.
2211 			 */
2212 			if (unlikely(page->mapping != mapping)) {
2213 continue_unlock:
2214 				unlock_page(page);
2215 				continue;
2216 			}
2217 
2218 			if (!PageDirty(page)) {
2219 				/* someone wrote it for us */
2220 				goto continue_unlock;
2221 			}
2222 
2223 			if (PageWriteback(page)) {
2224 				if (wbc->sync_mode != WB_SYNC_NONE)
2225 					wait_on_page_writeback(page);
2226 				else
2227 					goto continue_unlock;
2228 			}
2229 
2230 			BUG_ON(PageWriteback(page));
2231 			if (!clear_page_dirty_for_io(page))
2232 				goto continue_unlock;
2233 
2234 			trace_wbc_writepage(wbc, inode_to_bdi(mapping->host));
2235 			error = (*writepage)(page, wbc, data);
2236 			if (unlikely(error)) {
2237 				/*
2238 				 * Handle errors according to the type of
2239 				 * writeback. There's no need to continue for
2240 				 * background writeback. Just push done_index
2241 				 * past this page so media errors won't choke
2242 				 * writeout for the entire file. For integrity
2243 				 * writeback, we must process the entire dirty
2244 				 * set regardless of errors because the fs may
2245 				 * still have state to clear for each page. In
2246 				 * that case we continue processing and return
2247 				 * the first error.
2248 				 */
2249 				if (error == AOP_WRITEPAGE_ACTIVATE) {
2250 					unlock_page(page);
2251 					error = 0;
2252 				} else if (wbc->sync_mode != WB_SYNC_ALL) {
2253 					ret = error;
2254 					done_index = page->index + 1;
2255 					done = 1;
2256 					break;
2257 				}
2258 				if (!ret)
2259 					ret = error;
2260 			}
2261 
2262 			/*
2263 			 * We stop writing back only if we are not doing
2264 			 * integrity sync. In case of integrity sync we have to
2265 			 * keep going until we have written all the pages
2266 			 * we tagged for writeback prior to entering this loop.
2267 			 */
2268 			if (--wbc->nr_to_write <= 0 &&
2269 			    wbc->sync_mode == WB_SYNC_NONE) {
2270 				done = 1;
2271 				break;
2272 			}
2273 		}
2274 		pagevec_release(&pvec);
2275 		cond_resched();
2276 	}
2277 
2278 	/*
2279 	 * If we hit the last page and there is more work to be done: wrap
2280 	 * back the index back to the start of the file for the next
2281 	 * time we are called.
2282 	 */
2283 	if (wbc->range_cyclic && !done)
2284 		done_index = 0;
2285 	if (wbc->range_cyclic || (range_whole && wbc->nr_to_write > 0))
2286 		mapping->writeback_index = done_index;
2287 
2288 	return ret;
2289 }
2290 EXPORT_SYMBOL(write_cache_pages);
2291 
2292 /*
2293  * Function used by generic_writepages to call the real writepage
2294  * function and set the mapping flags on error
2295  */
2296 static int __writepage(struct page *page, struct writeback_control *wbc,
2297 		       void *data)
2298 {
2299 	struct address_space *mapping = data;
2300 	int ret = mapping->a_ops->writepage(page, wbc);
2301 	mapping_set_error(mapping, ret);
2302 	return ret;
2303 }
2304 
2305 /**
2306  * generic_writepages - walk the list of dirty pages of the given address space and writepage() all of them.
2307  * @mapping: address space structure to write
2308  * @wbc: subtract the number of written pages from *@wbc->nr_to_write
2309  *
2310  * This is a library function, which implements the writepages()
2311  * address_space_operation.
2312  *
2313  * Return: %0 on success, negative error code otherwise
2314  */
2315 int generic_writepages(struct address_space *mapping,
2316 		       struct writeback_control *wbc)
2317 {
2318 	struct blk_plug plug;
2319 	int ret;
2320 
2321 	/* deal with chardevs and other special file */
2322 	if (!mapping->a_ops->writepage)
2323 		return 0;
2324 
2325 	blk_start_plug(&plug);
2326 	ret = write_cache_pages(mapping, wbc, __writepage, mapping);
2327 	blk_finish_plug(&plug);
2328 	return ret;
2329 }
2330 
2331 EXPORT_SYMBOL(generic_writepages);
2332 
2333 int do_writepages(struct address_space *mapping, struct writeback_control *wbc)
2334 {
2335 	int ret;
2336 
2337 	if (wbc->nr_to_write <= 0)
2338 		return 0;
2339 	while (1) {
2340 		if (mapping->a_ops->writepages)
2341 			ret = mapping->a_ops->writepages(mapping, wbc);
2342 		else
2343 			ret = generic_writepages(mapping, wbc);
2344 		if ((ret != -ENOMEM) || (wbc->sync_mode != WB_SYNC_ALL))
2345 			break;
2346 		cond_resched();
2347 		congestion_wait(BLK_RW_ASYNC, HZ/50);
2348 	}
2349 	return ret;
2350 }
2351 
2352 /**
2353  * write_one_page - write out a single page and wait on I/O
2354  * @page: the page to write
2355  *
2356  * The page must be locked by the caller and will be unlocked upon return.
2357  *
2358  * Note that the mapping's AS_EIO/AS_ENOSPC flags will be cleared when this
2359  * function returns.
2360  *
2361  * Return: %0 on success, negative error code otherwise
2362  */
2363 int write_one_page(struct page *page)
2364 {
2365 	struct address_space *mapping = page->mapping;
2366 	int ret = 0;
2367 	struct writeback_control wbc = {
2368 		.sync_mode = WB_SYNC_ALL,
2369 		.nr_to_write = 1,
2370 	};
2371 
2372 	BUG_ON(!PageLocked(page));
2373 
2374 	wait_on_page_writeback(page);
2375 
2376 	if (clear_page_dirty_for_io(page)) {
2377 		get_page(page);
2378 		ret = mapping->a_ops->writepage(page, &wbc);
2379 		if (ret == 0)
2380 			wait_on_page_writeback(page);
2381 		put_page(page);
2382 	} else {
2383 		unlock_page(page);
2384 	}
2385 
2386 	if (!ret)
2387 		ret = filemap_check_errors(mapping);
2388 	return ret;
2389 }
2390 EXPORT_SYMBOL(write_one_page);
2391 
2392 /*
2393  * For address_spaces which do not use buffers nor write back.
2394  */
2395 int __set_page_dirty_no_writeback(struct page *page)
2396 {
2397 	if (!PageDirty(page))
2398 		return !TestSetPageDirty(page);
2399 	return 0;
2400 }
2401 
2402 /*
2403  * Helper function for set_page_dirty family.
2404  *
2405  * Caller must hold lock_page_memcg().
2406  *
2407  * NOTE: This relies on being atomic wrt interrupts.
2408  */
2409 void account_page_dirtied(struct page *page, struct address_space *mapping)
2410 {
2411 	struct inode *inode = mapping->host;
2412 
2413 	trace_writeback_dirty_page(page, mapping);
2414 
2415 	if (mapping_cap_account_dirty(mapping)) {
2416 		struct bdi_writeback *wb;
2417 
2418 		inode_attach_wb(inode, page);
2419 		wb = inode_to_wb(inode);
2420 
2421 		__inc_lruvec_page_state(page, NR_FILE_DIRTY);
2422 		__inc_zone_page_state(page, NR_ZONE_WRITE_PENDING);
2423 		__inc_node_page_state(page, NR_DIRTIED);
2424 		inc_wb_stat(wb, WB_RECLAIMABLE);
2425 		inc_wb_stat(wb, WB_DIRTIED);
2426 		task_io_account_write(PAGE_SIZE);
2427 		current->nr_dirtied++;
2428 		this_cpu_inc(bdp_ratelimits);
2429 	}
2430 }
2431 EXPORT_SYMBOL(account_page_dirtied);
2432 
2433 /*
2434  * Helper function for deaccounting dirty page without writeback.
2435  *
2436  * Caller must hold lock_page_memcg().
2437  */
2438 void account_page_cleaned(struct page *page, struct address_space *mapping,
2439 			  struct bdi_writeback *wb)
2440 {
2441 	if (mapping_cap_account_dirty(mapping)) {
2442 		dec_lruvec_page_state(page, NR_FILE_DIRTY);
2443 		dec_zone_page_state(page, NR_ZONE_WRITE_PENDING);
2444 		dec_wb_stat(wb, WB_RECLAIMABLE);
2445 		task_io_account_cancelled_write(PAGE_SIZE);
2446 	}
2447 }
2448 
2449 /*
2450  * For address_spaces which do not use buffers.  Just tag the page as dirty in
2451  * the xarray.
2452  *
2453  * This is also used when a single buffer is being dirtied: we want to set the
2454  * page dirty in that case, but not all the buffers.  This is a "bottom-up"
2455  * dirtying, whereas __set_page_dirty_buffers() is a "top-down" dirtying.
2456  *
2457  * The caller must ensure this doesn't race with truncation.  Most will simply
2458  * hold the page lock, but e.g. zap_pte_range() calls with the page mapped and
2459  * the pte lock held, which also locks out truncation.
2460  */
2461 int __set_page_dirty_nobuffers(struct page *page)
2462 {
2463 	lock_page_memcg(page);
2464 	if (!TestSetPageDirty(page)) {
2465 		struct address_space *mapping = page_mapping(page);
2466 		unsigned long flags;
2467 
2468 		if (!mapping) {
2469 			unlock_page_memcg(page);
2470 			return 1;
2471 		}
2472 
2473 		xa_lock_irqsave(&mapping->i_pages, flags);
2474 		BUG_ON(page_mapping(page) != mapping);
2475 		WARN_ON_ONCE(!PagePrivate(page) && !PageUptodate(page));
2476 		account_page_dirtied(page, mapping);
2477 		__xa_set_mark(&mapping->i_pages, page_index(page),
2478 				   PAGECACHE_TAG_DIRTY);
2479 		xa_unlock_irqrestore(&mapping->i_pages, flags);
2480 		unlock_page_memcg(page);
2481 
2482 		if (mapping->host) {
2483 			/* !PageAnon && !swapper_space */
2484 			__mark_inode_dirty(mapping->host, I_DIRTY_PAGES);
2485 		}
2486 		return 1;
2487 	}
2488 	unlock_page_memcg(page);
2489 	return 0;
2490 }
2491 EXPORT_SYMBOL(__set_page_dirty_nobuffers);
2492 
2493 /*
2494  * Call this whenever redirtying a page, to de-account the dirty counters
2495  * (NR_DIRTIED, WB_DIRTIED, tsk->nr_dirtied), so that they match the written
2496  * counters (NR_WRITTEN, WB_WRITTEN) in long term. The mismatches will lead to
2497  * systematic errors in balanced_dirty_ratelimit and the dirty pages position
2498  * control.
2499  */
2500 void account_page_redirty(struct page *page)
2501 {
2502 	struct address_space *mapping = page->mapping;
2503 
2504 	if (mapping && mapping_cap_account_dirty(mapping)) {
2505 		struct inode *inode = mapping->host;
2506 		struct bdi_writeback *wb;
2507 		struct wb_lock_cookie cookie = {};
2508 
2509 		wb = unlocked_inode_to_wb_begin(inode, &cookie);
2510 		current->nr_dirtied--;
2511 		dec_node_page_state(page, NR_DIRTIED);
2512 		dec_wb_stat(wb, WB_DIRTIED);
2513 		unlocked_inode_to_wb_end(inode, &cookie);
2514 	}
2515 }
2516 EXPORT_SYMBOL(account_page_redirty);
2517 
2518 /*
2519  * When a writepage implementation decides that it doesn't want to write this
2520  * page for some reason, it should redirty the locked page via
2521  * redirty_page_for_writepage() and it should then unlock the page and return 0
2522  */
2523 int redirty_page_for_writepage(struct writeback_control *wbc, struct page *page)
2524 {
2525 	int ret;
2526 
2527 	wbc->pages_skipped++;
2528 	ret = __set_page_dirty_nobuffers(page);
2529 	account_page_redirty(page);
2530 	return ret;
2531 }
2532 EXPORT_SYMBOL(redirty_page_for_writepage);
2533 
2534 /*
2535  * Dirty a page.
2536  *
2537  * For pages with a mapping this should be done under the page lock
2538  * for the benefit of asynchronous memory errors who prefer a consistent
2539  * dirty state. This rule can be broken in some special cases,
2540  * but should be better not to.
2541  *
2542  * If the mapping doesn't provide a set_page_dirty a_op, then
2543  * just fall through and assume that it wants buffer_heads.
2544  */
2545 int set_page_dirty(struct page *page)
2546 {
2547 	struct address_space *mapping = page_mapping(page);
2548 
2549 	page = compound_head(page);
2550 	if (likely(mapping)) {
2551 		int (*spd)(struct page *) = mapping->a_ops->set_page_dirty;
2552 		/*
2553 		 * readahead/lru_deactivate_page could remain
2554 		 * PG_readahead/PG_reclaim due to race with end_page_writeback
2555 		 * About readahead, if the page is written, the flags would be
2556 		 * reset. So no problem.
2557 		 * About lru_deactivate_page, if the page is redirty, the flag
2558 		 * will be reset. So no problem. but if the page is used by readahead
2559 		 * it will confuse readahead and make it restart the size rampup
2560 		 * process. But it's a trivial problem.
2561 		 */
2562 		if (PageReclaim(page))
2563 			ClearPageReclaim(page);
2564 #ifdef CONFIG_BLOCK
2565 		if (!spd)
2566 			spd = __set_page_dirty_buffers;
2567 #endif
2568 		return (*spd)(page);
2569 	}
2570 	if (!PageDirty(page)) {
2571 		if (!TestSetPageDirty(page))
2572 			return 1;
2573 	}
2574 	return 0;
2575 }
2576 EXPORT_SYMBOL(set_page_dirty);
2577 
2578 /*
2579  * set_page_dirty() is racy if the caller has no reference against
2580  * page->mapping->host, and if the page is unlocked.  This is because another
2581  * CPU could truncate the page off the mapping and then free the mapping.
2582  *
2583  * Usually, the page _is_ locked, or the caller is a user-space process which
2584  * holds a reference on the inode by having an open file.
2585  *
2586  * In other cases, the page should be locked before running set_page_dirty().
2587  */
2588 int set_page_dirty_lock(struct page *page)
2589 {
2590 	int ret;
2591 
2592 	lock_page(page);
2593 	ret = set_page_dirty(page);
2594 	unlock_page(page);
2595 	return ret;
2596 }
2597 EXPORT_SYMBOL(set_page_dirty_lock);
2598 
2599 /*
2600  * This cancels just the dirty bit on the kernel page itself, it does NOT
2601  * actually remove dirty bits on any mmap's that may be around. It also
2602  * leaves the page tagged dirty, so any sync activity will still find it on
2603  * the dirty lists, and in particular, clear_page_dirty_for_io() will still
2604  * look at the dirty bits in the VM.
2605  *
2606  * Doing this should *normally* only ever be done when a page is truncated,
2607  * and is not actually mapped anywhere at all. However, fs/buffer.c does
2608  * this when it notices that somebody has cleaned out all the buffers on a
2609  * page without actually doing it through the VM. Can you say "ext3 is
2610  * horribly ugly"? Thought you could.
2611  */
2612 void __cancel_dirty_page(struct page *page)
2613 {
2614 	struct address_space *mapping = page_mapping(page);
2615 
2616 	if (mapping_cap_account_dirty(mapping)) {
2617 		struct inode *inode = mapping->host;
2618 		struct bdi_writeback *wb;
2619 		struct wb_lock_cookie cookie = {};
2620 
2621 		lock_page_memcg(page);
2622 		wb = unlocked_inode_to_wb_begin(inode, &cookie);
2623 
2624 		if (TestClearPageDirty(page))
2625 			account_page_cleaned(page, mapping, wb);
2626 
2627 		unlocked_inode_to_wb_end(inode, &cookie);
2628 		unlock_page_memcg(page);
2629 	} else {
2630 		ClearPageDirty(page);
2631 	}
2632 }
2633 EXPORT_SYMBOL(__cancel_dirty_page);
2634 
2635 /*
2636  * Clear a page's dirty flag, while caring for dirty memory accounting.
2637  * Returns true if the page was previously dirty.
2638  *
2639  * This is for preparing to put the page under writeout.  We leave the page
2640  * tagged as dirty in the xarray so that a concurrent write-for-sync
2641  * can discover it via a PAGECACHE_TAG_DIRTY walk.  The ->writepage
2642  * implementation will run either set_page_writeback() or set_page_dirty(),
2643  * at which stage we bring the page's dirty flag and xarray dirty tag
2644  * back into sync.
2645  *
2646  * This incoherency between the page's dirty flag and xarray tag is
2647  * unfortunate, but it only exists while the page is locked.
2648  */
2649 int clear_page_dirty_for_io(struct page *page)
2650 {
2651 	struct address_space *mapping = page_mapping(page);
2652 	int ret = 0;
2653 
2654 	BUG_ON(!PageLocked(page));
2655 
2656 	if (mapping && mapping_cap_account_dirty(mapping)) {
2657 		struct inode *inode = mapping->host;
2658 		struct bdi_writeback *wb;
2659 		struct wb_lock_cookie cookie = {};
2660 
2661 		/*
2662 		 * Yes, Virginia, this is indeed insane.
2663 		 *
2664 		 * We use this sequence to make sure that
2665 		 *  (a) we account for dirty stats properly
2666 		 *  (b) we tell the low-level filesystem to
2667 		 *      mark the whole page dirty if it was
2668 		 *      dirty in a pagetable. Only to then
2669 		 *  (c) clean the page again and return 1 to
2670 		 *      cause the writeback.
2671 		 *
2672 		 * This way we avoid all nasty races with the
2673 		 * dirty bit in multiple places and clearing
2674 		 * them concurrently from different threads.
2675 		 *
2676 		 * Note! Normally the "set_page_dirty(page)"
2677 		 * has no effect on the actual dirty bit - since
2678 		 * that will already usually be set. But we
2679 		 * need the side effects, and it can help us
2680 		 * avoid races.
2681 		 *
2682 		 * We basically use the page "master dirty bit"
2683 		 * as a serialization point for all the different
2684 		 * threads doing their things.
2685 		 */
2686 		if (page_mkclean(page))
2687 			set_page_dirty(page);
2688 		/*
2689 		 * We carefully synchronise fault handlers against
2690 		 * installing a dirty pte and marking the page dirty
2691 		 * at this point.  We do this by having them hold the
2692 		 * page lock while dirtying the page, and pages are
2693 		 * always locked coming in here, so we get the desired
2694 		 * exclusion.
2695 		 */
2696 		wb = unlocked_inode_to_wb_begin(inode, &cookie);
2697 		if (TestClearPageDirty(page)) {
2698 			dec_lruvec_page_state(page, NR_FILE_DIRTY);
2699 			dec_zone_page_state(page, NR_ZONE_WRITE_PENDING);
2700 			dec_wb_stat(wb, WB_RECLAIMABLE);
2701 			ret = 1;
2702 		}
2703 		unlocked_inode_to_wb_end(inode, &cookie);
2704 		return ret;
2705 	}
2706 	return TestClearPageDirty(page);
2707 }
2708 EXPORT_SYMBOL(clear_page_dirty_for_io);
2709 
2710 int test_clear_page_writeback(struct page *page)
2711 {
2712 	struct address_space *mapping = page_mapping(page);
2713 	struct mem_cgroup *memcg;
2714 	struct lruvec *lruvec;
2715 	int ret;
2716 
2717 	memcg = lock_page_memcg(page);
2718 	lruvec = mem_cgroup_page_lruvec(page, page_pgdat(page));
2719 	if (mapping && mapping_use_writeback_tags(mapping)) {
2720 		struct inode *inode = mapping->host;
2721 		struct backing_dev_info *bdi = inode_to_bdi(inode);
2722 		unsigned long flags;
2723 
2724 		xa_lock_irqsave(&mapping->i_pages, flags);
2725 		ret = TestClearPageWriteback(page);
2726 		if (ret) {
2727 			__xa_clear_mark(&mapping->i_pages, page_index(page),
2728 						PAGECACHE_TAG_WRITEBACK);
2729 			if (bdi_cap_account_writeback(bdi)) {
2730 				struct bdi_writeback *wb = inode_to_wb(inode);
2731 
2732 				dec_wb_stat(wb, WB_WRITEBACK);
2733 				__wb_writeout_inc(wb);
2734 			}
2735 		}
2736 
2737 		if (mapping->host && !mapping_tagged(mapping,
2738 						     PAGECACHE_TAG_WRITEBACK))
2739 			sb_clear_inode_writeback(mapping->host);
2740 
2741 		xa_unlock_irqrestore(&mapping->i_pages, flags);
2742 	} else {
2743 		ret = TestClearPageWriteback(page);
2744 	}
2745 	/*
2746 	 * NOTE: Page might be free now! Writeback doesn't hold a page
2747 	 * reference on its own, it relies on truncation to wait for
2748 	 * the clearing of PG_writeback. The below can only access
2749 	 * page state that is static across allocation cycles.
2750 	 */
2751 	if (ret) {
2752 		dec_lruvec_state(lruvec, NR_WRITEBACK);
2753 		dec_zone_page_state(page, NR_ZONE_WRITE_PENDING);
2754 		inc_node_page_state(page, NR_WRITTEN);
2755 	}
2756 	__unlock_page_memcg(memcg);
2757 	return ret;
2758 }
2759 
2760 int __test_set_page_writeback(struct page *page, bool keep_write)
2761 {
2762 	struct address_space *mapping = page_mapping(page);
2763 	int ret;
2764 
2765 	lock_page_memcg(page);
2766 	if (mapping && mapping_use_writeback_tags(mapping)) {
2767 		XA_STATE(xas, &mapping->i_pages, page_index(page));
2768 		struct inode *inode = mapping->host;
2769 		struct backing_dev_info *bdi = inode_to_bdi(inode);
2770 		unsigned long flags;
2771 
2772 		xas_lock_irqsave(&xas, flags);
2773 		xas_load(&xas);
2774 		ret = TestSetPageWriteback(page);
2775 		if (!ret) {
2776 			bool on_wblist;
2777 
2778 			on_wblist = mapping_tagged(mapping,
2779 						   PAGECACHE_TAG_WRITEBACK);
2780 
2781 			xas_set_mark(&xas, PAGECACHE_TAG_WRITEBACK);
2782 			if (bdi_cap_account_writeback(bdi))
2783 				inc_wb_stat(inode_to_wb(inode), WB_WRITEBACK);
2784 
2785 			/*
2786 			 * We can come through here when swapping anonymous
2787 			 * pages, so we don't necessarily have an inode to track
2788 			 * for sync.
2789 			 */
2790 			if (mapping->host && !on_wblist)
2791 				sb_mark_inode_writeback(mapping->host);
2792 		}
2793 		if (!PageDirty(page))
2794 			xas_clear_mark(&xas, PAGECACHE_TAG_DIRTY);
2795 		if (!keep_write)
2796 			xas_clear_mark(&xas, PAGECACHE_TAG_TOWRITE);
2797 		xas_unlock_irqrestore(&xas, flags);
2798 	} else {
2799 		ret = TestSetPageWriteback(page);
2800 	}
2801 	if (!ret) {
2802 		inc_lruvec_page_state(page, NR_WRITEBACK);
2803 		inc_zone_page_state(page, NR_ZONE_WRITE_PENDING);
2804 	}
2805 	unlock_page_memcg(page);
2806 	return ret;
2807 
2808 }
2809 EXPORT_SYMBOL(__test_set_page_writeback);
2810 
2811 /*
2812  * Wait for a page to complete writeback
2813  */
2814 void wait_on_page_writeback(struct page *page)
2815 {
2816 	if (PageWriteback(page)) {
2817 		trace_wait_on_page_writeback(page, page_mapping(page));
2818 		wait_on_page_bit(page, PG_writeback);
2819 	}
2820 }
2821 EXPORT_SYMBOL_GPL(wait_on_page_writeback);
2822 
2823 /**
2824  * wait_for_stable_page() - wait for writeback to finish, if necessary.
2825  * @page:	The page to wait on.
2826  *
2827  * This function determines if the given page is related to a backing device
2828  * that requires page contents to be held stable during writeback.  If so, then
2829  * it will wait for any pending writeback to complete.
2830  */
2831 void wait_for_stable_page(struct page *page)
2832 {
2833 	if (bdi_cap_stable_pages_required(inode_to_bdi(page->mapping->host)))
2834 		wait_on_page_writeback(page);
2835 }
2836 EXPORT_SYMBOL_GPL(wait_for_stable_page);
2837