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Linux/mm/page-writeback.c

  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/notifier.h>
 31 #include <linux/smp.h>
 32 #include <linux/sysctl.h>
 33 #include <linux/cpu.h>
 34 #include <linux/syscalls.h>
 35 #include <linux/buffer_head.h> /* __set_page_dirty_buffers */
 36 #include <linux/pagevec.h>
 37 #include <linux/timer.h>
 38 #include <linux/sched/rt.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  * zone_dirtyable_memory - number of dirtyable pages in a zone
271  * @zone: the zone
272  *
273  * Returns the zone's number of pages potentially available for dirty
274  * page cache.  This is the base value for the per-zone dirty limits.
275  */
276 static unsigned long zone_dirtyable_memory(struct zone *zone)
277 {
278         unsigned long nr_pages;
279 
280         nr_pages = zone_page_state(zone, NR_FREE_PAGES);
281         /*
282          * Pages reserved for the kernel should not be considered
283          * dirtyable, to prevent a situation where reclaim has to
284          * clean pages in order to balance the zones.
285          */
286         nr_pages -= min(nr_pages, zone->totalreserve_pages);
287 
288         nr_pages += zone_page_state(zone, NR_INACTIVE_FILE);
289         nr_pages += zone_page_state(zone, NR_ACTIVE_FILE);
290 
291         return nr_pages;
292 }
293 
294 static unsigned long highmem_dirtyable_memory(unsigned long total)
295 {
296 #ifdef CONFIG_HIGHMEM
297         int node;
298         unsigned long x = 0;
299 
300         for_each_node_state(node, N_HIGH_MEMORY) {
301                 struct zone *z = &NODE_DATA(node)->node_zones[ZONE_HIGHMEM];
302 
303                 x += zone_dirtyable_memory(z);
304         }
305         /*
306          * Unreclaimable memory (kernel memory or anonymous memory
307          * without swap) can bring down the dirtyable pages below
308          * the zone's dirty balance reserve and the above calculation
309          * will underflow.  However we still want to add in nodes
310          * which are below threshold (negative values) to get a more
311          * accurate calculation but make sure that the total never
312          * underflows.
313          */
314         if ((long)x < 0)
315                 x = 0;
316 
317         /*
318          * Make sure that the number of highmem pages is never larger
319          * than the number of the total dirtyable memory. This can only
320          * occur in very strange VM situations but we want to make sure
321          * that this does not occur.
322          */
323         return min(x, total);
324 #else
325         return 0;
326 #endif
327 }
328 
329 /**
330  * global_dirtyable_memory - number of globally dirtyable pages
331  *
332  * Returns the global number of pages potentially available for dirty
333  * page cache.  This is the base value for the global dirty limits.
334  */
335 static unsigned long global_dirtyable_memory(void)
336 {
337         unsigned long x;
338 
339         x = global_page_state(NR_FREE_PAGES);
340         /*
341          * Pages reserved for the kernel should not be considered
342          * dirtyable, to prevent a situation where reclaim has to
343          * clean pages in order to balance the zones.
344          */
345         x -= min(x, totalreserve_pages);
346 
347         x += global_page_state(NR_INACTIVE_FILE);
348         x += global_page_state(NR_ACTIVE_FILE);
349 
350         if (!vm_highmem_is_dirtyable)
351                 x -= highmem_dirtyable_memory(x);
352 
353         return x + 1;   /* Ensure that we never return 0 */
354 }
355 
356 /**
357  * domain_dirty_limits - calculate thresh and bg_thresh for a wb_domain
358  * @dtc: dirty_throttle_control of interest
359  *
360  * Calculate @dtc->thresh and ->bg_thresh considering
361  * vm_dirty_{bytes|ratio} and dirty_background_{bytes|ratio}.  The caller
362  * must ensure that @dtc->avail is set before calling this function.  The
363  * dirty limits will be lifted by 1/4 for PF_LESS_THROTTLE (ie. nfsd) and
364  * real-time tasks.
365  */
366 static void domain_dirty_limits(struct dirty_throttle_control *dtc)
367 {
368         const unsigned long available_memory = dtc->avail;
369         struct dirty_throttle_control *gdtc = mdtc_gdtc(dtc);
370         unsigned long bytes = vm_dirty_bytes;
371         unsigned long bg_bytes = dirty_background_bytes;
372         unsigned long ratio = vm_dirty_ratio;
373         unsigned long bg_ratio = dirty_background_ratio;
374         unsigned long thresh;
375         unsigned long bg_thresh;
376         struct task_struct *tsk;
377 
378         /* gdtc is !NULL iff @dtc is for memcg domain */
379         if (gdtc) {
380                 unsigned long global_avail = gdtc->avail;
381 
382                 /*
383                  * The byte settings can't be applied directly to memcg
384                  * domains.  Convert them to ratios by scaling against
385                  * globally available memory.
386                  */
387                 if (bytes)
388                         ratio = min(DIV_ROUND_UP(bytes, PAGE_SIZE) * 100 /
389                                     global_avail, 100UL);
390                 if (bg_bytes)
391                         bg_ratio = min(DIV_ROUND_UP(bg_bytes, PAGE_SIZE) * 100 /
392                                        global_avail, 100UL);
393                 bytes = bg_bytes = 0;
394         }
395 
396         if (bytes)
397                 thresh = DIV_ROUND_UP(bytes, PAGE_SIZE);
398         else
399                 thresh = (ratio * available_memory) / 100;
400 
401         if (bg_bytes)
402                 bg_thresh = DIV_ROUND_UP(bg_bytes, PAGE_SIZE);
403         else
404                 bg_thresh = (bg_ratio * available_memory) / 100;
405 
406         if (bg_thresh >= thresh)
407                 bg_thresh = thresh / 2;
408         tsk = current;
409         if (tsk->flags & PF_LESS_THROTTLE || rt_task(tsk)) {
410                 bg_thresh += bg_thresh / 4;
411                 thresh += thresh / 4;
412         }
413         dtc->thresh = thresh;
414         dtc->bg_thresh = bg_thresh;
415 
416         /* we should eventually report the domain in the TP */
417         if (!gdtc)
418                 trace_global_dirty_state(bg_thresh, thresh);
419 }
420 
421 /**
422  * global_dirty_limits - background-writeback and dirty-throttling thresholds
423  * @pbackground: out parameter for bg_thresh
424  * @pdirty: out parameter for thresh
425  *
426  * Calculate bg_thresh and thresh for global_wb_domain.  See
427  * domain_dirty_limits() for details.
428  */
429 void global_dirty_limits(unsigned long *pbackground, unsigned long *pdirty)
430 {
431         struct dirty_throttle_control gdtc = { GDTC_INIT_NO_WB };
432 
433         gdtc.avail = global_dirtyable_memory();
434         domain_dirty_limits(&gdtc);
435 
436         *pbackground = gdtc.bg_thresh;
437         *pdirty = gdtc.thresh;
438 }
439 
440 /**
441  * zone_dirty_limit - maximum number of dirty pages allowed in a zone
442  * @zone: the zone
443  *
444  * Returns the maximum number of dirty pages allowed in a zone, based
445  * on the zone's dirtyable memory.
446  */
447 static unsigned long zone_dirty_limit(struct zone *zone)
448 {
449         unsigned long zone_memory = zone_dirtyable_memory(zone);
450         struct task_struct *tsk = current;
451         unsigned long dirty;
452 
453         if (vm_dirty_bytes)
454                 dirty = DIV_ROUND_UP(vm_dirty_bytes, PAGE_SIZE) *
455                         zone_memory / global_dirtyable_memory();
456         else
457                 dirty = vm_dirty_ratio * zone_memory / 100;
458 
459         if (tsk->flags & PF_LESS_THROTTLE || rt_task(tsk))
460                 dirty += dirty / 4;
461 
462         return dirty;
463 }
464 
465 /**
466  * zone_dirty_ok - tells whether a zone is within its dirty limits
467  * @zone: the zone to check
468  *
469  * Returns %true when the dirty pages in @zone are within the zone's
470  * dirty limit, %false if the limit is exceeded.
471  */
472 bool zone_dirty_ok(struct zone *zone)
473 {
474         unsigned long limit = zone_dirty_limit(zone);
475 
476         return zone_page_state(zone, NR_FILE_DIRTY) +
477                zone_page_state(zone, NR_UNSTABLE_NFS) +
478                zone_page_state(zone, NR_WRITEBACK) <= limit;
479 }
480 
481 int dirty_background_ratio_handler(struct ctl_table *table, int write,
482                 void __user *buffer, size_t *lenp,
483                 loff_t *ppos)
484 {
485         int ret;
486 
487         ret = proc_dointvec_minmax(table, write, buffer, lenp, ppos);
488         if (ret == 0 && write)
489                 dirty_background_bytes = 0;
490         return ret;
491 }
492 
493 int dirty_background_bytes_handler(struct ctl_table *table, int write,
494                 void __user *buffer, size_t *lenp,
495                 loff_t *ppos)
496 {
497         int ret;
498 
499         ret = proc_doulongvec_minmax(table, write, buffer, lenp, ppos);
500         if (ret == 0 && write)
501                 dirty_background_ratio = 0;
502         return ret;
503 }
504 
505 int dirty_ratio_handler(struct ctl_table *table, int write,
506                 void __user *buffer, size_t *lenp,
507                 loff_t *ppos)
508 {
509         int old_ratio = vm_dirty_ratio;
510         int ret;
511 
512         ret = proc_dointvec_minmax(table, write, buffer, lenp, ppos);
513         if (ret == 0 && write && vm_dirty_ratio != old_ratio) {
514                 writeback_set_ratelimit();
515                 vm_dirty_bytes = 0;
516         }
517         return ret;
518 }
519 
520 int dirty_bytes_handler(struct ctl_table *table, int write,
521                 void __user *buffer, size_t *lenp,
522                 loff_t *ppos)
523 {
524         unsigned long old_bytes = vm_dirty_bytes;
525         int ret;
526 
527         ret = proc_doulongvec_minmax(table, write, buffer, lenp, ppos);
528         if (ret == 0 && write && vm_dirty_bytes != old_bytes) {
529                 writeback_set_ratelimit();
530                 vm_dirty_ratio = 0;
531         }
532         return ret;
533 }
534 
535 static unsigned long wp_next_time(unsigned long cur_time)
536 {
537         cur_time += VM_COMPLETIONS_PERIOD_LEN;
538         /* 0 has a special meaning... */
539         if (!cur_time)
540                 return 1;
541         return cur_time;
542 }
543 
544 static void wb_domain_writeout_inc(struct wb_domain *dom,
545                                    struct fprop_local_percpu *completions,
546                                    unsigned int max_prop_frac)
547 {
548         __fprop_inc_percpu_max(&dom->completions, completions,
549                                max_prop_frac);
550         /* First event after period switching was turned off? */
551         if (!unlikely(dom->period_time)) {
552                 /*
553                  * We can race with other __bdi_writeout_inc calls here but
554                  * it does not cause any harm since the resulting time when
555                  * timer will fire and what is in writeout_period_time will be
556                  * roughly the same.
557                  */
558                 dom->period_time = wp_next_time(jiffies);
559                 mod_timer(&dom->period_timer, dom->period_time);
560         }
561 }
562 
563 /*
564  * Increment @wb's writeout completion count and the global writeout
565  * completion count. Called from test_clear_page_writeback().
566  */
567 static inline void __wb_writeout_inc(struct bdi_writeback *wb)
568 {
569         struct wb_domain *cgdom;
570 
571         __inc_wb_stat(wb, WB_WRITTEN);
572         wb_domain_writeout_inc(&global_wb_domain, &wb->completions,
573                                wb->bdi->max_prop_frac);
574 
575         cgdom = mem_cgroup_wb_domain(wb);
576         if (cgdom)
577                 wb_domain_writeout_inc(cgdom, wb_memcg_completions(wb),
578                                        wb->bdi->max_prop_frac);
579 }
580 
581 void wb_writeout_inc(struct bdi_writeback *wb)
582 {
583         unsigned long flags;
584 
585         local_irq_save(flags);
586         __wb_writeout_inc(wb);
587         local_irq_restore(flags);
588 }
589 EXPORT_SYMBOL_GPL(wb_writeout_inc);
590 
591 /*
592  * On idle system, we can be called long after we scheduled because we use
593  * deferred timers so count with missed periods.
594  */
595 static void writeout_period(unsigned long t)
596 {
597         struct wb_domain *dom = (void *)t;
598         int miss_periods = (jiffies - dom->period_time) /
599                                                  VM_COMPLETIONS_PERIOD_LEN;
600 
601         if (fprop_new_period(&dom->completions, miss_periods + 1)) {
602                 dom->period_time = wp_next_time(dom->period_time +
603                                 miss_periods * VM_COMPLETIONS_PERIOD_LEN);
604                 mod_timer(&dom->period_timer, dom->period_time);
605         } else {
606                 /*
607                  * Aging has zeroed all fractions. Stop wasting CPU on period
608                  * updates.
609                  */
610                 dom->period_time = 0;
611         }
612 }
613 
614 int wb_domain_init(struct wb_domain *dom, gfp_t gfp)
615 {
616         memset(dom, 0, sizeof(*dom));
617 
618         spin_lock_init(&dom->lock);
619 
620         init_timer_deferrable(&dom->period_timer);
621         dom->period_timer.function = writeout_period;
622         dom->period_timer.data = (unsigned long)dom;
623 
624         dom->dirty_limit_tstamp = jiffies;
625 
626         return fprop_global_init(&dom->completions, gfp);
627 }
628 
629 #ifdef CONFIG_CGROUP_WRITEBACK
630 void wb_domain_exit(struct wb_domain *dom)
631 {
632         del_timer_sync(&dom->period_timer);
633         fprop_global_destroy(&dom->completions);
634 }
635 #endif
636 
637 /*
638  * bdi_min_ratio keeps the sum of the minimum dirty shares of all
639  * registered backing devices, which, for obvious reasons, can not
640  * exceed 100%.
641  */
642 static unsigned int bdi_min_ratio;
643 
644 int bdi_set_min_ratio(struct backing_dev_info *bdi, unsigned int min_ratio)
645 {
646         int ret = 0;
647 
648         spin_lock_bh(&bdi_lock);
649         if (min_ratio > bdi->max_ratio) {
650                 ret = -EINVAL;
651         } else {
652                 min_ratio -= bdi->min_ratio;
653                 if (bdi_min_ratio + min_ratio < 100) {
654                         bdi_min_ratio += min_ratio;
655                         bdi->min_ratio += min_ratio;
656                 } else {
657                         ret = -EINVAL;
658                 }
659         }
660         spin_unlock_bh(&bdi_lock);
661 
662         return ret;
663 }
664 
665 int bdi_set_max_ratio(struct backing_dev_info *bdi, unsigned max_ratio)
666 {
667         int ret = 0;
668 
669         if (max_ratio > 100)
670                 return -EINVAL;
671 
672         spin_lock_bh(&bdi_lock);
673         if (bdi->min_ratio > max_ratio) {
674                 ret = -EINVAL;
675         } else {
676                 bdi->max_ratio = max_ratio;
677                 bdi->max_prop_frac = (FPROP_FRAC_BASE * max_ratio) / 100;
678         }
679         spin_unlock_bh(&bdi_lock);
680 
681         return ret;
682 }
683 EXPORT_SYMBOL(bdi_set_max_ratio);
684 
685 static unsigned long dirty_freerun_ceiling(unsigned long thresh,
686                                            unsigned long bg_thresh)
687 {
688         return (thresh + bg_thresh) / 2;
689 }
690 
691 static unsigned long hard_dirty_limit(struct wb_domain *dom,
692                                       unsigned long thresh)
693 {
694         return max(thresh, dom->dirty_limit);
695 }
696 
697 /*
698  * Memory which can be further allocated to a memcg domain is capped by
699  * system-wide clean memory excluding the amount being used in the domain.
700  */
701 static void mdtc_calc_avail(struct dirty_throttle_control *mdtc,
702                             unsigned long filepages, unsigned long headroom)
703 {
704         struct dirty_throttle_control *gdtc = mdtc_gdtc(mdtc);
705         unsigned long clean = filepages - min(filepages, mdtc->dirty);
706         unsigned long global_clean = gdtc->avail - min(gdtc->avail, gdtc->dirty);
707         unsigned long other_clean = global_clean - min(global_clean, clean);
708 
709         mdtc->avail = filepages + min(headroom, other_clean);
710 }
711 
712 /**
713  * __wb_calc_thresh - @wb's share of dirty throttling threshold
714  * @dtc: dirty_throttle_context of interest
715  *
716  * Returns @wb's dirty limit in pages. The term "dirty" in the context of
717  * dirty balancing includes all PG_dirty, PG_writeback and NFS unstable pages.
718  *
719  * Note that balance_dirty_pages() will only seriously take it as a hard limit
720  * when sleeping max_pause per page is not enough to keep the dirty pages under
721  * control. For example, when the device is completely stalled due to some error
722  * conditions, or when there are 1000 dd tasks writing to a slow 10MB/s USB key.
723  * In the other normal situations, it acts more gently by throttling the tasks
724  * more (rather than completely block them) when the wb dirty pages go high.
725  *
726  * It allocates high/low dirty limits to fast/slow devices, in order to prevent
727  * - starving fast devices
728  * - piling up dirty pages (that will take long time to sync) on slow devices
729  *
730  * The wb's share of dirty limit will be adapting to its throughput and
731  * bounded by the bdi->min_ratio and/or bdi->max_ratio parameters, if set.
732  */
733 static unsigned long __wb_calc_thresh(struct dirty_throttle_control *dtc)
734 {
735         struct wb_domain *dom = dtc_dom(dtc);
736         unsigned long thresh = dtc->thresh;
737         u64 wb_thresh;
738         long numerator, denominator;
739         unsigned long wb_min_ratio, wb_max_ratio;
740 
741         /*
742          * Calculate this BDI's share of the thresh ratio.
743          */
744         fprop_fraction_percpu(&dom->completions, dtc->wb_completions,
745                               &numerator, &denominator);
746 
747         wb_thresh = (thresh * (100 - bdi_min_ratio)) / 100;
748         wb_thresh *= numerator;
749         do_div(wb_thresh, denominator);
750 
751         wb_min_max_ratio(dtc->wb, &wb_min_ratio, &wb_max_ratio);
752 
753         wb_thresh += (thresh * wb_min_ratio) / 100;
754         if (wb_thresh > (thresh * wb_max_ratio) / 100)
755                 wb_thresh = thresh * wb_max_ratio / 100;
756 
757         return wb_thresh;
758 }
759 
760 unsigned long wb_calc_thresh(struct bdi_writeback *wb, unsigned long thresh)
761 {
762         struct dirty_throttle_control gdtc = { GDTC_INIT(wb),
763                                                .thresh = thresh };
764         return __wb_calc_thresh(&gdtc);
765 }
766 
767 /*
768  *                           setpoint - dirty 3
769  *        f(dirty) := 1.0 + (----------------)
770  *                           limit - setpoint
771  *
772  * it's a 3rd order polynomial that subjects to
773  *
774  * (1) f(freerun)  = 2.0 => rampup dirty_ratelimit reasonably fast
775  * (2) f(setpoint) = 1.0 => the balance point
776  * (3) f(limit)    = 0   => the hard limit
777  * (4) df/dx      <= 0   => negative feedback control
778  * (5) the closer to setpoint, the smaller |df/dx| (and the reverse)
779  *     => fast response on large errors; small oscillation near setpoint
780  */
781 static long long pos_ratio_polynom(unsigned long setpoint,
782                                           unsigned long dirty,
783                                           unsigned long limit)
784 {
785         long long pos_ratio;
786         long x;
787 
788         x = div64_s64(((s64)setpoint - (s64)dirty) << RATELIMIT_CALC_SHIFT,
789                       (limit - setpoint) | 1);
790         pos_ratio = x;
791         pos_ratio = pos_ratio * x >> RATELIMIT_CALC_SHIFT;
792         pos_ratio = pos_ratio * x >> RATELIMIT_CALC_SHIFT;
793         pos_ratio += 1 << RATELIMIT_CALC_SHIFT;
794 
795         return clamp(pos_ratio, 0LL, 2LL << RATELIMIT_CALC_SHIFT);
796 }
797 
798 /*
799  * Dirty position control.
800  *
801  * (o) global/bdi setpoints
802  *
803  * We want the dirty pages be balanced around the global/wb setpoints.
804  * When the number of dirty pages is higher/lower than the setpoint, the
805  * dirty position control ratio (and hence task dirty ratelimit) will be
806  * decreased/increased to bring the dirty pages back to the setpoint.
807  *
808  *     pos_ratio = 1 << RATELIMIT_CALC_SHIFT
809  *
810  *     if (dirty < setpoint) scale up   pos_ratio
811  *     if (dirty > setpoint) scale down pos_ratio
812  *
813  *     if (wb_dirty < wb_setpoint) scale up   pos_ratio
814  *     if (wb_dirty > wb_setpoint) scale down pos_ratio
815  *
816  *     task_ratelimit = dirty_ratelimit * pos_ratio >> RATELIMIT_CALC_SHIFT
817  *
818  * (o) global control line
819  *
820  *     ^ pos_ratio
821  *     |
822  *     |            |<===== global dirty control scope ======>|
823  * 2.0 .............*
824  *     |            .*
825  *     |            . *
826  *     |            .   *
827  *     |            .     *
828  *     |            .        *
829  *     |            .            *
830  * 1.0 ................................*
831  *     |            .                  .     *
832  *     |            .                  .          *
833  *     |            .                  .              *
834  *     |            .                  .                 *
835  *     |            .                  .                    *
836  *   0 +------------.------------------.----------------------*------------->
837  *           freerun^          setpoint^                 limit^   dirty pages
838  *
839  * (o) wb control line
840  *
841  *     ^ pos_ratio
842  *     |
843  *     |            *
844  *     |              *
845  *     |                *
846  *     |                  *
847  *     |                    * |<=========== span ============>|
848  * 1.0 .......................*
849  *     |                      . *
850  *     |                      .   *
851  *     |                      .     *
852  *     |                      .       *
853  *     |                      .         *
854  *     |                      .           *
855  *     |                      .             *
856  *     |                      .               *
857  *     |                      .                 *
858  *     |                      .                   *
859  *     |                      .                     *
860  * 1/4 ...............................................* * * * * * * * * * * *
861  *     |                      .                         .
862  *     |                      .                           .
863  *     |                      .                             .
864  *   0 +----------------------.-------------------------------.------------->
865  *                wb_setpoint^                    x_intercept^
866  *
867  * The wb control line won't drop below pos_ratio=1/4, so that wb_dirty can
868  * be smoothly throttled down to normal if it starts high in situations like
869  * - start writing to a slow SD card and a fast disk at the same time. The SD
870  *   card's wb_dirty may rush to many times higher than wb_setpoint.
871  * - the wb dirty thresh drops quickly due to change of JBOD workload
872  */
873 static void wb_position_ratio(struct dirty_throttle_control *dtc)
874 {
875         struct bdi_writeback *wb = dtc->wb;
876         unsigned long write_bw = wb->avg_write_bandwidth;
877         unsigned long freerun = dirty_freerun_ceiling(dtc->thresh, dtc->bg_thresh);
878         unsigned long limit = hard_dirty_limit(dtc_dom(dtc), dtc->thresh);
879         unsigned long wb_thresh = dtc->wb_thresh;
880         unsigned long x_intercept;
881         unsigned long setpoint;         /* dirty pages' target balance point */
882         unsigned long wb_setpoint;
883         unsigned long span;
884         long long pos_ratio;            /* for scaling up/down the rate limit */
885         long x;
886 
887         dtc->pos_ratio = 0;
888 
889         if (unlikely(dtc->dirty >= limit))
890                 return;
891 
892         /*
893          * global setpoint
894          *
895          * See comment for pos_ratio_polynom().
896          */
897         setpoint = (freerun + limit) / 2;
898         pos_ratio = pos_ratio_polynom(setpoint, dtc->dirty, limit);
899 
900         /*
901          * The strictlimit feature is a tool preventing mistrusted filesystems
902          * from growing a large number of dirty pages before throttling. For
903          * such filesystems balance_dirty_pages always checks wb counters
904          * against wb limits. Even if global "nr_dirty" is under "freerun".
905          * This is especially important for fuse which sets bdi->max_ratio to
906          * 1% by default. Without strictlimit feature, fuse writeback may
907          * consume arbitrary amount of RAM because it is accounted in
908          * NR_WRITEBACK_TEMP which is not involved in calculating "nr_dirty".
909          *
910          * Here, in wb_position_ratio(), we calculate pos_ratio based on
911          * two values: wb_dirty and wb_thresh. Let's consider an example:
912          * total amount of RAM is 16GB, bdi->max_ratio is equal to 1%, global
913          * limits are set by default to 10% and 20% (background and throttle).
914          * Then wb_thresh is 1% of 20% of 16GB. This amounts to ~8K pages.
915          * wb_calc_thresh(wb, bg_thresh) is about ~4K pages. wb_setpoint is
916          * about ~6K pages (as the average of background and throttle wb
917          * limits). The 3rd order polynomial will provide positive feedback if
918          * wb_dirty is under wb_setpoint and vice versa.
919          *
920          * Note, that we cannot use global counters in these calculations
921          * because we want to throttle process writing to a strictlimit wb
922          * much earlier than global "freerun" is reached (~23MB vs. ~2.3GB
923          * in the example above).
924          */
925         if (unlikely(wb->bdi->capabilities & BDI_CAP_STRICTLIMIT)) {
926                 long long wb_pos_ratio;
927 
928                 if (dtc->wb_dirty < 8) {
929                         dtc->pos_ratio = min_t(long long, pos_ratio * 2,
930                                            2 << RATELIMIT_CALC_SHIFT);
931                         return;
932                 }
933 
934                 if (dtc->wb_dirty >= wb_thresh)
935                         return;
936 
937                 wb_setpoint = dirty_freerun_ceiling(wb_thresh,
938                                                     dtc->wb_bg_thresh);
939 
940                 if (wb_setpoint == 0 || wb_setpoint == wb_thresh)
941                         return;
942 
943                 wb_pos_ratio = pos_ratio_polynom(wb_setpoint, dtc->wb_dirty,
944                                                  wb_thresh);
945 
946                 /*
947                  * Typically, for strictlimit case, wb_setpoint << setpoint
948                  * and pos_ratio >> wb_pos_ratio. In the other words global
949                  * state ("dirty") is not limiting factor and we have to
950                  * make decision based on wb counters. But there is an
951                  * important case when global pos_ratio should get precedence:
952                  * global limits are exceeded (e.g. due to activities on other
953                  * wb's) while given strictlimit wb is below limit.
954                  *
955                  * "pos_ratio * wb_pos_ratio" would work for the case above,
956                  * but it would look too non-natural for the case of all
957                  * activity in the system coming from a single strictlimit wb
958                  * with bdi->max_ratio == 100%.
959                  *
960                  * Note that min() below somewhat changes the dynamics of the
961                  * control system. Normally, pos_ratio value can be well over 3
962                  * (when globally we are at freerun and wb is well below wb
963                  * setpoint). Now the maximum pos_ratio in the same situation
964                  * is 2. We might want to tweak this if we observe the control
965                  * system is too slow to adapt.
966                  */
967                 dtc->pos_ratio = min(pos_ratio, wb_pos_ratio);
968                 return;
969         }
970 
971         /*
972          * We have computed basic pos_ratio above based on global situation. If
973          * the wb is over/under its share of dirty pages, we want to scale
974          * pos_ratio further down/up. That is done by the following mechanism.
975          */
976 
977         /*
978          * wb setpoint
979          *
980          *        f(wb_dirty) := 1.0 + k * (wb_dirty - wb_setpoint)
981          *
982          *                        x_intercept - wb_dirty
983          *                     := --------------------------
984          *                        x_intercept - wb_setpoint
985          *
986          * The main wb control line is a linear function that subjects to
987          *
988          * (1) f(wb_setpoint) = 1.0
989          * (2) k = - 1 / (8 * write_bw)  (in single wb case)
990          *     or equally: x_intercept = wb_setpoint + 8 * write_bw
991          *
992          * For single wb case, the dirty pages are observed to fluctuate
993          * regularly within range
994          *        [wb_setpoint - write_bw/2, wb_setpoint + write_bw/2]
995          * for various filesystems, where (2) can yield in a reasonable 12.5%
996          * fluctuation range for pos_ratio.
997          *
998          * For JBOD case, wb_thresh (not wb_dirty!) could fluctuate up to its
999          * own size, so move the slope over accordingly and choose a slope that
1000          * yields 100% pos_ratio fluctuation on suddenly doubled wb_thresh.
1001          */
1002         if (unlikely(wb_thresh > dtc->thresh))
1003                 wb_thresh = dtc->thresh;
1004         /*
1005          * It's very possible that wb_thresh is close to 0 not because the
1006          * device is slow, but that it has remained inactive for long time.
1007          * Honour such devices a reasonable good (hopefully IO efficient)
1008          * threshold, so that the occasional writes won't be blocked and active
1009          * writes can rampup the threshold quickly.
1010          */
1011         wb_thresh = max(wb_thresh, (limit - dtc->dirty) / 8);
1012         /*
1013          * scale global setpoint to wb's:
1014          *      wb_setpoint = setpoint * wb_thresh / thresh
1015          */
1016         x = div_u64((u64)wb_thresh << 16, dtc->thresh | 1);
1017         wb_setpoint = setpoint * (u64)x >> 16;
1018         /*
1019          * Use span=(8*write_bw) in single wb case as indicated by
1020          * (thresh - wb_thresh ~= 0) and transit to wb_thresh in JBOD case.
1021          *
1022          *        wb_thresh                    thresh - wb_thresh
1023          * span = --------- * (8 * write_bw) + ------------------ * wb_thresh
1024          *         thresh                           thresh
1025          */
1026         span = (dtc->thresh - wb_thresh + 8 * write_bw) * (u64)x >> 16;
1027         x_intercept = wb_setpoint + span;
1028 
1029         if (dtc->wb_dirty < x_intercept - span / 4) {
1030                 pos_ratio = div64_u64(pos_ratio * (x_intercept - dtc->wb_dirty),
1031                                       (x_intercept - wb_setpoint) | 1);
1032         } else
1033                 pos_ratio /= 4;
1034 
1035         /*
1036          * wb reserve area, safeguard against dirty pool underrun and disk idle
1037          * It may push the desired control point of global dirty pages higher
1038          * than setpoint.
1039          */
1040         x_intercept = wb_thresh / 2;
1041         if (dtc->wb_dirty < x_intercept) {
1042                 if (dtc->wb_dirty > x_intercept / 8)
1043                         pos_ratio = div_u64(pos_ratio * x_intercept,
1044                                             dtc->wb_dirty);
1045                 else
1046                         pos_ratio *= 8;
1047         }
1048 
1049         dtc->pos_ratio = pos_ratio;
1050 }
1051 
1052 static void wb_update_write_bandwidth(struct bdi_writeback *wb,
1053                                       unsigned long elapsed,
1054                                       unsigned long written)
1055 {
1056         const unsigned long period = roundup_pow_of_two(3 * HZ);
1057         unsigned long avg = wb->avg_write_bandwidth;
1058         unsigned long old = wb->write_bandwidth;
1059         u64 bw;
1060 
1061         /*
1062          * bw = written * HZ / elapsed
1063          *
1064          *                   bw * elapsed + write_bandwidth * (period - elapsed)
1065          * write_bandwidth = ---------------------------------------------------
1066          *                                          period
1067          *
1068          * @written may have decreased due to account_page_redirty().
1069          * Avoid underflowing @bw calculation.
1070          */
1071         bw = written - min(written, wb->written_stamp);
1072         bw *= HZ;
1073         if (unlikely(elapsed > period)) {
1074                 do_div(bw, elapsed);
1075                 avg = bw;
1076                 goto out;
1077         }
1078         bw += (u64)wb->write_bandwidth * (period - elapsed);
1079         bw >>= ilog2(period);
1080 
1081         /*
1082          * one more level of smoothing, for filtering out sudden spikes
1083          */
1084         if (avg > old && old >= (unsigned long)bw)
1085                 avg -= (avg - old) >> 3;
1086 
1087         if (avg < old && old <= (unsigned long)bw)
1088                 avg += (old - avg) >> 3;
1089 
1090 out:
1091         /* keep avg > 0 to guarantee that tot > 0 if there are dirty wbs */
1092         avg = max(avg, 1LU);
1093         if (wb_has_dirty_io(wb)) {
1094                 long delta = avg - wb->avg_write_bandwidth;
1095                 WARN_ON_ONCE(atomic_long_add_return(delta,
1096                                         &wb->bdi->tot_write_bandwidth) <= 0);
1097         }
1098         wb->write_bandwidth = bw;
1099         wb->avg_write_bandwidth = avg;
1100 }
1101 
1102 static void update_dirty_limit(struct dirty_throttle_control *dtc)
1103 {
1104         struct wb_domain *dom = dtc_dom(dtc);
1105         unsigned long thresh = dtc->thresh;
1106         unsigned long limit = dom->dirty_limit;
1107 
1108         /*
1109          * Follow up in one step.
1110          */
1111         if (limit < thresh) {
1112                 limit = thresh;
1113                 goto update;
1114         }
1115 
1116         /*
1117          * Follow down slowly. Use the higher one as the target, because thresh
1118          * may drop below dirty. This is exactly the reason to introduce
1119          * dom->dirty_limit which is guaranteed to lie above the dirty pages.
1120          */
1121         thresh = max(thresh, dtc->dirty);
1122         if (limit > thresh) {
1123                 limit -= (limit - thresh) >> 5;
1124                 goto update;
1125         }
1126         return;
1127 update:
1128         dom->dirty_limit = limit;
1129 }
1130 
1131 static void domain_update_bandwidth(struct dirty_throttle_control *dtc,
1132                                     unsigned long now)
1133 {
1134         struct wb_domain *dom = dtc_dom(dtc);
1135 
1136         /*
1137          * check locklessly first to optimize away locking for the most time
1138          */
1139         if (time_before(now, dom->dirty_limit_tstamp + BANDWIDTH_INTERVAL))
1140                 return;
1141 
1142         spin_lock(&dom->lock);
1143         if (time_after_eq(now, dom->dirty_limit_tstamp + BANDWIDTH_INTERVAL)) {
1144                 update_dirty_limit(dtc);
1145                 dom->dirty_limit_tstamp = now;
1146         }
1147         spin_unlock(&dom->lock);
1148 }
1149 
1150 /*
1151  * Maintain wb->dirty_ratelimit, the base dirty throttle rate.
1152  *
1153  * Normal wb tasks will be curbed at or below it in long term.
1154  * Obviously it should be around (write_bw / N) when there are N dd tasks.
1155  */
1156 static void wb_update_dirty_ratelimit(struct dirty_throttle_control *dtc,
1157                                       unsigned long dirtied,
1158                                       unsigned long elapsed)
1159 {
1160         struct bdi_writeback *wb = dtc->wb;
1161         unsigned long dirty = dtc->dirty;
1162         unsigned long freerun = dirty_freerun_ceiling(dtc->thresh, dtc->bg_thresh);
1163         unsigned long limit = hard_dirty_limit(dtc_dom(dtc), dtc->thresh);
1164         unsigned long setpoint = (freerun + limit) / 2;
1165         unsigned long write_bw = wb->avg_write_bandwidth;
1166         unsigned long dirty_ratelimit = wb->dirty_ratelimit;
1167         unsigned long dirty_rate;
1168         unsigned long task_ratelimit;
1169         unsigned long balanced_dirty_ratelimit;
1170         unsigned long step;
1171         unsigned long x;
1172 
1173         /*
1174          * The dirty rate will match the writeout rate in long term, except
1175          * when dirty pages are truncated by userspace or re-dirtied by FS.
1176          */
1177         dirty_rate = (dirtied - wb->dirtied_stamp) * HZ / elapsed;
1178 
1179         /*
1180          * task_ratelimit reflects each dd's dirty rate for the past 200ms.
1181          */
1182         task_ratelimit = (u64)dirty_ratelimit *
1183                                         dtc->pos_ratio >> RATELIMIT_CALC_SHIFT;
1184         task_ratelimit++; /* it helps rampup dirty_ratelimit from tiny values */
1185 
1186         /*
1187          * A linear estimation of the "balanced" throttle rate. The theory is,
1188          * if there are N dd tasks, each throttled at task_ratelimit, the wb's
1189          * dirty_rate will be measured to be (N * task_ratelimit). So the below
1190          * formula will yield the balanced rate limit (write_bw / N).
1191          *
1192          * Note that the expanded form is not a pure rate feedback:
1193          *      rate_(i+1) = rate_(i) * (write_bw / dirty_rate)              (1)
1194          * but also takes pos_ratio into account:
1195          *      rate_(i+1) = rate_(i) * (write_bw / dirty_rate) * pos_ratio  (2)
1196          *
1197          * (1) is not realistic because pos_ratio also takes part in balancing
1198          * the dirty rate.  Consider the state
1199          *      pos_ratio = 0.5                                              (3)
1200          *      rate = 2 * (write_bw / N)                                    (4)
1201          * If (1) is used, it will stuck in that state! Because each dd will
1202          * be throttled at
1203          *      task_ratelimit = pos_ratio * rate = (write_bw / N)           (5)
1204          * yielding
1205          *      dirty_rate = N * task_ratelimit = write_bw                   (6)
1206          * put (6) into (1) we get
1207          *      rate_(i+1) = rate_(i)                                        (7)
1208          *
1209          * So we end up using (2) to always keep
1210          *      rate_(i+1) ~= (write_bw / N)                                 (8)
1211          * regardless of the value of pos_ratio. As long as (8) is satisfied,
1212          * pos_ratio is able to drive itself to 1.0, which is not only where
1213          * the dirty count meet the setpoint, but also where the slope of
1214          * pos_ratio is most flat and hence task_ratelimit is least fluctuated.
1215          */
1216         balanced_dirty_ratelimit = div_u64((u64)task_ratelimit * write_bw,
1217                                            dirty_rate | 1);
1218         /*
1219          * balanced_dirty_ratelimit ~= (write_bw / N) <= write_bw
1220          */
1221         if (unlikely(balanced_dirty_ratelimit > write_bw))
1222                 balanced_dirty_ratelimit = write_bw;
1223 
1224         /*
1225          * We could safely do this and return immediately:
1226          *
1227          *      wb->dirty_ratelimit = balanced_dirty_ratelimit;
1228          *
1229          * However to get a more stable dirty_ratelimit, the below elaborated
1230          * code makes use of task_ratelimit to filter out singular points and
1231          * limit the step size.
1232          *
1233          * The below code essentially only uses the relative value of
1234          *
1235          *      task_ratelimit - dirty_ratelimit
1236          *      = (pos_ratio - 1) * dirty_ratelimit
1237          *
1238          * which reflects the direction and size of dirty position error.
1239          */
1240 
1241         /*
1242          * dirty_ratelimit will follow balanced_dirty_ratelimit iff
1243          * task_ratelimit is on the same side of dirty_ratelimit, too.
1244          * For example, when
1245          * - dirty_ratelimit > balanced_dirty_ratelimit
1246          * - dirty_ratelimit > task_ratelimit (dirty pages are above setpoint)
1247          * lowering dirty_ratelimit will help meet both the position and rate
1248          * control targets. Otherwise, don't update dirty_ratelimit if it will
1249          * only help meet the rate target. After all, what the users ultimately
1250          * feel and care are stable dirty rate and small position error.
1251          *
1252          * |task_ratelimit - dirty_ratelimit| is used to limit the step size
1253          * and filter out the singular points of balanced_dirty_ratelimit. Which
1254          * keeps jumping around randomly and can even leap far away at times
1255          * due to the small 200ms estimation period of dirty_rate (we want to
1256          * keep that period small to reduce time lags).
1257          */
1258         step = 0;
1259 
1260         /*
1261          * For strictlimit case, calculations above were based on wb counters
1262          * and limits (starting from pos_ratio = wb_position_ratio() and up to
1263          * balanced_dirty_ratelimit = task_ratelimit * write_bw / dirty_rate).
1264          * Hence, to calculate "step" properly, we have to use wb_dirty as
1265          * "dirty" and wb_setpoint as "setpoint".
1266          *
1267          * We rampup dirty_ratelimit forcibly if wb_dirty is low because
1268          * it's possible that wb_thresh is close to zero due to inactivity
1269          * of backing device.
1270          */
1271         if (unlikely(wb->bdi->capabilities & BDI_CAP_STRICTLIMIT)) {
1272                 dirty = dtc->wb_dirty;
1273                 if (dtc->wb_dirty < 8)
1274                         setpoint = dtc->wb_dirty + 1;
1275                 else
1276                         setpoint = (dtc->wb_thresh + dtc->wb_bg_thresh) / 2;
1277         }
1278 
1279         if (dirty < setpoint) {
1280                 x = min3(wb->balanced_dirty_ratelimit,
1281                          balanced_dirty_ratelimit, task_ratelimit);
1282                 if (dirty_ratelimit < x)
1283                         step = x - dirty_ratelimit;
1284         } else {
1285                 x = max3(wb->balanced_dirty_ratelimit,
1286                          balanced_dirty_ratelimit, task_ratelimit);
1287                 if (dirty_ratelimit > x)
1288                         step = dirty_ratelimit - x;
1289         }
1290 
1291         /*
1292          * Don't pursue 100% rate matching. It's impossible since the balanced
1293          * rate itself is constantly fluctuating. So decrease the track speed
1294          * when it gets close to the target. Helps eliminate pointless tremors.
1295          */
1296         step >>= dirty_ratelimit / (2 * step + 1);
1297         /*
1298          * Limit the tracking speed to avoid overshooting.
1299          */
1300         step = (step + 7) / 8;
1301 
1302         if (dirty_ratelimit < balanced_dirty_ratelimit)
1303                 dirty_ratelimit += step;
1304         else
1305                 dirty_ratelimit -= step;
1306 
1307         wb->dirty_ratelimit = max(dirty_ratelimit, 1UL);
1308         wb->balanced_dirty_ratelimit = balanced_dirty_ratelimit;
1309 
1310         trace_bdi_dirty_ratelimit(wb, dirty_rate, task_ratelimit);
1311 }
1312 
1313 static void __wb_update_bandwidth(struct dirty_throttle_control *gdtc,
1314                                   struct dirty_throttle_control *mdtc,
1315                                   unsigned long start_time,
1316                                   bool update_ratelimit)
1317 {
1318         struct bdi_writeback *wb = gdtc->wb;
1319         unsigned long now = jiffies;
1320         unsigned long elapsed = now - wb->bw_time_stamp;
1321         unsigned long dirtied;
1322         unsigned long written;
1323 
1324         lockdep_assert_held(&wb->list_lock);
1325 
1326         /*
1327          * rate-limit, only update once every 200ms.
1328          */
1329         if (elapsed < BANDWIDTH_INTERVAL)
1330                 return;
1331 
1332         dirtied = percpu_counter_read(&wb->stat[WB_DIRTIED]);
1333         written = percpu_counter_read(&wb->stat[WB_WRITTEN]);
1334 
1335         /*
1336          * Skip quiet periods when disk bandwidth is under-utilized.
1337          * (at least 1s idle time between two flusher runs)
1338          */
1339         if (elapsed > HZ && time_before(wb->bw_time_stamp, start_time))
1340                 goto snapshot;
1341 
1342         if (update_ratelimit) {
1343                 domain_update_bandwidth(gdtc, now);
1344                 wb_update_dirty_ratelimit(gdtc, dirtied, elapsed);
1345 
1346                 /*
1347                  * @mdtc is always NULL if !CGROUP_WRITEBACK but the
1348                  * compiler has no way to figure that out.  Help it.
1349                  */
1350                 if (IS_ENABLED(CONFIG_CGROUP_WRITEBACK) && mdtc) {
1351                         domain_update_bandwidth(mdtc, now);
1352                         wb_update_dirty_ratelimit(mdtc, dirtied, elapsed);
1353                 }
1354         }
1355         wb_update_write_bandwidth(wb, elapsed, written);
1356 
1357 snapshot:
1358         wb->dirtied_stamp = dirtied;
1359         wb->written_stamp = written;
1360         wb->bw_time_stamp = now;
1361 }
1362 
1363 void wb_update_bandwidth(struct bdi_writeback *wb, unsigned long start_time)
1364 {
1365         struct dirty_throttle_control gdtc = { GDTC_INIT(wb) };
1366 
1367         __wb_update_bandwidth(&gdtc, NULL, start_time, false);
1368 }
1369 
1370 /*
1371  * After a task dirtied this many pages, balance_dirty_pages_ratelimited()
1372  * will look to see if it needs to start dirty throttling.
1373  *
1374  * If dirty_poll_interval is too low, big NUMA machines will call the expensive
1375  * global_page_state() too often. So scale it near-sqrt to the safety margin
1376  * (the number of pages we may dirty without exceeding the dirty limits).
1377  */
1378 static unsigned long dirty_poll_interval(unsigned long dirty,
1379                                          unsigned long thresh)
1380 {
1381         if (thresh > dirty)
1382                 return 1UL << (ilog2(thresh - dirty) >> 1);
1383 
1384         return 1;
1385 }
1386 
1387 static unsigned long wb_max_pause(struct bdi_writeback *wb,
1388                                   unsigned long wb_dirty)
1389 {
1390         unsigned long bw = wb->avg_write_bandwidth;
1391         unsigned long t;
1392 
1393         /*
1394          * Limit pause time for small memory systems. If sleeping for too long
1395          * time, a small pool of dirty/writeback pages may go empty and disk go
1396          * idle.
1397          *
1398          * 8 serves as the safety ratio.
1399          */
1400         t = wb_dirty / (1 + bw / roundup_pow_of_two(1 + HZ / 8));
1401         t++;
1402 
1403         return min_t(unsigned long, t, MAX_PAUSE);
1404 }
1405 
1406 static long wb_min_pause(struct bdi_writeback *wb,
1407                          long max_pause,
1408                          unsigned long task_ratelimit,
1409                          unsigned long dirty_ratelimit,
1410                          int *nr_dirtied_pause)
1411 {
1412         long hi = ilog2(wb->avg_write_bandwidth);
1413         long lo = ilog2(wb->dirty_ratelimit);
1414         long t;         /* target pause */
1415         long pause;     /* estimated next pause */
1416         int pages;      /* target nr_dirtied_pause */
1417 
1418         /* target for 10ms pause on 1-dd case */
1419         t = max(1, HZ / 100);
1420 
1421         /*
1422          * Scale up pause time for concurrent dirtiers in order to reduce CPU
1423          * overheads.
1424          *
1425          * (N * 10ms) on 2^N concurrent tasks.
1426          */
1427         if (hi > lo)
1428                 t += (hi - lo) * (10 * HZ) / 1024;
1429 
1430         /*
1431          * This is a bit convoluted. We try to base the next nr_dirtied_pause
1432          * on the much more stable dirty_ratelimit. However the next pause time
1433          * will be computed based on task_ratelimit and the two rate limits may
1434          * depart considerably at some time. Especially if task_ratelimit goes
1435          * below dirty_ratelimit/2 and the target pause is max_pause, the next
1436          * pause time will be max_pause*2 _trimmed down_ to max_pause.  As a
1437          * result task_ratelimit won't be executed faithfully, which could
1438          * eventually bring down dirty_ratelimit.
1439          *
1440          * We apply two rules to fix it up:
1441          * 1) try to estimate the next pause time and if necessary, use a lower
1442          *    nr_dirtied_pause so as not to exceed max_pause. When this happens,
1443          *    nr_dirtied_pause will be "dancing" with task_ratelimit.
1444          * 2) limit the target pause time to max_pause/2, so that the normal
1445          *    small fluctuations of task_ratelimit won't trigger rule (1) and
1446          *    nr_dirtied_pause will remain as stable as dirty_ratelimit.
1447          */
1448         t = min(t, 1 + max_pause / 2);
1449         pages = dirty_ratelimit * t / roundup_pow_of_two(HZ);
1450 
1451         /*
1452          * Tiny nr_dirtied_pause is found to hurt I/O performance in the test
1453          * case fio-mmap-randwrite-64k, which does 16*{sync read, async write}.
1454          * When the 16 consecutive reads are often interrupted by some dirty
1455          * throttling pause during the async writes, cfq will go into idles
1456          * (deadline is fine). So push nr_dirtied_pause as high as possible
1457          * until reaches DIRTY_POLL_THRESH=32 pages.
1458          */
1459         if (pages < DIRTY_POLL_THRESH) {
1460                 t = max_pause;
1461                 pages = dirty_ratelimit * t / roundup_pow_of_two(HZ);
1462                 if (pages > DIRTY_POLL_THRESH) {
1463                         pages = DIRTY_POLL_THRESH;
1464                         t = HZ * DIRTY_POLL_THRESH / dirty_ratelimit;
1465                 }
1466         }
1467 
1468         pause = HZ * pages / (task_ratelimit + 1);
1469         if (pause > max_pause) {
1470                 t = max_pause;
1471                 pages = task_ratelimit * t / roundup_pow_of_two(HZ);
1472         }
1473 
1474         *nr_dirtied_pause = pages;
1475         /*
1476          * The minimal pause time will normally be half the target pause time.
1477          */
1478         return pages >= DIRTY_POLL_THRESH ? 1 + t / 2 : t;
1479 }
1480 
1481 static inline void wb_dirty_limits(struct dirty_throttle_control *dtc)
1482 {
1483         struct bdi_writeback *wb = dtc->wb;
1484         unsigned long wb_reclaimable;
1485 
1486         /*
1487          * wb_thresh is not treated as some limiting factor as
1488          * dirty_thresh, due to reasons
1489          * - in JBOD setup, wb_thresh can fluctuate a lot
1490          * - in a system with HDD and USB key, the USB key may somehow
1491          *   go into state (wb_dirty >> wb_thresh) either because
1492          *   wb_dirty starts high, or because wb_thresh drops low.
1493          *   In this case we don't want to hard throttle the USB key
1494          *   dirtiers for 100 seconds until wb_dirty drops under
1495          *   wb_thresh. Instead the auxiliary wb control line in
1496          *   wb_position_ratio() will let the dirtier task progress
1497          *   at some rate <= (write_bw / 2) for bringing down wb_dirty.
1498          */
1499         dtc->wb_thresh = __wb_calc_thresh(dtc);
1500         dtc->wb_bg_thresh = dtc->thresh ?
1501                 div_u64((u64)dtc->wb_thresh * dtc->bg_thresh, dtc->thresh) : 0;
1502 
1503         /*
1504          * In order to avoid the stacked BDI deadlock we need
1505          * to ensure we accurately count the 'dirty' pages when
1506          * the threshold is low.
1507          *
1508          * Otherwise it would be possible to get thresh+n pages
1509          * reported dirty, even though there are thresh-m pages
1510          * actually dirty; with m+n sitting in the percpu
1511          * deltas.
1512          */
1513         if (dtc->wb_thresh < 2 * wb_stat_error(wb)) {
1514                 wb_reclaimable = wb_stat_sum(wb, WB_RECLAIMABLE);
1515                 dtc->wb_dirty = wb_reclaimable + wb_stat_sum(wb, WB_WRITEBACK);
1516         } else {
1517                 wb_reclaimable = wb_stat(wb, WB_RECLAIMABLE);
1518                 dtc->wb_dirty = wb_reclaimable + wb_stat(wb, WB_WRITEBACK);
1519         }
1520 }
1521 
1522 /*
1523  * balance_dirty_pages() must be called by processes which are generating dirty
1524  * data.  It looks at the number of dirty pages in the machine and will force
1525  * the caller to wait once crossing the (background_thresh + dirty_thresh) / 2.
1526  * If we're over `background_thresh' then the writeback threads are woken to
1527  * perform some writeout.
1528  */
1529 static void balance_dirty_pages(struct address_space *mapping,
1530                                 struct bdi_writeback *wb,
1531                                 unsigned long pages_dirtied)
1532 {
1533         struct dirty_throttle_control gdtc_stor = { GDTC_INIT(wb) };
1534         struct dirty_throttle_control mdtc_stor = { MDTC_INIT(wb, &gdtc_stor) };
1535         struct dirty_throttle_control * const gdtc = &gdtc_stor;
1536         struct dirty_throttle_control * const mdtc = mdtc_valid(&mdtc_stor) ?
1537                                                      &mdtc_stor : NULL;
1538         struct dirty_throttle_control *sdtc;
1539         unsigned long nr_reclaimable;   /* = file_dirty + unstable_nfs */
1540         long period;
1541         long pause;
1542         long max_pause;
1543         long min_pause;
1544         int nr_dirtied_pause;
1545         bool dirty_exceeded = false;
1546         unsigned long task_ratelimit;
1547         unsigned long dirty_ratelimit;
1548         struct backing_dev_info *bdi = wb->bdi;
1549         bool strictlimit = bdi->capabilities & BDI_CAP_STRICTLIMIT;
1550         unsigned long start_time = jiffies;
1551 
1552         for (;;) {
1553                 unsigned long now = jiffies;
1554                 unsigned long dirty, thresh, bg_thresh;
1555                 unsigned long m_dirty = 0;      /* stop bogus uninit warnings */
1556                 unsigned long m_thresh = 0;
1557                 unsigned long m_bg_thresh = 0;
1558 
1559                 /*
1560                  * Unstable writes are a feature of certain networked
1561                  * filesystems (i.e. NFS) in which data may have been
1562                  * written to the server's write cache, but has not yet
1563                  * been flushed to permanent storage.
1564                  */
1565                 nr_reclaimable = global_page_state(NR_FILE_DIRTY) +
1566                                         global_page_state(NR_UNSTABLE_NFS);
1567                 gdtc->avail = global_dirtyable_memory();
1568                 gdtc->dirty = nr_reclaimable + global_page_state(NR_WRITEBACK);
1569 
1570                 domain_dirty_limits(gdtc);
1571 
1572                 if (unlikely(strictlimit)) {
1573                         wb_dirty_limits(gdtc);
1574 
1575                         dirty = gdtc->wb_dirty;
1576                         thresh = gdtc->wb_thresh;
1577                         bg_thresh = gdtc->wb_bg_thresh;
1578                 } else {
1579                         dirty = gdtc->dirty;
1580                         thresh = gdtc->thresh;
1581                         bg_thresh = gdtc->bg_thresh;
1582                 }
1583 
1584                 if (mdtc) {
1585                         unsigned long filepages, headroom, writeback;
1586 
1587                         /*
1588                          * If @wb belongs to !root memcg, repeat the same
1589                          * basic calculations for the memcg domain.
1590                          */
1591                         mem_cgroup_wb_stats(wb, &filepages, &headroom,
1592                                             &mdtc->dirty, &writeback);
1593                         mdtc->dirty += writeback;
1594                         mdtc_calc_avail(mdtc, filepages, headroom);
1595 
1596                         domain_dirty_limits(mdtc);
1597 
1598                         if (unlikely(strictlimit)) {
1599                                 wb_dirty_limits(mdtc);
1600                                 m_dirty = mdtc->wb_dirty;
1601                                 m_thresh = mdtc->wb_thresh;
1602                                 m_bg_thresh = mdtc->wb_bg_thresh;
1603                         } else {
1604                                 m_dirty = mdtc->dirty;
1605                                 m_thresh = mdtc->thresh;
1606                                 m_bg_thresh = mdtc->bg_thresh;
1607                         }
1608                 }
1609 
1610                 /*
1611                  * Throttle it only when the background writeback cannot
1612                  * catch-up. This avoids (excessively) small writeouts
1613                  * when the wb limits are ramping up in case of !strictlimit.
1614                  *
1615                  * In strictlimit case make decision based on the wb counters
1616                  * and limits. Small writeouts when the wb limits are ramping
1617                  * up are the price we consciously pay for strictlimit-ing.
1618                  *
1619                  * If memcg domain is in effect, @dirty should be under
1620                  * both global and memcg freerun ceilings.
1621                  */
1622                 if (dirty <= dirty_freerun_ceiling(thresh, bg_thresh) &&
1623                     (!mdtc ||
1624                      m_dirty <= dirty_freerun_ceiling(m_thresh, m_bg_thresh))) {
1625                         unsigned long intv = dirty_poll_interval(dirty, thresh);
1626                         unsigned long m_intv = ULONG_MAX;
1627 
1628                         current->dirty_paused_when = now;
1629                         current->nr_dirtied = 0;
1630                         if (mdtc)
1631                                 m_intv = dirty_poll_interval(m_dirty, m_thresh);
1632                         current->nr_dirtied_pause = min(intv, m_intv);
1633                         break;
1634                 }
1635 
1636                 if (unlikely(!writeback_in_progress(wb)))
1637                         wb_start_background_writeback(wb);
1638 
1639                 /*
1640                  * Calculate global domain's pos_ratio and select the
1641                  * global dtc by default.
1642                  */
1643                 if (!strictlimit)
1644                         wb_dirty_limits(gdtc);
1645 
1646                 dirty_exceeded = (gdtc->wb_dirty > gdtc->wb_thresh) &&
1647                         ((gdtc->dirty > gdtc->thresh) || strictlimit);
1648 
1649                 wb_position_ratio(gdtc);
1650                 sdtc = gdtc;
1651 
1652                 if (mdtc) {
1653                         /*
1654                          * If memcg domain is in effect, calculate its
1655                          * pos_ratio.  @wb should satisfy constraints from
1656                          * both global and memcg domains.  Choose the one
1657                          * w/ lower pos_ratio.
1658                          */
1659                         if (!strictlimit)
1660                                 wb_dirty_limits(mdtc);
1661 
1662                         dirty_exceeded |= (mdtc->wb_dirty > mdtc->wb_thresh) &&
1663                                 ((mdtc->dirty > mdtc->thresh) || strictlimit);
1664 
1665                         wb_position_ratio(mdtc);
1666                         if (mdtc->pos_ratio < gdtc->pos_ratio)
1667                                 sdtc = mdtc;
1668                 }
1669 
1670                 if (dirty_exceeded && !wb->dirty_exceeded)
1671                         wb->dirty_exceeded = 1;
1672 
1673                 if (time_is_before_jiffies(wb->bw_time_stamp +
1674                                            BANDWIDTH_INTERVAL)) {
1675                         spin_lock(&wb->list_lock);
1676                         __wb_update_bandwidth(gdtc, mdtc, start_time, true);
1677                         spin_unlock(&wb->list_lock);
1678                 }
1679 
1680                 /* throttle according to the chosen dtc */
1681                 dirty_ratelimit = wb->dirty_ratelimit;
1682                 task_ratelimit = ((u64)dirty_ratelimit * sdtc->pos_ratio) >>
1683                                                         RATELIMIT_CALC_SHIFT;
1684                 max_pause = wb_max_pause(wb, sdtc->wb_dirty);
1685                 min_pause = wb_min_pause(wb, max_pause,
1686                                          task_ratelimit, dirty_ratelimit,
1687                                          &nr_dirtied_pause);
1688 
1689                 if (unlikely(task_ratelimit == 0)) {
1690                         period = max_pause;
1691                         pause = max_pause;
1692                         goto pause;
1693                 }
1694                 period = HZ * pages_dirtied / task_ratelimit;
1695                 pause = period;
1696                 if (current->dirty_paused_when)
1697                         pause -= now - current->dirty_paused_when;
1698                 /*
1699                  * For less than 1s think time (ext3/4 may block the dirtier
1700                  * for up to 800ms from time to time on 1-HDD; so does xfs,
1701                  * however at much less frequency), try to compensate it in
1702                  * future periods by updating the virtual time; otherwise just
1703                  * do a reset, as it may be a light dirtier.
1704                  */
1705                 if (pause < min_pause) {
1706                         trace_balance_dirty_pages(wb,
1707                                                   sdtc->thresh,
1708                                                   sdtc->bg_thresh,
1709                                                   sdtc->dirty,
1710                                                   sdtc->wb_thresh,
1711                                                   sdtc->wb_dirty,
1712                                                   dirty_ratelimit,
1713                                                   task_ratelimit,
1714                                                   pages_dirtied,
1715                                                   period,
1716                                                   min(pause, 0L),
1717                                                   start_time);
1718                         if (pause < -HZ) {
1719                                 current->dirty_paused_when = now;
1720                                 current->nr_dirtied = 0;
1721                         } else if (period) {
1722                                 current->dirty_paused_when += period;
1723                                 current->nr_dirtied = 0;
1724                         } else if (current->nr_dirtied_pause <= pages_dirtied)
1725                                 current->nr_dirtied_pause += pages_dirtied;
1726                         break;
1727                 }
1728                 if (unlikely(pause > max_pause)) {
1729                         /* for occasional dropped task_ratelimit */
1730                         now += min(pause - max_pause, max_pause);
1731                         pause = max_pause;
1732                 }
1733 
1734 pause:
1735                 trace_balance_dirty_pages(wb,
1736                                           sdtc->thresh,
1737                                           sdtc->bg_thresh,
1738                                           sdtc->dirty,
1739                                           sdtc->wb_thresh,
1740                                           sdtc->wb_dirty,
1741                                           dirty_ratelimit,
1742                                           task_ratelimit,
1743                                           pages_dirtied,
1744                                           period,
1745                                           pause,
1746                                           start_time);
1747                 __set_current_state(TASK_KILLABLE);
1748                 io_schedule_timeout(pause);
1749 
1750                 current->dirty_paused_when = now + pause;
1751                 current->nr_dirtied = 0;
1752                 current->nr_dirtied_pause = nr_dirtied_pause;
1753 
1754                 /*
1755                  * This is typically equal to (dirty < thresh) and can also
1756                  * keep "1000+ dd on a slow USB stick" under control.
1757                  */
1758                 if (task_ratelimit)
1759                         break;
1760 
1761                 /*
1762                  * In the case of an unresponding NFS server and the NFS dirty
1763                  * pages exceeds dirty_thresh, give the other good wb's a pipe
1764                  * to go through, so that tasks on them still remain responsive.
1765                  *
1766                  * In theory 1 page is enough to keep the comsumer-producer
1767                  * pipe going: the flusher cleans 1 page => the task dirties 1
1768                  * more page. However wb_dirty has accounting errors.  So use
1769                  * the larger and more IO friendly wb_stat_error.
1770                  */
1771                 if (sdtc->wb_dirty <= wb_stat_error(wb))
1772                         break;
1773 
1774                 if (fatal_signal_pending(current))
1775                         break;
1776         }
1777 
1778         if (!dirty_exceeded && wb->dirty_exceeded)
1779                 wb->dirty_exceeded = 0;
1780 
1781         if (writeback_in_progress(wb))
1782                 return;
1783 
1784         /*
1785          * In laptop mode, we wait until hitting the higher threshold before
1786          * starting background writeout, and then write out all the way down
1787          * to the lower threshold.  So slow writers cause minimal disk activity.
1788          *
1789          * In normal mode, we start background writeout at the lower
1790          * background_thresh, to keep the amount of dirty memory low.
1791          */
1792         if (laptop_mode)
1793                 return;
1794 
1795         if (nr_reclaimable > gdtc->bg_thresh)
1796                 wb_start_background_writeback(wb);
1797 }
1798 
1799 static DEFINE_PER_CPU(int, bdp_ratelimits);
1800 
1801 /*
1802  * Normal tasks are throttled by
1803  *      loop {
1804  *              dirty tsk->nr_dirtied_pause pages;
1805  *              take a snap in balance_dirty_pages();
1806  *      }
1807  * However there is a worst case. If every task exit immediately when dirtied
1808  * (tsk->nr_dirtied_pause - 1) pages, balance_dirty_pages() will never be
1809  * called to throttle the page dirties. The solution is to save the not yet
1810  * throttled page dirties in dirty_throttle_leaks on task exit and charge them
1811  * randomly into the running tasks. This works well for the above worst case,
1812  * as the new task will pick up and accumulate the old task's leaked dirty
1813  * count and eventually get throttled.
1814  */
1815 DEFINE_PER_CPU(int, dirty_throttle_leaks) = 0;
1816 
1817 /**
1818  * balance_dirty_pages_ratelimited - balance dirty memory state
1819  * @mapping: address_space which was dirtied
1820  *
1821  * Processes which are dirtying memory should call in here once for each page
1822  * which was newly dirtied.  The function will periodically check the system's
1823  * dirty state and will initiate writeback if needed.
1824  *
1825  * On really big machines, get_writeback_state is expensive, so try to avoid
1826  * calling it too often (ratelimiting).  But once we're over the dirty memory
1827  * limit we decrease the ratelimiting by a lot, to prevent individual processes
1828  * from overshooting the limit by (ratelimit_pages) each.
1829  */
1830 void balance_dirty_pages_ratelimited(struct address_space *mapping)
1831 {
1832         struct inode *inode = mapping->host;
1833         struct backing_dev_info *bdi = inode_to_bdi(inode);
1834         struct bdi_writeback *wb = NULL;
1835         int ratelimit;
1836         int *p;
1837 
1838         if (!bdi_cap_account_dirty(bdi))
1839                 return;
1840 
1841         if (inode_cgwb_enabled(inode))
1842                 wb = wb_get_create_current(bdi, GFP_KERNEL);
1843         if (!wb)
1844                 wb = &bdi->wb;
1845 
1846         ratelimit = current->nr_dirtied_pause;
1847         if (wb->dirty_exceeded)
1848                 ratelimit = min(ratelimit, 32 >> (PAGE_SHIFT - 10));
1849 
1850         preempt_disable();
1851         /*
1852          * This prevents one CPU to accumulate too many dirtied pages without
1853          * calling into balance_dirty_pages(), which can happen when there are
1854          * 1000+ tasks, all of them start dirtying pages at exactly the same
1855          * time, hence all honoured too large initial task->nr_dirtied_pause.
1856          */
1857         p =  this_cpu_ptr(&bdp_ratelimits);
1858         if (unlikely(current->nr_dirtied >= ratelimit))
1859                 *p = 0;
1860         else if (unlikely(*p >= ratelimit_pages)) {
1861                 *p = 0;
1862                 ratelimit = 0;
1863         }
1864         /*
1865          * Pick up the dirtied pages by the exited tasks. This avoids lots of
1866          * short-lived tasks (eg. gcc invocations in a kernel build) escaping
1867          * the dirty throttling and livelock other long-run dirtiers.
1868          */
1869         p = this_cpu_ptr(&dirty_throttle_leaks);
1870         if (*p > 0 && current->nr_dirtied < ratelimit) {
1871                 unsigned long nr_pages_dirtied;
1872                 nr_pages_dirtied = min(*p, ratelimit - current->nr_dirtied);
1873                 *p -= nr_pages_dirtied;
1874                 current->nr_dirtied += nr_pages_dirtied;
1875         }
1876         preempt_enable();
1877 
1878         if (unlikely(current->nr_dirtied >= ratelimit))
1879                 balance_dirty_pages(mapping, wb, current->nr_dirtied);
1880 
1881         wb_put(wb);
1882 }
1883 EXPORT_SYMBOL(balance_dirty_pages_ratelimited);
1884 
1885 /**
1886  * wb_over_bg_thresh - does @wb need to be written back?
1887  * @wb: bdi_writeback of interest
1888  *
1889  * Determines whether background writeback should keep writing @wb or it's
1890  * clean enough.  Returns %true if writeback should continue.
1891  */
1892 bool wb_over_bg_thresh(struct bdi_writeback *wb)
1893 {
1894         struct dirty_throttle_control gdtc_stor = { GDTC_INIT(wb) };
1895         struct dirty_throttle_control mdtc_stor = { MDTC_INIT(wb, &gdtc_stor) };
1896         struct dirty_throttle_control * const gdtc = &gdtc_stor;
1897         struct dirty_throttle_control * const mdtc = mdtc_valid(&mdtc_stor) ?
1898                                                      &mdtc_stor : NULL;
1899 
1900         /*
1901          * Similar to balance_dirty_pages() but ignores pages being written
1902          * as we're trying to decide whether to put more under writeback.
1903          */
1904         gdtc->avail = global_dirtyable_memory();
1905         gdtc->dirty = global_page_state(NR_FILE_DIRTY) +
1906                       global_page_state(NR_UNSTABLE_NFS);
1907         domain_dirty_limits(gdtc);
1908 
1909         if (gdtc->dirty > gdtc->bg_thresh)
1910                 return true;
1911 
1912         if (wb_stat(wb, WB_RECLAIMABLE) > __wb_calc_thresh(gdtc))
1913                 return true;
1914 
1915         if (mdtc) {
1916                 unsigned long filepages, headroom, writeback;
1917 
1918                 mem_cgroup_wb_stats(wb, &filepages, &headroom, &mdtc->dirty,
1919                                     &writeback);
1920                 mdtc_calc_avail(mdtc, filepages, headroom);
1921                 domain_dirty_limits(mdtc);      /* ditto, ignore writeback */
1922 
1923                 if (mdtc->dirty > mdtc->bg_thresh)
1924                         return true;
1925 
1926                 if (wb_stat(wb, WB_RECLAIMABLE) > __wb_calc_thresh(mdtc))
1927                         return true;
1928         }
1929 
1930         return false;
1931 }
1932 
1933 void throttle_vm_writeout(gfp_t gfp_mask)
1934 {
1935         unsigned long background_thresh;
1936         unsigned long dirty_thresh;
1937 
1938         for ( ; ; ) {
1939                 global_dirty_limits(&background_thresh, &dirty_thresh);
1940                 dirty_thresh = hard_dirty_limit(&global_wb_domain, dirty_thresh);
1941 
1942                 /*
1943                  * Boost the allowable dirty threshold a bit for page
1944                  * allocators so they don't get DoS'ed by heavy writers
1945                  */
1946                 dirty_thresh += dirty_thresh / 10;      /* wheeee... */
1947 
1948                 if (global_page_state(NR_UNSTABLE_NFS) +
1949                         global_page_state(NR_WRITEBACK) <= dirty_thresh)
1950                                 break;
1951                 congestion_wait(BLK_RW_ASYNC, HZ/10);
1952 
1953                 /*
1954                  * The caller might hold locks which can prevent IO completion
1955                  * or progress in the filesystem.  So we cannot just sit here
1956                  * waiting for IO to complete.
1957                  */
1958                 if ((gfp_mask & (__GFP_FS|__GFP_IO)) != (__GFP_FS|__GFP_IO))
1959                         break;
1960         }
1961 }
1962 
1963 /*
1964  * sysctl handler for /proc/sys/vm/dirty_writeback_centisecs
1965  */
1966 int dirty_writeback_centisecs_handler(struct ctl_table *table, int write,
1967         void __user *buffer, size_t *length, loff_t *ppos)
1968 {
1969         proc_dointvec(table, write, buffer, length, ppos);
1970         return 0;
1971 }
1972 
1973 #ifdef CONFIG_BLOCK
1974 void laptop_mode_timer_fn(unsigned long data)
1975 {
1976         struct request_queue *q = (struct request_queue *)data;
1977         int nr_pages = global_page_state(NR_FILE_DIRTY) +
1978                 global_page_state(NR_UNSTABLE_NFS);
1979         struct bdi_writeback *wb;
1980 
1981         /*
1982          * We want to write everything out, not just down to the dirty
1983          * threshold
1984          */
1985         if (!bdi_has_dirty_io(&q->backing_dev_info))
1986                 return;
1987 
1988         rcu_read_lock();
1989         list_for_each_entry_rcu(wb, &q->backing_dev_info.wb_list, bdi_node)
1990                 if (wb_has_dirty_io(wb))
1991                         wb_start_writeback(wb, nr_pages, true,
1992                                            WB_REASON_LAPTOP_TIMER);
1993         rcu_read_unlock();
1994 }
1995 
1996 /*
1997  * We've spun up the disk and we're in laptop mode: schedule writeback
1998  * of all dirty data a few seconds from now.  If the flush is already scheduled
1999  * then push it back - the user is still using the disk.
2000  */
2001 void laptop_io_completion(struct backing_dev_info *info)
2002 {
2003         mod_timer(&info->laptop_mode_wb_timer, jiffies + laptop_mode);
2004 }
2005 
2006 /*
2007  * We're in laptop mode and we've just synced. The sync's writes will have
2008  * caused another writeback to be scheduled by laptop_io_completion.
2009  * Nothing needs to be written back anymore, so we unschedule the writeback.
2010  */
2011 void laptop_sync_completion(void)
2012 {
2013         struct backing_dev_info *bdi;
2014 
2015         rcu_read_lock();
2016 
2017         list_for_each_entry_rcu(bdi, &bdi_list, bdi_list)
2018                 del_timer(&bdi->laptop_mode_wb_timer);
2019 
2020         rcu_read_unlock();
2021 }
2022 #endif
2023 
2024 /*
2025  * If ratelimit_pages is too high then we can get into dirty-data overload
2026  * if a large number of processes all perform writes at the same time.
2027  * If it is too low then SMP machines will call the (expensive)
2028  * get_writeback_state too often.
2029  *
2030  * Here we set ratelimit_pages to a level which ensures that when all CPUs are
2031  * dirtying in parallel, we cannot go more than 3% (1/32) over the dirty memory
2032  * thresholds.
2033  */
2034 
2035 void writeback_set_ratelimit(void)
2036 {
2037         struct wb_domain *dom = &global_wb_domain;
2038         unsigned long background_thresh;
2039         unsigned long dirty_thresh;
2040 
2041         global_dirty_limits(&background_thresh, &dirty_thresh);
2042         dom->dirty_limit = dirty_thresh;
2043         ratelimit_pages = dirty_thresh / (num_online_cpus() * 32);
2044         if (ratelimit_pages < 16)
2045                 ratelimit_pages = 16;
2046 }
2047 
2048 static int
2049 ratelimit_handler(struct notifier_block *self, unsigned long action,
2050                   void *hcpu)
2051 {
2052 
2053         switch (action & ~CPU_TASKS_FROZEN) {
2054         case CPU_ONLINE:
2055         case CPU_DEAD:
2056                 writeback_set_ratelimit();
2057                 return NOTIFY_OK;
2058         default:
2059                 return NOTIFY_DONE;
2060         }
2061 }
2062 
2063 static struct notifier_block ratelimit_nb = {
2064         .notifier_call  = ratelimit_handler,
2065         .next           = NULL,
2066 };
2067 
2068 /*
2069  * Called early on to tune the page writeback dirty limits.
2070  *
2071  * We used to scale dirty pages according to how total memory
2072  * related to pages that could be allocated for buffers (by
2073  * comparing nr_free_buffer_pages() to vm_total_pages.
2074  *
2075  * However, that was when we used "dirty_ratio" to scale with
2076  * all memory, and we don't do that any more. "dirty_ratio"
2077  * is now applied to total non-HIGHPAGE memory (by subtracting
2078  * totalhigh_pages from vm_total_pages), and as such we can't
2079  * get into the old insane situation any more where we had
2080  * large amounts of dirty pages compared to a small amount of
2081  * non-HIGHMEM memory.
2082  *
2083  * But we might still want to scale the dirty_ratio by how
2084  * much memory the box has..
2085  */
2086 void __init page_writeback_init(void)
2087 {
2088         BUG_ON(wb_domain_init(&global_wb_domain, GFP_KERNEL));
2089 
2090         writeback_set_ratelimit();
2091         register_cpu_notifier(&ratelimit_nb);
2092 }
2093 
2094 /**
2095  * tag_pages_for_writeback - tag pages to be written by write_cache_pages
2096  * @mapping: address space structure to write
2097  * @start: starting page index
2098  * @end: ending page index (inclusive)
2099  *
2100  * This function scans the page range from @start to @end (inclusive) and tags
2101  * all pages that have DIRTY tag set with a special TOWRITE tag. The idea is
2102  * that write_cache_pages (or whoever calls this function) will then use
2103  * TOWRITE tag to identify pages eligible for writeback.  This mechanism is
2104  * used to avoid livelocking of writeback by a process steadily creating new
2105  * dirty pages in the file (thus it is important for this function to be quick
2106  * so that it can tag pages faster than a dirtying process can create them).
2107  */
2108 /*
2109  * We tag pages in batches of WRITEBACK_TAG_BATCH to reduce tree_lock latency.
2110  */
2111 void tag_pages_for_writeback(struct address_space *mapping,
2112                              pgoff_t start, pgoff_t end)
2113 {
2114 #define WRITEBACK_TAG_BATCH 4096
2115         unsigned long tagged;
2116 
2117         do {
2118                 spin_lock_irq(&mapping->tree_lock);
2119                 tagged = radix_tree_range_tag_if_tagged(&mapping->page_tree,
2120                                 &start, end, WRITEBACK_TAG_BATCH,
2121                                 PAGECACHE_TAG_DIRTY, PAGECACHE_TAG_TOWRITE);
2122                 spin_unlock_irq(&mapping->tree_lock);
2123                 WARN_ON_ONCE(tagged > WRITEBACK_TAG_BATCH);
2124                 cond_resched();
2125                 /* We check 'start' to handle wrapping when end == ~0UL */
2126         } while (tagged >= WRITEBACK_TAG_BATCH && start);
2127 }
2128 EXPORT_SYMBOL(tag_pages_for_writeback);
2129 
2130 /**
2131  * write_cache_pages - walk the list of dirty pages of the given address space and write all of them.
2132  * @mapping: address space structure to write
2133  * @wbc: subtract the number of written pages from *@wbc->nr_to_write
2134  * @writepage: function called for each page
2135  * @data: data passed to writepage function
2136  *
2137  * If a page is already under I/O, write_cache_pages() skips it, even
2138  * if it's dirty.  This is desirable behaviour for memory-cleaning writeback,
2139  * but it is INCORRECT for data-integrity system calls such as fsync().  fsync()
2140  * and msync() need to guarantee that all the data which was dirty at the time
2141  * the call was made get new I/O started against them.  If wbc->sync_mode is
2142  * WB_SYNC_ALL then we were called for data integrity and we must wait for
2143  * existing IO to complete.
2144  *
2145  * To avoid livelocks (when other process dirties new pages), we first tag
2146  * pages which should be written back with TOWRITE tag and only then start
2147  * writing them. For data-integrity sync we have to be careful so that we do
2148  * not miss some pages (e.g., because some other process has cleared TOWRITE
2149  * tag we set). The rule we follow is that TOWRITE tag can be cleared only
2150  * by the process clearing the DIRTY tag (and submitting the page for IO).
2151  */
2152 int write_cache_pages(struct address_space *mapping,
2153                       struct writeback_control *wbc, writepage_t writepage,
2154                       void *data)
2155 {
2156         int ret = 0;
2157         int done = 0;
2158         struct pagevec pvec;
2159         int nr_pages;
2160         pgoff_t uninitialized_var(writeback_index);
2161         pgoff_t index;
2162         pgoff_t end;            /* Inclusive */
2163         pgoff_t done_index;
2164         int cycled;
2165         int range_whole = 0;
2166         int tag;
2167 
2168         pagevec_init(&pvec, 0);
2169         if (wbc->range_cyclic) {
2170                 writeback_index = mapping->writeback_index; /* prev offset */
2171                 index = writeback_index;
2172                 if (index == 0)
2173                         cycled = 1;
2174                 else
2175                         cycled = 0;
2176                 end = -1;
2177         } else {
2178                 index = wbc->range_start >> PAGE_CACHE_SHIFT;
2179                 end = wbc->range_end >> PAGE_CACHE_SHIFT;
2180                 if (wbc->range_start == 0 && wbc->range_end == LLONG_MAX)
2181                         range_whole = 1;
2182                 cycled = 1; /* ignore range_cyclic tests */
2183         }
2184         if (wbc->sync_mode == WB_SYNC_ALL || wbc->tagged_writepages)
2185                 tag = PAGECACHE_TAG_TOWRITE;
2186         else
2187                 tag = PAGECACHE_TAG_DIRTY;
2188 retry:
2189         if (wbc->sync_mode == WB_SYNC_ALL || wbc->tagged_writepages)
2190                 tag_pages_for_writeback(mapping, index, end);
2191         done_index = index;
2192         while (!done && (index <= end)) {
2193                 int i;
2194 
2195                 nr_pages = pagevec_lookup_tag(&pvec, mapping, &index, tag,
2196                               min(end - index, (pgoff_t)PAGEVEC_SIZE-1) + 1);
2197                 if (nr_pages == 0)
2198                         break;
2199 
2200                 for (i = 0; i < nr_pages; i++) {
2201                         struct page *page = pvec.pages[i];
2202 
2203                         /*
2204                          * At this point, the page may be truncated or
2205                          * invalidated (changing page->mapping to NULL), or
2206                          * even swizzled back from swapper_space to tmpfs file
2207                          * mapping. However, page->index will not change
2208                          * because we have a reference on the page.
2209                          */
2210                         if (page->index > end) {
2211                                 /*
2212                                  * can't be range_cyclic (1st pass) because
2213                                  * end == -1 in that case.
2214                                  */
2215                                 done = 1;
2216                                 break;
2217                         }
2218 
2219                         done_index = page->index;
2220 
2221                         lock_page(page);
2222 
2223                         /*
2224                          * Page truncated or invalidated. We can freely skip it
2225                          * then, even for data integrity operations: the page
2226                          * has disappeared concurrently, so there could be no
2227                          * real expectation of this data interity operation
2228                          * even if there is now a new, dirty page at the same
2229                          * pagecache address.
2230                          */
2231                         if (unlikely(page->mapping != mapping)) {
2232 continue_unlock:
2233                                 unlock_page(page);
2234                                 continue;
2235                         }
2236 
2237                         if (!PageDirty(page)) {
2238                                 /* someone wrote it for us */
2239                                 goto continue_unlock;
2240                         }
2241 
2242                         if (PageWriteback(page)) {
2243                                 if (wbc->sync_mode != WB_SYNC_NONE)
2244                                         wait_on_page_writeback(page);
2245                                 else
2246                                         goto continue_unlock;
2247                         }
2248 
2249                         BUG_ON(PageWriteback(page));
2250                         if (!clear_page_dirty_for_io(page))
2251                                 goto continue_unlock;
2252 
2253                         trace_wbc_writepage(wbc, inode_to_bdi(mapping->host));
2254                         ret = (*writepage)(page, wbc, data);
2255                         if (unlikely(ret)) {
2256                                 if (ret == AOP_WRITEPAGE_ACTIVATE) {
2257                                         unlock_page(page);
2258                                         ret = 0;
2259                                 } else {
2260                                         /*
2261                                          * done_index is set past this page,
2262                                          * so media errors will not choke
2263                                          * background writeout for the entire
2264                                          * file. This has consequences for
2265                                          * range_cyclic semantics (ie. it may
2266                                          * not be suitable for data integrity
2267                                          * writeout).
2268                                          */
2269                                         done_index = page->index + 1;
2270                                         done = 1;
2271                                         break;
2272                                 }
2273                         }
2274 
2275                         /*
2276                          * We stop writing back only if we are not doing
2277                          * integrity sync. In case of integrity sync we have to
2278                          * keep going until we have written all the pages
2279                          * we tagged for writeback prior to entering this loop.
2280                          */
2281                         if (--wbc->nr_to_write <= 0 &&
2282                             wbc->sync_mode == WB_SYNC_NONE) {
2283                                 done = 1;
2284                                 break;
2285                         }
2286                 }
2287                 pagevec_release(&pvec);
2288                 cond_resched();
2289         }
2290         if (!cycled && !done) {
2291                 /*
2292                  * range_cyclic:
2293                  * We hit the last page and there is more work to be done: wrap
2294                  * back to the start of the file
2295                  */
2296                 cycled = 1;
2297                 index = 0;
2298                 end = writeback_index - 1;
2299                 goto retry;
2300         }
2301         if (wbc->range_cyclic || (range_whole && wbc->nr_to_write > 0))
2302                 mapping->writeback_index = done_index;
2303 
2304         return ret;
2305 }
2306 EXPORT_SYMBOL(write_cache_pages);
2307 
2308 /*
2309  * Function used by generic_writepages to call the real writepage
2310  * function and set the mapping flags on error
2311  */
2312 static int __writepage(struct page *page, struct writeback_control *wbc,
2313                        void *data)
2314 {
2315         struct address_space *mapping = data;
2316         int ret = mapping->a_ops->writepage(page, wbc);
2317         mapping_set_error(mapping, ret);
2318         return ret;
2319 }
2320 
2321 /**
2322  * generic_writepages - walk the list of dirty pages of the given address space and writepage() all of them.
2323  * @mapping: address space structure to write
2324  * @wbc: subtract the number of written pages from *@wbc->nr_to_write
2325  *
2326  * This is a library function, which implements the writepages()
2327  * address_space_operation.
2328  */
2329 int generic_writepages(struct address_space *mapping,
2330                        struct writeback_control *wbc)
2331 {
2332         struct blk_plug plug;
2333         int ret;
2334 
2335         /* deal with chardevs and other special file */
2336         if (!mapping->a_ops->writepage)
2337                 return 0;
2338 
2339         blk_start_plug(&plug);
2340         ret = write_cache_pages(mapping, wbc, __writepage, mapping);
2341         blk_finish_plug(&plug);
2342         return ret;
2343 }
2344 
2345 EXPORT_SYMBOL(generic_writepages);
2346 
2347 int do_writepages(struct address_space *mapping, struct writeback_control *wbc)
2348 {
2349         int ret;
2350 
2351         if (wbc->nr_to_write <= 0)
2352                 return 0;
2353         if (mapping->a_ops->writepages)
2354                 ret = mapping->a_ops->writepages(mapping, wbc);
2355         else
2356                 ret = generic_writepages(mapping, wbc);
2357         return ret;
2358 }
2359 
2360 /**
2361  * write_one_page - write out a single page and optionally wait on I/O
2362  * @page: the page to write
2363  * @wait: if true, wait on writeout
2364  *
2365  * The page must be locked by the caller and will be unlocked upon return.
2366  *
2367  * write_one_page() returns a negative error code if I/O failed.
2368  */
2369 int write_one_page(struct page *page, int wait)
2370 {
2371         struct address_space *mapping = page->mapping;
2372         int ret = 0;
2373         struct writeback_control wbc = {
2374                 .sync_mode = WB_SYNC_ALL,
2375                 .nr_to_write = 1,
2376         };
2377 
2378         BUG_ON(!PageLocked(page));
2379 
2380         if (wait)
2381                 wait_on_page_writeback(page);
2382 
2383         if (clear_page_dirty_for_io(page)) {
2384                 page_cache_get(page);
2385                 ret = mapping->a_ops->writepage(page, &wbc);
2386                 if (ret == 0 && wait) {
2387                         wait_on_page_writeback(page);
2388                         if (PageError(page))
2389                                 ret = -EIO;
2390                 }
2391                 page_cache_release(page);
2392         } else {
2393                 unlock_page(page);
2394         }
2395         return ret;
2396 }
2397 EXPORT_SYMBOL(write_one_page);
2398 
2399 /*
2400  * For address_spaces which do not use buffers nor write back.
2401  */
2402 int __set_page_dirty_no_writeback(struct page *page)
2403 {
2404         if (!PageDirty(page))
2405                 return !TestSetPageDirty(page);
2406         return 0;
2407 }
2408 
2409 /*
2410  * Helper function for set_page_dirty family.
2411  *
2412  * Caller must hold mem_cgroup_begin_page_stat().
2413  *
2414  * NOTE: This relies on being atomic wrt interrupts.
2415  */
2416 void account_page_dirtied(struct page *page, struct address_space *mapping,
2417                           struct mem_cgroup *memcg)
2418 {
2419         struct inode *inode = mapping->host;
2420 
2421         trace_writeback_dirty_page(page, mapping);
2422 
2423         if (mapping_cap_account_dirty(mapping)) {
2424                 struct bdi_writeback *wb;
2425 
2426                 inode_attach_wb(inode, page);
2427                 wb = inode_to_wb(inode);
2428 
2429                 mem_cgroup_inc_page_stat(memcg, MEM_CGROUP_STAT_DIRTY);
2430                 __inc_zone_page_state(page, NR_FILE_DIRTY);
2431                 __inc_zone_page_state(page, NR_DIRTIED);
2432                 __inc_wb_stat(wb, WB_RECLAIMABLE);
2433                 __inc_wb_stat(wb, WB_DIRTIED);
2434                 task_io_account_write(PAGE_CACHE_SIZE);
2435                 current->nr_dirtied++;
2436                 this_cpu_inc(bdp_ratelimits);
2437         }
2438 }
2439 EXPORT_SYMBOL(account_page_dirtied);
2440 
2441 /*
2442  * Helper function for deaccounting dirty page without writeback.
2443  *
2444  * Caller must hold mem_cgroup_begin_page_stat().
2445  */
2446 void account_page_cleaned(struct page *page, struct address_space *mapping,
2447                           struct mem_cgroup *memcg, struct bdi_writeback *wb)
2448 {
2449         if (mapping_cap_account_dirty(mapping)) {
2450                 mem_cgroup_dec_page_stat(memcg, MEM_CGROUP_STAT_DIRTY);
2451                 dec_zone_page_state(page, NR_FILE_DIRTY);
2452                 dec_wb_stat(wb, WB_RECLAIMABLE);
2453                 task_io_account_cancelled_write(PAGE_CACHE_SIZE);
2454         }
2455 }
2456 
2457 /*
2458  * For address_spaces which do not use buffers.  Just tag the page as dirty in
2459  * its radix tree.
2460  *
2461  * This is also used when a single buffer is being dirtied: we want to set the
2462  * page dirty in that case, but not all the buffers.  This is a "bottom-up"
2463  * dirtying, whereas __set_page_dirty_buffers() is a "top-down" dirtying.
2464  *
2465  * The caller must ensure this doesn't race with truncation.  Most will simply
2466  * hold the page lock, but e.g. zap_pte_range() calls with the page mapped and
2467  * the pte lock held, which also locks out truncation.
2468  */
2469 int __set_page_dirty_nobuffers(struct page *page)
2470 {
2471         struct mem_cgroup *memcg;
2472 
2473         memcg = mem_cgroup_begin_page_stat(page);
2474         if (!TestSetPageDirty(page)) {
2475                 struct address_space *mapping = page_mapping(page);
2476                 unsigned long flags;
2477 
2478                 if (!mapping) {
2479                         mem_cgroup_end_page_stat(memcg);
2480                         return 1;
2481                 }
2482 
2483                 spin_lock_irqsave(&mapping->tree_lock, flags);
2484                 BUG_ON(page_mapping(page) != mapping);
2485                 WARN_ON_ONCE(!PagePrivate(page) && !PageUptodate(page));
2486                 account_page_dirtied(page, mapping, memcg);
2487                 radix_tree_tag_set(&mapping->page_tree, page_index(page),
2488                                    PAGECACHE_TAG_DIRTY);
2489                 spin_unlock_irqrestore(&mapping->tree_lock, flags);
2490                 mem_cgroup_end_page_stat(memcg);
2491 
2492                 if (mapping->host) {
2493                         /* !PageAnon && !swapper_space */
2494                         __mark_inode_dirty(mapping->host, I_DIRTY_PAGES);
2495                 }
2496                 return 1;
2497         }
2498         mem_cgroup_end_page_stat(memcg);
2499         return 0;
2500 }
2501 EXPORT_SYMBOL(__set_page_dirty_nobuffers);
2502 
2503 /*
2504  * Call this whenever redirtying a page, to de-account the dirty counters
2505  * (NR_DIRTIED, BDI_DIRTIED, tsk->nr_dirtied), so that they match the written
2506  * counters (NR_WRITTEN, BDI_WRITTEN) in long term. The mismatches will lead to
2507  * systematic errors in balanced_dirty_ratelimit and the dirty pages position
2508  * control.
2509  */
2510 void account_page_redirty(struct page *page)
2511 {
2512         struct address_space *mapping = page->mapping;
2513 
2514         if (mapping && mapping_cap_account_dirty(mapping)) {
2515                 struct inode *inode = mapping->host;
2516                 struct bdi_writeback *wb;
2517                 bool locked;
2518 
2519                 wb = unlocked_inode_to_wb_begin(inode, &locked);
2520                 current->nr_dirtied--;
2521                 dec_zone_page_state(page, NR_DIRTIED);
2522                 dec_wb_stat(wb, WB_DIRTIED);
2523                 unlocked_inode_to_wb_end(inode, locked);
2524         }
2525 }
2526 EXPORT_SYMBOL(account_page_redirty);
2527 
2528 /*
2529  * When a writepage implementation decides that it doesn't want to write this
2530  * page for some reason, it should redirty the locked page via
2531  * redirty_page_for_writepage() and it should then unlock the page and return 0
2532  */
2533 int redirty_page_for_writepage(struct writeback_control *wbc, struct page *page)
2534 {
2535         int ret;
2536 
2537         wbc->pages_skipped++;
2538         ret = __set_page_dirty_nobuffers(page);
2539         account_page_redirty(page);
2540         return ret;
2541 }
2542 EXPORT_SYMBOL(redirty_page_for_writepage);
2543 
2544 /*
2545  * Dirty a page.
2546  *
2547  * For pages with a mapping this should be done under the page lock
2548  * for the benefit of asynchronous memory errors who prefer a consistent
2549  * dirty state. This rule can be broken in some special cases,
2550  * but should be better not to.
2551  *
2552  * If the mapping doesn't provide a set_page_dirty a_op, then
2553  * just fall through and assume that it wants buffer_heads.
2554  */
2555 int set_page_dirty(struct page *page)
2556 {
2557         struct address_space *mapping = page_mapping(page);
2558 
2559         if (likely(mapping)) {
2560                 int (*spd)(struct page *) = mapping->a_ops->set_page_dirty;
2561                 /*
2562                  * readahead/lru_deactivate_page could remain
2563                  * PG_readahead/PG_reclaim due to race with end_page_writeback
2564                  * About readahead, if the page is written, the flags would be
2565                  * reset. So no problem.
2566                  * About lru_deactivate_page, if the page is redirty, the flag
2567                  * will be reset. So no problem. but if the page is used by readahead
2568                  * it will confuse readahead and make it restart the size rampup
2569                  * process. But it's a trivial problem.
2570                  */
2571                 if (PageReclaim(page))
2572                         ClearPageReclaim(page);
2573 #ifdef CONFIG_BLOCK
2574                 if (!spd)
2575                         spd = __set_page_dirty_buffers;
2576 #endif
2577                 return (*spd)(page);
2578         }
2579         if (!PageDirty(page)) {
2580                 if (!TestSetPageDirty(page))
2581                         return 1;
2582         }
2583         return 0;
2584 }
2585 EXPORT_SYMBOL(set_page_dirty);
2586 
2587 /*
2588  * set_page_dirty() is racy if the caller has no reference against
2589  * page->mapping->host, and if the page is unlocked.  This is because another
2590  * CPU could truncate the page off the mapping and then free the mapping.
2591  *
2592  * Usually, the page _is_ locked, or the caller is a user-space process which
2593  * holds a reference on the inode by having an open file.
2594  *
2595  * In other cases, the page should be locked before running set_page_dirty().
2596  */
2597 int set_page_dirty_lock(struct page *page)
2598 {
2599         int ret;
2600 
2601         lock_page(page);
2602         ret = set_page_dirty(page);
2603         unlock_page(page);
2604         return ret;
2605 }
2606 EXPORT_SYMBOL(set_page_dirty_lock);
2607 
2608 /*
2609  * This cancels just the dirty bit on the kernel page itself, it does NOT
2610  * actually remove dirty bits on any mmap's that may be around. It also
2611  * leaves the page tagged dirty, so any sync activity will still find it on
2612  * the dirty lists, and in particular, clear_page_dirty_for_io() will still
2613  * look at the dirty bits in the VM.
2614  *
2615  * Doing this should *normally* only ever be done when a page is truncated,
2616  * and is not actually mapped anywhere at all. However, fs/buffer.c does
2617  * this when it notices that somebody has cleaned out all the buffers on a
2618  * page without actually doing it through the VM. Can you say "ext3 is
2619  * horribly ugly"? Thought you could.
2620  */
2621 void cancel_dirty_page(struct page *page)
2622 {
2623         struct address_space *mapping = page_mapping(page);
2624 
2625         if (mapping_cap_account_dirty(mapping)) {
2626                 struct inode *inode = mapping->host;
2627                 struct bdi_writeback *wb;
2628                 struct mem_cgroup *memcg;
2629                 bool locked;
2630 
2631                 memcg = mem_cgroup_begin_page_stat(page);
2632                 wb = unlocked_inode_to_wb_begin(inode, &locked);
2633 
2634                 if (TestClearPageDirty(page))
2635                         account_page_cleaned(page, mapping, memcg, wb);
2636 
2637                 unlocked_inode_to_wb_end(inode, locked);
2638                 mem_cgroup_end_page_stat(memcg);
2639         } else {
2640                 ClearPageDirty(page);
2641         }
2642 }
2643 EXPORT_SYMBOL(cancel_dirty_page);
2644 
2645 /*
2646  * Clear a page's dirty flag, while caring for dirty memory accounting.
2647  * Returns true if the page was previously dirty.
2648  *
2649  * This is for preparing to put the page under writeout.  We leave the page
2650  * tagged as dirty in the radix tree so that a concurrent write-for-sync
2651  * can discover it via a PAGECACHE_TAG_DIRTY walk.  The ->writepage
2652  * implementation will run either set_page_writeback() or set_page_dirty(),
2653  * at which stage we bring the page's dirty flag and radix-tree dirty tag
2654  * back into sync.
2655  *
2656  * This incoherency between the page's dirty flag and radix-tree tag is
2657  * unfortunate, but it only exists while the page is locked.
2658  */
2659 int clear_page_dirty_for_io(struct page *page)
2660 {
2661         struct address_space *mapping = page_mapping(page);
2662         int ret = 0;
2663 
2664         BUG_ON(!PageLocked(page));
2665 
2666         if (mapping && mapping_cap_account_dirty(mapping)) {
2667                 struct inode *inode = mapping->host;
2668                 struct bdi_writeback *wb;
2669                 struct mem_cgroup *memcg;
2670                 bool locked;
2671 
2672                 /*
2673                  * Yes, Virginia, this is indeed insane.
2674                  *
2675                  * We use this sequence to make sure that
2676                  *  (a) we account for dirty stats properly
2677                  *  (b) we tell the low-level filesystem to
2678                  *      mark the whole page dirty if it was
2679                  *      dirty in a pagetable. Only to then
2680                  *  (c) clean the page again and return 1 to
2681                  *      cause the writeback.
2682                  *
2683                  * This way we avoid all nasty races with the
2684                  * dirty bit in multiple places and clearing
2685                  * them concurrently from different threads.
2686                  *
2687                  * Note! Normally the "set_page_dirty(page)"
2688                  * has no effect on the actual dirty bit - since
2689                  * that will already usually be set. But we
2690                  * need the side effects, and it can help us
2691                  * avoid races.
2692                  *
2693                  * We basically use the page "master dirty bit"
2694                  * as a serialization point for all the different
2695                  * threads doing their things.
2696                  */
2697                 if (page_mkclean(page))
2698                         set_page_dirty(page);
2699                 /*
2700                  * We carefully synchronise fault handlers against
2701                  * installing a dirty pte and marking the page dirty
2702                  * at this point.  We do this by having them hold the
2703                  * page lock while dirtying the page, and pages are
2704                  * always locked coming in here, so we get the desired
2705                  * exclusion.
2706                  */
2707                 memcg = mem_cgroup_begin_page_stat(page);
2708                 wb = unlocked_inode_to_wb_begin(inode, &locked);
2709                 if (TestClearPageDirty(page)) {
2710                         mem_cgroup_dec_page_stat(memcg, MEM_CGROUP_STAT_DIRTY);
2711                         dec_zone_page_state(page, NR_FILE_DIRTY);
2712                         dec_wb_stat(wb, WB_RECLAIMABLE);
2713                         ret = 1;
2714                 }
2715                 unlocked_inode_to_wb_end(inode, locked);
2716                 mem_cgroup_end_page_stat(memcg);
2717                 return ret;
2718         }
2719         return TestClearPageDirty(page);
2720 }
2721 EXPORT_SYMBOL(clear_page_dirty_for_io);
2722 
2723 int test_clear_page_writeback(struct page *page)
2724 {
2725         struct address_space *mapping = page_mapping(page);
2726         struct mem_cgroup *memcg;
2727         int ret;
2728 
2729         memcg = mem_cgroup_begin_page_stat(page);
2730         if (mapping) {
2731                 struct inode *inode = mapping->host;
2732                 struct backing_dev_info *bdi = inode_to_bdi(inode);
2733                 unsigned long flags;
2734 
2735                 spin_lock_irqsave(&mapping->tree_lock, flags);
2736                 ret = TestClearPageWriteback(page);
2737                 if (ret) {
2738                         radix_tree_tag_clear(&mapping->page_tree,
2739                                                 page_index(page),
2740                                                 PAGECACHE_TAG_WRITEBACK);
2741                         if (bdi_cap_account_writeback(bdi)) {
2742                                 struct bdi_writeback *wb = inode_to_wb(inode);
2743 
2744                                 __dec_wb_stat(wb, WB_WRITEBACK);
2745                                 __wb_writeout_inc(wb);
2746                         }
2747                 }
2748                 spin_unlock_irqrestore(&mapping->tree_lock, flags);
2749         } else {
2750                 ret = TestClearPageWriteback(page);
2751         }
2752         if (ret) {
2753                 mem_cgroup_dec_page_stat(memcg, MEM_CGROUP_STAT_WRITEBACK);
2754                 dec_zone_page_state(page, NR_WRITEBACK);
2755                 inc_zone_page_state(page, NR_WRITTEN);
2756         }
2757         mem_cgroup_end_page_stat(memcg);
2758         return ret;
2759 }
2760 
2761 int __test_set_page_writeback(struct page *page, bool keep_write)
2762 {
2763         struct address_space *mapping = page_mapping(page);
2764         struct mem_cgroup *memcg;
2765         int ret;
2766 
2767         memcg = mem_cgroup_begin_page_stat(page);
2768         if (mapping) {
2769                 struct inode *inode = mapping->host;
2770                 struct backing_dev_info *bdi = inode_to_bdi(inode);
2771                 unsigned long flags;
2772 
2773                 spin_lock_irqsave(&mapping->tree_lock, flags);
2774                 ret = TestSetPageWriteback(page);
2775                 if (!ret) {
2776                         radix_tree_tag_set(&mapping->page_tree,
2777                                                 page_index(page),
2778                                                 PAGECACHE_TAG_WRITEBACK);
2779                         if (bdi_cap_account_writeback(bdi))
2780                                 __inc_wb_stat(inode_to_wb(inode), WB_WRITEBACK);
2781                 }
2782                 if (!PageDirty(page))
2783                         radix_tree_tag_clear(&mapping->page_tree,
2784                                                 page_index(page),
2785                                                 PAGECACHE_TAG_DIRTY);
2786                 if (!keep_write)
2787                         radix_tree_tag_clear(&mapping->page_tree,
2788                                                 page_index(page),
2789                                                 PAGECACHE_TAG_TOWRITE);
2790                 spin_unlock_irqrestore(&mapping->tree_lock, flags);
2791         } else {
2792                 ret = TestSetPageWriteback(page);
2793         }
2794         if (!ret) {
2795                 mem_cgroup_inc_page_stat(memcg, MEM_CGROUP_STAT_WRITEBACK);
2796                 inc_zone_page_state(page, NR_WRITEBACK);
2797         }
2798         mem_cgroup_end_page_stat(memcg);
2799         return ret;
2800 
2801 }
2802 EXPORT_SYMBOL(__test_set_page_writeback);
2803 
2804 /*
2805  * Return true if any of the pages in the mapping are marked with the
2806  * passed tag.
2807  */
2808 int mapping_tagged(struct address_space *mapping, int tag)
2809 {
2810         return radix_tree_tagged(&mapping->page_tree, tag);
2811 }
2812 EXPORT_SYMBOL(mapping_tagged);
2813 
2814 /**
2815  * wait_for_stable_page() - wait for writeback to finish, if necessary.
2816  * @page:       The page to wait on.
2817  *
2818  * This function determines if the given page is related to a backing device
2819  * that requires page contents to be held stable during writeback.  If so, then
2820  * it will wait for any pending writeback to complete.
2821  */
2822 void wait_for_stable_page(struct page *page)
2823 {
2824         if (bdi_cap_stable_pages_required(inode_to_bdi(page->mapping->host)))
2825                 wait_on_page_writeback(page);
2826 }
2827 EXPORT_SYMBOL_GPL(wait_for_stable_page);
2828 

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