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

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