Version:  2.0.40 2.2.26 2.4.37 3.0 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9 3.10 3.11 3.12 3.13 3.14 3.15 3.16

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 <pzijlstr@redhat.com>
  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 unsigned long global_dirty_limit;
126 
127 /*
128  * Scale the writeback cache size proportional to the relative writeout speeds.
129  *
130  * We do this by keeping a floating proportion between BDIs, based on page
131  * writeback completions [end_page_writeback()]. Those devices that write out
132  * pages fastest will get the larger share, while the slower will get a smaller
133  * share.
134  *
135  * We use page writeout completions because we are interested in getting rid of
136  * dirty pages. Having them written out is the primary goal.
137  *
138  * We introduce a concept of time, a period over which we measure these events,
139  * because demand can/will vary over time. The length of this period itself is
140  * measured in page writeback completions.
141  *
142  */
143 static struct fprop_global writeout_completions;
144 
145 static void writeout_period(unsigned long t);
146 /* Timer for aging of writeout_completions */
147 static struct timer_list writeout_period_timer =
148                 TIMER_DEFERRED_INITIALIZER(writeout_period, 0, 0);
149 static unsigned long writeout_period_time = 0;
150 
151 /*
152  * Length of period for aging writeout fractions of bdis. This is an
153  * arbitrarily chosen number. The longer the period, the slower fractions will
154  * reflect changes in current writeout rate.
155  */
156 #define VM_COMPLETIONS_PERIOD_LEN (3*HZ)
157 
158 /*
159  * In a memory zone, there is a certain amount of pages we consider
160  * available for the page cache, which is essentially the number of
161  * free and reclaimable pages, minus some zone reserves to protect
162  * lowmem and the ability to uphold the zone's watermarks without
163  * requiring writeback.
164  *
165  * This number of dirtyable pages is the base value of which the
166  * user-configurable dirty ratio is the effictive number of pages that
167  * are allowed to be actually dirtied.  Per individual zone, or
168  * globally by using the sum of dirtyable pages over all zones.
169  *
170  * Because the user is allowed to specify the dirty limit globally as
171  * absolute number of bytes, calculating the per-zone dirty limit can
172  * require translating the configured limit into a percentage of
173  * global dirtyable memory first.
174  */
175 
176 /**
177  * zone_dirtyable_memory - number of dirtyable pages in a zone
178  * @zone: the zone
179  *
180  * Returns the zone's number of pages potentially available for dirty
181  * page cache.  This is the base value for the per-zone dirty limits.
182  */
183 static unsigned long zone_dirtyable_memory(struct zone *zone)
184 {
185         unsigned long nr_pages;
186 
187         nr_pages = zone_page_state(zone, NR_FREE_PAGES);
188         nr_pages -= min(nr_pages, zone->dirty_balance_reserve);
189 
190         nr_pages += zone_page_state(zone, NR_INACTIVE_FILE);
191         nr_pages += zone_page_state(zone, NR_ACTIVE_FILE);
192 
193         return nr_pages;
194 }
195 
196 static unsigned long highmem_dirtyable_memory(unsigned long total)
197 {
198 #ifdef CONFIG_HIGHMEM
199         int node;
200         unsigned long x = 0;
201 
202         for_each_node_state(node, N_HIGH_MEMORY) {
203                 struct zone *z = &NODE_DATA(node)->node_zones[ZONE_HIGHMEM];
204 
205                 x += zone_dirtyable_memory(z);
206         }
207         /*
208          * Unreclaimable memory (kernel memory or anonymous memory
209          * without swap) can bring down the dirtyable pages below
210          * the zone's dirty balance reserve and the above calculation
211          * will underflow.  However we still want to add in nodes
212          * which are below threshold (negative values) to get a more
213          * accurate calculation but make sure that the total never
214          * underflows.
215          */
216         if ((long)x < 0)
217                 x = 0;
218 
219         /*
220          * Make sure that the number of highmem pages is never larger
221          * than the number of the total dirtyable memory. This can only
222          * occur in very strange VM situations but we want to make sure
223          * that this does not occur.
224          */
225         return min(x, total);
226 #else
227         return 0;
228 #endif
229 }
230 
231 /**
232  * global_dirtyable_memory - number of globally dirtyable pages
233  *
234  * Returns the global number of pages potentially available for dirty
235  * page cache.  This is the base value for the global dirty limits.
236  */
237 static unsigned long global_dirtyable_memory(void)
238 {
239         unsigned long x;
240 
241         x = global_page_state(NR_FREE_PAGES);
242         x -= min(x, dirty_balance_reserve);
243 
244         x += global_page_state(NR_INACTIVE_FILE);
245         x += global_page_state(NR_ACTIVE_FILE);
246 
247         if (!vm_highmem_is_dirtyable)
248                 x -= highmem_dirtyable_memory(x);
249 
250         return x + 1;   /* Ensure that we never return 0 */
251 }
252 
253 /*
254  * global_dirty_limits - background-writeback and dirty-throttling thresholds
255  *
256  * Calculate the dirty thresholds based on sysctl parameters
257  * - vm.dirty_background_ratio  or  vm.dirty_background_bytes
258  * - vm.dirty_ratio             or  vm.dirty_bytes
259  * The dirty limits will be lifted by 1/4 for PF_LESS_THROTTLE (ie. nfsd) and
260  * real-time tasks.
261  */
262 void global_dirty_limits(unsigned long *pbackground, unsigned long *pdirty)
263 {
264         unsigned long background;
265         unsigned long dirty;
266         unsigned long uninitialized_var(available_memory);
267         struct task_struct *tsk;
268 
269         if (!vm_dirty_bytes || !dirty_background_bytes)
270                 available_memory = global_dirtyable_memory();
271 
272         if (vm_dirty_bytes)
273                 dirty = DIV_ROUND_UP(vm_dirty_bytes, PAGE_SIZE);
274         else
275                 dirty = (vm_dirty_ratio * available_memory) / 100;
276 
277         if (dirty_background_bytes)
278                 background = DIV_ROUND_UP(dirty_background_bytes, PAGE_SIZE);
279         else
280                 background = (dirty_background_ratio * available_memory) / 100;
281 
282         if (background >= dirty)
283                 background = dirty / 2;
284         tsk = current;
285         if (tsk->flags & PF_LESS_THROTTLE || rt_task(tsk)) {
286                 background += background / 4;
287                 dirty += dirty / 4;
288         }
289         *pbackground = background;
290         *pdirty = dirty;
291         trace_global_dirty_state(background, dirty);
292 }
293 
294 /**
295  * zone_dirty_limit - maximum number of dirty pages allowed in a zone
296  * @zone: the zone
297  *
298  * Returns the maximum number of dirty pages allowed in a zone, based
299  * on the zone's dirtyable memory.
300  */
301 static unsigned long zone_dirty_limit(struct zone *zone)
302 {
303         unsigned long zone_memory = zone_dirtyable_memory(zone);
304         struct task_struct *tsk = current;
305         unsigned long dirty;
306 
307         if (vm_dirty_bytes)
308                 dirty = DIV_ROUND_UP(vm_dirty_bytes, PAGE_SIZE) *
309                         zone_memory / global_dirtyable_memory();
310         else
311                 dirty = vm_dirty_ratio * zone_memory / 100;
312 
313         if (tsk->flags & PF_LESS_THROTTLE || rt_task(tsk))
314                 dirty += dirty / 4;
315 
316         return dirty;
317 }
318 
319 /**
320  * zone_dirty_ok - tells whether a zone is within its dirty limits
321  * @zone: the zone to check
322  *
323  * Returns %true when the dirty pages in @zone are within the zone's
324  * dirty limit, %false if the limit is exceeded.
325  */
326 bool zone_dirty_ok(struct zone *zone)
327 {
328         unsigned long limit = zone_dirty_limit(zone);
329 
330         return zone_page_state(zone, NR_FILE_DIRTY) +
331                zone_page_state(zone, NR_UNSTABLE_NFS) +
332                zone_page_state(zone, NR_WRITEBACK) <= limit;
333 }
334 
335 int dirty_background_ratio_handler(struct ctl_table *table, int write,
336                 void __user *buffer, size_t *lenp,
337                 loff_t *ppos)
338 {
339         int ret;
340 
341         ret = proc_dointvec_minmax(table, write, buffer, lenp, ppos);
342         if (ret == 0 && write)
343                 dirty_background_bytes = 0;
344         return ret;
345 }
346 
347 int dirty_background_bytes_handler(struct ctl_table *table, int write,
348                 void __user *buffer, size_t *lenp,
349                 loff_t *ppos)
350 {
351         int ret;
352 
353         ret = proc_doulongvec_minmax(table, write, buffer, lenp, ppos);
354         if (ret == 0 && write)
355                 dirty_background_ratio = 0;
356         return ret;
357 }
358 
359 int dirty_ratio_handler(struct ctl_table *table, int write,
360                 void __user *buffer, size_t *lenp,
361                 loff_t *ppos)
362 {
363         int old_ratio = vm_dirty_ratio;
364         int ret;
365 
366         ret = proc_dointvec_minmax(table, write, buffer, lenp, ppos);
367         if (ret == 0 && write && vm_dirty_ratio != old_ratio) {
368                 writeback_set_ratelimit();
369                 vm_dirty_bytes = 0;
370         }
371         return ret;
372 }
373 
374 int dirty_bytes_handler(struct ctl_table *table, int write,
375                 void __user *buffer, size_t *lenp,
376                 loff_t *ppos)
377 {
378         unsigned long old_bytes = vm_dirty_bytes;
379         int ret;
380 
381         ret = proc_doulongvec_minmax(table, write, buffer, lenp, ppos);
382         if (ret == 0 && write && vm_dirty_bytes != old_bytes) {
383                 writeback_set_ratelimit();
384                 vm_dirty_ratio = 0;
385         }
386         return ret;
387 }
388 
389 static unsigned long wp_next_time(unsigned long cur_time)
390 {
391         cur_time += VM_COMPLETIONS_PERIOD_LEN;
392         /* 0 has a special meaning... */
393         if (!cur_time)
394                 return 1;
395         return cur_time;
396 }
397 
398 /*
399  * Increment the BDI's writeout completion count and the global writeout
400  * completion count. Called from test_clear_page_writeback().
401  */
402 static inline void __bdi_writeout_inc(struct backing_dev_info *bdi)
403 {
404         __inc_bdi_stat(bdi, BDI_WRITTEN);
405         __fprop_inc_percpu_max(&writeout_completions, &bdi->completions,
406                                bdi->max_prop_frac);
407         /* First event after period switching was turned off? */
408         if (!unlikely(writeout_period_time)) {
409                 /*
410                  * We can race with other __bdi_writeout_inc calls here but
411                  * it does not cause any harm since the resulting time when
412                  * timer will fire and what is in writeout_period_time will be
413                  * roughly the same.
414                  */
415                 writeout_period_time = wp_next_time(jiffies);
416                 mod_timer(&writeout_period_timer, writeout_period_time);
417         }
418 }
419 
420 void bdi_writeout_inc(struct backing_dev_info *bdi)
421 {
422         unsigned long flags;
423 
424         local_irq_save(flags);
425         __bdi_writeout_inc(bdi);
426         local_irq_restore(flags);
427 }
428 EXPORT_SYMBOL_GPL(bdi_writeout_inc);
429 
430 /*
431  * Obtain an accurate fraction of the BDI's portion.
432  */
433 static void bdi_writeout_fraction(struct backing_dev_info *bdi,
434                 long *numerator, long *denominator)
435 {
436         fprop_fraction_percpu(&writeout_completions, &bdi->completions,
437                                 numerator, denominator);
438 }
439 
440 /*
441  * On idle system, we can be called long after we scheduled because we use
442  * deferred timers so count with missed periods.
443  */
444 static void writeout_period(unsigned long t)
445 {
446         int miss_periods = (jiffies - writeout_period_time) /
447                                                  VM_COMPLETIONS_PERIOD_LEN;
448 
449         if (fprop_new_period(&writeout_completions, miss_periods + 1)) {
450                 writeout_period_time = wp_next_time(writeout_period_time +
451                                 miss_periods * VM_COMPLETIONS_PERIOD_LEN);
452                 mod_timer(&writeout_period_timer, writeout_period_time);
453         } else {
454                 /*
455                  * Aging has zeroed all fractions. Stop wasting CPU on period
456                  * updates.
457                  */
458                 writeout_period_time = 0;
459         }
460 }
461 
462 /*
463  * bdi_min_ratio keeps the sum of the minimum dirty shares of all
464  * registered backing devices, which, for obvious reasons, can not
465  * exceed 100%.
466  */
467 static unsigned int bdi_min_ratio;
468 
469 int bdi_set_min_ratio(struct backing_dev_info *bdi, unsigned int min_ratio)
470 {
471         int ret = 0;
472 
473         spin_lock_bh(&bdi_lock);
474         if (min_ratio > bdi->max_ratio) {
475                 ret = -EINVAL;
476         } else {
477                 min_ratio -= bdi->min_ratio;
478                 if (bdi_min_ratio + min_ratio < 100) {
479                         bdi_min_ratio += min_ratio;
480                         bdi->min_ratio += min_ratio;
481                 } else {
482                         ret = -EINVAL;
483                 }
484         }
485         spin_unlock_bh(&bdi_lock);
486 
487         return ret;
488 }
489 
490 int bdi_set_max_ratio(struct backing_dev_info *bdi, unsigned max_ratio)
491 {
492         int ret = 0;
493 
494         if (max_ratio > 100)
495                 return -EINVAL;
496 
497         spin_lock_bh(&bdi_lock);
498         if (bdi->min_ratio > max_ratio) {
499                 ret = -EINVAL;
500         } else {
501                 bdi->max_ratio = max_ratio;
502                 bdi->max_prop_frac = (FPROP_FRAC_BASE * max_ratio) / 100;
503         }
504         spin_unlock_bh(&bdi_lock);
505 
506         return ret;
507 }
508 EXPORT_SYMBOL(bdi_set_max_ratio);
509 
510 static unsigned long dirty_freerun_ceiling(unsigned long thresh,
511                                            unsigned long bg_thresh)
512 {
513         return (thresh + bg_thresh) / 2;
514 }
515 
516 static unsigned long hard_dirty_limit(unsigned long thresh)
517 {
518         return max(thresh, global_dirty_limit);
519 }
520 
521 /**
522  * bdi_dirty_limit - @bdi's share of dirty throttling threshold
523  * @bdi: the backing_dev_info to query
524  * @dirty: global dirty limit in pages
525  *
526  * Returns @bdi's dirty limit in pages. The term "dirty" in the context of
527  * dirty balancing includes all PG_dirty, PG_writeback and NFS unstable pages.
528  *
529  * Note that balance_dirty_pages() will only seriously take it as a hard limit
530  * when sleeping max_pause per page is not enough to keep the dirty pages under
531  * control. For example, when the device is completely stalled due to some error
532  * conditions, or when there are 1000 dd tasks writing to a slow 10MB/s USB key.
533  * In the other normal situations, it acts more gently by throttling the tasks
534  * more (rather than completely block them) when the bdi dirty pages go high.
535  *
536  * It allocates high/low dirty limits to fast/slow devices, in order to prevent
537  * - starving fast devices
538  * - piling up dirty pages (that will take long time to sync) on slow devices
539  *
540  * The bdi's share of dirty limit will be adapting to its throughput and
541  * bounded by the bdi->min_ratio and/or bdi->max_ratio parameters, if set.
542  */
543 unsigned long bdi_dirty_limit(struct backing_dev_info *bdi, unsigned long dirty)
544 {
545         u64 bdi_dirty;
546         long numerator, denominator;
547 
548         /*
549          * Calculate this BDI's share of the dirty ratio.
550          */
551         bdi_writeout_fraction(bdi, &numerator, &denominator);
552 
553         bdi_dirty = (dirty * (100 - bdi_min_ratio)) / 100;
554         bdi_dirty *= numerator;
555         do_div(bdi_dirty, denominator);
556 
557         bdi_dirty += (dirty * bdi->min_ratio) / 100;
558         if (bdi_dirty > (dirty * bdi->max_ratio) / 100)
559                 bdi_dirty = dirty * bdi->max_ratio / 100;
560 
561         return bdi_dirty;
562 }
563 
564 /*
565  *                           setpoint - dirty 3
566  *        f(dirty) := 1.0 + (----------------)
567  *                           limit - setpoint
568  *
569  * it's a 3rd order polynomial that subjects to
570  *
571  * (1) f(freerun)  = 2.0 => rampup dirty_ratelimit reasonably fast
572  * (2) f(setpoint) = 1.0 => the balance point
573  * (3) f(limit)    = 0   => the hard limit
574  * (4) df/dx      <= 0   => negative feedback control
575  * (5) the closer to setpoint, the smaller |df/dx| (and the reverse)
576  *     => fast response on large errors; small oscillation near setpoint
577  */
578 static long long pos_ratio_polynom(unsigned long setpoint,
579                                           unsigned long dirty,
580                                           unsigned long limit)
581 {
582         long long pos_ratio;
583         long x;
584 
585         x = div64_s64(((s64)setpoint - (s64)dirty) << RATELIMIT_CALC_SHIFT,
586                     limit - setpoint + 1);
587         pos_ratio = x;
588         pos_ratio = pos_ratio * x >> RATELIMIT_CALC_SHIFT;
589         pos_ratio = pos_ratio * x >> RATELIMIT_CALC_SHIFT;
590         pos_ratio += 1 << RATELIMIT_CALC_SHIFT;
591 
592         return clamp(pos_ratio, 0LL, 2LL << RATELIMIT_CALC_SHIFT);
593 }
594 
595 /*
596  * Dirty position control.
597  *
598  * (o) global/bdi setpoints
599  *
600  * We want the dirty pages be balanced around the global/bdi setpoints.
601  * When the number of dirty pages is higher/lower than the setpoint, the
602  * dirty position control ratio (and hence task dirty ratelimit) will be
603  * decreased/increased to bring the dirty pages back to the setpoint.
604  *
605  *     pos_ratio = 1 << RATELIMIT_CALC_SHIFT
606  *
607  *     if (dirty < setpoint) scale up   pos_ratio
608  *     if (dirty > setpoint) scale down pos_ratio
609  *
610  *     if (bdi_dirty < bdi_setpoint) scale up   pos_ratio
611  *     if (bdi_dirty > bdi_setpoint) scale down pos_ratio
612  *
613  *     task_ratelimit = dirty_ratelimit * pos_ratio >> RATELIMIT_CALC_SHIFT
614  *
615  * (o) global control line
616  *
617  *     ^ pos_ratio
618  *     |
619  *     |            |<===== global dirty control scope ======>|
620  * 2.0 .............*
621  *     |            .*
622  *     |            . *
623  *     |            .   *
624  *     |            .     *
625  *     |            .        *
626  *     |            .            *
627  * 1.0 ................................*
628  *     |            .                  .     *
629  *     |            .                  .          *
630  *     |            .                  .              *
631  *     |            .                  .                 *
632  *     |            .                  .                    *
633  *   0 +------------.------------------.----------------------*------------->
634  *           freerun^          setpoint^                 limit^   dirty pages
635  *
636  * (o) bdi control line
637  *
638  *     ^ pos_ratio
639  *     |
640  *     |            *
641  *     |              *
642  *     |                *
643  *     |                  *
644  *     |                    * |<=========== span ============>|
645  * 1.0 .......................*
646  *     |                      . *
647  *     |                      .   *
648  *     |                      .     *
649  *     |                      .       *
650  *     |                      .         *
651  *     |                      .           *
652  *     |                      .             *
653  *     |                      .               *
654  *     |                      .                 *
655  *     |                      .                   *
656  *     |                      .                     *
657  * 1/4 ...............................................* * * * * * * * * * * *
658  *     |                      .                         .
659  *     |                      .                           .
660  *     |                      .                             .
661  *   0 +----------------------.-------------------------------.------------->
662  *                bdi_setpoint^                    x_intercept^
663  *
664  * The bdi control line won't drop below pos_ratio=1/4, so that bdi_dirty can
665  * be smoothly throttled down to normal if it starts high in situations like
666  * - start writing to a slow SD card and a fast disk at the same time. The SD
667  *   card's bdi_dirty may rush to many times higher than bdi_setpoint.
668  * - the bdi dirty thresh drops quickly due to change of JBOD workload
669  */
670 static unsigned long bdi_position_ratio(struct backing_dev_info *bdi,
671                                         unsigned long thresh,
672                                         unsigned long bg_thresh,
673                                         unsigned long dirty,
674                                         unsigned long bdi_thresh,
675                                         unsigned long bdi_dirty)
676 {
677         unsigned long write_bw = bdi->avg_write_bandwidth;
678         unsigned long freerun = dirty_freerun_ceiling(thresh, bg_thresh);
679         unsigned long limit = hard_dirty_limit(thresh);
680         unsigned long x_intercept;
681         unsigned long setpoint;         /* dirty pages' target balance point */
682         unsigned long bdi_setpoint;
683         unsigned long span;
684         long long pos_ratio;            /* for scaling up/down the rate limit */
685         long x;
686 
687         if (unlikely(dirty >= limit))
688                 return 0;
689 
690         /*
691          * global setpoint
692          *
693          * See comment for pos_ratio_polynom().
694          */
695         setpoint = (freerun + limit) / 2;
696         pos_ratio = pos_ratio_polynom(setpoint, dirty, limit);
697 
698         /*
699          * The strictlimit feature is a tool preventing mistrusted filesystems
700          * from growing a large number of dirty pages before throttling. For
701          * such filesystems balance_dirty_pages always checks bdi counters
702          * against bdi limits. Even if global "nr_dirty" is under "freerun".
703          * This is especially important for fuse which sets bdi->max_ratio to
704          * 1% by default. Without strictlimit feature, fuse writeback may
705          * consume arbitrary amount of RAM because it is accounted in
706          * NR_WRITEBACK_TEMP which is not involved in calculating "nr_dirty".
707          *
708          * Here, in bdi_position_ratio(), we calculate pos_ratio based on
709          * two values: bdi_dirty and bdi_thresh. Let's consider an example:
710          * total amount of RAM is 16GB, bdi->max_ratio is equal to 1%, global
711          * limits are set by default to 10% and 20% (background and throttle).
712          * Then bdi_thresh is 1% of 20% of 16GB. This amounts to ~8K pages.
713          * bdi_dirty_limit(bdi, bg_thresh) is about ~4K pages. bdi_setpoint is
714          * about ~6K pages (as the average of background and throttle bdi
715          * limits). The 3rd order polynomial will provide positive feedback if
716          * bdi_dirty is under bdi_setpoint and vice versa.
717          *
718          * Note, that we cannot use global counters in these calculations
719          * because we want to throttle process writing to a strictlimit BDI
720          * much earlier than global "freerun" is reached (~23MB vs. ~2.3GB
721          * in the example above).
722          */
723         if (unlikely(bdi->capabilities & BDI_CAP_STRICTLIMIT)) {
724                 long long bdi_pos_ratio;
725                 unsigned long bdi_bg_thresh;
726 
727                 if (bdi_dirty < 8)
728                         return min_t(long long, pos_ratio * 2,
729                                      2 << RATELIMIT_CALC_SHIFT);
730 
731                 if (bdi_dirty >= bdi_thresh)
732                         return 0;
733 
734                 bdi_bg_thresh = div_u64((u64)bdi_thresh * bg_thresh, thresh);
735                 bdi_setpoint = dirty_freerun_ceiling(bdi_thresh,
736                                                      bdi_bg_thresh);
737 
738                 if (bdi_setpoint == 0 || bdi_setpoint == bdi_thresh)
739                         return 0;
740 
741                 bdi_pos_ratio = pos_ratio_polynom(bdi_setpoint, bdi_dirty,
742                                                   bdi_thresh);
743 
744                 /*
745                  * Typically, for strictlimit case, bdi_setpoint << setpoint
746                  * and pos_ratio >> bdi_pos_ratio. In the other words global
747                  * state ("dirty") is not limiting factor and we have to
748                  * make decision based on bdi counters. But there is an
749                  * important case when global pos_ratio should get precedence:
750                  * global limits are exceeded (e.g. due to activities on other
751                  * BDIs) while given strictlimit BDI is below limit.
752                  *
753                  * "pos_ratio * bdi_pos_ratio" would work for the case above,
754                  * but it would look too non-natural for the case of all
755                  * activity in the system coming from a single strictlimit BDI
756                  * with bdi->max_ratio == 100%.
757                  *
758                  * Note that min() below somewhat changes the dynamics of the
759                  * control system. Normally, pos_ratio value can be well over 3
760                  * (when globally we are at freerun and bdi is well below bdi
761                  * setpoint). Now the maximum pos_ratio in the same situation
762                  * is 2. We might want to tweak this if we observe the control
763                  * system is too slow to adapt.
764                  */
765                 return min(pos_ratio, bdi_pos_ratio);
766         }
767 
768         /*
769          * We have computed basic pos_ratio above based on global situation. If
770          * the bdi is over/under its share of dirty pages, we want to scale
771          * pos_ratio further down/up. That is done by the following mechanism.
772          */
773 
774         /*
775          * bdi setpoint
776          *
777          *        f(bdi_dirty) := 1.0 + k * (bdi_dirty - bdi_setpoint)
778          *
779          *                        x_intercept - bdi_dirty
780          *                     := --------------------------
781          *                        x_intercept - bdi_setpoint
782          *
783          * The main bdi control line is a linear function that subjects to
784          *
785          * (1) f(bdi_setpoint) = 1.0
786          * (2) k = - 1 / (8 * write_bw)  (in single bdi case)
787          *     or equally: x_intercept = bdi_setpoint + 8 * write_bw
788          *
789          * For single bdi case, the dirty pages are observed to fluctuate
790          * regularly within range
791          *        [bdi_setpoint - write_bw/2, bdi_setpoint + write_bw/2]
792          * for various filesystems, where (2) can yield in a reasonable 12.5%
793          * fluctuation range for pos_ratio.
794          *
795          * For JBOD case, bdi_thresh (not bdi_dirty!) could fluctuate up to its
796          * own size, so move the slope over accordingly and choose a slope that
797          * yields 100% pos_ratio fluctuation on suddenly doubled bdi_thresh.
798          */
799         if (unlikely(bdi_thresh > thresh))
800                 bdi_thresh = thresh;
801         /*
802          * It's very possible that bdi_thresh is close to 0 not because the
803          * device is slow, but that it has remained inactive for long time.
804          * Honour such devices a reasonable good (hopefully IO efficient)
805          * threshold, so that the occasional writes won't be blocked and active
806          * writes can rampup the threshold quickly.
807          */
808         bdi_thresh = max(bdi_thresh, (limit - dirty) / 8);
809         /*
810          * scale global setpoint to bdi's:
811          *      bdi_setpoint = setpoint * bdi_thresh / thresh
812          */
813         x = div_u64((u64)bdi_thresh << 16, thresh + 1);
814         bdi_setpoint = setpoint * (u64)x >> 16;
815         /*
816          * Use span=(8*write_bw) in single bdi case as indicated by
817          * (thresh - bdi_thresh ~= 0) and transit to bdi_thresh in JBOD case.
818          *
819          *        bdi_thresh                    thresh - bdi_thresh
820          * span = ---------- * (8 * write_bw) + ------------------- * bdi_thresh
821          *          thresh                            thresh
822          */
823         span = (thresh - bdi_thresh + 8 * write_bw) * (u64)x >> 16;
824         x_intercept = bdi_setpoint + span;
825 
826         if (bdi_dirty < x_intercept - span / 4) {
827                 pos_ratio = div64_u64(pos_ratio * (x_intercept - bdi_dirty),
828                                     x_intercept - bdi_setpoint + 1);
829         } else
830                 pos_ratio /= 4;
831 
832         /*
833          * bdi reserve area, safeguard against dirty pool underrun and disk idle
834          * It may push the desired control point of global dirty pages higher
835          * than setpoint.
836          */
837         x_intercept = bdi_thresh / 2;
838         if (bdi_dirty < x_intercept) {
839                 if (bdi_dirty > x_intercept / 8)
840                         pos_ratio = div_u64(pos_ratio * x_intercept, bdi_dirty);
841                 else
842                         pos_ratio *= 8;
843         }
844 
845         return pos_ratio;
846 }
847 
848 static void bdi_update_write_bandwidth(struct backing_dev_info *bdi,
849                                        unsigned long elapsed,
850                                        unsigned long written)
851 {
852         const unsigned long period = roundup_pow_of_two(3 * HZ);
853         unsigned long avg = bdi->avg_write_bandwidth;
854         unsigned long old = bdi->write_bandwidth;
855         u64 bw;
856 
857         /*
858          * bw = written * HZ / elapsed
859          *
860          *                   bw * elapsed + write_bandwidth * (period - elapsed)
861          * write_bandwidth = ---------------------------------------------------
862          *                                          period
863          */
864         bw = written - bdi->written_stamp;
865         bw *= HZ;
866         if (unlikely(elapsed > period)) {
867                 do_div(bw, elapsed);
868                 avg = bw;
869                 goto out;
870         }
871         bw += (u64)bdi->write_bandwidth * (period - elapsed);
872         bw >>= ilog2(period);
873 
874         /*
875          * one more level of smoothing, for filtering out sudden spikes
876          */
877         if (avg > old && old >= (unsigned long)bw)
878                 avg -= (avg - old) >> 3;
879 
880         if (avg < old && old <= (unsigned long)bw)
881                 avg += (old - avg) >> 3;
882 
883 out:
884         bdi->write_bandwidth = bw;
885         bdi->avg_write_bandwidth = avg;
886 }
887 
888 /*
889  * The global dirtyable memory and dirty threshold could be suddenly knocked
890  * down by a large amount (eg. on the startup of KVM in a swapless system).
891  * This may throw the system into deep dirty exceeded state and throttle
892  * heavy/light dirtiers alike. To retain good responsiveness, maintain
893  * global_dirty_limit for tracking slowly down to the knocked down dirty
894  * threshold.
895  */
896 static void update_dirty_limit(unsigned long thresh, unsigned long dirty)
897 {
898         unsigned long limit = global_dirty_limit;
899 
900         /*
901          * Follow up in one step.
902          */
903         if (limit < thresh) {
904                 limit = thresh;
905                 goto update;
906         }
907 
908         /*
909          * Follow down slowly. Use the higher one as the target, because thresh
910          * may drop below dirty. This is exactly the reason to introduce
911          * global_dirty_limit which is guaranteed to lie above the dirty pages.
912          */
913         thresh = max(thresh, dirty);
914         if (limit > thresh) {
915                 limit -= (limit - thresh) >> 5;
916                 goto update;
917         }
918         return;
919 update:
920         global_dirty_limit = limit;
921 }
922 
923 static void global_update_bandwidth(unsigned long thresh,
924                                     unsigned long dirty,
925                                     unsigned long now)
926 {
927         static DEFINE_SPINLOCK(dirty_lock);
928         static unsigned long update_time;
929 
930         /*
931          * check locklessly first to optimize away locking for the most time
932          */
933         if (time_before(now, update_time + BANDWIDTH_INTERVAL))
934                 return;
935 
936         spin_lock(&dirty_lock);
937         if (time_after_eq(now, update_time + BANDWIDTH_INTERVAL)) {
938                 update_dirty_limit(thresh, dirty);
939                 update_time = now;
940         }
941         spin_unlock(&dirty_lock);
942 }
943 
944 /*
945  * Maintain bdi->dirty_ratelimit, the base dirty throttle rate.
946  *
947  * Normal bdi tasks will be curbed at or below it in long term.
948  * Obviously it should be around (write_bw / N) when there are N dd tasks.
949  */
950 static void bdi_update_dirty_ratelimit(struct backing_dev_info *bdi,
951                                        unsigned long thresh,
952                                        unsigned long bg_thresh,
953                                        unsigned long dirty,
954                                        unsigned long bdi_thresh,
955                                        unsigned long bdi_dirty,
956                                        unsigned long dirtied,
957                                        unsigned long elapsed)
958 {
959         unsigned long freerun = dirty_freerun_ceiling(thresh, bg_thresh);
960         unsigned long limit = hard_dirty_limit(thresh);
961         unsigned long setpoint = (freerun + limit) / 2;
962         unsigned long write_bw = bdi->avg_write_bandwidth;
963         unsigned long dirty_ratelimit = bdi->dirty_ratelimit;
964         unsigned long dirty_rate;
965         unsigned long task_ratelimit;
966         unsigned long balanced_dirty_ratelimit;
967         unsigned long pos_ratio;
968         unsigned long step;
969         unsigned long x;
970 
971         /*
972          * The dirty rate will match the writeout rate in long term, except
973          * when dirty pages are truncated by userspace or re-dirtied by FS.
974          */
975         dirty_rate = (dirtied - bdi->dirtied_stamp) * HZ / elapsed;
976 
977         pos_ratio = bdi_position_ratio(bdi, thresh, bg_thresh, dirty,
978                                        bdi_thresh, bdi_dirty);
979         /*
980          * task_ratelimit reflects each dd's dirty rate for the past 200ms.
981          */
982         task_ratelimit = (u64)dirty_ratelimit *
983                                         pos_ratio >> RATELIMIT_CALC_SHIFT;
984         task_ratelimit++; /* it helps rampup dirty_ratelimit from tiny values */
985 
986         /*
987          * A linear estimation of the "balanced" throttle rate. The theory is,
988          * if there are N dd tasks, each throttled at task_ratelimit, the bdi's
989          * dirty_rate will be measured to be (N * task_ratelimit). So the below
990          * formula will yield the balanced rate limit (write_bw / N).
991          *
992          * Note that the expanded form is not a pure rate feedback:
993          *      rate_(i+1) = rate_(i) * (write_bw / dirty_rate)              (1)
994          * but also takes pos_ratio into account:
995          *      rate_(i+1) = rate_(i) * (write_bw / dirty_rate) * pos_ratio  (2)
996          *
997          * (1) is not realistic because pos_ratio also takes part in balancing
998          * the dirty rate.  Consider the state
999          *      pos_ratio = 0.5                                              (3)
1000          *      rate = 2 * (write_bw / N)                                    (4)
1001          * If (1) is used, it will stuck in that state! Because each dd will
1002          * be throttled at
1003          *      task_ratelimit = pos_ratio * rate = (write_bw / N)           (5)
1004          * yielding
1005          *      dirty_rate = N * task_ratelimit = write_bw                   (6)
1006          * put (6) into (1) we get
1007          *      rate_(i+1) = rate_(i)                                        (7)
1008          *
1009          * So we end up using (2) to always keep
1010          *      rate_(i+1) ~= (write_bw / N)                                 (8)
1011          * regardless of the value of pos_ratio. As long as (8) is satisfied,
1012          * pos_ratio is able to drive itself to 1.0, which is not only where
1013          * the dirty count meet the setpoint, but also where the slope of
1014          * pos_ratio is most flat and hence task_ratelimit is least fluctuated.
1015          */
1016         balanced_dirty_ratelimit = div_u64((u64)task_ratelimit * write_bw,
1017                                            dirty_rate | 1);
1018         /*
1019          * balanced_dirty_ratelimit ~= (write_bw / N) <= write_bw
1020          */
1021         if (unlikely(balanced_dirty_ratelimit > write_bw))
1022                 balanced_dirty_ratelimit = write_bw;
1023 
1024         /*
1025          * We could safely do this and return immediately:
1026          *
1027          *      bdi->dirty_ratelimit = balanced_dirty_ratelimit;
1028          *
1029          * However to get a more stable dirty_ratelimit, the below elaborated
1030          * code makes use of task_ratelimit to filter out singular points and
1031          * limit the step size.
1032          *
1033          * The below code essentially only uses the relative value of
1034          *
1035          *      task_ratelimit - dirty_ratelimit
1036          *      = (pos_ratio - 1) * dirty_ratelimit
1037          *
1038          * which reflects the direction and size of dirty position error.
1039          */
1040 
1041         /*
1042          * dirty_ratelimit will follow balanced_dirty_ratelimit iff
1043          * task_ratelimit is on the same side of dirty_ratelimit, too.
1044          * For example, when
1045          * - dirty_ratelimit > balanced_dirty_ratelimit
1046          * - dirty_ratelimit > task_ratelimit (dirty pages are above setpoint)
1047          * lowering dirty_ratelimit will help meet both the position and rate
1048          * control targets. Otherwise, don't update dirty_ratelimit if it will
1049          * only help meet the rate target. After all, what the users ultimately
1050          * feel and care are stable dirty rate and small position error.
1051          *
1052          * |task_ratelimit - dirty_ratelimit| is used to limit the step size
1053          * and filter out the singular points of balanced_dirty_ratelimit. Which
1054          * keeps jumping around randomly and can even leap far away at times
1055          * due to the small 200ms estimation period of dirty_rate (we want to
1056          * keep that period small to reduce time lags).
1057          */
1058         step = 0;
1059 
1060         /*
1061          * For strictlimit case, calculations above were based on bdi counters
1062          * and limits (starting from pos_ratio = bdi_position_ratio() and up to
1063          * balanced_dirty_ratelimit = task_ratelimit * write_bw / dirty_rate).
1064          * Hence, to calculate "step" properly, we have to use bdi_dirty as
1065          * "dirty" and bdi_setpoint as "setpoint".
1066          *
1067          * We rampup dirty_ratelimit forcibly if bdi_dirty is low because
1068          * it's possible that bdi_thresh is close to zero due to inactivity
1069          * of backing device (see the implementation of bdi_dirty_limit()).
1070          */
1071         if (unlikely(bdi->capabilities & BDI_CAP_STRICTLIMIT)) {
1072                 dirty = bdi_dirty;
1073                 if (bdi_dirty < 8)
1074                         setpoint = bdi_dirty + 1;
1075                 else
1076                         setpoint = (bdi_thresh +
1077                                     bdi_dirty_limit(bdi, bg_thresh)) / 2;
1078         }
1079 
1080         if (dirty < setpoint) {
1081                 x = min(bdi->balanced_dirty_ratelimit,
1082                          min(balanced_dirty_ratelimit, task_ratelimit));
1083                 if (dirty_ratelimit < x)
1084                         step = x - dirty_ratelimit;
1085         } else {
1086                 x = max(bdi->balanced_dirty_ratelimit,
1087                          max(balanced_dirty_ratelimit, task_ratelimit));
1088                 if (dirty_ratelimit > x)
1089                         step = dirty_ratelimit - x;
1090         }
1091 
1092         /*
1093          * Don't pursue 100% rate matching. It's impossible since the balanced
1094          * rate itself is constantly fluctuating. So decrease the track speed
1095          * when it gets close to the target. Helps eliminate pointless tremors.
1096          */
1097         step >>= dirty_ratelimit / (2 * step + 1);
1098         /*
1099          * Limit the tracking speed to avoid overshooting.
1100          */
1101         step = (step + 7) / 8;
1102 
1103         if (dirty_ratelimit < balanced_dirty_ratelimit)
1104                 dirty_ratelimit += step;
1105         else
1106                 dirty_ratelimit -= step;
1107 
1108         bdi->dirty_ratelimit = max(dirty_ratelimit, 1UL);
1109         bdi->balanced_dirty_ratelimit = balanced_dirty_ratelimit;
1110 
1111         trace_bdi_dirty_ratelimit(bdi, dirty_rate, task_ratelimit);
1112 }
1113 
1114 void __bdi_update_bandwidth(struct backing_dev_info *bdi,
1115                             unsigned long thresh,
1116                             unsigned long bg_thresh,
1117                             unsigned long dirty,
1118                             unsigned long bdi_thresh,
1119                             unsigned long bdi_dirty,
1120                             unsigned long start_time)
1121 {
1122         unsigned long now = jiffies;
1123         unsigned long elapsed = now - bdi->bw_time_stamp;
1124         unsigned long dirtied;
1125         unsigned long written;
1126 
1127         /*
1128          * rate-limit, only update once every 200ms.
1129          */
1130         if (elapsed < BANDWIDTH_INTERVAL)
1131                 return;
1132 
1133         dirtied = percpu_counter_read(&bdi->bdi_stat[BDI_DIRTIED]);
1134         written = percpu_counter_read(&bdi->bdi_stat[BDI_WRITTEN]);
1135 
1136         /*
1137          * Skip quiet periods when disk bandwidth is under-utilized.
1138          * (at least 1s idle time between two flusher runs)
1139          */
1140         if (elapsed > HZ && time_before(bdi->bw_time_stamp, start_time))
1141                 goto snapshot;
1142 
1143         if (thresh) {
1144                 global_update_bandwidth(thresh, dirty, now);
1145                 bdi_update_dirty_ratelimit(bdi, thresh, bg_thresh, dirty,
1146                                            bdi_thresh, bdi_dirty,
1147                                            dirtied, elapsed);
1148         }
1149         bdi_update_write_bandwidth(bdi, elapsed, written);
1150 
1151 snapshot:
1152         bdi->dirtied_stamp = dirtied;
1153         bdi->written_stamp = written;
1154         bdi->bw_time_stamp = now;
1155 }
1156 
1157 static void bdi_update_bandwidth(struct backing_dev_info *bdi,
1158                                  unsigned long thresh,
1159                                  unsigned long bg_thresh,
1160                                  unsigned long dirty,
1161                                  unsigned long bdi_thresh,
1162                                  unsigned long bdi_dirty,
1163                                  unsigned long start_time)
1164 {
1165         if (time_is_after_eq_jiffies(bdi->bw_time_stamp + BANDWIDTH_INTERVAL))
1166                 return;
1167         spin_lock(&bdi->wb.list_lock);
1168         __bdi_update_bandwidth(bdi, thresh, bg_thresh, dirty,
1169                                bdi_thresh, bdi_dirty, start_time);
1170         spin_unlock(&bdi->wb.list_lock);
1171 }
1172 
1173 /*
1174  * After a task dirtied this many pages, balance_dirty_pages_ratelimited()
1175  * will look to see if it needs to start dirty throttling.
1176  *
1177  * If dirty_poll_interval is too low, big NUMA machines will call the expensive
1178  * global_page_state() too often. So scale it near-sqrt to the safety margin
1179  * (the number of pages we may dirty without exceeding the dirty limits).
1180  */
1181 static unsigned long dirty_poll_interval(unsigned long dirty,
1182                                          unsigned long thresh)
1183 {
1184         if (thresh > dirty)
1185                 return 1UL << (ilog2(thresh - dirty) >> 1);
1186 
1187         return 1;
1188 }
1189 
1190 static unsigned long bdi_max_pause(struct backing_dev_info *bdi,
1191                                    unsigned long bdi_dirty)
1192 {
1193         unsigned long bw = bdi->avg_write_bandwidth;
1194         unsigned long t;
1195 
1196         /*
1197          * Limit pause time for small memory systems. If sleeping for too long
1198          * time, a small pool of dirty/writeback pages may go empty and disk go
1199          * idle.
1200          *
1201          * 8 serves as the safety ratio.
1202          */
1203         t = bdi_dirty / (1 + bw / roundup_pow_of_two(1 + HZ / 8));
1204         t++;
1205 
1206         return min_t(unsigned long, t, MAX_PAUSE);
1207 }
1208 
1209 static long bdi_min_pause(struct backing_dev_info *bdi,
1210                           long max_pause,
1211                           unsigned long task_ratelimit,
1212                           unsigned long dirty_ratelimit,
1213                           int *nr_dirtied_pause)
1214 {
1215         long hi = ilog2(bdi->avg_write_bandwidth);
1216         long lo = ilog2(bdi->dirty_ratelimit);
1217         long t;         /* target pause */
1218         long pause;     /* estimated next pause */
1219         int pages;      /* target nr_dirtied_pause */
1220 
1221         /* target for 10ms pause on 1-dd case */
1222         t = max(1, HZ / 100);
1223 
1224         /*
1225          * Scale up pause time for concurrent dirtiers in order to reduce CPU
1226          * overheads.
1227          *
1228          * (N * 10ms) on 2^N concurrent tasks.
1229          */
1230         if (hi > lo)
1231                 t += (hi - lo) * (10 * HZ) / 1024;
1232 
1233         /*
1234          * This is a bit convoluted. We try to base the next nr_dirtied_pause
1235          * on the much more stable dirty_ratelimit. However the next pause time
1236          * will be computed based on task_ratelimit and the two rate limits may
1237          * depart considerably at some time. Especially if task_ratelimit goes
1238          * below dirty_ratelimit/2 and the target pause is max_pause, the next
1239          * pause time will be max_pause*2 _trimmed down_ to max_pause.  As a
1240          * result task_ratelimit won't be executed faithfully, which could
1241          * eventually bring down dirty_ratelimit.
1242          *
1243          * We apply two rules to fix it up:
1244          * 1) try to estimate the next pause time and if necessary, use a lower
1245          *    nr_dirtied_pause so as not to exceed max_pause. When this happens,
1246          *    nr_dirtied_pause will be "dancing" with task_ratelimit.
1247          * 2) limit the target pause time to max_pause/2, so that the normal
1248          *    small fluctuations of task_ratelimit won't trigger rule (1) and
1249          *    nr_dirtied_pause will remain as stable as dirty_ratelimit.
1250          */
1251         t = min(t, 1 + max_pause / 2);
1252         pages = dirty_ratelimit * t / roundup_pow_of_two(HZ);
1253 
1254         /*
1255          * Tiny nr_dirtied_pause is found to hurt I/O performance in the test
1256          * case fio-mmap-randwrite-64k, which does 16*{sync read, async write}.
1257          * When the 16 consecutive reads are often interrupted by some dirty
1258          * throttling pause during the async writes, cfq will go into idles
1259          * (deadline is fine). So push nr_dirtied_pause as high as possible
1260          * until reaches DIRTY_POLL_THRESH=32 pages.
1261          */
1262         if (pages < DIRTY_POLL_THRESH) {
1263                 t = max_pause;
1264                 pages = dirty_ratelimit * t / roundup_pow_of_two(HZ);
1265                 if (pages > DIRTY_POLL_THRESH) {
1266                         pages = DIRTY_POLL_THRESH;
1267                         t = HZ * DIRTY_POLL_THRESH / dirty_ratelimit;
1268                 }
1269         }
1270 
1271         pause = HZ * pages / (task_ratelimit + 1);
1272         if (pause > max_pause) {
1273                 t = max_pause;
1274                 pages = task_ratelimit * t / roundup_pow_of_two(HZ);
1275         }
1276 
1277         *nr_dirtied_pause = pages;
1278         /*
1279          * The minimal pause time will normally be half the target pause time.
1280          */
1281         return pages >= DIRTY_POLL_THRESH ? 1 + t / 2 : t;
1282 }
1283 
1284 static inline void bdi_dirty_limits(struct backing_dev_info *bdi,
1285                                     unsigned long dirty_thresh,
1286                                     unsigned long background_thresh,
1287                                     unsigned long *bdi_dirty,
1288                                     unsigned long *bdi_thresh,
1289                                     unsigned long *bdi_bg_thresh)
1290 {
1291         unsigned long bdi_reclaimable;
1292 
1293         /*
1294          * bdi_thresh is not treated as some limiting factor as
1295          * dirty_thresh, due to reasons
1296          * - in JBOD setup, bdi_thresh can fluctuate a lot
1297          * - in a system with HDD and USB key, the USB key may somehow
1298          *   go into state (bdi_dirty >> bdi_thresh) either because
1299          *   bdi_dirty starts high, or because bdi_thresh drops low.
1300          *   In this case we don't want to hard throttle the USB key
1301          *   dirtiers for 100 seconds until bdi_dirty drops under
1302          *   bdi_thresh. Instead the auxiliary bdi control line in
1303          *   bdi_position_ratio() will let the dirtier task progress
1304          *   at some rate <= (write_bw / 2) for bringing down bdi_dirty.
1305          */
1306         *bdi_thresh = bdi_dirty_limit(bdi, dirty_thresh);
1307 
1308         if (bdi_bg_thresh)
1309                 *bdi_bg_thresh = dirty_thresh ? div_u64((u64)*bdi_thresh *
1310                                                         background_thresh,
1311                                                         dirty_thresh) : 0;
1312 
1313         /*
1314          * In order to avoid the stacked BDI deadlock we need
1315          * to ensure we accurately count the 'dirty' pages when
1316          * the threshold is low.
1317          *
1318          * Otherwise it would be possible to get thresh+n pages
1319          * reported dirty, even though there are thresh-m pages
1320          * actually dirty; with m+n sitting in the percpu
1321          * deltas.
1322          */
1323         if (*bdi_thresh < 2 * bdi_stat_error(bdi)) {
1324                 bdi_reclaimable = bdi_stat_sum(bdi, BDI_RECLAIMABLE);
1325                 *bdi_dirty = bdi_reclaimable +
1326                         bdi_stat_sum(bdi, BDI_WRITEBACK);
1327         } else {
1328                 bdi_reclaimable = bdi_stat(bdi, BDI_RECLAIMABLE);
1329                 *bdi_dirty = bdi_reclaimable +
1330                         bdi_stat(bdi, BDI_WRITEBACK);
1331         }
1332 }
1333 
1334 /*
1335  * balance_dirty_pages() must be called by processes which are generating dirty
1336  * data.  It looks at the number of dirty pages in the machine and will force
1337  * the caller to wait once crossing the (background_thresh + dirty_thresh) / 2.
1338  * If we're over `background_thresh' then the writeback threads are woken to
1339  * perform some writeout.
1340  */
1341 static void balance_dirty_pages(struct address_space *mapping,
1342                                 unsigned long pages_dirtied)
1343 {
1344         unsigned long nr_reclaimable;   /* = file_dirty + unstable_nfs */
1345         unsigned long nr_dirty;  /* = file_dirty + writeback + unstable_nfs */
1346         unsigned long background_thresh;
1347         unsigned long dirty_thresh;
1348         long period;
1349         long pause;
1350         long max_pause;
1351         long min_pause;
1352         int nr_dirtied_pause;
1353         bool dirty_exceeded = false;
1354         unsigned long task_ratelimit;
1355         unsigned long dirty_ratelimit;
1356         unsigned long pos_ratio;
1357         struct backing_dev_info *bdi = mapping->backing_dev_info;
1358         bool strictlimit = bdi->capabilities & BDI_CAP_STRICTLIMIT;
1359         unsigned long start_time = jiffies;
1360 
1361         for (;;) {
1362                 unsigned long now = jiffies;
1363                 unsigned long uninitialized_var(bdi_thresh);
1364                 unsigned long thresh;
1365                 unsigned long uninitialized_var(bdi_dirty);
1366                 unsigned long dirty;
1367                 unsigned long bg_thresh;
1368 
1369                 /*
1370                  * Unstable writes are a feature of certain networked
1371                  * filesystems (i.e. NFS) in which data may have been
1372                  * written to the server's write cache, but has not yet
1373                  * been flushed to permanent storage.
1374                  */
1375                 nr_reclaimable = global_page_state(NR_FILE_DIRTY) +
1376                                         global_page_state(NR_UNSTABLE_NFS);
1377                 nr_dirty = nr_reclaimable + global_page_state(NR_WRITEBACK);
1378 
1379                 global_dirty_limits(&background_thresh, &dirty_thresh);
1380 
1381                 if (unlikely(strictlimit)) {
1382                         bdi_dirty_limits(bdi, dirty_thresh, background_thresh,
1383                                          &bdi_dirty, &bdi_thresh, &bg_thresh);
1384 
1385                         dirty = bdi_dirty;
1386                         thresh = bdi_thresh;
1387                 } else {
1388                         dirty = nr_dirty;
1389                         thresh = dirty_thresh;
1390                         bg_thresh = background_thresh;
1391                 }
1392 
1393                 /*
1394                  * Throttle it only when the background writeback cannot
1395                  * catch-up. This avoids (excessively) small writeouts
1396                  * when the bdi limits are ramping up in case of !strictlimit.
1397                  *
1398                  * In strictlimit case make decision based on the bdi counters
1399                  * and limits. Small writeouts when the bdi limits are ramping
1400                  * up are the price we consciously pay for strictlimit-ing.
1401                  */
1402                 if (dirty <= dirty_freerun_ceiling(thresh, bg_thresh)) {
1403                         current->dirty_paused_when = now;
1404                         current->nr_dirtied = 0;
1405                         current->nr_dirtied_pause =
1406                                 dirty_poll_interval(dirty, thresh);
1407                         break;
1408                 }
1409 
1410                 if (unlikely(!writeback_in_progress(bdi)))
1411                         bdi_start_background_writeback(bdi);
1412 
1413                 if (!strictlimit)
1414                         bdi_dirty_limits(bdi, dirty_thresh, background_thresh,
1415                                          &bdi_dirty, &bdi_thresh, NULL);
1416 
1417                 dirty_exceeded = (bdi_dirty > bdi_thresh) &&
1418                                  ((nr_dirty > dirty_thresh) || strictlimit);
1419                 if (dirty_exceeded && !bdi->dirty_exceeded)
1420                         bdi->dirty_exceeded = 1;
1421 
1422                 bdi_update_bandwidth(bdi, dirty_thresh, background_thresh,
1423                                      nr_dirty, bdi_thresh, bdi_dirty,
1424                                      start_time);
1425 
1426                 dirty_ratelimit = bdi->dirty_ratelimit;
1427                 pos_ratio = bdi_position_ratio(bdi, dirty_thresh,
1428                                                background_thresh, nr_dirty,
1429                                                bdi_thresh, bdi_dirty);
1430                 task_ratelimit = ((u64)dirty_ratelimit * pos_ratio) >>
1431                                                         RATELIMIT_CALC_SHIFT;
1432                 max_pause = bdi_max_pause(bdi, bdi_dirty);
1433                 min_pause = bdi_min_pause(bdi, max_pause,
1434                                           task_ratelimit, dirty_ratelimit,
1435                                           &nr_dirtied_pause);
1436 
1437                 if (unlikely(task_ratelimit == 0)) {
1438                         period = max_pause;
1439                         pause = max_pause;
1440                         goto pause;
1441                 }
1442                 period = HZ * pages_dirtied / task_ratelimit;
1443                 pause = period;
1444                 if (current->dirty_paused_when)
1445                         pause -= now - current->dirty_paused_when;
1446                 /*
1447                  * For less than 1s think time (ext3/4 may block the dirtier
1448                  * for up to 800ms from time to time on 1-HDD; so does xfs,
1449                  * however at much less frequency), try to compensate it in
1450                  * future periods by updating the virtual time; otherwise just
1451                  * do a reset, as it may be a light dirtier.
1452                  */
1453                 if (pause < min_pause) {
1454                         trace_balance_dirty_pages(bdi,
1455                                                   dirty_thresh,
1456                                                   background_thresh,
1457                                                   nr_dirty,
1458                                                   bdi_thresh,
1459                                                   bdi_dirty,
1460                                                   dirty_ratelimit,
1461                                                   task_ratelimit,
1462                                                   pages_dirtied,
1463                                                   period,
1464                                                   min(pause, 0L),
1465                                                   start_time);
1466                         if (pause < -HZ) {
1467                                 current->dirty_paused_when = now;
1468                                 current->nr_dirtied = 0;
1469                         } else if (period) {
1470                                 current->dirty_paused_when += period;
1471                                 current->nr_dirtied = 0;
1472                         } else if (current->nr_dirtied_pause <= pages_dirtied)
1473                                 current->nr_dirtied_pause += pages_dirtied;
1474                         break;
1475                 }
1476                 if (unlikely(pause > max_pause)) {
1477                         /* for occasional dropped task_ratelimit */
1478                         now += min(pause - max_pause, max_pause);
1479                         pause = max_pause;
1480                 }
1481 
1482 pause:
1483                 trace_balance_dirty_pages(bdi,
1484                                           dirty_thresh,
1485                                           background_thresh,
1486                                           nr_dirty,
1487                                           bdi_thresh,
1488                                           bdi_dirty,
1489                                           dirty_ratelimit,
1490                                           task_ratelimit,
1491                                           pages_dirtied,
1492                                           period,
1493                                           pause,
1494                                           start_time);
1495                 __set_current_state(TASK_KILLABLE);
1496                 io_schedule_timeout(pause);
1497 
1498                 current->dirty_paused_when = now + pause;
1499                 current->nr_dirtied = 0;
1500                 current->nr_dirtied_pause = nr_dirtied_pause;
1501 
1502                 /*
1503                  * This is typically equal to (nr_dirty < dirty_thresh) and can
1504                  * also keep "1000+ dd on a slow USB stick" under control.
1505                  */
1506                 if (task_ratelimit)
1507                         break;
1508 
1509                 /*
1510                  * In the case of an unresponding NFS server and the NFS dirty
1511                  * pages exceeds dirty_thresh, give the other good bdi's a pipe
1512                  * to go through, so that tasks on them still remain responsive.
1513                  *
1514                  * In theory 1 page is enough to keep the comsumer-producer
1515                  * pipe going: the flusher cleans 1 page => the task dirties 1
1516                  * more page. However bdi_dirty has accounting errors.  So use
1517                  * the larger and more IO friendly bdi_stat_error.
1518                  */
1519                 if (bdi_dirty <= bdi_stat_error(bdi))
1520                         break;
1521 
1522                 if (fatal_signal_pending(current))
1523                         break;
1524         }
1525 
1526         if (!dirty_exceeded && bdi->dirty_exceeded)
1527                 bdi->dirty_exceeded = 0;
1528 
1529         if (writeback_in_progress(bdi))
1530                 return;
1531 
1532         /*
1533          * In laptop mode, we wait until hitting the higher threshold before
1534          * starting background writeout, and then write out all the way down
1535          * to the lower threshold.  So slow writers cause minimal disk activity.
1536          *
1537          * In normal mode, we start background writeout at the lower
1538          * background_thresh, to keep the amount of dirty memory low.
1539          */
1540         if (laptop_mode)
1541                 return;
1542 
1543         if (nr_reclaimable > background_thresh)
1544                 bdi_start_background_writeback(bdi);
1545 }
1546 
1547 void set_page_dirty_balance(struct page *page)
1548 {
1549         if (set_page_dirty(page)) {
1550                 struct address_space *mapping = page_mapping(page);
1551 
1552                 if (mapping)
1553                         balance_dirty_pages_ratelimited(mapping);
1554         }
1555 }
1556 
1557 static DEFINE_PER_CPU(int, bdp_ratelimits);
1558 
1559 /*
1560  * Normal tasks are throttled by
1561  *      loop {
1562  *              dirty tsk->nr_dirtied_pause pages;
1563  *              take a snap in balance_dirty_pages();
1564  *      }
1565  * However there is a worst case. If every task exit immediately when dirtied
1566  * (tsk->nr_dirtied_pause - 1) pages, balance_dirty_pages() will never be
1567  * called to throttle the page dirties. The solution is to save the not yet
1568  * throttled page dirties in dirty_throttle_leaks on task exit and charge them
1569  * randomly into the running tasks. This works well for the above worst case,
1570  * as the new task will pick up and accumulate the old task's leaked dirty
1571  * count and eventually get throttled.
1572  */
1573 DEFINE_PER_CPU(int, dirty_throttle_leaks) = 0;
1574 
1575 /**
1576  * balance_dirty_pages_ratelimited - balance dirty memory state
1577  * @mapping: address_space which was dirtied
1578  *
1579  * Processes which are dirtying memory should call in here once for each page
1580  * which was newly dirtied.  The function will periodically check the system's
1581  * dirty state and will initiate writeback if needed.
1582  *
1583  * On really big machines, get_writeback_state is expensive, so try to avoid
1584  * calling it too often (ratelimiting).  But once we're over the dirty memory
1585  * limit we decrease the ratelimiting by a lot, to prevent individual processes
1586  * from overshooting the limit by (ratelimit_pages) each.
1587  */
1588 void balance_dirty_pages_ratelimited(struct address_space *mapping)
1589 {
1590         struct backing_dev_info *bdi = mapping->backing_dev_info;
1591         int ratelimit;
1592         int *p;
1593 
1594         if (!bdi_cap_account_dirty(bdi))
1595                 return;
1596 
1597         ratelimit = current->nr_dirtied_pause;
1598         if (bdi->dirty_exceeded)
1599                 ratelimit = min(ratelimit, 32 >> (PAGE_SHIFT - 10));
1600 
1601         preempt_disable();
1602         /*
1603          * This prevents one CPU to accumulate too many dirtied pages without
1604          * calling into balance_dirty_pages(), which can happen when there are
1605          * 1000+ tasks, all of them start dirtying pages at exactly the same
1606          * time, hence all honoured too large initial task->nr_dirtied_pause.
1607          */
1608         p =  this_cpu_ptr(&bdp_ratelimits);
1609         if (unlikely(current->nr_dirtied >= ratelimit))
1610                 *p = 0;
1611         else if (unlikely(*p >= ratelimit_pages)) {
1612                 *p = 0;
1613                 ratelimit = 0;
1614         }
1615         /*
1616          * Pick up the dirtied pages by the exited tasks. This avoids lots of
1617          * short-lived tasks (eg. gcc invocations in a kernel build) escaping
1618          * the dirty throttling and livelock other long-run dirtiers.
1619          */
1620         p = this_cpu_ptr(&dirty_throttle_leaks);
1621         if (*p > 0 && current->nr_dirtied < ratelimit) {
1622                 unsigned long nr_pages_dirtied;
1623                 nr_pages_dirtied = min(*p, ratelimit - current->nr_dirtied);
1624                 *p -= nr_pages_dirtied;
1625                 current->nr_dirtied += nr_pages_dirtied;
1626         }
1627         preempt_enable();
1628 
1629         if (unlikely(current->nr_dirtied >= ratelimit))
1630                 balance_dirty_pages(mapping, current->nr_dirtied);
1631 }
1632 EXPORT_SYMBOL(balance_dirty_pages_ratelimited);
1633 
1634 void throttle_vm_writeout(gfp_t gfp_mask)
1635 {
1636         unsigned long background_thresh;
1637         unsigned long dirty_thresh;
1638 
1639         for ( ; ; ) {
1640                 global_dirty_limits(&background_thresh, &dirty_thresh);
1641                 dirty_thresh = hard_dirty_limit(dirty_thresh);
1642 
1643                 /*
1644                  * Boost the allowable dirty threshold a bit for page
1645                  * allocators so they don't get DoS'ed by heavy writers
1646                  */
1647                 dirty_thresh += dirty_thresh / 10;      /* wheeee... */
1648 
1649                 if (global_page_state(NR_UNSTABLE_NFS) +
1650                         global_page_state(NR_WRITEBACK) <= dirty_thresh)
1651                                 break;
1652                 congestion_wait(BLK_RW_ASYNC, HZ/10);
1653 
1654                 /*
1655                  * The caller might hold locks which can prevent IO completion
1656                  * or progress in the filesystem.  So we cannot just sit here
1657                  * waiting for IO to complete.
1658                  */
1659                 if ((gfp_mask & (__GFP_FS|__GFP_IO)) != (__GFP_FS|__GFP_IO))
1660                         break;
1661         }
1662 }
1663 
1664 /*
1665  * sysctl handler for /proc/sys/vm/dirty_writeback_centisecs
1666  */
1667 int dirty_writeback_centisecs_handler(struct ctl_table *table, int write,
1668         void __user *buffer, size_t *length, loff_t *ppos)
1669 {
1670         proc_dointvec(table, write, buffer, length, ppos);
1671         return 0;
1672 }
1673 
1674 #ifdef CONFIG_BLOCK
1675 void laptop_mode_timer_fn(unsigned long data)
1676 {
1677         struct request_queue *q = (struct request_queue *)data;
1678         int nr_pages = global_page_state(NR_FILE_DIRTY) +
1679                 global_page_state(NR_UNSTABLE_NFS);
1680 
1681         /*
1682          * We want to write everything out, not just down to the dirty
1683          * threshold
1684          */
1685         if (bdi_has_dirty_io(&q->backing_dev_info))
1686                 bdi_start_writeback(&q->backing_dev_info, nr_pages,
1687                                         WB_REASON_LAPTOP_TIMER);
1688 }
1689 
1690 /*
1691  * We've spun up the disk and we're in laptop mode: schedule writeback
1692  * of all dirty data a few seconds from now.  If the flush is already scheduled
1693  * then push it back - the user is still using the disk.
1694  */
1695 void laptop_io_completion(struct backing_dev_info *info)
1696 {
1697         mod_timer(&info->laptop_mode_wb_timer, jiffies + laptop_mode);
1698 }
1699 
1700 /*
1701  * We're in laptop mode and we've just synced. The sync's writes will have
1702  * caused another writeback to be scheduled by laptop_io_completion.
1703  * Nothing needs to be written back anymore, so we unschedule the writeback.
1704  */
1705 void laptop_sync_completion(void)
1706 {
1707         struct backing_dev_info *bdi;
1708 
1709         rcu_read_lock();
1710 
1711         list_for_each_entry_rcu(bdi, &bdi_list, bdi_list)
1712                 del_timer(&bdi->laptop_mode_wb_timer);
1713 
1714         rcu_read_unlock();
1715 }
1716 #endif
1717 
1718 /*
1719  * If ratelimit_pages is too high then we can get into dirty-data overload
1720  * if a large number of processes all perform writes at the same time.
1721  * If it is too low then SMP machines will call the (expensive)
1722  * get_writeback_state too often.
1723  *
1724  * Here we set ratelimit_pages to a level which ensures that when all CPUs are
1725  * dirtying in parallel, we cannot go more than 3% (1/32) over the dirty memory
1726  * thresholds.
1727  */
1728 
1729 void writeback_set_ratelimit(void)
1730 {
1731         unsigned long background_thresh;
1732         unsigned long dirty_thresh;
1733         global_dirty_limits(&background_thresh, &dirty_thresh);
1734         global_dirty_limit = dirty_thresh;
1735         ratelimit_pages = dirty_thresh / (num_online_cpus() * 32);
1736         if (ratelimit_pages < 16)
1737                 ratelimit_pages = 16;
1738 }
1739 
1740 static int
1741 ratelimit_handler(struct notifier_block *self, unsigned long action,
1742                   void *hcpu)
1743 {
1744 
1745         switch (action & ~CPU_TASKS_FROZEN) {
1746         case CPU_ONLINE:
1747         case CPU_DEAD:
1748                 writeback_set_ratelimit();
1749                 return NOTIFY_OK;
1750         default:
1751                 return NOTIFY_DONE;
1752         }
1753 }
1754 
1755 static struct notifier_block ratelimit_nb = {
1756         .notifier_call  = ratelimit_handler,
1757         .next           = NULL,
1758 };
1759 
1760 /*
1761  * Called early on to tune the page writeback dirty limits.
1762  *
1763  * We used to scale dirty pages according to how total memory
1764  * related to pages that could be allocated for buffers (by
1765  * comparing nr_free_buffer_pages() to vm_total_pages.
1766  *
1767  * However, that was when we used "dirty_ratio" to scale with
1768  * all memory, and we don't do that any more. "dirty_ratio"
1769  * is now applied to total non-HIGHPAGE memory (by subtracting
1770  * totalhigh_pages from vm_total_pages), and as such we can't
1771  * get into the old insane situation any more where we had
1772  * large amounts of dirty pages compared to a small amount of
1773  * non-HIGHMEM memory.
1774  *
1775  * But we might still want to scale the dirty_ratio by how
1776  * much memory the box has..
1777  */
1778 void __init page_writeback_init(void)
1779 {
1780         writeback_set_ratelimit();
1781         register_cpu_notifier(&ratelimit_nb);
1782 
1783         fprop_global_init(&writeout_completions);
1784 }
1785 
1786 /**
1787  * tag_pages_for_writeback - tag pages to be written by write_cache_pages
1788  * @mapping: address space structure to write
1789  * @start: starting page index
1790  * @end: ending page index (inclusive)
1791  *
1792  * This function scans the page range from @start to @end (inclusive) and tags
1793  * all pages that have DIRTY tag set with a special TOWRITE tag. The idea is
1794  * that write_cache_pages (or whoever calls this function) will then use
1795  * TOWRITE tag to identify pages eligible for writeback.  This mechanism is
1796  * used to avoid livelocking of writeback by a process steadily creating new
1797  * dirty pages in the file (thus it is important for this function to be quick
1798  * so that it can tag pages faster than a dirtying process can create them).
1799  */
1800 /*
1801  * We tag pages in batches of WRITEBACK_TAG_BATCH to reduce tree_lock latency.
1802  */
1803 void tag_pages_for_writeback(struct address_space *mapping,
1804                              pgoff_t start, pgoff_t end)
1805 {
1806 #define WRITEBACK_TAG_BATCH 4096
1807         unsigned long tagged;
1808 
1809         do {
1810                 spin_lock_irq(&mapping->tree_lock);
1811                 tagged = radix_tree_range_tag_if_tagged(&mapping->page_tree,
1812                                 &start, end, WRITEBACK_TAG_BATCH,
1813                                 PAGECACHE_TAG_DIRTY, PAGECACHE_TAG_TOWRITE);
1814                 spin_unlock_irq(&mapping->tree_lock);
1815                 WARN_ON_ONCE(tagged > WRITEBACK_TAG_BATCH);
1816                 cond_resched();
1817                 /* We check 'start' to handle wrapping when end == ~0UL */
1818         } while (tagged >= WRITEBACK_TAG_BATCH && start);
1819 }
1820 EXPORT_SYMBOL(tag_pages_for_writeback);
1821 
1822 /**
1823  * write_cache_pages - walk the list of dirty pages of the given address space and write all of them.
1824  * @mapping: address space structure to write
1825  * @wbc: subtract the number of written pages from *@wbc->nr_to_write
1826  * @writepage: function called for each page
1827  * @data: data passed to writepage function
1828  *
1829  * If a page is already under I/O, write_cache_pages() skips it, even
1830  * if it's dirty.  This is desirable behaviour for memory-cleaning writeback,
1831  * but it is INCORRECT for data-integrity system calls such as fsync().  fsync()
1832  * and msync() need to guarantee that all the data which was dirty at the time
1833  * the call was made get new I/O started against them.  If wbc->sync_mode is
1834  * WB_SYNC_ALL then we were called for data integrity and we must wait for
1835  * existing IO to complete.
1836  *
1837  * To avoid livelocks (when other process dirties new pages), we first tag
1838  * pages which should be written back with TOWRITE tag and only then start
1839  * writing them. For data-integrity sync we have to be careful so that we do
1840  * not miss some pages (e.g., because some other process has cleared TOWRITE
1841  * tag we set). The rule we follow is that TOWRITE tag can be cleared only
1842  * by the process clearing the DIRTY tag (and submitting the page for IO).
1843  */
1844 int write_cache_pages(struct address_space *mapping,
1845                       struct writeback_control *wbc, writepage_t writepage,
1846                       void *data)
1847 {
1848         int ret = 0;
1849         int done = 0;
1850         struct pagevec pvec;
1851         int nr_pages;
1852         pgoff_t uninitialized_var(writeback_index);
1853         pgoff_t index;
1854         pgoff_t end;            /* Inclusive */
1855         pgoff_t done_index;
1856         int cycled;
1857         int range_whole = 0;
1858         int tag;
1859 
1860         pagevec_init(&pvec, 0);
1861         if (wbc->range_cyclic) {
1862                 writeback_index = mapping->writeback_index; /* prev offset */
1863                 index = writeback_index;
1864                 if (index == 0)
1865                         cycled = 1;
1866                 else
1867                         cycled = 0;
1868                 end = -1;
1869         } else {
1870                 index = wbc->range_start >> PAGE_CACHE_SHIFT;
1871                 end = wbc->range_end >> PAGE_CACHE_SHIFT;
1872                 if (wbc->range_start == 0 && wbc->range_end == LLONG_MAX)
1873                         range_whole = 1;
1874                 cycled = 1; /* ignore range_cyclic tests */
1875         }
1876         if (wbc->sync_mode == WB_SYNC_ALL || wbc->tagged_writepages)
1877                 tag = PAGECACHE_TAG_TOWRITE;
1878         else
1879                 tag = PAGECACHE_TAG_DIRTY;
1880 retry:
1881         if (wbc->sync_mode == WB_SYNC_ALL || wbc->tagged_writepages)
1882                 tag_pages_for_writeback(mapping, index, end);
1883         done_index = index;
1884         while (!done && (index <= end)) {
1885                 int i;
1886 
1887                 nr_pages = pagevec_lookup_tag(&pvec, mapping, &index, tag,
1888                               min(end - index, (pgoff_t)PAGEVEC_SIZE-1) + 1);
1889                 if (nr_pages == 0)
1890                         break;
1891 
1892                 for (i = 0; i < nr_pages; i++) {
1893                         struct page *page = pvec.pages[i];
1894 
1895                         /*
1896                          * At this point, the page may be truncated or
1897                          * invalidated (changing page->mapping to NULL), or
1898                          * even swizzled back from swapper_space to tmpfs file
1899                          * mapping. However, page->index will not change
1900                          * because we have a reference on the page.
1901                          */
1902                         if (page->index > end) {
1903                                 /*
1904                                  * can't be range_cyclic (1st pass) because
1905                                  * end == -1 in that case.
1906                                  */
1907                                 done = 1;
1908                                 break;
1909                         }
1910 
1911                         done_index = page->index;
1912 
1913                         lock_page(page);
1914 
1915                         /*
1916                          * Page truncated or invalidated. We can freely skip it
1917                          * then, even for data integrity operations: the page
1918                          * has disappeared concurrently, so there could be no
1919                          * real expectation of this data interity operation
1920                          * even if there is now a new, dirty page at the same
1921                          * pagecache address.
1922                          */
1923                         if (unlikely(page->mapping != mapping)) {
1924 continue_unlock:
1925                                 unlock_page(page);
1926                                 continue;
1927                         }
1928 
1929                         if (!PageDirty(page)) {
1930                                 /* someone wrote it for us */
1931                                 goto continue_unlock;
1932                         }
1933 
1934                         if (PageWriteback(page)) {
1935                                 if (wbc->sync_mode != WB_SYNC_NONE)
1936                                         wait_on_page_writeback(page);
1937                                 else
1938                                         goto continue_unlock;
1939                         }
1940 
1941                         BUG_ON(PageWriteback(page));
1942                         if (!clear_page_dirty_for_io(page))
1943                                 goto continue_unlock;
1944 
1945                         trace_wbc_writepage(wbc, mapping->backing_dev_info);
1946                         ret = (*writepage)(page, wbc, data);
1947                         if (unlikely(ret)) {
1948                                 if (ret == AOP_WRITEPAGE_ACTIVATE) {
1949                                         unlock_page(page);
1950                                         ret = 0;
1951                                 } else {
1952                                         /*
1953                                          * done_index is set past this page,
1954                                          * so media errors will not choke
1955                                          * background writeout for the entire
1956                                          * file. This has consequences for
1957                                          * range_cyclic semantics (ie. it may
1958                                          * not be suitable for data integrity
1959                                          * writeout).
1960                                          */
1961                                         done_index = page->index + 1;
1962                                         done = 1;
1963                                         break;
1964                                 }
1965                         }
1966 
1967                         /*
1968                          * We stop writing back only if we are not doing
1969                          * integrity sync. In case of integrity sync we have to
1970                          * keep going until we have written all the pages
1971                          * we tagged for writeback prior to entering this loop.
1972                          */
1973                         if (--wbc->nr_to_write <= 0 &&
1974                             wbc->sync_mode == WB_SYNC_NONE) {
1975                                 done = 1;
1976                                 break;
1977                         }
1978                 }
1979                 pagevec_release(&pvec);
1980                 cond_resched();
1981         }
1982         if (!cycled && !done) {
1983                 /*
1984                  * range_cyclic:
1985                  * We hit the last page and there is more work to be done: wrap
1986                  * back to the start of the file
1987                  */
1988                 cycled = 1;
1989                 index = 0;
1990                 end = writeback_index - 1;
1991                 goto retry;
1992         }
1993         if (wbc->range_cyclic || (range_whole && wbc->nr_to_write > 0))
1994                 mapping->writeback_index = done_index;
1995 
1996         return ret;
1997 }
1998 EXPORT_SYMBOL(write_cache_pages);
1999 
2000 /*
2001  * Function used by generic_writepages to call the real writepage
2002  * function and set the mapping flags on error
2003  */
2004 static int __writepage(struct page *page, struct writeback_control *wbc,
2005                        void *data)
2006 {
2007         struct address_space *mapping = data;
2008         int ret = mapping->a_ops->writepage(page, wbc);
2009         mapping_set_error(mapping, ret);
2010         return ret;
2011 }
2012 
2013 /**
2014  * generic_writepages - walk the list of dirty pages of the given address space and writepage() all of them.
2015  * @mapping: address space structure to write
2016  * @wbc: subtract the number of written pages from *@wbc->nr_to_write
2017  *
2018  * This is a library function, which implements the writepages()
2019  * address_space_operation.
2020  */
2021 int generic_writepages(struct address_space *mapping,
2022                        struct writeback_control *wbc)
2023 {
2024         struct blk_plug plug;
2025         int ret;
2026 
2027         /* deal with chardevs and other special file */
2028         if (!mapping->a_ops->writepage)
2029                 return 0;
2030 
2031         blk_start_plug(&plug);
2032         ret = write_cache_pages(mapping, wbc, __writepage, mapping);
2033         blk_finish_plug(&plug);
2034         return ret;
2035 }
2036 
2037 EXPORT_SYMBOL(generic_writepages);
2038 
2039 int do_writepages(struct address_space *mapping, struct writeback_control *wbc)
2040 {
2041         int ret;
2042 
2043         if (wbc->nr_to_write <= 0)
2044                 return 0;
2045         if (mapping->a_ops->writepages)
2046                 ret = mapping->a_ops->writepages(mapping, wbc);
2047         else
2048                 ret = generic_writepages(mapping, wbc);
2049         return ret;
2050 }
2051 
2052 /**
2053  * write_one_page - write out a single page and optionally wait on I/O
2054  * @page: the page to write
2055  * @wait: if true, wait on writeout
2056  *
2057  * The page must be locked by the caller and will be unlocked upon return.
2058  *
2059  * write_one_page() returns a negative error code if I/O failed.
2060  */
2061 int write_one_page(struct page *page, int wait)
2062 {
2063         struct address_space *mapping = page->mapping;
2064         int ret = 0;
2065         struct writeback_control wbc = {
2066                 .sync_mode = WB_SYNC_ALL,
2067                 .nr_to_write = 1,
2068         };
2069 
2070         BUG_ON(!PageLocked(page));
2071 
2072         if (wait)
2073                 wait_on_page_writeback(page);
2074 
2075         if (clear_page_dirty_for_io(page)) {
2076                 page_cache_get(page);
2077                 ret = mapping->a_ops->writepage(page, &wbc);
2078                 if (ret == 0 && wait) {
2079                         wait_on_page_writeback(page);
2080                         if (PageError(page))
2081                                 ret = -EIO;
2082                 }
2083                 page_cache_release(page);
2084         } else {
2085                 unlock_page(page);
2086         }
2087         return ret;
2088 }
2089 EXPORT_SYMBOL(write_one_page);
2090 
2091 /*
2092  * For address_spaces which do not use buffers nor write back.
2093  */
2094 int __set_page_dirty_no_writeback(struct page *page)
2095 {
2096         if (!PageDirty(page))
2097                 return !TestSetPageDirty(page);
2098         return 0;
2099 }
2100 
2101 /*
2102  * Helper function for set_page_dirty family.
2103  * NOTE: This relies on being atomic wrt interrupts.
2104  */
2105 void account_page_dirtied(struct page *page, struct address_space *mapping)
2106 {
2107         trace_writeback_dirty_page(page, mapping);
2108 
2109         if (mapping_cap_account_dirty(mapping)) {
2110                 __inc_zone_page_state(page, NR_FILE_DIRTY);
2111                 __inc_zone_page_state(page, NR_DIRTIED);
2112                 __inc_bdi_stat(mapping->backing_dev_info, BDI_RECLAIMABLE);
2113                 __inc_bdi_stat(mapping->backing_dev_info, BDI_DIRTIED);
2114                 task_io_account_write(PAGE_CACHE_SIZE);
2115                 current->nr_dirtied++;
2116                 this_cpu_inc(bdp_ratelimits);
2117         }
2118 }
2119 EXPORT_SYMBOL(account_page_dirtied);
2120 
2121 /*
2122  * Helper function for set_page_writeback family.
2123  *
2124  * The caller must hold mem_cgroup_begin/end_update_page_stat() lock
2125  * while calling this function.
2126  * See test_set_page_writeback for example.
2127  *
2128  * NOTE: Unlike account_page_dirtied this does not rely on being atomic
2129  * wrt interrupts.
2130  */
2131 void account_page_writeback(struct page *page)
2132 {
2133         mem_cgroup_inc_page_stat(page, MEM_CGROUP_STAT_WRITEBACK);
2134         inc_zone_page_state(page, NR_WRITEBACK);
2135 }
2136 EXPORT_SYMBOL(account_page_writeback);
2137 
2138 /*
2139  * For address_spaces which do not use buffers.  Just tag the page as dirty in
2140  * its radix tree.
2141  *
2142  * This is also used when a single buffer is being dirtied: we want to set the
2143  * page dirty in that case, but not all the buffers.  This is a "bottom-up"
2144  * dirtying, whereas __set_page_dirty_buffers() is a "top-down" dirtying.
2145  *
2146  * Most callers have locked the page, which pins the address_space in memory.
2147  * But zap_pte_range() does not lock the page, however in that case the
2148  * mapping is pinned by the vma's ->vm_file reference.
2149  *
2150  * We take care to handle the case where the page was truncated from the
2151  * mapping by re-checking page_mapping() inside tree_lock.
2152  */
2153 int __set_page_dirty_nobuffers(struct page *page)
2154 {
2155         if (!TestSetPageDirty(page)) {
2156                 struct address_space *mapping = page_mapping(page);
2157                 struct address_space *mapping2;
2158                 unsigned long flags;
2159 
2160                 if (!mapping)
2161                         return 1;
2162 
2163                 spin_lock_irqsave(&mapping->tree_lock, flags);
2164                 mapping2 = page_mapping(page);
2165                 if (mapping2) { /* Race with truncate? */
2166                         BUG_ON(mapping2 != mapping);
2167                         WARN_ON_ONCE(!PagePrivate(page) && !PageUptodate(page));
2168                         account_page_dirtied(page, mapping);
2169                         radix_tree_tag_set(&mapping->page_tree,
2170                                 page_index(page), PAGECACHE_TAG_DIRTY);
2171                 }
2172                 spin_unlock_irqrestore(&mapping->tree_lock, flags);
2173                 if (mapping->host) {
2174                         /* !PageAnon && !swapper_space */
2175                         __mark_inode_dirty(mapping->host, I_DIRTY_PAGES);
2176                 }
2177                 return 1;
2178         }
2179         return 0;
2180 }
2181 EXPORT_SYMBOL(__set_page_dirty_nobuffers);
2182 
2183 /*
2184  * Call this whenever redirtying a page, to de-account the dirty counters
2185  * (NR_DIRTIED, BDI_DIRTIED, tsk->nr_dirtied), so that they match the written
2186  * counters (NR_WRITTEN, BDI_WRITTEN) in long term. The mismatches will lead to
2187  * systematic errors in balanced_dirty_ratelimit and the dirty pages position
2188  * control.
2189  */
2190 void account_page_redirty(struct page *page)
2191 {
2192         struct address_space *mapping = page->mapping;
2193         if (mapping && mapping_cap_account_dirty(mapping)) {
2194                 current->nr_dirtied--;
2195                 dec_zone_page_state(page, NR_DIRTIED);
2196                 dec_bdi_stat(mapping->backing_dev_info, BDI_DIRTIED);
2197         }
2198 }
2199 EXPORT_SYMBOL(account_page_redirty);
2200 
2201 /*
2202  * When a writepage implementation decides that it doesn't want to write this
2203  * page for some reason, it should redirty the locked page via
2204  * redirty_page_for_writepage() and it should then unlock the page and return 0
2205  */
2206 int redirty_page_for_writepage(struct writeback_control *wbc, struct page *page)
2207 {
2208         wbc->pages_skipped++;
2209         account_page_redirty(page);
2210         return __set_page_dirty_nobuffers(page);
2211 }
2212 EXPORT_SYMBOL(redirty_page_for_writepage);
2213 
2214 /*
2215  * Dirty a page.
2216  *
2217  * For pages with a mapping this should be done under the page lock
2218  * for the benefit of asynchronous memory errors who prefer a consistent
2219  * dirty state. This rule can be broken in some special cases,
2220  * but should be better not to.
2221  *
2222  * If the mapping doesn't provide a set_page_dirty a_op, then
2223  * just fall through and assume that it wants buffer_heads.
2224  */
2225 int set_page_dirty(struct page *page)
2226 {
2227         struct address_space *mapping = page_mapping(page);
2228 
2229         if (likely(mapping)) {
2230                 int (*spd)(struct page *) = mapping->a_ops->set_page_dirty;
2231                 /*
2232                  * readahead/lru_deactivate_page could remain
2233                  * PG_readahead/PG_reclaim due to race with end_page_writeback
2234                  * About readahead, if the page is written, the flags would be
2235                  * reset. So no problem.
2236                  * About lru_deactivate_page, if the page is redirty, the flag
2237                  * will be reset. So no problem. but if the page is used by readahead
2238                  * it will confuse readahead and make it restart the size rampup
2239                  * process. But it's a trivial problem.
2240                  */
2241                 ClearPageReclaim(page);
2242 #ifdef CONFIG_BLOCK
2243                 if (!spd)
2244                         spd = __set_page_dirty_buffers;
2245 #endif
2246                 return (*spd)(page);
2247         }
2248         if (!PageDirty(page)) {
2249                 if (!TestSetPageDirty(page))
2250                         return 1;
2251         }
2252         return 0;
2253 }
2254 EXPORT_SYMBOL(set_page_dirty);
2255 
2256 /*
2257  * set_page_dirty() is racy if the caller has no reference against
2258  * page->mapping->host, and if the page is unlocked.  This is because another
2259  * CPU could truncate the page off the mapping and then free the mapping.
2260  *
2261  * Usually, the page _is_ locked, or the caller is a user-space process which
2262  * holds a reference on the inode by having an open file.
2263  *
2264  * In other cases, the page should be locked before running set_page_dirty().
2265  */
2266 int set_page_dirty_lock(struct page *page)
2267 {
2268         int ret;
2269 
2270         lock_page(page);
2271         ret = set_page_dirty(page);
2272         unlock_page(page);
2273         return ret;
2274 }
2275 EXPORT_SYMBOL(set_page_dirty_lock);
2276 
2277 /*
2278  * Clear a page's dirty flag, while caring for dirty memory accounting.
2279  * Returns true if the page was previously dirty.
2280  *
2281  * This is for preparing to put the page under writeout.  We leave the page
2282  * tagged as dirty in the radix tree so that a concurrent write-for-sync
2283  * can discover it via a PAGECACHE_TAG_DIRTY walk.  The ->writepage
2284  * implementation will run either set_page_writeback() or set_page_dirty(),
2285  * at which stage we bring the page's dirty flag and radix-tree dirty tag
2286  * back into sync.
2287  *
2288  * This incoherency between the page's dirty flag and radix-tree tag is
2289  * unfortunate, but it only exists while the page is locked.
2290  */
2291 int clear_page_dirty_for_io(struct page *page)
2292 {
2293         struct address_space *mapping = page_mapping(page);
2294 
2295         BUG_ON(!PageLocked(page));
2296 
2297         if (mapping && mapping_cap_account_dirty(mapping)) {
2298                 /*
2299                  * Yes, Virginia, this is indeed insane.
2300                  *
2301                  * We use this sequence to make sure that
2302                  *  (a) we account for dirty stats properly
2303                  *  (b) we tell the low-level filesystem to
2304                  *      mark the whole page dirty if it was
2305                  *      dirty in a pagetable. Only to then
2306                  *  (c) clean the page again and return 1 to
2307                  *      cause the writeback.
2308                  *
2309                  * This way we avoid all nasty races with the
2310                  * dirty bit in multiple places and clearing
2311                  * them concurrently from different threads.
2312                  *
2313                  * Note! Normally the "set_page_dirty(page)"
2314                  * has no effect on the actual dirty bit - since
2315                  * that will already usually be set. But we
2316                  * need the side effects, and it can help us
2317                  * avoid races.
2318                  *
2319                  * We basically use the page "master dirty bit"
2320                  * as a serialization point for all the different
2321                  * threads doing their things.
2322                  */
2323                 if (page_mkclean(page))
2324                         set_page_dirty(page);
2325                 /*
2326                  * We carefully synchronise fault handlers against
2327                  * installing a dirty pte and marking the page dirty
2328                  * at this point. We do this by having them hold the
2329                  * page lock at some point after installing their
2330                  * pte, but before marking the page dirty.
2331                  * Pages are always locked coming in here, so we get
2332                  * the desired exclusion. See mm/memory.c:do_wp_page()
2333                  * for more comments.
2334                  */
2335                 if (TestClearPageDirty(page)) {
2336                         dec_zone_page_state(page, NR_FILE_DIRTY);
2337                         dec_bdi_stat(mapping->backing_dev_info,
2338                                         BDI_RECLAIMABLE);
2339                         return 1;
2340                 }
2341                 return 0;
2342         }
2343         return TestClearPageDirty(page);
2344 }
2345 EXPORT_SYMBOL(clear_page_dirty_for_io);
2346 
2347 int test_clear_page_writeback(struct page *page)
2348 {
2349         struct address_space *mapping = page_mapping(page);
2350         int ret;
2351         bool locked;
2352         unsigned long memcg_flags;
2353 
2354         mem_cgroup_begin_update_page_stat(page, &locked, &memcg_flags);
2355         if (mapping) {
2356                 struct backing_dev_info *bdi = mapping->backing_dev_info;
2357                 unsigned long flags;
2358 
2359                 spin_lock_irqsave(&mapping->tree_lock, flags);
2360                 ret = TestClearPageWriteback(page);
2361                 if (ret) {
2362                         radix_tree_tag_clear(&mapping->page_tree,
2363                                                 page_index(page),
2364                                                 PAGECACHE_TAG_WRITEBACK);
2365                         if (bdi_cap_account_writeback(bdi)) {
2366                                 __dec_bdi_stat(bdi, BDI_WRITEBACK);
2367                                 __bdi_writeout_inc(bdi);
2368                         }
2369                 }
2370                 spin_unlock_irqrestore(&mapping->tree_lock, flags);
2371         } else {
2372                 ret = TestClearPageWriteback(page);
2373         }
2374         if (ret) {
2375                 mem_cgroup_dec_page_stat(page, MEM_CGROUP_STAT_WRITEBACK);
2376                 dec_zone_page_state(page, NR_WRITEBACK);
2377                 inc_zone_page_state(page, NR_WRITTEN);
2378         }
2379         mem_cgroup_end_update_page_stat(page, &locked, &memcg_flags);
2380         return ret;
2381 }
2382 
2383 int __test_set_page_writeback(struct page *page, bool keep_write)
2384 {
2385         struct address_space *mapping = page_mapping(page);
2386         int ret;
2387         bool locked;
2388         unsigned long memcg_flags;
2389 
2390         mem_cgroup_begin_update_page_stat(page, &locked, &memcg_flags);
2391         if (mapping) {
2392                 struct backing_dev_info *bdi = mapping->backing_dev_info;
2393                 unsigned long flags;
2394 
2395                 spin_lock_irqsave(&mapping->tree_lock, flags);
2396                 ret = TestSetPageWriteback(page);
2397                 if (!ret) {
2398                         radix_tree_tag_set(&mapping->page_tree,
2399                                                 page_index(page),
2400                                                 PAGECACHE_TAG_WRITEBACK);
2401                         if (bdi_cap_account_writeback(bdi))
2402                                 __inc_bdi_stat(bdi, BDI_WRITEBACK);
2403                 }
2404                 if (!PageDirty(page))
2405                         radix_tree_tag_clear(&mapping->page_tree,
2406                                                 page_index(page),
2407                                                 PAGECACHE_TAG_DIRTY);
2408                 if (!keep_write)
2409                         radix_tree_tag_clear(&mapping->page_tree,
2410                                                 page_index(page),
2411                                                 PAGECACHE_TAG_TOWRITE);
2412                 spin_unlock_irqrestore(&mapping->tree_lock, flags);
2413         } else {
2414                 ret = TestSetPageWriteback(page);
2415         }
2416         if (!ret)
2417                 account_page_writeback(page);
2418         mem_cgroup_end_update_page_stat(page, &locked, &memcg_flags);
2419         return ret;
2420 
2421 }
2422 EXPORT_SYMBOL(__test_set_page_writeback);
2423 
2424 /*
2425  * Return true if any of the pages in the mapping are marked with the
2426  * passed tag.
2427  */
2428 int mapping_tagged(struct address_space *mapping, int tag)
2429 {
2430         return radix_tree_tagged(&mapping->page_tree, tag);
2431 }
2432 EXPORT_SYMBOL(mapping_tagged);
2433 
2434 /**
2435  * wait_for_stable_page() - wait for writeback to finish, if necessary.
2436  * @page:       The page to wait on.
2437  *
2438  * This function determines if the given page is related to a backing device
2439  * that requires page contents to be held stable during writeback.  If so, then
2440  * it will wait for any pending writeback to complete.
2441  */
2442 void wait_for_stable_page(struct page *page)
2443 {
2444         struct address_space *mapping = page_mapping(page);
2445         struct backing_dev_info *bdi = mapping->backing_dev_info;
2446 
2447         if (!bdi_cap_stable_pages_required(bdi))
2448                 return;
2449 
2450         wait_on_page_writeback(page);
2451 }
2452 EXPORT_SYMBOL_GPL(wait_for_stable_page);
2453 

This page was automatically generated by LXR 0.3.1 (source).  •  Linux is a registered trademark of Linus Torvalds  •  Contact us