75d7f48b79bba537d522cbd709138d48196d6782
[linux-3.10.git] / mm / page-writeback.c
1 /*
2  * mm/page-writeback.c.
3  *
4  * Copyright (C) 2002, Linus Torvalds.
5  *
6  * Contains functions related to writing back dirty pages at the
7  * address_space level.
8  *
9  * 10Apr2002    akpm@zip.com.au
10  *              Initial version
11  */
12
13 #include <linux/kernel.h>
14 #include <linux/module.h>
15 #include <linux/spinlock.h>
16 #include <linux/fs.h>
17 #include <linux/mm.h>
18 #include <linux/swap.h>
19 #include <linux/slab.h>
20 #include <linux/pagemap.h>
21 #include <linux/writeback.h>
22 #include <linux/init.h>
23 #include <linux/backing-dev.h>
24 #include <linux/blkdev.h>
25 #include <linux/mpage.h>
26 #include <linux/percpu.h>
27 #include <linux/notifier.h>
28 #include <linux/smp.h>
29 #include <linux/sysctl.h>
30 #include <linux/cpu.h>
31 #include <linux/syscalls.h>
32
33 /*
34  * The maximum number of pages to writeout in a single bdflush/kupdate
35  * operation.  We do this so we don't hold I_LOCK against an inode for
36  * enormous amounts of time, which would block a userspace task which has
37  * been forced to throttle against that inode.  Also, the code reevaluates
38  * the dirty each time it has written this many pages.
39  */
40 #define MAX_WRITEBACK_PAGES     1024
41
42 /*
43  * After a CPU has dirtied this many pages, balance_dirty_pages_ratelimited
44  * will look to see if it needs to force writeback or throttling.
45  */
46 static long ratelimit_pages = 32;
47
48 static long total_pages;        /* The total number of pages in the machine. */
49 static int dirty_exceeded __cacheline_aligned_in_smp;   /* Dirty mem may be over limit */
50
51 /*
52  * When balance_dirty_pages decides that the caller needs to perform some
53  * non-background writeback, this is how many pages it will attempt to write.
54  * It should be somewhat larger than RATELIMIT_PAGES to ensure that reasonably
55  * large amounts of I/O are submitted.
56  */
57 static inline long sync_writeback_pages(void)
58 {
59         return ratelimit_pages + ratelimit_pages / 2;
60 }
61
62 /* The following parameters are exported via /proc/sys/vm */
63
64 /*
65  * Start background writeback (via pdflush) at this percentage
66  */
67 int dirty_background_ratio = 10;
68
69 /*
70  * The generator of dirty data starts writeback at this percentage
71  */
72 int vm_dirty_ratio = 40;
73
74 /*
75  * The interval between `kupdate'-style writebacks, in jiffies
76  */
77 int dirty_writeback_interval = 5 * HZ;
78
79 /*
80  * The longest number of jiffies for which data is allowed to remain dirty
81  */
82 int dirty_expire_interval = 30 * HZ;
83
84 /*
85  * Flag that makes the machine dump writes/reads and block dirtyings.
86  */
87 int block_dump;
88
89 /*
90  * Flag that puts the machine in "laptop mode". Doubles as a timeout in jiffies:
91  * a full sync is triggered after this time elapses without any disk activity.
92  */
93 int laptop_mode;
94
95 EXPORT_SYMBOL(laptop_mode);
96
97 /* End of sysctl-exported parameters */
98
99
100 static void background_writeout(unsigned long _min_pages);
101
102 struct writeback_state
103 {
104         unsigned long nr_dirty;
105         unsigned long nr_unstable;
106         unsigned long nr_mapped;
107         unsigned long nr_writeback;
108 };
109
110 static void get_writeback_state(struct writeback_state *wbs)
111 {
112         wbs->nr_dirty = read_page_state(nr_dirty);
113         wbs->nr_unstable = read_page_state(nr_unstable);
114         wbs->nr_mapped = read_page_state(nr_mapped);
115         wbs->nr_writeback = read_page_state(nr_writeback);
116 }
117
118 /*
119  * Work out the current dirty-memory clamping and background writeout
120  * thresholds.
121  *
122  * The main aim here is to lower them aggressively if there is a lot of mapped
123  * memory around.  To avoid stressing page reclaim with lots of unreclaimable
124  * pages.  It is better to clamp down on writers than to start swapping, and
125  * performing lots of scanning.
126  *
127  * We only allow 1/2 of the currently-unmapped memory to be dirtied.
128  *
129  * We don't permit the clamping level to fall below 5% - that is getting rather
130  * excessive.
131  *
132  * We make sure that the background writeout level is below the adjusted
133  * clamping level.
134  */
135 static void
136 get_dirty_limits(struct writeback_state *wbs, long *pbackground, long *pdirty,
137                 struct address_space *mapping)
138 {
139         int background_ratio;           /* Percentages */
140         int dirty_ratio;
141         int unmapped_ratio;
142         long background;
143         long dirty;
144         unsigned long available_memory = total_pages;
145         struct task_struct *tsk;
146
147         get_writeback_state(wbs);
148
149 #ifdef CONFIG_HIGHMEM
150         /*
151          * If this mapping can only allocate from low memory,
152          * we exclude high memory from our count.
153          */
154         if (mapping && !(mapping_gfp_mask(mapping) & __GFP_HIGHMEM))
155                 available_memory -= totalhigh_pages;
156 #endif
157
158
159         unmapped_ratio = 100 - (wbs->nr_mapped * 100) / total_pages;
160
161         dirty_ratio = vm_dirty_ratio;
162         if (dirty_ratio > unmapped_ratio / 2)
163                 dirty_ratio = unmapped_ratio / 2;
164
165         if (dirty_ratio < 5)
166                 dirty_ratio = 5;
167
168         background_ratio = dirty_background_ratio;
169         if (background_ratio >= dirty_ratio)
170                 background_ratio = dirty_ratio / 2;
171
172         background = (background_ratio * available_memory) / 100;
173         dirty = (dirty_ratio * available_memory) / 100;
174         tsk = current;
175         if (tsk->flags & PF_LESS_THROTTLE || rt_task(tsk)) {
176                 background += background / 4;
177                 dirty += dirty / 4;
178         }
179         *pbackground = background;
180         *pdirty = dirty;
181 }
182
183 /*
184  * balance_dirty_pages() must be called by processes which are generating dirty
185  * data.  It looks at the number of dirty pages in the machine and will force
186  * the caller to perform writeback if the system is over `vm_dirty_ratio'.
187  * If we're over `background_thresh' then pdflush is woken to perform some
188  * writeout.
189  */
190 static void balance_dirty_pages(struct address_space *mapping)
191 {
192         struct writeback_state wbs;
193         long nr_reclaimable;
194         long background_thresh;
195         long dirty_thresh;
196         unsigned long pages_written = 0;
197         unsigned long write_chunk = sync_writeback_pages();
198
199         struct backing_dev_info *bdi = mapping->backing_dev_info;
200
201         for (;;) {
202                 struct writeback_control wbc = {
203                         .bdi            = bdi,
204                         .sync_mode      = WB_SYNC_NONE,
205                         .older_than_this = NULL,
206                         .nr_to_write    = write_chunk,
207                 };
208
209                 get_dirty_limits(&wbs, &background_thresh,
210                                         &dirty_thresh, mapping);
211                 nr_reclaimable = wbs.nr_dirty + wbs.nr_unstable;
212                 if (nr_reclaimable + wbs.nr_writeback <= dirty_thresh)
213                         break;
214
215                 if (!dirty_exceeded)
216                         dirty_exceeded = 1;
217
218                 /* Note: nr_reclaimable denotes nr_dirty + nr_unstable.
219                  * Unstable writes are a feature of certain networked
220                  * filesystems (i.e. NFS) in which data may have been
221                  * written to the server's write cache, but has not yet
222                  * been flushed to permanent storage.
223                  */
224                 if (nr_reclaimable) {
225                         writeback_inodes(&wbc);
226                         get_dirty_limits(&wbs, &background_thresh,
227                                         &dirty_thresh, mapping);
228                         nr_reclaimable = wbs.nr_dirty + wbs.nr_unstable;
229                         if (nr_reclaimable + wbs.nr_writeback <= dirty_thresh)
230                                 break;
231                         pages_written += write_chunk - wbc.nr_to_write;
232                         if (pages_written >= write_chunk)
233                                 break;          /* We've done our duty */
234                 }
235                 blk_congestion_wait(WRITE, HZ/10);
236         }
237
238         if (nr_reclaimable + wbs.nr_writeback <= dirty_thresh && dirty_exceeded)
239                 dirty_exceeded = 0;
240
241         if (writeback_in_progress(bdi))
242                 return;         /* pdflush is already working this queue */
243
244         /*
245          * In laptop mode, we wait until hitting the higher threshold before
246          * starting background writeout, and then write out all the way down
247          * to the lower threshold.  So slow writers cause minimal disk activity.
248          *
249          * In normal mode, we start background writeout at the lower
250          * background_thresh, to keep the amount of dirty memory low.
251          */
252         if ((laptop_mode && pages_written) ||
253              (!laptop_mode && (nr_reclaimable > background_thresh)))
254                 pdflush_operation(background_writeout, 0);
255 }
256
257 /**
258  * balance_dirty_pages_ratelimited_nr - balance dirty memory state
259  * @mapping: address_space which was dirtied
260  * @nr_pages_dirtied: number of pages which the caller has just dirtied
261  *
262  * Processes which are dirtying memory should call in here once for each page
263  * which was newly dirtied.  The function will periodically check the system's
264  * dirty state and will initiate writeback if needed.
265  *
266  * On really big machines, get_writeback_state is expensive, so try to avoid
267  * calling it too often (ratelimiting).  But once we're over the dirty memory
268  * limit we decrease the ratelimiting by a lot, to prevent individual processes
269  * from overshooting the limit by (ratelimit_pages) each.
270  */
271 void balance_dirty_pages_ratelimited_nr(struct address_space *mapping,
272                                         unsigned long nr_pages_dirtied)
273 {
274         static DEFINE_PER_CPU(unsigned long, ratelimits) = 0;
275         unsigned long ratelimit;
276         unsigned long *p;
277
278         ratelimit = ratelimit_pages;
279         if (dirty_exceeded)
280                 ratelimit = 8;
281
282         /*
283          * Check the rate limiting. Also, we do not want to throttle real-time
284          * tasks in balance_dirty_pages(). Period.
285          */
286         preempt_disable();
287         p =  &__get_cpu_var(ratelimits);
288         *p += nr_pages_dirtied;
289         if (unlikely(*p >= ratelimit)) {
290                 *p = 0;
291                 preempt_enable();
292                 balance_dirty_pages(mapping);
293                 return;
294         }
295         preempt_enable();
296 }
297 EXPORT_SYMBOL(balance_dirty_pages_ratelimited_nr);
298
299 void throttle_vm_writeout(void)
300 {
301         struct writeback_state wbs;
302         long background_thresh;
303         long dirty_thresh;
304
305         for ( ; ; ) {
306                 get_dirty_limits(&wbs, &background_thresh, &dirty_thresh, NULL);
307
308                 /*
309                  * Boost the allowable dirty threshold a bit for page
310                  * allocators so they don't get DoS'ed by heavy writers
311                  */
312                 dirty_thresh += dirty_thresh / 10;      /* wheeee... */
313
314                 if (wbs.nr_unstable + wbs.nr_writeback <= dirty_thresh)
315                         break;
316                 blk_congestion_wait(WRITE, HZ/10);
317         }
318 }
319
320
321 /*
322  * writeback at least _min_pages, and keep writing until the amount of dirty
323  * memory is less than the background threshold, or until we're all clean.
324  */
325 static void background_writeout(unsigned long _min_pages)
326 {
327         long min_pages = _min_pages;
328         struct writeback_control wbc = {
329                 .bdi            = NULL,
330                 .sync_mode      = WB_SYNC_NONE,
331                 .older_than_this = NULL,
332                 .nr_to_write    = 0,
333                 .nonblocking    = 1,
334         };
335
336         for ( ; ; ) {
337                 struct writeback_state wbs;
338                 long background_thresh;
339                 long dirty_thresh;
340
341                 get_dirty_limits(&wbs, &background_thresh, &dirty_thresh, NULL);
342                 if (wbs.nr_dirty + wbs.nr_unstable < background_thresh
343                                 && min_pages <= 0)
344                         break;
345                 wbc.encountered_congestion = 0;
346                 wbc.nr_to_write = MAX_WRITEBACK_PAGES;
347                 wbc.pages_skipped = 0;
348                 writeback_inodes(&wbc);
349                 min_pages -= MAX_WRITEBACK_PAGES - wbc.nr_to_write;
350                 if (wbc.nr_to_write > 0 || wbc.pages_skipped > 0) {
351                         /* Wrote less than expected */
352                         blk_congestion_wait(WRITE, HZ/10);
353                         if (!wbc.encountered_congestion)
354                                 break;
355                 }
356         }
357 }
358
359 /*
360  * Start writeback of `nr_pages' pages.  If `nr_pages' is zero, write back
361  * the whole world.  Returns 0 if a pdflush thread was dispatched.  Returns
362  * -1 if all pdflush threads were busy.
363  */
364 int wakeup_pdflush(long nr_pages)
365 {
366         if (nr_pages == 0) {
367                 struct writeback_state wbs;
368
369                 get_writeback_state(&wbs);
370                 nr_pages = wbs.nr_dirty + wbs.nr_unstable;
371         }
372         return pdflush_operation(background_writeout, nr_pages);
373 }
374
375 static void wb_timer_fn(unsigned long unused);
376 static void laptop_timer_fn(unsigned long unused);
377
378 static DEFINE_TIMER(wb_timer, wb_timer_fn, 0, 0);
379 static DEFINE_TIMER(laptop_mode_wb_timer, laptop_timer_fn, 0, 0);
380
381 /*
382  * Periodic writeback of "old" data.
383  *
384  * Define "old": the first time one of an inode's pages is dirtied, we mark the
385  * dirtying-time in the inode's address_space.  So this periodic writeback code
386  * just walks the superblock inode list, writing back any inodes which are
387  * older than a specific point in time.
388  *
389  * Try to run once per dirty_writeback_interval.  But if a writeback event
390  * takes longer than a dirty_writeback_interval interval, then leave a
391  * one-second gap.
392  *
393  * older_than_this takes precedence over nr_to_write.  So we'll only write back
394  * all dirty pages if they are all attached to "old" mappings.
395  */
396 static void wb_kupdate(unsigned long arg)
397 {
398         unsigned long oldest_jif;
399         unsigned long start_jif;
400         unsigned long next_jif;
401         long nr_to_write;
402         struct writeback_state wbs;
403         struct writeback_control wbc = {
404                 .bdi            = NULL,
405                 .sync_mode      = WB_SYNC_NONE,
406                 .older_than_this = &oldest_jif,
407                 .nr_to_write    = 0,
408                 .nonblocking    = 1,
409                 .for_kupdate    = 1,
410         };
411
412         sync_supers();
413
414         get_writeback_state(&wbs);
415         oldest_jif = jiffies - dirty_expire_interval;
416         start_jif = jiffies;
417         next_jif = start_jif + dirty_writeback_interval;
418         nr_to_write = wbs.nr_dirty + wbs.nr_unstable +
419                         (inodes_stat.nr_inodes - inodes_stat.nr_unused);
420         while (nr_to_write > 0) {
421                 wbc.encountered_congestion = 0;
422                 wbc.nr_to_write = MAX_WRITEBACK_PAGES;
423                 writeback_inodes(&wbc);
424                 if (wbc.nr_to_write > 0) {
425                         if (wbc.encountered_congestion)
426                                 blk_congestion_wait(WRITE, HZ/10);
427                         else
428                                 break;  /* All the old data is written */
429                 }
430                 nr_to_write -= MAX_WRITEBACK_PAGES - wbc.nr_to_write;
431         }
432         if (time_before(next_jif, jiffies + HZ))
433                 next_jif = jiffies + HZ;
434         if (dirty_writeback_interval)
435                 mod_timer(&wb_timer, next_jif);
436 }
437
438 /*
439  * sysctl handler for /proc/sys/vm/dirty_writeback_centisecs
440  */
441 int dirty_writeback_centisecs_handler(ctl_table *table, int write,
442                 struct file *file, void __user *buffer, size_t *length, loff_t *ppos)
443 {
444         proc_dointvec_userhz_jiffies(table, write, file, buffer, length, ppos);
445         if (dirty_writeback_interval) {
446                 mod_timer(&wb_timer,
447                         jiffies + dirty_writeback_interval);
448                 } else {
449                 del_timer(&wb_timer);
450         }
451         return 0;
452 }
453
454 static void wb_timer_fn(unsigned long unused)
455 {
456         if (pdflush_operation(wb_kupdate, 0) < 0)
457                 mod_timer(&wb_timer, jiffies + HZ); /* delay 1 second */
458 }
459
460 static void laptop_flush(unsigned long unused)
461 {
462         sys_sync();
463 }
464
465 static void laptop_timer_fn(unsigned long unused)
466 {
467         pdflush_operation(laptop_flush, 0);
468 }
469
470 /*
471  * We've spun up the disk and we're in laptop mode: schedule writeback
472  * of all dirty data a few seconds from now.  If the flush is already scheduled
473  * then push it back - the user is still using the disk.
474  */
475 void laptop_io_completion(void)
476 {
477         mod_timer(&laptop_mode_wb_timer, jiffies + laptop_mode);
478 }
479
480 /*
481  * We're in laptop mode and we've just synced. The sync's writes will have
482  * caused another writeback to be scheduled by laptop_io_completion.
483  * Nothing needs to be written back anymore, so we unschedule the writeback.
484  */
485 void laptop_sync_completion(void)
486 {
487         del_timer(&laptop_mode_wb_timer);
488 }
489
490 /*
491  * If ratelimit_pages is too high then we can get into dirty-data overload
492  * if a large number of processes all perform writes at the same time.
493  * If it is too low then SMP machines will call the (expensive)
494  * get_writeback_state too often.
495  *
496  * Here we set ratelimit_pages to a level which ensures that when all CPUs are
497  * dirtying in parallel, we cannot go more than 3% (1/32) over the dirty memory
498  * thresholds before writeback cuts in.
499  *
500  * But the limit should not be set too high.  Because it also controls the
501  * amount of memory which the balance_dirty_pages() caller has to write back.
502  * If this is too large then the caller will block on the IO queue all the
503  * time.  So limit it to four megabytes - the balance_dirty_pages() caller
504  * will write six megabyte chunks, max.
505  */
506
507 static void set_ratelimit(void)
508 {
509         ratelimit_pages = total_pages / (num_online_cpus() * 32);
510         if (ratelimit_pages < 16)
511                 ratelimit_pages = 16;
512         if (ratelimit_pages * PAGE_CACHE_SIZE > 4096 * 1024)
513                 ratelimit_pages = (4096 * 1024) / PAGE_CACHE_SIZE;
514 }
515
516 static int
517 ratelimit_handler(struct notifier_block *self, unsigned long u, void *v)
518 {
519         set_ratelimit();
520         return 0;
521 }
522
523 static struct notifier_block ratelimit_nb = {
524         .notifier_call  = ratelimit_handler,
525         .next           = NULL,
526 };
527
528 /*
529  * If the machine has a large highmem:lowmem ratio then scale back the default
530  * dirty memory thresholds: allowing too much dirty highmem pins an excessive
531  * number of buffer_heads.
532  */
533 void __init page_writeback_init(void)
534 {
535         long buffer_pages = nr_free_buffer_pages();
536         long correction;
537
538         total_pages = nr_free_pagecache_pages();
539
540         correction = (100 * 4 * buffer_pages) / total_pages;
541
542         if (correction < 100) {
543                 dirty_background_ratio *= correction;
544                 dirty_background_ratio /= 100;
545                 vm_dirty_ratio *= correction;
546                 vm_dirty_ratio /= 100;
547
548                 if (dirty_background_ratio <= 0)
549                         dirty_background_ratio = 1;
550                 if (vm_dirty_ratio <= 0)
551                         vm_dirty_ratio = 1;
552         }
553         mod_timer(&wb_timer, jiffies + dirty_writeback_interval);
554         set_ratelimit();
555         register_cpu_notifier(&ratelimit_nb);
556 }
557
558 int do_writepages(struct address_space *mapping, struct writeback_control *wbc)
559 {
560         int ret;
561
562         if (wbc->nr_to_write <= 0)
563                 return 0;
564         wbc->for_writepages = 1;
565         if (mapping->a_ops->writepages)
566                 ret =  mapping->a_ops->writepages(mapping, wbc);
567         else
568                 ret = generic_writepages(mapping, wbc);
569         wbc->for_writepages = 0;
570         return ret;
571 }
572
573 /**
574  * write_one_page - write out a single page and optionally wait on I/O
575  *
576  * @page: the page to write
577  * @wait: if true, wait on writeout
578  *
579  * The page must be locked by the caller and will be unlocked upon return.
580  *
581  * write_one_page() returns a negative error code if I/O failed.
582  */
583 int write_one_page(struct page *page, int wait)
584 {
585         struct address_space *mapping = page->mapping;
586         int ret = 0;
587         struct writeback_control wbc = {
588                 .sync_mode = WB_SYNC_ALL,
589                 .nr_to_write = 1,
590         };
591
592         BUG_ON(!PageLocked(page));
593
594         if (wait)
595                 wait_on_page_writeback(page);
596
597         if (clear_page_dirty_for_io(page)) {
598                 page_cache_get(page);
599                 ret = mapping->a_ops->writepage(page, &wbc);
600                 if (ret == 0 && wait) {
601                         wait_on_page_writeback(page);
602                         if (PageError(page))
603                                 ret = -EIO;
604                 }
605                 page_cache_release(page);
606         } else {
607                 unlock_page(page);
608         }
609         return ret;
610 }
611 EXPORT_SYMBOL(write_one_page);
612
613 /*
614  * For address_spaces which do not use buffers.  Just tag the page as dirty in
615  * its radix tree.
616  *
617  * This is also used when a single buffer is being dirtied: we want to set the
618  * page dirty in that case, but not all the buffers.  This is a "bottom-up"
619  * dirtying, whereas __set_page_dirty_buffers() is a "top-down" dirtying.
620  *
621  * Most callers have locked the page, which pins the address_space in memory.
622  * But zap_pte_range() does not lock the page, however in that case the
623  * mapping is pinned by the vma's ->vm_file reference.
624  *
625  * We take care to handle the case where the page was truncated from the
626  * mapping by re-checking page_mapping() insode tree_lock.
627  */
628 int __set_page_dirty_nobuffers(struct page *page)
629 {
630         if (!TestSetPageDirty(page)) {
631                 struct address_space *mapping = page_mapping(page);
632                 struct address_space *mapping2;
633
634                 if (mapping) {
635                         write_lock_irq(&mapping->tree_lock);
636                         mapping2 = page_mapping(page);
637                         if (mapping2) { /* Race with truncate? */
638                                 BUG_ON(mapping2 != mapping);
639                                 if (mapping_cap_account_dirty(mapping))
640                                         inc_page_state(nr_dirty);
641                                 radix_tree_tag_set(&mapping->page_tree,
642                                         page_index(page), PAGECACHE_TAG_DIRTY);
643                         }
644                         write_unlock_irq(&mapping->tree_lock);
645                         if (mapping->host) {
646                                 /* !PageAnon && !swapper_space */
647                                 __mark_inode_dirty(mapping->host,
648                                                         I_DIRTY_PAGES);
649                         }
650                 }
651                 return 1;
652         }
653         return 0;
654 }
655 EXPORT_SYMBOL(__set_page_dirty_nobuffers);
656
657 /*
658  * When a writepage implementation decides that it doesn't want to write this
659  * page for some reason, it should redirty the locked page via
660  * redirty_page_for_writepage() and it should then unlock the page and return 0
661  */
662 int redirty_page_for_writepage(struct writeback_control *wbc, struct page *page)
663 {
664         wbc->pages_skipped++;
665         return __set_page_dirty_nobuffers(page);
666 }
667 EXPORT_SYMBOL(redirty_page_for_writepage);
668
669 /*
670  * If the mapping doesn't provide a set_page_dirty a_op, then
671  * just fall through and assume that it wants buffer_heads.
672  */
673 int fastcall set_page_dirty(struct page *page)
674 {
675         struct address_space *mapping = page_mapping(page);
676
677         if (likely(mapping)) {
678                 int (*spd)(struct page *) = mapping->a_ops->set_page_dirty;
679                 if (spd)
680                         return (*spd)(page);
681                 return __set_page_dirty_buffers(page);
682         }
683         if (!PageDirty(page)) {
684                 if (!TestSetPageDirty(page))
685                         return 1;
686         }
687         return 0;
688 }
689 EXPORT_SYMBOL(set_page_dirty);
690
691 /*
692  * set_page_dirty() is racy if the caller has no reference against
693  * page->mapping->host, and if the page is unlocked.  This is because another
694  * CPU could truncate the page off the mapping and then free the mapping.
695  *
696  * Usually, the page _is_ locked, or the caller is a user-space process which
697  * holds a reference on the inode by having an open file.
698  *
699  * In other cases, the page should be locked before running set_page_dirty().
700  */
701 int set_page_dirty_lock(struct page *page)
702 {
703         int ret;
704
705         lock_page(page);
706         ret = set_page_dirty(page);
707         unlock_page(page);
708         return ret;
709 }
710 EXPORT_SYMBOL(set_page_dirty_lock);
711
712 /*
713  * Clear a page's dirty flag, while caring for dirty memory accounting. 
714  * Returns true if the page was previously dirty.
715  */
716 int test_clear_page_dirty(struct page *page)
717 {
718         struct address_space *mapping = page_mapping(page);
719         unsigned long flags;
720
721         if (mapping) {
722                 write_lock_irqsave(&mapping->tree_lock, flags);
723                 if (TestClearPageDirty(page)) {
724                         radix_tree_tag_clear(&mapping->page_tree,
725                                                 page_index(page),
726                                                 PAGECACHE_TAG_DIRTY);
727                         write_unlock_irqrestore(&mapping->tree_lock, flags);
728                         if (mapping_cap_account_dirty(mapping))
729                                 dec_page_state(nr_dirty);
730                         return 1;
731                 }
732                 write_unlock_irqrestore(&mapping->tree_lock, flags);
733                 return 0;
734         }
735         return TestClearPageDirty(page);
736 }
737 EXPORT_SYMBOL(test_clear_page_dirty);
738
739 /*
740  * Clear a page's dirty flag, while caring for dirty memory accounting.
741  * Returns true if the page was previously dirty.
742  *
743  * This is for preparing to put the page under writeout.  We leave the page
744  * tagged as dirty in the radix tree so that a concurrent write-for-sync
745  * can discover it via a PAGECACHE_TAG_DIRTY walk.  The ->writepage
746  * implementation will run either set_page_writeback() or set_page_dirty(),
747  * at which stage we bring the page's dirty flag and radix-tree dirty tag
748  * back into sync.
749  *
750  * This incoherency between the page's dirty flag and radix-tree tag is
751  * unfortunate, but it only exists while the page is locked.
752  */
753 int clear_page_dirty_for_io(struct page *page)
754 {
755         struct address_space *mapping = page_mapping(page);
756
757         if (mapping) {
758                 if (TestClearPageDirty(page)) {
759                         if (mapping_cap_account_dirty(mapping))
760                                 dec_page_state(nr_dirty);
761                         return 1;
762                 }
763                 return 0;
764         }
765         return TestClearPageDirty(page);
766 }
767 EXPORT_SYMBOL(clear_page_dirty_for_io);
768
769 int test_clear_page_writeback(struct page *page)
770 {
771         struct address_space *mapping = page_mapping(page);
772         int ret;
773
774         if (mapping) {
775                 unsigned long flags;
776
777                 write_lock_irqsave(&mapping->tree_lock, flags);
778                 ret = TestClearPageWriteback(page);
779                 if (ret)
780                         radix_tree_tag_clear(&mapping->page_tree,
781                                                 page_index(page),
782                                                 PAGECACHE_TAG_WRITEBACK);
783                 write_unlock_irqrestore(&mapping->tree_lock, flags);
784         } else {
785                 ret = TestClearPageWriteback(page);
786         }
787         return ret;
788 }
789
790 int test_set_page_writeback(struct page *page)
791 {
792         struct address_space *mapping = page_mapping(page);
793         int ret;
794
795         if (mapping) {
796                 unsigned long flags;
797
798                 write_lock_irqsave(&mapping->tree_lock, flags);
799                 ret = TestSetPageWriteback(page);
800                 if (!ret)
801                         radix_tree_tag_set(&mapping->page_tree,
802                                                 page_index(page),
803                                                 PAGECACHE_TAG_WRITEBACK);
804                 if (!PageDirty(page))
805                         radix_tree_tag_clear(&mapping->page_tree,
806                                                 page_index(page),
807                                                 PAGECACHE_TAG_DIRTY);
808                 write_unlock_irqrestore(&mapping->tree_lock, flags);
809         } else {
810                 ret = TestSetPageWriteback(page);
811         }
812         return ret;
813
814 }
815 EXPORT_SYMBOL(test_set_page_writeback);
816
817 /*
818  * Return true if any of the pages in the mapping are marged with the
819  * passed tag.
820  */
821 int mapping_tagged(struct address_space *mapping, int tag)
822 {
823         unsigned long flags;
824         int ret;
825
826         read_lock_irqsave(&mapping->tree_lock, flags);
827         ret = radix_tree_tagged(&mapping->page_tree, tag);
828         read_unlock_irqrestore(&mapping->tree_lock, flags);
829         return ret;
830 }
831 EXPORT_SYMBOL(mapping_tagged);