Documentation: fix minor kernel-doc warnings
[linux-2.6.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 centiseconds
76  * (hundredths of a second)
77  */
78 int dirty_writeback_interval = 5 * HZ;
79
80 /*
81  * The longest number of centiseconds for which data is allowed to remain dirty
82  */
83 int dirty_expire_interval = 30 * HZ;
84
85 /*
86  * Flag that makes the machine dump writes/reads and block dirtyings.
87  */
88 int block_dump;
89
90 /*
91  * Flag that puts the machine in "laptop mode". Doubles as a timeout in jiffies:
92  * a full sync is triggered after this time elapses without any disk activity.
93  */
94 int laptop_mode;
95
96 EXPORT_SYMBOL(laptop_mode);
97
98 /* End of sysctl-exported parameters */
99
100
101 static void background_writeout(unsigned long _min_pages);
102
103 struct writeback_state
104 {
105         unsigned long nr_dirty;
106         unsigned long nr_unstable;
107         unsigned long nr_mapped;
108         unsigned long nr_writeback;
109 };
110
111 static void get_writeback_state(struct writeback_state *wbs)
112 {
113         wbs->nr_dirty = read_page_state(nr_dirty);
114         wbs->nr_unstable = read_page_state(nr_unstable);
115         wbs->nr_mapped = read_page_state(nr_mapped);
116         wbs->nr_writeback = read_page_state(nr_writeback);
117 }
118
119 /*
120  * Work out the current dirty-memory clamping and background writeout
121  * thresholds.
122  *
123  * The main aim here is to lower them aggressively if there is a lot of mapped
124  * memory around.  To avoid stressing page reclaim with lots of unreclaimable
125  * pages.  It is better to clamp down on writers than to start swapping, and
126  * performing lots of scanning.
127  *
128  * We only allow 1/2 of the currently-unmapped memory to be dirtied.
129  *
130  * We don't permit the clamping level to fall below 5% - that is getting rather
131  * excessive.
132  *
133  * We make sure that the background writeout level is below the adjusted
134  * clamping level.
135  */
136 static void
137 get_dirty_limits(struct writeback_state *wbs, long *pbackground, long *pdirty,
138                 struct address_space *mapping)
139 {
140         int background_ratio;           /* Percentages */
141         int dirty_ratio;
142         int unmapped_ratio;
143         long background;
144         long dirty;
145         unsigned long available_memory = total_pages;
146         struct task_struct *tsk;
147
148         get_writeback_state(wbs);
149
150 #ifdef CONFIG_HIGHMEM
151         /*
152          * If this mapping can only allocate from low memory,
153          * we exclude high memory from our count.
154          */
155         if (mapping && !(mapping_gfp_mask(mapping) & __GFP_HIGHMEM))
156                 available_memory -= totalhigh_pages;
157 #endif
158
159
160         unmapped_ratio = 100 - (wbs->nr_mapped * 100) / total_pages;
161
162         dirty_ratio = vm_dirty_ratio;
163         if (dirty_ratio > unmapped_ratio / 2)
164                 dirty_ratio = unmapped_ratio / 2;
165
166         if (dirty_ratio < 5)
167                 dirty_ratio = 5;
168
169         background_ratio = dirty_background_ratio;
170         if (background_ratio >= dirty_ratio)
171                 background_ratio = dirty_ratio / 2;
172
173         background = (background_ratio * available_memory) / 100;
174         dirty = (dirty_ratio * available_memory) / 100;
175         tsk = current;
176         if (tsk->flags & PF_LESS_THROTTLE || rt_task(tsk)) {
177                 background += background / 4;
178                 dirty += dirty / 4;
179         }
180         *pbackground = background;
181         *pdirty = dirty;
182 }
183
184 /*
185  * balance_dirty_pages() must be called by processes which are generating dirty
186  * data.  It looks at the number of dirty pages in the machine and will force
187  * the caller to perform writeback if the system is over `vm_dirty_ratio'.
188  * If we're over `background_thresh' then pdflush is woken to perform some
189  * writeout.
190  */
191 static void balance_dirty_pages(struct address_space *mapping)
192 {
193         struct writeback_state wbs;
194         long nr_reclaimable;
195         long background_thresh;
196         long dirty_thresh;
197         unsigned long pages_written = 0;
198         unsigned long write_chunk = sync_writeback_pages();
199
200         struct backing_dev_info *bdi = mapping->backing_dev_info;
201
202         for (;;) {
203                 struct writeback_control wbc = {
204                         .bdi            = bdi,
205                         .sync_mode      = WB_SYNC_NONE,
206                         .older_than_this = NULL,
207                         .nr_to_write    = write_chunk,
208                 };
209
210                 get_dirty_limits(&wbs, &background_thresh,
211                                         &dirty_thresh, mapping);
212                 nr_reclaimable = wbs.nr_dirty + wbs.nr_unstable;
213                 if (nr_reclaimable + wbs.nr_writeback <= dirty_thresh)
214                         break;
215
216                 if (!dirty_exceeded)
217                         dirty_exceeded = 1;
218
219                 /* Note: nr_reclaimable denotes nr_dirty + nr_unstable.
220                  * Unstable writes are a feature of certain networked
221                  * filesystems (i.e. NFS) in which data may have been
222                  * written to the server's write cache, but has not yet
223                  * been flushed to permanent storage.
224                  */
225                 if (nr_reclaimable) {
226                         writeback_inodes(&wbc);
227                         get_dirty_limits(&wbs, &background_thresh,
228                                         &dirty_thresh, mapping);
229                         nr_reclaimable = wbs.nr_dirty + wbs.nr_unstable;
230                         if (nr_reclaimable + wbs.nr_writeback <= dirty_thresh)
231                                 break;
232                         pages_written += write_chunk - wbc.nr_to_write;
233                         if (pages_written >= write_chunk)
234                                 break;          /* We've done our duty */
235                 }
236                 blk_congestion_wait(WRITE, HZ/10);
237         }
238
239         if (nr_reclaimable + wbs.nr_writeback <= dirty_thresh && dirty_exceeded)
240                 dirty_exceeded = 0;
241
242         if (writeback_in_progress(bdi))
243                 return;         /* pdflush is already working this queue */
244
245         /*
246          * In laptop mode, we wait until hitting the higher threshold before
247          * starting background writeout, and then write out all the way down
248          * to the lower threshold.  So slow writers cause minimal disk activity.
249          *
250          * In normal mode, we start background writeout at the lower
251          * background_thresh, to keep the amount of dirty memory low.
252          */
253         if ((laptop_mode && pages_written) ||
254              (!laptop_mode && (nr_reclaimable > background_thresh)))
255                 pdflush_operation(background_writeout, 0);
256 }
257
258 /**
259  * balance_dirty_pages_ratelimited_nr - balance dirty memory state
260  * @mapping: address_space which was dirtied
261  * @nr_pages_dirtied: number of pages which the caller has just dirtied
262  *
263  * Processes which are dirtying memory should call in here once for each page
264  * which was newly dirtied.  The function will periodically check the system's
265  * dirty state and will initiate writeback if needed.
266  *
267  * On really big machines, get_writeback_state is expensive, so try to avoid
268  * calling it too often (ratelimiting).  But once we're over the dirty memory
269  * limit we decrease the ratelimiting by a lot, to prevent individual processes
270  * from overshooting the limit by (ratelimit_pages) each.
271  */
272 void balance_dirty_pages_ratelimited_nr(struct address_space *mapping,
273                                         unsigned long nr_pages_dirtied)
274 {
275         static DEFINE_PER_CPU(unsigned long, ratelimits) = 0;
276         unsigned long ratelimit;
277         unsigned long *p;
278
279         ratelimit = ratelimit_pages;
280         if (dirty_exceeded)
281                 ratelimit = 8;
282
283         /*
284          * Check the rate limiting. Also, we do not want to throttle real-time
285          * tasks in balance_dirty_pages(). Period.
286          */
287         preempt_disable();
288         p =  &__get_cpu_var(ratelimits);
289         *p += nr_pages_dirtied;
290         if (unlikely(*p >= ratelimit)) {
291                 *p = 0;
292                 preempt_enable();
293                 balance_dirty_pages(mapping);
294                 return;
295         }
296         preempt_enable();
297 }
298 EXPORT_SYMBOL(balance_dirty_pages_ratelimited_nr);
299
300 void throttle_vm_writeout(void)
301 {
302         struct writeback_state wbs;
303         long background_thresh;
304         long dirty_thresh;
305
306         for ( ; ; ) {
307                 get_dirty_limits(&wbs, &background_thresh, &dirty_thresh, NULL);
308
309                 /*
310                  * Boost the allowable dirty threshold a bit for page
311                  * allocators so they don't get DoS'ed by heavy writers
312                  */
313                 dirty_thresh += dirty_thresh / 10;      /* wheeee... */
314
315                 if (wbs.nr_unstable + wbs.nr_writeback <= dirty_thresh)
316                         break;
317                 blk_congestion_wait(WRITE, HZ/10);
318         }
319 }
320
321
322 /*
323  * writeback at least _min_pages, and keep writing until the amount of dirty
324  * memory is less than the background threshold, or until we're all clean.
325  */
326 static void background_writeout(unsigned long _min_pages)
327 {
328         long min_pages = _min_pages;
329         struct writeback_control wbc = {
330                 .bdi            = NULL,
331                 .sync_mode      = WB_SYNC_NONE,
332                 .older_than_this = NULL,
333                 .nr_to_write    = 0,
334                 .nonblocking    = 1,
335         };
336
337         for ( ; ; ) {
338                 struct writeback_state wbs;
339                 long background_thresh;
340                 long dirty_thresh;
341
342                 get_dirty_limits(&wbs, &background_thresh, &dirty_thresh, NULL);
343                 if (wbs.nr_dirty + wbs.nr_unstable < background_thresh
344                                 && min_pages <= 0)
345                         break;
346                 wbc.encountered_congestion = 0;
347                 wbc.nr_to_write = MAX_WRITEBACK_PAGES;
348                 wbc.pages_skipped = 0;
349                 writeback_inodes(&wbc);
350                 min_pages -= MAX_WRITEBACK_PAGES - wbc.nr_to_write;
351                 if (wbc.nr_to_write > 0 || wbc.pages_skipped > 0) {
352                         /* Wrote less than expected */
353                         blk_congestion_wait(WRITE, HZ/10);
354                         if (!wbc.encountered_congestion)
355                                 break;
356                 }
357         }
358 }
359
360 /*
361  * Start writeback of `nr_pages' pages.  If `nr_pages' is zero, write back
362  * the whole world.  Returns 0 if a pdflush thread was dispatched.  Returns
363  * -1 if all pdflush threads were busy.
364  */
365 int wakeup_pdflush(long nr_pages)
366 {
367         if (nr_pages == 0) {
368                 struct writeback_state wbs;
369
370                 get_writeback_state(&wbs);
371                 nr_pages = wbs.nr_dirty + wbs.nr_unstable;
372         }
373         return pdflush_operation(background_writeout, nr_pages);
374 }
375
376 static void wb_timer_fn(unsigned long unused);
377 static void laptop_timer_fn(unsigned long unused);
378
379 static DEFINE_TIMER(wb_timer, wb_timer_fn, 0, 0);
380 static DEFINE_TIMER(laptop_mode_wb_timer, laptop_timer_fn, 0, 0);
381
382 /*
383  * Periodic writeback of "old" data.
384  *
385  * Define "old": the first time one of an inode's pages is dirtied, we mark the
386  * dirtying-time in the inode's address_space.  So this periodic writeback code
387  * just walks the superblock inode list, writing back any inodes which are
388  * older than a specific point in time.
389  *
390  * Try to run once per dirty_writeback_interval.  But if a writeback event
391  * takes longer than a dirty_writeback_interval interval, then leave a
392  * one-second gap.
393  *
394  * older_than_this takes precedence over nr_to_write.  So we'll only write back
395  * all dirty pages if they are all attached to "old" mappings.
396  */
397 static void wb_kupdate(unsigned long arg)
398 {
399         unsigned long oldest_jif;
400         unsigned long start_jif;
401         unsigned long next_jif;
402         long nr_to_write;
403         struct writeback_state wbs;
404         struct writeback_control wbc = {
405                 .bdi            = NULL,
406                 .sync_mode      = WB_SYNC_NONE,
407                 .older_than_this = &oldest_jif,
408                 .nr_to_write    = 0,
409                 .nonblocking    = 1,
410                 .for_kupdate    = 1,
411         };
412
413         sync_supers();
414
415         get_writeback_state(&wbs);
416         oldest_jif = jiffies - dirty_expire_interval;
417         start_jif = jiffies;
418         next_jif = start_jif + dirty_writeback_interval;
419         nr_to_write = wbs.nr_dirty + wbs.nr_unstable +
420                         (inodes_stat.nr_inodes - inodes_stat.nr_unused);
421         while (nr_to_write > 0) {
422                 wbc.encountered_congestion = 0;
423                 wbc.nr_to_write = MAX_WRITEBACK_PAGES;
424                 writeback_inodes(&wbc);
425                 if (wbc.nr_to_write > 0) {
426                         if (wbc.encountered_congestion)
427                                 blk_congestion_wait(WRITE, HZ/10);
428                         else
429                                 break;  /* All the old data is written */
430                 }
431                 nr_to_write -= MAX_WRITEBACK_PAGES - wbc.nr_to_write;
432         }
433         if (time_before(next_jif, jiffies + HZ))
434                 next_jif = jiffies + HZ;
435         if (dirty_writeback_interval)
436                 mod_timer(&wb_timer, next_jif);
437 }
438
439 /*
440  * sysctl handler for /proc/sys/vm/dirty_writeback_centisecs
441  */
442 int dirty_writeback_centisecs_handler(ctl_table *table, int write,
443                 struct file *file, void __user *buffer, size_t *length, loff_t *ppos)
444 {
445         proc_dointvec_userhz_jiffies(table, write, file, buffer, length, ppos);
446         if (dirty_writeback_interval) {
447                 mod_timer(&wb_timer,
448                         jiffies + dirty_writeback_interval);
449                 } else {
450                 del_timer(&wb_timer);
451         }
452         return 0;
453 }
454
455 static void wb_timer_fn(unsigned long unused)
456 {
457         if (pdflush_operation(wb_kupdate, 0) < 0)
458                 mod_timer(&wb_timer, jiffies + HZ); /* delay 1 second */
459 }
460
461 static void laptop_flush(unsigned long unused)
462 {
463         sys_sync();
464 }
465
466 static void laptop_timer_fn(unsigned long unused)
467 {
468         pdflush_operation(laptop_flush, 0);
469 }
470
471 /*
472  * We've spun up the disk and we're in laptop mode: schedule writeback
473  * of all dirty data a few seconds from now.  If the flush is already scheduled
474  * then push it back - the user is still using the disk.
475  */
476 void laptop_io_completion(void)
477 {
478         mod_timer(&laptop_mode_wb_timer, jiffies + laptop_mode);
479 }
480
481 /*
482  * We're in laptop mode and we've just synced. The sync's writes will have
483  * caused another writeback to be scheduled by laptop_io_completion.
484  * Nothing needs to be written back anymore, so we unschedule the writeback.
485  */
486 void laptop_sync_completion(void)
487 {
488         del_timer(&laptop_mode_wb_timer);
489 }
490
491 /*
492  * If ratelimit_pages is too high then we can get into dirty-data overload
493  * if a large number of processes all perform writes at the same time.
494  * If it is too low then SMP machines will call the (expensive)
495  * get_writeback_state too often.
496  *
497  * Here we set ratelimit_pages to a level which ensures that when all CPUs are
498  * dirtying in parallel, we cannot go more than 3% (1/32) over the dirty memory
499  * thresholds before writeback cuts in.
500  *
501  * But the limit should not be set too high.  Because it also controls the
502  * amount of memory which the balance_dirty_pages() caller has to write back.
503  * If this is too large then the caller will block on the IO queue all the
504  * time.  So limit it to four megabytes - the balance_dirty_pages() caller
505  * will write six megabyte chunks, max.
506  */
507
508 static void set_ratelimit(void)
509 {
510         ratelimit_pages = total_pages / (num_online_cpus() * 32);
511         if (ratelimit_pages < 16)
512                 ratelimit_pages = 16;
513         if (ratelimit_pages * PAGE_CACHE_SIZE > 4096 * 1024)
514                 ratelimit_pages = (4096 * 1024) / PAGE_CACHE_SIZE;
515 }
516
517 static int
518 ratelimit_handler(struct notifier_block *self, unsigned long u, void *v)
519 {
520         set_ratelimit();
521         return 0;
522 }
523
524 static struct notifier_block ratelimit_nb = {
525         .notifier_call  = ratelimit_handler,
526         .next           = NULL,
527 };
528
529 /*
530  * If the machine has a large highmem:lowmem ratio then scale back the default
531  * dirty memory thresholds: allowing too much dirty highmem pins an excessive
532  * number of buffer_heads.
533  */
534 void __init page_writeback_init(void)
535 {
536         long buffer_pages = nr_free_buffer_pages();
537         long correction;
538
539         total_pages = nr_free_pagecache_pages();
540
541         correction = (100 * 4 * buffer_pages) / total_pages;
542
543         if (correction < 100) {
544                 dirty_background_ratio *= correction;
545                 dirty_background_ratio /= 100;
546                 vm_dirty_ratio *= correction;
547                 vm_dirty_ratio /= 100;
548
549                 if (dirty_background_ratio <= 0)
550                         dirty_background_ratio = 1;
551                 if (vm_dirty_ratio <= 0)
552                         vm_dirty_ratio = 1;
553         }
554         mod_timer(&wb_timer, jiffies + dirty_writeback_interval);
555         set_ratelimit();
556         register_cpu_notifier(&ratelimit_nb);
557 }
558
559 int do_writepages(struct address_space *mapping, struct writeback_control *wbc)
560 {
561         int ret;
562
563         if (wbc->nr_to_write <= 0)
564                 return 0;
565         wbc->for_writepages = 1;
566         if (mapping->a_ops->writepages)
567                 ret =  mapping->a_ops->writepages(mapping, wbc);
568         else
569                 ret = generic_writepages(mapping, wbc);
570         wbc->for_writepages = 0;
571         return ret;
572 }
573
574 /**
575  * write_one_page - write out a single page and optionally wait on I/O
576  *
577  * @page: the page to write
578  * @wait: if true, wait on writeout
579  *
580  * The page must be locked by the caller and will be unlocked upon return.
581  *
582  * write_one_page() returns a negative error code if I/O failed.
583  */
584 int write_one_page(struct page *page, int wait)
585 {
586         struct address_space *mapping = page->mapping;
587         int ret = 0;
588         struct writeback_control wbc = {
589                 .sync_mode = WB_SYNC_ALL,
590                 .nr_to_write = 1,
591         };
592
593         BUG_ON(!PageLocked(page));
594
595         if (wait)
596                 wait_on_page_writeback(page);
597
598         if (clear_page_dirty_for_io(page)) {
599                 page_cache_get(page);
600                 ret = mapping->a_ops->writepage(page, &wbc);
601                 if (ret == 0 && wait) {
602                         wait_on_page_writeback(page);
603                         if (PageError(page))
604                                 ret = -EIO;
605                 }
606                 page_cache_release(page);
607         } else {
608                 unlock_page(page);
609         }
610         return ret;
611 }
612 EXPORT_SYMBOL(write_one_page);
613
614 /*
615  * For address_spaces which do not use buffers.  Just tag the page as dirty in
616  * its radix tree.
617  *
618  * This is also used when a single buffer is being dirtied: we want to set the
619  * page dirty in that case, but not all the buffers.  This is a "bottom-up"
620  * dirtying, whereas __set_page_dirty_buffers() is a "top-down" dirtying.
621  *
622  * Most callers have locked the page, which pins the address_space in memory.
623  * But zap_pte_range() does not lock the page, however in that case the
624  * mapping is pinned by the vma's ->vm_file reference.
625  *
626  * We take care to handle the case where the page was truncated from the
627  * mapping by re-checking page_mapping() insode tree_lock.
628  */
629 int __set_page_dirty_nobuffers(struct page *page)
630 {
631         if (!TestSetPageDirty(page)) {
632                 struct address_space *mapping = page_mapping(page);
633                 struct address_space *mapping2;
634
635                 if (mapping) {
636                         write_lock_irq(&mapping->tree_lock);
637                         mapping2 = page_mapping(page);
638                         if (mapping2) { /* Race with truncate? */
639                                 BUG_ON(mapping2 != mapping);
640                                 if (mapping_cap_account_dirty(mapping))
641                                         inc_page_state(nr_dirty);
642                                 radix_tree_tag_set(&mapping->page_tree,
643                                         page_index(page), PAGECACHE_TAG_DIRTY);
644                         }
645                         write_unlock_irq(&mapping->tree_lock);
646                         if (mapping->host) {
647                                 /* !PageAnon && !swapper_space */
648                                 __mark_inode_dirty(mapping->host,
649                                                         I_DIRTY_PAGES);
650                         }
651                 }
652                 return 1;
653         }
654         return 0;
655 }
656 EXPORT_SYMBOL(__set_page_dirty_nobuffers);
657
658 /*
659  * When a writepage implementation decides that it doesn't want to write this
660  * page for some reason, it should redirty the locked page via
661  * redirty_page_for_writepage() and it should then unlock the page and return 0
662  */
663 int redirty_page_for_writepage(struct writeback_control *wbc, struct page *page)
664 {
665         wbc->pages_skipped++;
666         return __set_page_dirty_nobuffers(page);
667 }
668 EXPORT_SYMBOL(redirty_page_for_writepage);
669
670 /*
671  * If the mapping doesn't provide a set_page_dirty a_op, then
672  * just fall through and assume that it wants buffer_heads.
673  */
674 int fastcall set_page_dirty(struct page *page)
675 {
676         struct address_space *mapping = page_mapping(page);
677
678         if (likely(mapping)) {
679                 int (*spd)(struct page *) = mapping->a_ops->set_page_dirty;
680                 if (spd)
681                         return (*spd)(page);
682                 return __set_page_dirty_buffers(page);
683         }
684         if (!PageDirty(page)) {
685                 if (!TestSetPageDirty(page))
686                         return 1;
687         }
688         return 0;
689 }
690 EXPORT_SYMBOL(set_page_dirty);
691
692 /*
693  * set_page_dirty() is racy if the caller has no reference against
694  * page->mapping->host, and if the page is unlocked.  This is because another
695  * CPU could truncate the page off the mapping and then free the mapping.
696  *
697  * Usually, the page _is_ locked, or the caller is a user-space process which
698  * holds a reference on the inode by having an open file.
699  *
700  * In other cases, the page should be locked before running set_page_dirty().
701  */
702 int set_page_dirty_lock(struct page *page)
703 {
704         int ret;
705
706         lock_page(page);
707         ret = set_page_dirty(page);
708         unlock_page(page);
709         return ret;
710 }
711 EXPORT_SYMBOL(set_page_dirty_lock);
712
713 /*
714  * Clear a page's dirty flag, while caring for dirty memory accounting. 
715  * Returns true if the page was previously dirty.
716  */
717 int test_clear_page_dirty(struct page *page)
718 {
719         struct address_space *mapping = page_mapping(page);
720         unsigned long flags;
721
722         if (mapping) {
723                 write_lock_irqsave(&mapping->tree_lock, flags);
724                 if (TestClearPageDirty(page)) {
725                         radix_tree_tag_clear(&mapping->page_tree,
726                                                 page_index(page),
727                                                 PAGECACHE_TAG_DIRTY);
728                         write_unlock_irqrestore(&mapping->tree_lock, flags);
729                         if (mapping_cap_account_dirty(mapping))
730                                 dec_page_state(nr_dirty);
731                         return 1;
732                 }
733                 write_unlock_irqrestore(&mapping->tree_lock, flags);
734                 return 0;
735         }
736         return TestClearPageDirty(page);
737 }
738 EXPORT_SYMBOL(test_clear_page_dirty);
739
740 /*
741  * Clear a page's dirty flag, while caring for dirty memory accounting.
742  * Returns true if the page was previously dirty.
743  *
744  * This is for preparing to put the page under writeout.  We leave the page
745  * tagged as dirty in the radix tree so that a concurrent write-for-sync
746  * can discover it via a PAGECACHE_TAG_DIRTY walk.  The ->writepage
747  * implementation will run either set_page_writeback() or set_page_dirty(),
748  * at which stage we bring the page's dirty flag and radix-tree dirty tag
749  * back into sync.
750  *
751  * This incoherency between the page's dirty flag and radix-tree tag is
752  * unfortunate, but it only exists while the page is locked.
753  */
754 int clear_page_dirty_for_io(struct page *page)
755 {
756         struct address_space *mapping = page_mapping(page);
757
758         if (mapping) {
759                 if (TestClearPageDirty(page)) {
760                         if (mapping_cap_account_dirty(mapping))
761                                 dec_page_state(nr_dirty);
762                         return 1;
763                 }
764                 return 0;
765         }
766         return TestClearPageDirty(page);
767 }
768 EXPORT_SYMBOL(clear_page_dirty_for_io);
769
770 int test_clear_page_writeback(struct page *page)
771 {
772         struct address_space *mapping = page_mapping(page);
773         int ret;
774
775         if (mapping) {
776                 unsigned long flags;
777
778                 write_lock_irqsave(&mapping->tree_lock, flags);
779                 ret = TestClearPageWriteback(page);
780                 if (ret)
781                         radix_tree_tag_clear(&mapping->page_tree,
782                                                 page_index(page),
783                                                 PAGECACHE_TAG_WRITEBACK);
784                 write_unlock_irqrestore(&mapping->tree_lock, flags);
785         } else {
786                 ret = TestClearPageWriteback(page);
787         }
788         return ret;
789 }
790
791 int test_set_page_writeback(struct page *page)
792 {
793         struct address_space *mapping = page_mapping(page);
794         int ret;
795
796         if (mapping) {
797                 unsigned long flags;
798
799                 write_lock_irqsave(&mapping->tree_lock, flags);
800                 ret = TestSetPageWriteback(page);
801                 if (!ret)
802                         radix_tree_tag_set(&mapping->page_tree,
803                                                 page_index(page),
804                                                 PAGECACHE_TAG_WRITEBACK);
805                 if (!PageDirty(page))
806                         radix_tree_tag_clear(&mapping->page_tree,
807                                                 page_index(page),
808                                                 PAGECACHE_TAG_DIRTY);
809                 write_unlock_irqrestore(&mapping->tree_lock, flags);
810         } else {
811                 ret = TestSetPageWriteback(page);
812         }
813         return ret;
814
815 }
816 EXPORT_SYMBOL(test_set_page_writeback);
817
818 /*
819  * Return true if any of the pages in the mapping are marged with the
820  * passed tag.
821  */
822 int mapping_tagged(struct address_space *mapping, int tag)
823 {
824         unsigned long flags;
825         int ret;
826
827         read_lock_irqsave(&mapping->tree_lock, flags);
828         ret = radix_tree_tagged(&mapping->page_tree, tag);
829         read_unlock_irqrestore(&mapping->tree_lock, flags);
830         return ret;
831 }
832 EXPORT_SYMBOL(mapping_tagged);