aboutsummaryrefslogtreecommitdiff
path: root/fs/cachefiles/rdwr.c
blob: 3304646dae843b78d6041b5e95d0d6bf654383c4 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
/* Storage object read/write
 *
 * Copyright (C) 2007 Red Hat, Inc. All Rights Reserved.
 * Written by David Howells (dhowells@redhat.com)
 *
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public Licence
 * as published by the Free Software Foundation; either version
 * 2 of the Licence, or (at your option) any later version.
 */

#include <linux/mount.h>
#include <linux/file.h>
#include "internal.h"

/*
 * detect wake up events generated by the unlocking of pages in which we're
 * interested
 * - we use this to detect read completion of backing pages
 * - the caller holds the waitqueue lock
 */
static int cachefiles_read_waiter(wait_queue_t *wait, unsigned mode,
				  int sync, void *_key)
{
	struct cachefiles_one_read *monitor =
		container_of(wait, struct cachefiles_one_read, monitor);
	struct cachefiles_object *object;
	struct wait_bit_key *key = _key;
	struct page *page = wait->private;

	ASSERT(key);

	_enter("{%lu},%u,%d,{%p,%u}",
	       monitor->netfs_page->index, mode, sync,
	       key->flags, key->bit_nr);

	if (key->flags != &page->flags ||
	    key->bit_nr != PG_locked)
		return 0;

	_debug("--- monitor %p %lx ---", page, page->flags);

	if (!PageUptodate(page) && !PageError(page))
		dump_stack();

	/* remove from the waitqueue */
	list_del(&wait->task_list);

	/* move onto the action list and queue for FS-Cache thread pool */
	ASSERT(monitor->op);

	object = container_of(monitor->op->op.object,
			      struct cachefiles_object, fscache);

	spin_lock(&object->work_lock);
	list_add_tail(&monitor->op_link, &monitor->op->to_do);
	spin_unlock(&object->work_lock);

	fscache_enqueue_retrieval(monitor->op);
	return 0;
}

/*
 * copy data from backing pages to netfs pages to complete a read operation
 * - driven by FS-Cache's thread pool
 */
static void cachefiles_read_copier(struct fscache_operation *_op)
{
	struct cachefiles_one_read *monitor;
	struct cachefiles_object *object;
	struct fscache_retrieval *op;
	struct pagevec pagevec;
	int error, max;

	op = container_of(_op, struct fscache_retrieval, op);
	object = container_of(op->op.object,
			      struct cachefiles_object, fscache);

	_enter("{ino=%lu}", object->backer->d_inode->i_ino);

	pagevec_init(&pagevec, 0);

	max = 8;
	spin_lock_irq(&object->work_lock);

	while (!list_empty(&op->to_do)) {
		monitor = list_entry(op->to_do.next,
				     struct cachefiles_one_read, op_link);
		list_del(&monitor->op_link);

		spin_unlock_irq(&object->work_lock);

		_debug("- copy {%lu}", monitor->back_page->index);

		error = -EIO;
		if (PageUptodate(monitor->back_page)) {
			copy_highpage(monitor->netfs_page, monitor->back_page);

			pagevec_add(&pagevec, monitor->netfs_page);
			fscache_mark_pages_cached(monitor->op, &pagevec);
			error = 0;
		}

		if (error)
			cachefiles_io_error_obj(
				object,
				"Readpage failed on backing file %lx",
				(unsigned long) monitor->back_page->flags);

		page_cache_release(monitor->back_page);

		fscache_end_io(op, monitor->netfs_page, error);
		page_cache_release(monitor->netfs_page);
		fscache_put_retrieval(op);
		kfree(monitor);

		/* let the thread pool have some air occasionally */
		max--;
		if (max < 0 || need_resched()) {
			if (!list_empty(&op->to_do))
				fscache_enqueue_retrieval(op);
			_leave(" [maxed out]");
			return;
		}

		spin_lock_irq(&object->work_lock);
	}

	spin_unlock_irq(&object->work_lock);
	_leave("");
}

/*
 * read the corresponding page to the given set from the backing file
 * - an uncertain page is simply discarded, to be tried again another time
 */
static int cachefiles_read_backing_file_one(struct cachefiles_object *object,
					    struct fscache_retrieval *op,
					    struct page *netpage,
					    struct pagevec *pagevec)
{
	struct cachefiles_one_read *monitor;
	struct address_space *bmapping;
	struct page *newpage, *backpage;
	int ret;

	_enter("");

	pagevec_reinit(pagevec);

	_debug("read back %p{%lu,%d}",
	       netpage, netpage->index, page_count(netpage));

	monitor = kzalloc(sizeof(*monitor), GFP_KERNEL);
	if (!monitor)
		goto nomem;

	monitor->netfs_page = netpage;
	monitor->op = fscache_get_retrieval(op);

	init_waitqueue_func_entry(&monitor->monitor, cachefiles_read_waiter);

	/* attempt to get hold of the backing page */
	bmapping = object->backer->d_inode->i_mapping;
	newpage = NULL;

	for (;;) {
		backpage = find_get_page(bmapping, netpage->index);
		if (backpage)
			goto backing_page_already_present;

		if (!newpage) {
			newpage = page_cache_alloc_cold(bmapping);
			if (!newpage)
				goto nomem_monitor;
		}

		ret = add_to_page_cache(newpage, bmapping,
					netpage->index, GFP_KERNEL);
		if (ret == 0)
			goto installed_new_backing_page;
		if (ret != -EEXIST)
			goto nomem_page;
	}

	/* we've installed a new backing page, so now we need to add it
	 * to the LRU list and start it reading */
installed_new_backing_page:
	_debug("- new %p", newpage);

	backpage = newpage;
	newpage = NULL;

	page_cache_get(backpage);
	pagevec_add(pagevec, backpage);
	__pagevec_lru_add_file(pagevec);

read_backing_page:
	ret = bmapping->a_ops->readpage(NULL, backpage);
	if (ret < 0)
		goto read_error;

	/* set the monitor to transfer the data across */
monitor_backing_page:
	_debug("- monitor add");

	/* install the monitor */
	page_cache_get(monitor->netfs_page);
	page_cache_get(backpage);
	monitor->back_page = backpage;
	monitor->monitor.private = backpage;
	add_page_wait_queue(backpage, &monitor->monitor);
	monitor = NULL;

	/* but the page may have been read before the monitor was installed, so
	 * the monitor may miss the event - so we have to ensure that we do get
	 * one in such a case */
	if (trylock_page(backpage)) {
		_debug("jumpstart %p {%lx}", backpage, backpage->flags);
		unlock_page(backpage);
	}
	goto success;

	/* if the backing page is already present, it can be in one of
	 * three states: read in progress, read failed or read okay */
backing_page_already_present:
	_debug("- present");

	if (newpage) {
		page_cache_release(newpage);
		newpage = NULL;
	}

	if (PageError(backpage))
		goto io_error;

	if (PageUptodate(backpage))
		goto backing_page_already_uptodate;

	if (!trylock_page(backpage))
		goto monitor_backing_page;
	_debug("read %p {%lx}", backpage, backpage->flags);
	goto read_backing_page;

	/* the backing page is already up to date, attach the netfs
	 * page to the pagecache and LRU and copy the data across */
backing_page_already_uptodate:
	_debug("- uptodate");

	pagevec_add(pagevec, netpage);
	fscache_mark_pages_cached(op, pagevec);

	copy_highpage(netpage, backpage);
	fscache_end_io(op, netpage, 0);

success:
	_debug("success");
	ret = 0;

out:
	if (backpage)
		page_cache_release(backpage);
	if (monitor) {
		fscache_put_retrieval(monitor->op);
		kfree(monitor);
	}
	_leave(" = %d", ret);
	return ret;

read_error:
	_debug("read error %d", ret);
	if (ret == -ENOMEM)
		goto out;
io_error:
	cachefiles_io_error_obj(object, "Page read error on backing file");
	ret = -ENOBUFS;
	goto out;

nomem_page:
	page_cache_release(newpage);
nomem_monitor:
	fscache_put_retrieval(monitor->op);
	kfree(monitor);
nomem:
	_leave(" = -ENOMEM");
	return -ENOMEM;
}

/*
 * read a page from the cache or allocate a block in which to store it
 * - cache withdrawal is prevented by the caller
 * - returns -EINTR if interrupted
 * - returns -ENOMEM if ran out of memory
 * - returns -ENOBUFS if no buffers can be made available
 * - returns -ENOBUFS if page is beyond EOF
 * - if the page is backed by a block in the cache:
 *   - a read will be started which will call the callback on completion
 *   - 0 will be returned
 * - else if the page is unbacked:
 *   - the metadata will be retained
 *   - -ENODATA will be returned
 */
int cachefiles_read_or_alloc_page(struct fscache_retrieval *op,
				  struct page *page,
				  gfp_t gfp)
{
	struct cachefiles_object *object;
	struct cachefiles_cache *cache;
	struct pagevec pagevec;
	struct inode *inode;
	sector_t block0, block;
	unsigned shift;
	int ret;

	object = container_of(op->op.object,
			      struct cachefiles_object, fscache);
	cache = container_of(object->fscache.cache,
			     struct cachefiles_cache, cache);

	_enter("{%p},{%lx},,,", object, page->index);

	if (!object->backer)
		return -ENOBUFS;

	inode = object->backer->d_inode;
	ASSERT(S_ISREG(inode->i_mode));
	ASSERT(inode->i_mapping->a_ops->bmap);
	ASSERT(inode->i_mapping->a_ops->readpages);

	/* calculate the shift required to use bmap */
	if (inode->i_sb->s_blocksize > PAGE_SIZE)
		return -ENOBUFS;

	shift = PAGE_SHIFT - inode->i_sb->s_blocksize_bits;

	op->op.flags &= FSCACHE_OP_KEEP_FLAGS;
	op->op.flags |= FSCACHE_OP_FAST;
	op->op.processor = cachefiles_read_copier;

	pagevec_init(&pagevec, 0);

	/* we assume the absence or presence of the first block is a good
	 * enough indication for the page as a whole
	 * - TODO: don't use bmap() for this as it is _not_ actually good
	 *   enough for this as it doesn't indicate errors, but it's all we've
	 *   got for the moment
	 */
	block0 = page->index;
	block0 <<= shift;

	block = inode->i_mapping->a_ops->bmap(inode->i_mapping, block0);
	_debug("%llx -> %llx",
	       (unsigned long long) block0,
	       (unsigned long long) block);

	if (block) {
		/* submit the apparently valid page to the backing fs to be
		 * read from disk */
		ret = cachefiles_read_backing_file_one(object, op, page,
						       &pagevec);
	} else if (cachefiles_has_space(cache, 0, 1) == 0) {
		/* there's space in the cache we can use */
		pagevec_add(&pagevec, page);
		fscache_mark_pages_cached(op, &pagevec);
		ret = -ENODATA;
	} else {
		ret = -ENOBUFS;
	}

	_leave(" = %d", ret);
	return ret;
}

/*
 * read the corresponding pages to the given set from the backing file
 * - any uncertain pages are simply discarded, to be tried again another time
 */
static int cachefiles_read_backing_file(struct cachefiles_object *object,
					struct fscache_retrieval *op,
					struct list_head *list,
					struct pagevec *mark_pvec)
{
	struct cachefiles_one_read *monitor = NULL;
	struct address_space *bmapping = object->backer->d_inode->i_mapping;
	struct pagevec lru_pvec;
	struct page *newpage = NULL, *netpage, *_n, *backpage = NULL;
	int ret = 0;

	_enter("");

	pagevec_init(&lru_pvec, 0);

	list_for_each_entry_safe(netpage, _n, list, lru) {
		list_del(&netpage->lru);

		_debug("read back %p{%lu,%d}",
		       netpage, netpage->index, page_count(netpage));

		if (!monitor) {
			monitor = kzalloc(sizeof(*monitor), GFP_KERNEL);
			if (!monitor)
				goto nomem;

			monitor->op = fscache_get_retrieval(op);
			init_waitqueue_func_entry(&monitor->monitor,
						  cachefiles_read_waiter);
		}

		for (;;) {
			backpage = find_get_page(bmapping, netpage->index);
			if (backpage)
				goto backing_page_already_present;

			if (!newpage) {
				newpage = page_cache_alloc_cold(bmapping);
				if (!newpage)
					goto nomem;
			}

			ret = add_to_page_cache(newpage, bmapping,
						netpage->index, GFP_KERNEL);
			if (ret == 0)
				goto installed_new_backing_page;
			if (ret != -EEXIST)
				goto nomem;
		}

		/* we've installed a new backing page, so now we need to add it
		 * to the LRU list and start it reading */
	installed_new_backing_page:
		_debug("- new %p", newpage);

		backpage = newpage;
		newpage = NULL;

		page_cache_get(backpage);
		if (!pagevec_add(&lru_pvec, backpage))
			__pagevec_lru_add_file(&lru_pvec);

	reread_backing_page:
		ret = bmapping->a_ops->readpage(NULL, backpage);
		if (ret < 0)
			goto read_error;

		/* add the netfs page to the pagecache and LRU, and set the
		 * monitor to transfer the data across */
	monitor_backing_page:
		_debug("- monitor add");

		ret = add_to_page_cache(netpage, op->mapping, netpage->index,
					GFP_KERNEL);
		if (ret < 0) {
			if (ret == -EEXIST) {
				page_cache_release(netpage);
				continue;
			}
			goto nomem;
		}

		page_cache_get(netpage);
		if (!pagevec_add(&lru_pvec, netpage))
			__pagevec_lru_add_file(&lru_pvec);

		/* install a monitor */
		page_cache_get(netpage);
		monitor->netfs_page = netpage;

		page_cache_get(backpage);
		monitor->back_page = backpage;
		monitor->monitor.private = backpage;
		add_page_wait_queue(backpage, &monitor->monitor);
		monitor = NULL;

		/* but the page may have been read before the monitor was
		 * installed, so the monitor may miss the event - so we have to
		 * ensure that we do get one in such a case */
		if (trylock_page(backpage)) {
			_debug("2unlock %p {%lx}", backpage, backpage->flags);
			unlock_page(backpage);
		}

		page_cache_release(backpage);
		backpage = NULL;

		page_cache_release(netpage);
		netpage = NULL;
		continue;

		/* if the backing page is already present, it can be in one of
		 * three states: read in progress, read failed or read okay */
	backing_page_already_present:
		_debug("- present %p", backpage);

		if (PageError(backpage))
			goto io_error;

		if (PageUptodate(backpage))
			goto backing_page_already_uptodate;

		_debug("- not ready %p{%lx}", backpage, backpage->flags);

		if (!trylock_page(backpage))
			goto monitor_backing_page;

		if (PageError(backpage)) {
			_debug("error %lx", backpage->flags);
			unlock_page(backpage);
			goto io_error;
		}

		if (PageUptodate(backpage))
			goto backing_page_already_uptodate_unlock;

		/* we've locked a page that's neither up to date nor erroneous,
		 * so we need to attempt to read it again */
		goto reread_backing_page;

		/* the backing page is already up to date, attach the netfs
		 * page to the pagecache and LRU and copy the data across */
	backing_page_already_uptodate_unlock:
		_debug("uptodate %lx", backpage->flags);
		unlock_page(backpage);
	backing_page_already_uptodate:
		_debug("- uptodate");

		ret = add_to_page_cache(netpage, op->mapping, netpage->index,
					GFP_KERNEL);
		if (ret < 0) {
			if (ret == -EEXIST) {
				page_cache_release(netpage);
				continue;
			}
			goto nomem;
		}

		copy_highpage(netpage, backpage);

		page_cache_release(backpage);
		backpage = NULL;

		if (!pagevec_add(mark_pvec, netpage))
			fscache_mark_pages_cached(op, mark_pvec);

		page_cache_get(netpage);
		if (!pagevec_add(&lru_pvec, netpage))
			__pagevec_lru_add_file(&lru_pvec);

		fscache_end_io(op, netpage, 0);
		page_cache_release(netpage);
		netpage = NULL;
		continue;
	}

	netpage = NULL;

	_debug("out");

out:
	/* tidy up */
	pagevec_lru_add_file(&lru_pvec);

	if (newpage)
		page_cache_release(newpage);
	if (netpage)
		page_cache_release(netpage);
	if (backpage)
		page_cache_release(backpage);
	if (monitor) {
		fscache_put_retrieval(op);
		kfree(monitor);
	}

	list_for_each_entry_safe(netpage, _n, list, lru) {
		list_del(&netpage->lru);
		page_cache_release(netpage);
	}

	_leave(" = %d", ret);
	return ret;

nomem:
	_debug("nomem");
	ret = -ENOMEM;
	goto out;

read_error:
	_debug("read error %d", ret);
	if (ret == -ENOMEM)
		goto out;
io_error:
	cachefiles_io_error_obj(object, "Page read error on backing file");
	ret = -ENOBUFS;
	goto out;
}

/*
 * read a list of pages from the cache or allocate blocks in which to store
 * them
 */
int cachefiles_read_or_alloc_pages(struct fscache_retrieval *op,
				   struct list_head *pages,
				   unsigned *nr_pages,
				   gfp_t gfp)
{
	struct cachefiles_object *object;
	struct cachefiles_cache *cache;
	struct list_head backpages;
	struct pagevec pagevec;
	struct inode *inode;
	struct page *page, *_n;
	unsigned shift, nrbackpages;
	int ret, ret2, space;

	object = container_of(op->op.object,
			      struct cachefiles_object, fscache);
	cache = container_of(object->fscache.cache,
			     struct cachefiles_cache, cache);

	_enter("{OBJ%x,%d},,%d,,",
	       object->fscache.debug_id, atomic_read(&op->op.usage),
	       *nr_pages);

	if (!object->backer)
		return -ENOBUFS;

	space = 1;
	if (cachefiles_has_space(cache, 0, *nr_pages) < 0)
		space = 0;

	inode = object->backer->d_inode;
	ASSERT(S_ISREG(inode->i_mode));
	ASSERT(inode->i_mapping->a_ops->bmap);
	ASSERT(inode->i_mapping->a_ops->readpages);

	/* calculate the shift required to use bmap */
	if (inode->i_sb->s_blocksize > PAGE_SIZE)
		return -ENOBUFS;

	shift = PAGE_SHIFT - inode->i_sb->s_blocksize_bits;

	pagevec_init(&pagevec, 0);

	op->op.flags &= FSCACHE_OP_KEEP_FLAGS;
	op->op.flags |= FSCACHE_OP_FAST;
	op->op.processor = cachefiles_read_copier;

	INIT_LIST_HEAD(&backpages);
	nrbackpages = 0;

	ret = space ? -ENODATA : -ENOBUFS;
	list_for_each_entry_safe(page, _n, pages, lru) {
		sector_t block0, block;

		/* we assume the absence or presence of the first block is a
		 * good enough indication for the page as a whole
		 * - TODO: don't use bmap() for this as it is _not_ actually
		 *   good enough for this as it doesn't indicate errors, but
		 *   it's all we've got for the moment
		 */
		block0 = page->index;
		block0 <<= shift;

		block = inode->i_mapping->a_ops->bmap(inode->i_mapping,
						      block0);
		_debug("%llx -> %llx",
		       (unsigned long long) block0,
		       (unsigned long long) block);

		if (block) {
			/* we have data - add it to the list to give to the
			 * backing fs */
			list_move(&page->lru, &backpages);
			(*nr_pages)--;
			nrbackpages++;
		} else if (space && pagevec_add(&pagevec, page) == 0) {
			fscache_mark_pages_cached(op, &pagevec);
			ret = -ENODATA;
		}
	}

	if (pagevec_count(&pagevec) > 0)
		fscache_mark_pages_cached(op, &pagevec);

	if (list_empty(pages))
		ret = 0;

	/* submit the apparently valid pages to the backing fs to be read from
	 * disk */
	if (nrbackpages > 0) {
		ret2 = cachefiles_read_backing_file(object, op, &backpages,
						    &pagevec);
		if (ret2 == -ENOMEM || ret2 == -EINTR)
			ret = ret2;
	}

	if (pagevec_count(&pagevec) > 0)
		fscache_mark_pages_cached(op, &pagevec);

	_leave(" = %d [nr=%u%s]",
	       ret, *nr_pages, list_empty(pages) ? " empty" : "");
	return ret;
}

/*
 * allocate a block in the cache in which to store a page
 * - cache withdrawal is prevented by the caller
 * - returns -EINTR if interrupted
 * - returns -ENOMEM if ran out of memory
 * - returns -ENOBUFS if no buffers can be made available
 * - returns -ENOBUFS if page is beyond EOF
 * - otherwise:
 *   - the metadata will be retained
 *   - 0 will be returned
 */
int cachefiles_allocate_page(struct fscache_retrieval *op,
			     struct page *page,
			     gfp_t gfp)
{
	struct cachefiles_object *object;
	struct cachefiles_cache *cache;
	struct pagevec pagevec;
	int ret;

	object = container_of(op->op.object,
			      struct cachefiles_object, fscache);
	cache = container_of(object->fscache.cache,
			     struct cachefiles_cache, cache);

	_enter("%p,{%lx},", object, page->index);

	ret = cachefiles_has_space(cache, 0, 1);
	if (ret == 0) {
		pagevec_init(&pagevec, 0);
		pagevec_add(&pagevec, page);
		fscache_mark_pages_cached(op, &pagevec);
	} else {
		ret = -ENOBUFS;
	}

	_leave(" = %d", ret);
	return ret;
}

/*
 * allocate blocks in the cache in which to store a set of pages
 * - cache withdrawal is prevented by the caller
 * - returns -EINTR if interrupted
 * - returns -ENOMEM if ran out of memory
 * - returns -ENOBUFS if some buffers couldn't be made available
 * - returns -ENOBUFS if some pages are beyond EOF
 * - otherwise:
 *   - -ENODATA will be returned
 * - metadata will be retained for any page marked
 */
int cachefiles_allocate_pages(struct fscache_retrieval *op,
			      struct list_head *pages,
			      unsigned *nr_pages,
			      gfp_t gfp)
{
	struct cachefiles_object *object;
	struct cachefiles_cache *cache;
	struct pagevec pagevec;
	struct page *page;
	int ret;

	object = container_of(op->op.object,
			      struct cachefiles_object, fscache);
	cache = container_of(object->fscache.cache,
			     struct cachefiles_cache, cache);

	_enter("%p,,,%d,", object, *nr_pages);

	ret = cachefiles_has_space(cache, 0, *nr_pages);
	if (ret == 0) {
		pagevec_init(&pagevec, 0);

		list_for_each_entry(page, pages, lru) {
			if (pagevec_add(&pagevec, page) == 0)
				fscache_mark_pages_cached(op, &pagevec);
		}

		if (pagevec_count(&pagevec) > 0)
			fscache_mark_pages_cached(op, &pagevec);
		ret = -ENODATA;
	} else {
		ret = -ENOBUFS;
	}

	_leave(" = %d", ret);
	return ret;
}

/*
 * request a page be stored in the cache
 * - cache withdrawal is prevented by the caller
 * - this request may be ignored if there's no cache block available, in which
 *   case -ENOBUFS will be returned
 * - if the op is in progress, 0 will be returned
 */
int cachefiles_write_page(struct fscache_storage *op, struct page *page)
{
	struct cachefiles_object *object;
	struct cachefiles_cache *cache;
	mm_segment_t old_fs;
	struct file *file;
	loff_t pos;
	void *data;
	int ret;

	ASSERT(op != NULL);
	ASSERT(page != NULL);

	object = container_of(op->op.object,
			      struct cachefiles_object, fscache);

	_enter("%p,%p{%lx},,,", object, page, page->index);

	if (!object->backer) {
		_leave(" = -ENOBUFS");
		return -ENOBUFS;
	}

	ASSERT(S_ISREG(object->backer->d_inode->i_mode));

	cache = container_of(object->fscache.cache,
			     struct cachefiles_cache, cache);

	/* write the page to the backing filesystem and let it store it in its
	 * own time */
	dget(object->backer);
	mntget(cache->mnt);
	file = dentry_open(object->backer, cache->mnt, O_RDWR,
			   cache->cache_cred);
	if (IS_ERR(file)) {
		ret = PTR_ERR(file);
	} else {
		ret = -EIO;
		if (file->f_op->write) {
			pos = (loff_t) page->index << PAGE_SHIFT;
			data = kmap(page);
			old_fs = get_fs();
			set_fs(KERNEL_DS);
			ret = file->f_op->write(
				file, (const void __user *) data, PAGE_SIZE,
				&pos);
			set_fs(old_fs);
			kunmap(page);
			if (ret != PAGE_SIZE)
				ret = -EIO;
		}
		fput(file);
	}

	if (ret < 0) {
		if (ret == -EIO)
			cachefiles_io_error_obj(
				object, "Write page to backing file failed");
		ret = -ENOBUFS;
	}

	_leave(" = %d", ret);
	return ret;
}

/*
 * detach a backing block from a page
 * - cache withdrawal is prevented by the caller
 */
void cachefiles_uncache_page(struct fscache_object *_object, struct page *page)
{
	struct cachefiles_object *object;
	struct cachefiles_cache *cache;

	object = container_of(_object, struct cachefiles_object, fscache);
	cache = container_of(object->fscache.cache,
			     struct cachefiles_cache, cache);

	_enter("%p,{%lu}", object, page->index);

	spin_unlock(&object->fscache.cookie->lock);
}