aboutsummaryrefslogblamecommitdiffstats
path: root/fs/btrfs/backref.c
blob: 22c64fff1bd524b213ce8b13a233f861680d8424 (plain) (tree)
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





































































































































































































































































































































































































































































































































































































































































































                                                                                
                                                                  





                                                                          
                                                                   













                                                                                
                              
                                                                               
                                                                           








































































                                                                               
/*
 * Copyright (C) 2011 STRATO.  All rights reserved.
 *
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public
 * License v2 as published by the Free Software Foundation.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 * General Public License for more details.
 *
 * You should have received a copy of the GNU General Public
 * License along with this program; if not, write to the
 * Free Software Foundation, Inc., 59 Temple Place - Suite 330,
 * Boston, MA 021110-1307, USA.
 */

#include "ctree.h"
#include "disk-io.h"
#include "backref.h"

struct __data_ref {
	struct list_head list;
	u64 inum;
	u64 root;
	u64 extent_data_item_offset;
};

struct __shared_ref {
	struct list_head list;
	u64 disk_byte;
};

static int __inode_info(u64 inum, u64 ioff, u8 key_type,
			struct btrfs_root *fs_root, struct btrfs_path *path,
			struct btrfs_key *found_key)
{
	int ret;
	struct btrfs_key key;
	struct extent_buffer *eb;

	key.type = key_type;
	key.objectid = inum;
	key.offset = ioff;

	ret = btrfs_search_slot(NULL, fs_root, &key, path, 0, 0);
	if (ret < 0)
		return ret;

	eb = path->nodes[0];
	if (ret && path->slots[0] >= btrfs_header_nritems(eb)) {
		ret = btrfs_next_leaf(fs_root, path);
		if (ret)
			return ret;
		eb = path->nodes[0];
	}

	btrfs_item_key_to_cpu(eb, found_key, path->slots[0]);
	if (found_key->type != key.type || found_key->objectid != key.objectid)
		return 1;

	return 0;
}

/*
 * this makes the path point to (inum INODE_ITEM ioff)
 */
int inode_item_info(u64 inum, u64 ioff, struct btrfs_root *fs_root,
			struct btrfs_path *path)
{
	struct btrfs_key key;
	return __inode_info(inum, ioff, BTRFS_INODE_ITEM_KEY, fs_root, path,
				&key);
}

static int inode_ref_info(u64 inum, u64 ioff, struct btrfs_root *fs_root,
				struct btrfs_path *path,
				struct btrfs_key *found_key)
{
	return __inode_info(inum, ioff, BTRFS_INODE_REF_KEY, fs_root, path,
				found_key);
}

/*
 * this iterates to turn a btrfs_inode_ref into a full filesystem path. elements
 * of the path are separated by '/' and the path is guaranteed to be
 * 0-terminated. the path is only given within the current file system.
 * Therefore, it never starts with a '/'. the caller is responsible to provide
 * "size" bytes in "dest". the dest buffer will be filled backwards. finally,
 * the start point of the resulting string is returned. this pointer is within
 * dest, normally.
 * in case the path buffer would overflow, the pointer is decremented further
 * as if output was written to the buffer, though no more output is actually
 * generated. that way, the caller can determine how much space would be
 * required for the path to fit into the buffer. in that case, the returned
 * value will be smaller than dest. callers must check this!
 */
static char *iref_to_path(struct btrfs_root *fs_root, struct btrfs_path *path,
				struct btrfs_inode_ref *iref,
				struct extent_buffer *eb_in, u64 parent,
				char *dest, u32 size)
{
	u32 len;
	int slot;
	u64 next_inum;
	int ret;
	s64 bytes_left = size - 1;
	struct extent_buffer *eb = eb_in;
	struct btrfs_key found_key;

	if (bytes_left >= 0)
		dest[bytes_left] = '\0';

	while (1) {
		len = btrfs_inode_ref_name_len(eb, iref);
		bytes_left -= len;
		if (bytes_left >= 0)
			read_extent_buffer(eb, dest + bytes_left,
						(unsigned long)(iref + 1), len);
		if (eb != eb_in)
			free_extent_buffer(eb);
		ret = inode_ref_info(parent, 0, fs_root, path, &found_key);
		if (ret)
			break;
		next_inum = found_key.offset;

		/* regular exit ahead */
		if (parent == next_inum)
			break;

		slot = path->slots[0];
		eb = path->nodes[0];
		/* make sure we can use eb after releasing the path */
		if (eb != eb_in)
			atomic_inc(&eb->refs);
		btrfs_release_path(path);

		iref = btrfs_item_ptr(eb, slot, struct btrfs_inode_ref);
		parent = next_inum;
		--bytes_left;
		if (bytes_left >= 0)
			dest[bytes_left] = '/';
	}

	btrfs_release_path(path);

	if (ret)
		return ERR_PTR(ret);

	return dest + bytes_left;
}

/*
 * this makes the path point to (logical EXTENT_ITEM *)
 * returns BTRFS_EXTENT_FLAG_DATA for data, BTRFS_EXTENT_FLAG_TREE_BLOCK for
 * tree blocks and <0 on error.
 */
int extent_from_logical(struct btrfs_fs_info *fs_info, u64 logical,
			struct btrfs_path *path, struct btrfs_key *found_key)
{
	int ret;
	u64 flags;
	u32 item_size;
	struct extent_buffer *eb;
	struct btrfs_extent_item *ei;
	struct btrfs_key key;

	key.type = BTRFS_EXTENT_ITEM_KEY;
	key.objectid = logical;
	key.offset = (u64)-1;

	ret = btrfs_search_slot(NULL, fs_info->extent_root, &key, path, 0, 0);
	if (ret < 0)
		return ret;
	ret = btrfs_previous_item(fs_info->extent_root, path,
					0, BTRFS_EXTENT_ITEM_KEY);
	if (ret < 0)
		return ret;

	btrfs_item_key_to_cpu(path->nodes[0], found_key, path->slots[0]);
	if (found_key->type != BTRFS_EXTENT_ITEM_KEY ||
	    found_key->objectid > logical ||
	    found_key->objectid + found_key->offset <= logical)
		return -ENOENT;

	eb = path->nodes[0];
	item_size = btrfs_item_size_nr(eb, path->slots[0]);
	BUG_ON(item_size < sizeof(*ei));

	ei = btrfs_item_ptr(eb, path->slots[0], struct btrfs_extent_item);
	flags = btrfs_extent_flags(eb, ei);

	if (flags & BTRFS_EXTENT_FLAG_TREE_BLOCK)
		return BTRFS_EXTENT_FLAG_TREE_BLOCK;
	if (flags & BTRFS_EXTENT_FLAG_DATA)
		return BTRFS_EXTENT_FLAG_DATA;

	return -EIO;
}

/*
 * helper function to iterate extent inline refs. ptr must point to a 0 value
 * for the first call and may be modified. it is used to track state.
 * if more refs exist, 0 is returned and the next call to
 * __get_extent_inline_ref must pass the modified ptr parameter to get the
 * next ref. after the last ref was processed, 1 is returned.
 * returns <0 on error
 */
static int __get_extent_inline_ref(unsigned long *ptr, struct extent_buffer *eb,
				struct btrfs_extent_item *ei, u32 item_size,
				struct btrfs_extent_inline_ref **out_eiref,
				int *out_type)
{
	unsigned long end;
	u64 flags;
	struct btrfs_tree_block_info *info;

	if (!*ptr) {
		/* first call */
		flags = btrfs_extent_flags(eb, ei);
		if (flags & BTRFS_EXTENT_FLAG_TREE_BLOCK) {
			info = (struct btrfs_tree_block_info *)(ei + 1);
			*out_eiref =
				(struct btrfs_extent_inline_ref *)(info + 1);
		} else {
			*out_eiref = (struct btrfs_extent_inline_ref *)(ei + 1);
		}
		*ptr = (unsigned long)*out_eiref;
		if ((void *)*ptr >= (void *)ei + item_size)
			return -ENOENT;
	}

	end = (unsigned long)ei + item_size;
	*out_eiref = (struct btrfs_extent_inline_ref *)*ptr;
	*out_type = btrfs_extent_inline_ref_type(eb, *out_eiref);

	*ptr += btrfs_extent_inline_ref_size(*out_type);
	WARN_ON(*ptr > end);
	if (*ptr == end)
		return 1; /* last */

	return 0;
}

/*
 * reads the tree block backref for an extent. tree level and root are returned
 * through out_level and out_root. ptr must point to a 0 value for the first
 * call and may be modified (see __get_extent_inline_ref comment).
 * returns 0 if data was provided, 1 if there was no more data to provide or
 * <0 on error.
 */
int tree_backref_for_extent(unsigned long *ptr, struct extent_buffer *eb,
				struct btrfs_extent_item *ei, u32 item_size,
				u64 *out_root, u8 *out_level)
{
	int ret;
	int type;
	struct btrfs_tree_block_info *info;
	struct btrfs_extent_inline_ref *eiref;

	if (*ptr == (unsigned long)-1)
		return 1;

	while (1) {
		ret = __get_extent_inline_ref(ptr, eb, ei, item_size,
						&eiref, &type);
		if (ret < 0)
			return ret;

		if (type == BTRFS_TREE_BLOCK_REF_KEY ||
		    type == BTRFS_SHARED_BLOCK_REF_KEY)
			break;

		if (ret == 1)
			return 1;
	}

	/* we can treat both ref types equally here */
	info = (struct btrfs_tree_block_info *)(ei + 1);
	*out_root = btrfs_extent_inline_ref_offset(eb, eiref);
	*out_level = btrfs_tree_block_level(eb, info);

	if (ret == 1)
		*ptr = (unsigned long)-1;

	return 0;
}

static int __data_list_add(struct list_head *head, u64 inum,
				u64 extent_data_item_offset, u64 root)
{
	struct __data_ref *ref;

	ref = kmalloc(sizeof(*ref), GFP_NOFS);
	if (!ref)
		return -ENOMEM;

	ref->inum = inum;
	ref->extent_data_item_offset = extent_data_item_offset;
	ref->root = root;
	list_add_tail(&ref->list, head);

	return 0;
}

static int __data_list_add_eb(struct list_head *head, struct extent_buffer *eb,
				struct btrfs_extent_data_ref *dref)
{
	return __data_list_add(head, btrfs_extent_data_ref_objectid(eb, dref),
				btrfs_extent_data_ref_offset(eb, dref),
				btrfs_extent_data_ref_root(eb, dref));
}

static int __shared_list_add(struct list_head *head, u64 disk_byte)
{
	struct __shared_ref *ref;

	ref = kmalloc(sizeof(*ref), GFP_NOFS);
	if (!ref)
		return -ENOMEM;

	ref->disk_byte = disk_byte;
	list_add_tail(&ref->list, head);

	return 0;
}

static int __iter_shared_inline_ref_inodes(struct btrfs_fs_info *fs_info,
					   u64 logical, u64 inum,
					   u64 extent_data_item_offset,
					   u64 extent_offset,
					   struct btrfs_path *path,
					   struct list_head *data_refs,
					   iterate_extent_inodes_t *iterate,
					   void *ctx)
{
	u64 ref_root;
	u32 item_size;
	struct btrfs_key key;
	struct extent_buffer *eb;
	struct btrfs_extent_item *ei;
	struct btrfs_extent_inline_ref *eiref;
	struct __data_ref *ref;
	int ret;
	int type;
	int last;
	unsigned long ptr = 0;

	WARN_ON(!list_empty(data_refs));
	ret = extent_from_logical(fs_info, logical, path, &key);
	if (ret & BTRFS_EXTENT_FLAG_DATA)
		ret = -EIO;
	if (ret < 0)
		goto out;

	eb = path->nodes[0];
	ei = btrfs_item_ptr(eb, path->slots[0], struct btrfs_extent_item);
	item_size = btrfs_item_size_nr(eb, path->slots[0]);

	ret = 0;
	ref_root = 0;
	/*
	 * as done in iterate_extent_inodes, we first build a list of refs to
	 * iterate, then free the path and then iterate them to avoid deadlocks.
	 */
	do {
		last = __get_extent_inline_ref(&ptr, eb, ei, item_size,
						&eiref, &type);
		if (last < 0) {
			ret = last;
			goto out;
		}
		if (type == BTRFS_TREE_BLOCK_REF_KEY ||
		    type == BTRFS_SHARED_BLOCK_REF_KEY) {
			ref_root = btrfs_extent_inline_ref_offset(eb, eiref);
			ret = __data_list_add(data_refs, inum,
						extent_data_item_offset,
						ref_root);
		}
	} while (!ret && !last);

	btrfs_release_path(path);

	if (ref_root == 0) {
		printk(KERN_ERR "btrfs: failed to find tree block ref "
			"for shared data backref %llu\n", logical);
		WARN_ON(1);
		ret = -EIO;
	}

out:
	while (!list_empty(data_refs)) {
		ref = list_first_entry(data_refs, struct __data_ref, list);
		list_del(&ref->list);
		if (!ret)
			ret = iterate(ref->inum, extent_offset +
					ref->extent_data_item_offset,
					ref->root, ctx);
		kfree(ref);
	}

	return ret;
}

static int __iter_shared_inline_ref(struct btrfs_fs_info *fs_info,
				    u64 logical, u64 orig_extent_item_objectid,
				    u64 extent_offset, struct btrfs_path *path,
				    struct list_head *data_refs,
				    iterate_extent_inodes_t *iterate,
				    void *ctx)
{
	u64 disk_byte;
	struct btrfs_key key;
	struct btrfs_file_extent_item *fi;
	struct extent_buffer *eb;
	int slot;
	int nritems;
	int ret;
	int found = 0;

	eb = read_tree_block(fs_info->tree_root, logical,
				fs_info->tree_root->leafsize, 0);
	if (!eb)
		return -EIO;

	/*
	 * from the shared data ref, we only have the leaf but we need
	 * the key. thus, we must look into all items and see that we
	 * find one (some) with a reference to our extent item.
	 */
	nritems = btrfs_header_nritems(eb);
	for (slot = 0; slot < nritems; ++slot) {
		btrfs_item_key_to_cpu(eb, &key, slot);
		if (key.type != BTRFS_EXTENT_DATA_KEY)
			continue;
		fi = btrfs_item_ptr(eb, slot, struct btrfs_file_extent_item);
		if (!fi) {
			free_extent_buffer(eb);
			return -EIO;
		}
		disk_byte = btrfs_file_extent_disk_bytenr(eb, fi);
		if (disk_byte != orig_extent_item_objectid) {
			if (found)
				break;
			else
				continue;
		}
		++found;
		ret = __iter_shared_inline_ref_inodes(fs_info, logical,
							key.objectid,
							key.offset,
							extent_offset, path,
							data_refs,
							iterate, ctx);
		if (ret)
			break;
	}

	if (!found) {
		printk(KERN_ERR "btrfs: failed to follow shared data backref "
			"to parent %llu\n", logical);
		WARN_ON(1);
		ret = -EIO;
	}

	free_extent_buffer(eb);
	return ret;
}

/*
 * calls iterate() for every inode that references the extent identified by
 * the given parameters. will use the path given as a parameter and return it
 * released.
 * when the iterator function returns a non-zero value, iteration stops.
 */
int iterate_extent_inodes(struct btrfs_fs_info *fs_info,
				struct btrfs_path *path,
				u64 extent_item_objectid,
				u64 extent_offset,
				iterate_extent_inodes_t *iterate, void *ctx)
{
	unsigned long ptr = 0;
	int last;
	int ret;
	int type;
	u64 logical;
	u32 item_size;
	struct btrfs_extent_inline_ref *eiref;
	struct btrfs_extent_data_ref *dref;
	struct extent_buffer *eb;
	struct btrfs_extent_item *ei;
	struct btrfs_key key;
	struct list_head data_refs = LIST_HEAD_INIT(data_refs);
	struct list_head shared_refs = LIST_HEAD_INIT(shared_refs);
	struct __data_ref *ref_d;
	struct __shared_ref *ref_s;

	eb = path->nodes[0];
	ei = btrfs_item_ptr(eb, path->slots[0], struct btrfs_extent_item);
	item_size = btrfs_item_size_nr(eb, path->slots[0]);

	/* first we iterate the inline refs, ... */
	do {
		last = __get_extent_inline_ref(&ptr, eb, ei, item_size,
						&eiref, &type);
		if (last == -ENOENT) {
			ret = 0;
			break;
		}
		if (last < 0) {
			ret = last;
			break;
		}

		if (type == BTRFS_EXTENT_DATA_REF_KEY) {
			dref = (struct btrfs_extent_data_ref *)(&eiref->offset);
			ret = __data_list_add_eb(&data_refs, eb, dref);
		} else if (type == BTRFS_SHARED_DATA_REF_KEY) {
			logical = btrfs_extent_inline_ref_offset(eb, eiref);
			ret = __shared_list_add(&shared_refs, logical);
		}
	} while (!ret && !last);

	/* ... then we proceed to in-tree references and ... */
	while (!ret) {
		++path->slots[0];
		if (path->slots[0] > btrfs_header_nritems(eb)) {
			ret = btrfs_next_leaf(fs_info->extent_root, path);
			if (ret) {
				if (ret == 1)
					ret = 0; /* we're done */
				break;
			}
			eb = path->nodes[0];
		}
		btrfs_item_key_to_cpu(eb, &key, path->slots[0]);
		if (key.objectid != extent_item_objectid)
			break;
		if (key.type == BTRFS_EXTENT_DATA_REF_KEY) {
			dref = btrfs_item_ptr(eb, path->slots[0],
						struct btrfs_extent_data_ref);
			ret = __data_list_add_eb(&data_refs, eb, dref);
		} else if (key.type == BTRFS_SHARED_DATA_REF_KEY) {
			ret = __shared_list_add(&shared_refs, key.offset);
		}
	}

	btrfs_release_path(path);

	/*
	 * ... only at the very end we can process the refs we found. this is
	 * because the iterator function we call is allowed to make tree lookups
	 * and we have to avoid deadlocks. additionally, we need more tree
	 * lookups ourselves for shared data refs.
	 */
	while (!list_empty(&data_refs)) {
		ref_d = list_first_entry(&data_refs, struct __data_ref, list);
		list_del(&ref_d->list);
		if (!ret)
			ret = iterate(ref_d->inum, extent_offset +
					ref_d->extent_data_item_offset,
					ref_d->root, ctx);
		kfree(ref_d);
	}

	while (!list_empty(&shared_refs)) {
		ref_s = list_first_entry(&shared_refs, struct __shared_ref,
					list);
		list_del(&ref_s->list);
		if (!ret)
			ret = __iter_shared_inline_ref(fs_info,
							ref_s->disk_byte,
							extent_item_objectid,
							extent_offset, path,
							&data_refs,
							iterate, ctx);
		kfree(ref_s);
	}

	return ret;
}

int iterate_inodes_from_logical(u64 logical, struct btrfs_fs_info *fs_info,
				struct btrfs_path *path,
				iterate_extent_inodes_t *iterate, void *ctx)
{
	int ret;
	u64 offset;
	struct btrfs_key found_key;

	ret = extent_from_logical(fs_info, logical, path,
					&found_key);
	if (ret & BTRFS_EXTENT_FLAG_TREE_BLOCK)
		ret = -EINVAL;
	if (ret < 0)
		return ret;

	offset = logical - found_key.objectid;
	ret = iterate_extent_inodes(fs_info, path, found_key.objectid,
					offset, iterate, ctx);

	return ret;
}

static int iterate_irefs(u64 inum, struct btrfs_root *fs_root,
				struct btrfs_path *path,
				iterate_irefs_t *iterate, void *ctx)
{
	int ret;
	int slot;
	u32 cur;
	u32 len;
	u32 name_len;
	u64 parent = 0;
	int found = 0;
	struct extent_buffer *eb;
	struct btrfs_item *item;
	struct btrfs_inode_ref *iref;
	struct btrfs_key found_key;

	while (1) {
		ret = inode_ref_info(inum, parent ? parent+1 : 0, fs_root, path,
					&found_key);
		if (ret < 0)
			break;
		if (ret) {
			ret = found ? 0 : -ENOENT;
			break;
		}
		++found;

		parent = found_key.offset;
		slot = path->slots[0];
		eb = path->nodes[0];
		/* make sure we can use eb after releasing the path */
		atomic_inc(&eb->refs);
		btrfs_release_path(path);

		item = btrfs_item_nr(eb, slot);
		iref = btrfs_item_ptr(eb, slot, struct btrfs_inode_ref);

		for (cur = 0; cur < btrfs_item_size(eb, item); cur += len) {
			name_len = btrfs_inode_ref_name_len(eb, iref);
			/* path must be released before calling iterate()! */
			ret = iterate(parent, iref, eb, ctx);
			if (ret) {
				free_extent_buffer(eb);
				break;
			}
			len = sizeof(*iref) + name_len;
			iref = (struct btrfs_inode_ref *)((char *)iref + len);
		}
		free_extent_buffer(eb);
	}

	btrfs_release_path(path);

	return ret;
}

/*
 * returns 0 if the path could be dumped (probably truncated)
 * returns <0 in case of an error
 */
static int inode_to_path(u64 inum, struct btrfs_inode_ref *iref,
				struct extent_buffer *eb, void *ctx)
{
	struct inode_fs_paths *ipath = ctx;
	char *fspath;
	char *fspath_min;
	int i = ipath->fspath->elem_cnt;
	const int s_ptr = sizeof(char *);
	u32 bytes_left;

	bytes_left = ipath->fspath->bytes_left > s_ptr ?
					ipath->fspath->bytes_left - s_ptr : 0;

	fspath_min = (char *)ipath->fspath->val + (i + 1) * s_ptr;
	fspath = iref_to_path(ipath->fs_root, ipath->btrfs_path, iref, eb,
				inum, fspath_min, bytes_left);
	if (IS_ERR(fspath))
		return PTR_ERR(fspath);

	if (fspath > fspath_min) {
		ipath->fspath->val[i] = (u64)(unsigned long)fspath;
		++ipath->fspath->elem_cnt;
		ipath->fspath->bytes_left = fspath - fspath_min;
	} else {
		++ipath->fspath->elem_missed;
		ipath->fspath->bytes_missing += fspath_min - fspath;
		ipath->fspath->bytes_left = 0;
	}

	return 0;
}

/*
 * this dumps all file system paths to the inode into the ipath struct, provided
 * is has been created large enough. each path is zero-terminated and accessed
 * from ipath->fspath->val[i].
 * when it returns, there are ipath->fspath->elem_cnt number of paths available
 * in ipath->fspath->val[]. when the allocated space wasn't sufficient, the
 * number of missed paths in recored in ipath->fspath->elem_missed, otherwise,
 * it's zero. ipath->fspath->bytes_missing holds the number of bytes that would
 * have been needed to return all paths.
 */
int paths_from_inode(u64 inum, struct inode_fs_paths *ipath)
{
	return iterate_irefs(inum, ipath->fs_root, ipath->btrfs_path,
				inode_to_path, ipath);
}

/*
 * allocates space to return multiple file system paths for an inode.
 * total_bytes to allocate are passed, note that space usable for actual path
 * information will be total_bytes - sizeof(struct inode_fs_paths).
 * the returned pointer must be freed with free_ipath() in the end.
 */
struct btrfs_data_container *init_data_container(u32 total_bytes)
{
	struct btrfs_data_container *data;
	size_t alloc_bytes;

	alloc_bytes = max_t(size_t, total_bytes, sizeof(*data));
	data = kmalloc(alloc_bytes, GFP_NOFS);
	if (!data)
		return ERR_PTR(-ENOMEM);

	if (total_bytes >= sizeof(*data)) {
		data->bytes_left = total_bytes - sizeof(*data);
		data->bytes_missing = 0;
	} else {
		data->bytes_missing = sizeof(*data) - total_bytes;
		data->bytes_left = 0;
	}

	data->elem_cnt = 0;
	data->elem_missed = 0;

	return data;
}

/*
 * allocates space to return multiple file system paths for an inode.
 * total_bytes to allocate are passed, note that space usable for actual path
 * information will be total_bytes - sizeof(struct inode_fs_paths).
 * the returned pointer must be freed with free_ipath() in the end.
 */
struct inode_fs_paths *init_ipath(s32 total_bytes, struct btrfs_root *fs_root,
					struct btrfs_path *path)
{
	struct inode_fs_paths *ifp;
	struct btrfs_data_container *fspath;

	fspath = init_data_container(total_bytes);
	if (IS_ERR(fspath))
		return (void *)fspath;

	ifp = kmalloc(sizeof(*ifp), GFP_NOFS);
	if (!ifp) {
		kfree(fspath);
		return ERR_PTR(-ENOMEM);
	}

	ifp->btrfs_path = path;
	ifp->fspath = fspath;
	ifp->fs_root = fs_root;

	return ifp;
}

void free_ipath(struct inode_fs_paths *ipath)
{
	kfree(ipath);
}