aboutsummaryrefslogtreecommitdiff
path: root/fs/btrfs/extent-tree.c
blob: d785b721b461b41398a3cda6b2f873ee51a6ee2c (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
#include <linux/module.h>
#include "ctree.h"
#include "disk-io.h"
#include "print-tree.h"
#include "transaction.h"

static int find_free_extent(struct btrfs_trans_handle *trans, struct btrfs_root
			    *orig_root, u64 num_blocks, u64 search_start, u64
			    search_end, struct btrfs_key *ins);
static int finish_current_insert(struct btrfs_trans_handle *trans, struct
				 btrfs_root *extent_root);
static int del_pending_extents(struct btrfs_trans_handle *trans, struct
			       btrfs_root *extent_root);

static int inc_block_ref(struct btrfs_trans_handle *trans, struct btrfs_root
			 *root, u64 blocknr, u64 num_blocks)
{
	struct btrfs_path *path;
	int ret;
	struct btrfs_key key;
	struct btrfs_leaf *l;
	struct btrfs_extent_item *item;
	struct btrfs_key ins;
	u32 refs;

	find_free_extent(trans, root->fs_info->extent_root, 0, 0, (u64)-1,
			 &ins);
	path = btrfs_alloc_path();
	BUG_ON(!path);
	btrfs_init_path(path);
	key.objectid = blocknr;
	key.flags = 0;
	btrfs_set_key_type(&key, BTRFS_EXTENT_ITEM_KEY);
	key.offset = num_blocks;
	ret = btrfs_search_slot(trans, root->fs_info->extent_root, &key, path,
				0, 1);
	if (ret != 0)
		BUG();
	BUG_ON(ret != 0);
	l = btrfs_buffer_leaf(path->nodes[0]);
	item = btrfs_item_ptr(l, path->slots[0], struct btrfs_extent_item);
	refs = btrfs_extent_refs(item);
	btrfs_set_extent_refs(item, refs + 1);
	btrfs_mark_buffer_dirty(path->nodes[0]);

	btrfs_release_path(root->fs_info->extent_root, path);
	btrfs_free_path(path);
	finish_current_insert(trans, root->fs_info->extent_root);
	del_pending_extents(trans, root->fs_info->extent_root);
	return 0;
}

static int lookup_block_ref(struct btrfs_trans_handle *trans, struct btrfs_root
			    *root, u64 blocknr, u64 num_blocks, u32 *refs)
{
	struct btrfs_path *path;
	int ret;
	struct btrfs_key key;
	struct btrfs_leaf *l;
	struct btrfs_extent_item *item;

	path = btrfs_alloc_path();
	btrfs_init_path(path);
	key.objectid = blocknr;
	key.offset = num_blocks;
	key.flags = 0;
	btrfs_set_key_type(&key, BTRFS_EXTENT_ITEM_KEY);
	ret = btrfs_search_slot(trans, root->fs_info->extent_root, &key, path,
				0, 0);
	if (ret != 0)
		BUG();
	l = btrfs_buffer_leaf(path->nodes[0]);
	item = btrfs_item_ptr(l, path->slots[0], struct btrfs_extent_item);
	*refs = btrfs_extent_refs(item);
	btrfs_release_path(root->fs_info->extent_root, path);
	btrfs_free_path(path);
	return 0;
}

int btrfs_inc_ref(struct btrfs_trans_handle *trans, struct btrfs_root *root,
		  struct buffer_head *buf)
{
	u64 blocknr;
	struct btrfs_node *buf_node;
	struct btrfs_leaf *buf_leaf;
	struct btrfs_disk_key *key;
	struct btrfs_file_extent_item *fi;
	int i;
	int leaf;
	int ret;

	if (!root->ref_cows)
		return 0;
	buf_node = btrfs_buffer_node(buf);
	leaf = btrfs_is_leaf(buf_node);
	buf_leaf = btrfs_buffer_leaf(buf);
	for (i = 0; i < btrfs_header_nritems(&buf_node->header); i++) {
		if (leaf) {
			key = &buf_leaf->items[i].key;
			if (btrfs_disk_key_type(key) != BTRFS_EXTENT_DATA_KEY)
				continue;
			fi = btrfs_item_ptr(buf_leaf, i,
					    struct btrfs_file_extent_item);
			ret = inc_block_ref(trans, root,
				    btrfs_file_extent_disk_blocknr(fi),
				    btrfs_file_extent_disk_num_blocks(fi));
			BUG_ON(ret);
		} else {
			blocknr = btrfs_node_blockptr(buf_node, i);
			ret = inc_block_ref(trans, root, blocknr, 1);
			BUG_ON(ret);
		}
	}
	return 0;
}

int btrfs_finish_extent_commit(struct btrfs_trans_handle *trans, struct
			       btrfs_root *root)
{
	unsigned long gang[8];
	u64 first = 0;
	int ret;
	int i;
	struct radix_tree_root *pinned_radix = &root->fs_info->pinned_radix;

	while(1) {
		ret = find_first_radix_bit(pinned_radix, gang,
					   ARRAY_SIZE(gang));
		if (!ret)
			break;
		if (!first)
			first = gang[0];
		for (i = 0; i < ret; i++) {
			clear_radix_bit(pinned_radix, gang[i]);
		}
	}
	if (root->fs_info->last_insert.objectid > first)
		root->fs_info->last_insert.objectid = first;
	root->fs_info->last_insert.offset = 0;
	return 0;
}

static int finish_current_insert(struct btrfs_trans_handle *trans, struct
				 btrfs_root *extent_root)
{
	struct btrfs_key ins;
	struct btrfs_extent_item extent_item;
	int i;
	int ret;
	u64 super_blocks_used;
	struct btrfs_fs_info *info = extent_root->fs_info;

	btrfs_set_extent_refs(&extent_item, 1);
	btrfs_set_extent_owner(&extent_item,
		btrfs_header_parentid(btrfs_buffer_header(extent_root->node)));
	ins.offset = 1;
	ins.flags = 0;
	btrfs_set_key_type(&ins, BTRFS_EXTENT_ITEM_KEY);

	for (i = 0; i < extent_root->fs_info->current_insert.flags; i++) {
		ins.objectid = extent_root->fs_info->current_insert.objectid +
				i;
		super_blocks_used = btrfs_super_blocks_used(info->disk_super);
		btrfs_set_super_blocks_used(info->disk_super,
					    super_blocks_used + 1);
		ret = btrfs_insert_item(trans, extent_root, &ins, &extent_item,
					sizeof(extent_item));
		BUG_ON(ret);
	}
	extent_root->fs_info->current_insert.offset = 0;
	return 0;
}

static int pin_down_block(struct btrfs_root *root, u64 blocknr, int pending)
{
	int err;
	struct btrfs_header *header;
	struct buffer_head *bh;

	if (!pending) {
		bh = btrfs_find_tree_block(root, blocknr);
		if (bh) {
			if (buffer_uptodate(bh)) {
				u64 transid =
				    root->fs_info->running_transaction->transid;
				header = btrfs_buffer_header(bh);
				if (btrfs_header_generation(header) ==
				    transid) {
					btrfs_block_release(root, bh);
					return 0;
				}
			}
			btrfs_block_release(root, bh);
		}
		err = set_radix_bit(&root->fs_info->pinned_radix, blocknr);
	} else {
		err = set_radix_bit(&root->fs_info->pending_del_radix, blocknr);
	}
	BUG_ON(err);
	return 0;
}

/*
 * remove an extent from the root, returns 0 on success
 */
static int __free_extent(struct btrfs_trans_handle *trans, struct btrfs_root
			 *root, u64 blocknr, u64 num_blocks, int pin)
{
	struct btrfs_path *path;
	struct btrfs_key key;
	struct btrfs_fs_info *info = root->fs_info;
	struct btrfs_root *extent_root = info->extent_root;
	int ret;
	struct btrfs_extent_item *ei;
	struct btrfs_key ins;
	u32 refs;

	key.objectid = blocknr;
	key.flags = 0;
	btrfs_set_key_type(&key, BTRFS_EXTENT_ITEM_KEY);
	key.offset = num_blocks;

	find_free_extent(trans, root, 0, 0, (u64)-1, &ins);
	path = btrfs_alloc_path();
	BUG_ON(!path);
	btrfs_init_path(path);
	ret = btrfs_search_slot(trans, extent_root, &key, path, -1, 1);
	if (ret) {
		printk("failed to find %Lu\n", key.objectid);
		btrfs_print_tree(extent_root, extent_root->node);
		printk("failed to find %Lu\n", key.objectid);
		BUG();
	}
	ei = btrfs_item_ptr(btrfs_buffer_leaf(path->nodes[0]), path->slots[0],
			    struct btrfs_extent_item);
	BUG_ON(ei->refs == 0);
	refs = btrfs_extent_refs(ei) - 1;
	btrfs_set_extent_refs(ei, refs);
	btrfs_mark_buffer_dirty(path->nodes[0]);
	if (refs == 0) {
		u64 super_blocks_used;

		if (pin) {
			ret = pin_down_block(root, blocknr, 0);
			BUG_ON(ret);
		}

		super_blocks_used = btrfs_super_blocks_used(info->disk_super);
		btrfs_set_super_blocks_used(info->disk_super,
					    super_blocks_used - num_blocks);
		ret = btrfs_del_item(trans, extent_root, path);
		if (extent_root->fs_info->last_insert.objectid > blocknr)
			extent_root->fs_info->last_insert.objectid = blocknr;
		if (ret)
			BUG();
	}
	btrfs_release_path(extent_root, path);
	btrfs_free_path(path);
	finish_current_insert(trans, extent_root);
	return ret;
}

/*
 * find all the blocks marked as pending in the radix tree and remove
 * them from the extent map
 */
static int del_pending_extents(struct btrfs_trans_handle *trans, struct
			       btrfs_root *extent_root)
{
	int ret;
	int wret;
	int err = 0;
	unsigned long gang[4];
	int i;
	struct radix_tree_root *pending_radix;
	struct radix_tree_root *pinned_radix;

	pending_radix = &extent_root->fs_info->pending_del_radix;
	pinned_radix = &extent_root->fs_info->pinned_radix;

	while(1) {
		ret = find_first_radix_bit(pending_radix, gang,
					   ARRAY_SIZE(gang));
		if (!ret)
			break;
		for (i = 0; i < ret; i++) {
			wret = set_radix_bit(pinned_radix, gang[i]);
			BUG_ON(wret);
			wret = clear_radix_bit(pending_radix, gang[i]);
			BUG_ON(wret);
			wret = __free_extent(trans, extent_root,
					     gang[i], 1, 0);
			if (wret)
				err = wret;
		}
	}
	return err;
}

/*
 * remove an extent from the root, returns 0 on success
 */
int btrfs_free_extent(struct btrfs_trans_handle *trans, struct btrfs_root
		      *root, u64 blocknr, u64 num_blocks, int pin)
{
	struct btrfs_root *extent_root = root->fs_info->extent_root;
	int pending_ret;
	int ret;

	if (root == extent_root) {
		pin_down_block(root, blocknr, 1);
		return 0;
	}
	ret = __free_extent(trans, root, blocknr, num_blocks, pin);
	pending_ret = del_pending_extents(trans, root->fs_info->extent_root);
	return ret ? ret : pending_ret;
}

/*
 * walks the btree of allocated extents and find a hole of a given size.
 * The key ins is changed to record the hole:
 * ins->objectid == block start
 * ins->flags = BTRFS_EXTENT_ITEM_KEY
 * ins->offset == number of blocks
 * Any available blocks before search_start are skipped.
 */
static int find_free_extent(struct btrfs_trans_handle *trans, struct btrfs_root
			    *orig_root, u64 num_blocks, u64 search_start, u64
			    search_end, struct btrfs_key *ins)
{
	struct btrfs_path *path;
	struct btrfs_key key;
	int ret;
	u64 hole_size = 0;
	int slot = 0;
	u64 last_block = 0;
	u64 test_block;
	int start_found;
	struct btrfs_leaf *l;
	struct btrfs_root * root = orig_root->fs_info->extent_root;
	int total_needed = num_blocks;
	int level;

	level = btrfs_header_level(btrfs_buffer_header(root->node));
	total_needed += (level + 1) * 3;
	if (root->fs_info->last_insert.objectid > search_start)
		search_start = root->fs_info->last_insert.objectid;

	ins->flags = 0;
	btrfs_set_key_type(ins, BTRFS_EXTENT_ITEM_KEY);
	path = btrfs_alloc_path();

check_failed:
	btrfs_init_path(path);
	ins->objectid = search_start;
	ins->offset = 0;
	start_found = 0;
	ret = btrfs_search_slot(trans, root, ins, path, 0, 0);
	if (ret < 0)
		goto error;

	if (path->slots[0] > 0)
		path->slots[0]--;

	while (1) {
		l = btrfs_buffer_leaf(path->nodes[0]);
		slot = path->slots[0];
		if (slot >= btrfs_header_nritems(&l->header)) {
			ret = btrfs_next_leaf(root, path);
			if (ret == 0)
				continue;
			if (ret < 0)
				goto error;
			if (!start_found) {
				ins->objectid = search_start;
				ins->offset = (u64)-1;
				start_found = 1;
				goto check_pending;
			}
			ins->objectid = last_block > search_start ?
					last_block : search_start;
			ins->offset = (u64)-1;
			goto check_pending;
		}
		btrfs_disk_key_to_cpu(&key, &l->items[slot].key);
		if (key.objectid >= search_start) {
			if (start_found) {
				if (last_block < search_start)
					last_block = search_start;
				hole_size = key.objectid - last_block;
				if (hole_size > total_needed) {
					ins->objectid = last_block;
					ins->offset = hole_size;
					goto check_pending;
				}
			}
		}
		start_found = 1;
		last_block = key.objectid + key.offset;
		path->slots[0]++;
	}
	// FIXME -ENOSPC
check_pending:
	/* we have to make sure we didn't find an extent that has already
	 * been allocated by the map tree or the original allocation
	 */
	btrfs_release_path(root, path);
	BUG_ON(ins->objectid < search_start);
	for (test_block = ins->objectid;
	     test_block < ins->objectid + total_needed; test_block++) {
		if (test_radix_bit(&root->fs_info->pinned_radix,
				      test_block)) {
			search_start = test_block + 1;
			goto check_failed;
		}
	}
	BUG_ON(root->fs_info->current_insert.offset);
	root->fs_info->current_insert.offset = total_needed - num_blocks;
	root->fs_info->current_insert.objectid = ins->objectid + num_blocks;
	root->fs_info->current_insert.flags = 0;
	root->fs_info->last_insert.objectid = ins->objectid;
	ins->offset = num_blocks;
	btrfs_free_path(path);
	return 0;
error:
	btrfs_release_path(root, path);
	btrfs_free_path(path);
	return ret;
}

/*
 * finds a free extent and does all the dirty work required for allocation
 * returns the key for the extent through ins, and a tree buffer for
 * the first block of the extent through buf.
 *
 * returns 0 if everything worked, non-zero otherwise.
 */
int btrfs_alloc_extent(struct btrfs_trans_handle *trans, struct btrfs_root
			*root, u64 num_blocks, u64 search_start, u64
			search_end, u64 owner, struct btrfs_key *ins)
{
	int ret;
	int pending_ret;
	u64 super_blocks_used;
	struct btrfs_fs_info *info = root->fs_info;
	struct btrfs_root *extent_root = info->extent_root;
	struct btrfs_extent_item extent_item;

	btrfs_set_extent_refs(&extent_item, 1);
	btrfs_set_extent_owner(&extent_item, owner);

	if (root == extent_root) {
		BUG_ON(extent_root->fs_info->current_insert.offset == 0);
		BUG_ON(num_blocks != 1);
		BUG_ON(extent_root->fs_info->current_insert.flags ==
		       extent_root->fs_info->current_insert.offset);
		ins->offset = 1;
		ins->objectid = extent_root->fs_info->current_insert.objectid +
				extent_root->fs_info->current_insert.flags++;
		return 0;
	}
	ret = find_free_extent(trans, root, num_blocks, search_start,
			       search_end, ins);
	if (ret)
		return ret;

	super_blocks_used = btrfs_super_blocks_used(info->disk_super);
	btrfs_set_super_blocks_used(info->disk_super, super_blocks_used +
				    num_blocks);
	ret = btrfs_insert_item(trans, extent_root, ins, &extent_item,
				sizeof(extent_item));

	finish_current_insert(trans, extent_root);
	pending_ret = del_pending_extents(trans, extent_root);
	if (ret)
		return ret;
	if (pending_ret)
		return pending_ret;
	return 0;
}

/*
 * helper function to allocate a block for a given tree
 * returns the tree buffer or NULL.
 */
struct buffer_head *btrfs_alloc_free_block(struct btrfs_trans_handle *trans,
					    struct btrfs_root *root)
{
	struct btrfs_key ins;
	int ret;
	struct buffer_head *buf;

	ret = btrfs_alloc_extent(trans, root, 1, 0, (unsigned long)-1,
		btrfs_header_parentid(btrfs_buffer_header(root->node)), &ins);
	if (ret) {
		BUG();
		return NULL;
	}
	buf = btrfs_find_create_tree_block(root, ins.objectid);
	set_buffer_uptodate(buf);
	return buf;
}

static int drop_leaf_ref(struct btrfs_trans_handle *trans,
			 struct btrfs_root *root, struct buffer_head *cur)
{
	struct btrfs_disk_key *key;
	struct btrfs_leaf *leaf;
	struct btrfs_file_extent_item *fi;
	int i;
	int nritems;
	int ret;

	BUG_ON(!btrfs_is_leaf(btrfs_buffer_node(cur)));
	leaf = btrfs_buffer_leaf(cur);
	nritems = btrfs_header_nritems(&leaf->header);
	for (i = 0; i < nritems; i++) {
		key = &leaf->items[i].key;
		if (btrfs_disk_key_type(key) != BTRFS_EXTENT_DATA_KEY)
			continue;
		fi = btrfs_item_ptr(leaf, i, struct btrfs_file_extent_item);
		/*
		 * FIXME make sure to insert a trans record that
		 * repeats the snapshot del on crash
		 */
		ret = btrfs_free_extent(trans, root,
					btrfs_file_extent_disk_blocknr(fi),
					btrfs_file_extent_disk_num_blocks(fi),
					0);
		BUG_ON(ret);
	}
	return 0;
}

/*
 * helper function for drop_snapshot, this walks down the tree dropping ref
 * counts as it goes.
 */
static int walk_down_tree(struct btrfs_trans_handle *trans, struct btrfs_root
			  *root, struct btrfs_path *path, int *level)
{
	struct buffer_head *next;
	struct buffer_head *cur;
	u64 blocknr;
	int ret;
	u32 refs;

	WARN_ON(*level < 0);
	WARN_ON(*level >= BTRFS_MAX_LEVEL);
	ret = lookup_block_ref(trans, root, path->nodes[*level]->b_blocknr,
			       1, &refs);
	BUG_ON(ret);
	if (refs > 1)
		goto out;
	/*
	 * walk down to the last node level and free all the leaves
	 */
	while(*level >= 0) {
		WARN_ON(*level < 0);
		WARN_ON(*level >= BTRFS_MAX_LEVEL);
		cur = path->nodes[*level];
		if (btrfs_header_level(btrfs_buffer_header(cur)) != *level)
			WARN_ON(1);
		if (path->slots[*level] >=
		    btrfs_header_nritems(btrfs_buffer_header(cur)))
			break;
		if (*level == 0) {
			ret = drop_leaf_ref(trans, root, cur);
			BUG_ON(ret);
			break;
		}
		blocknr = btrfs_node_blockptr(btrfs_buffer_node(cur),
					      path->slots[*level]);
		ret = lookup_block_ref(trans, root, blocknr, 1, &refs);
		BUG_ON(ret);
		if (refs != 1) {
			path->slots[*level]++;
			ret = btrfs_free_extent(trans, root, blocknr, 1, 1);
			BUG_ON(ret);
			continue;
		}
		next = read_tree_block(root, blocknr);
		WARN_ON(*level <= 0);
		if (path->nodes[*level-1])
			btrfs_block_release(root, path->nodes[*level-1]);
		path->nodes[*level-1] = next;
		*level = btrfs_header_level(btrfs_buffer_header(next));
		path->slots[*level] = 0;
	}
out:
	WARN_ON(*level < 0);
	WARN_ON(*level >= BTRFS_MAX_LEVEL);
	ret = btrfs_free_extent(trans, root,
				path->nodes[*level]->b_blocknr, 1, 1);
	btrfs_block_release(root, path->nodes[*level]);
	path->nodes[*level] = NULL;
	*level += 1;
	BUG_ON(ret);
	return 0;
}

/*
 * helper for dropping snapshots.  This walks back up the tree in the path
 * to find the first node higher up where we haven't yet gone through
 * all the slots
 */
static int walk_up_tree(struct btrfs_trans_handle *trans, struct btrfs_root
			*root, struct btrfs_path *path, int *level)
{
	int i;
	int slot;
	int ret;
	for(i = *level; i < BTRFS_MAX_LEVEL - 1 && path->nodes[i]; i++) {
		slot = path->slots[i];
		if (slot < btrfs_header_nritems(
		    btrfs_buffer_header(path->nodes[i])) - 1) {
			path->slots[i]++;
			*level = i;
			return 0;
		} else {
			ret = btrfs_free_extent(trans, root,
						path->nodes[*level]->b_blocknr,
						1, 1);
			BUG_ON(ret);
			btrfs_block_release(root, path->nodes[*level]);
			path->nodes[*level] = NULL;
			*level = i + 1;
		}
	}
	return 1;
}

/*
 * drop the reference count on the tree rooted at 'snap'.  This traverses
 * the tree freeing any blocks that have a ref count of zero after being
 * decremented.
 */
int btrfs_drop_snapshot(struct btrfs_trans_handle *trans, struct btrfs_root
			*root, struct buffer_head *snap)
{
	int ret = 0;
	int wret;
	int level;
	struct btrfs_path *path;
	int i;
	int orig_level;

	path = btrfs_alloc_path();
	BUG_ON(!path);
	btrfs_init_path(path);

	level = btrfs_header_level(btrfs_buffer_header(snap));
	orig_level = level;
	path->nodes[level] = snap;
	path->slots[level] = 0;
	while(1) {
		wret = walk_down_tree(trans, root, path, &level);
		if (wret > 0)
			break;
		if (wret < 0)
			ret = wret;

		wret = walk_up_tree(trans, root, path, &level);
		if (wret > 0)
			break;
		if (wret < 0)
			ret = wret;
	}
	for (i = 0; i <= orig_level; i++) {
		if (path->nodes[i]) {
			btrfs_block_release(root, path->nodes[i]);
		}
	}
	btrfs_free_path(path);
	return ret;
}