Btrfs: calc file extent num_bytes correctly in file clone
[linux-2.6.git] / fs / btrfs / delayed-ref.c
1 /*
2  * Copyright (C) 2009 Oracle.  All rights reserved.
3  *
4  * This program is free software; you can redistribute it and/or
5  * modify it under the terms of the GNU General Public
6  * License v2 as published by the Free Software Foundation.
7  *
8  * This program is distributed in the hope that it will be useful,
9  * but WITHOUT ANY WARRANTY; without even the implied warranty of
10  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
11  * General Public License for more details.
12  *
13  * You should have received a copy of the GNU General Public
14  * License along with this program; if not, write to the
15  * Free Software Foundation, Inc., 59 Temple Place - Suite 330,
16  * Boston, MA 021110-1307, USA.
17  */
18
19 #include <linux/sched.h>
20 #include <linux/slab.h>
21 #include <linux/sort.h>
22 #include "ctree.h"
23 #include "delayed-ref.h"
24 #include "transaction.h"
25
26 /*
27  * delayed back reference update tracking.  For subvolume trees
28  * we queue up extent allocations and backref maintenance for
29  * delayed processing.   This avoids deep call chains where we
30  * add extents in the middle of btrfs_search_slot, and it allows
31  * us to buffer up frequently modified backrefs in an rb tree instead
32  * of hammering updates on the extent allocation tree.
33  */
34
35 /*
36  * compare two delayed tree backrefs with same bytenr and type
37  */
38 static int comp_tree_refs(struct btrfs_delayed_tree_ref *ref2,
39                           struct btrfs_delayed_tree_ref *ref1)
40 {
41         if (ref1->node.type == BTRFS_TREE_BLOCK_REF_KEY) {
42                 if (ref1->root < ref2->root)
43                         return -1;
44                 if (ref1->root > ref2->root)
45                         return 1;
46         } else {
47                 if (ref1->parent < ref2->parent)
48                         return -1;
49                 if (ref1->parent > ref2->parent)
50                         return 1;
51         }
52         return 0;
53 }
54
55 /*
56  * compare two delayed data backrefs with same bytenr and type
57  */
58 static int comp_data_refs(struct btrfs_delayed_data_ref *ref2,
59                           struct btrfs_delayed_data_ref *ref1)
60 {
61         if (ref1->node.type == BTRFS_EXTENT_DATA_REF_KEY) {
62                 if (ref1->root < ref2->root)
63                         return -1;
64                 if (ref1->root > ref2->root)
65                         return 1;
66                 if (ref1->objectid < ref2->objectid)
67                         return -1;
68                 if (ref1->objectid > ref2->objectid)
69                         return 1;
70                 if (ref1->offset < ref2->offset)
71                         return -1;
72                 if (ref1->offset > ref2->offset)
73                         return 1;
74         } else {
75                 if (ref1->parent < ref2->parent)
76                         return -1;
77                 if (ref1->parent > ref2->parent)
78                         return 1;
79         }
80         return 0;
81 }
82
83 /*
84  * entries in the rb tree are ordered by the byte number of the extent,
85  * type of the delayed backrefs and content of delayed backrefs.
86  */
87 static int comp_entry(struct btrfs_delayed_ref_node *ref2,
88                       struct btrfs_delayed_ref_node *ref1)
89 {
90         if (ref1->bytenr < ref2->bytenr)
91                 return -1;
92         if (ref1->bytenr > ref2->bytenr)
93                 return 1;
94         if (ref1->is_head && ref2->is_head)
95                 return 0;
96         if (ref2->is_head)
97                 return -1;
98         if (ref1->is_head)
99                 return 1;
100         if (ref1->type < ref2->type)
101                 return -1;
102         if (ref1->type > ref2->type)
103                 return 1;
104         if (ref1->type == BTRFS_TREE_BLOCK_REF_KEY ||
105             ref1->type == BTRFS_SHARED_BLOCK_REF_KEY) {
106                 return comp_tree_refs(btrfs_delayed_node_to_tree_ref(ref2),
107                                       btrfs_delayed_node_to_tree_ref(ref1));
108         } else if (ref1->type == BTRFS_EXTENT_DATA_REF_KEY ||
109                    ref1->type == BTRFS_SHARED_DATA_REF_KEY) {
110                 return comp_data_refs(btrfs_delayed_node_to_data_ref(ref2),
111                                       btrfs_delayed_node_to_data_ref(ref1));
112         }
113         BUG();
114         return 0;
115 }
116
117 /*
118  * insert a new ref into the rbtree.  This returns any existing refs
119  * for the same (bytenr,parent) tuple, or NULL if the new node was properly
120  * inserted.
121  */
122 static struct btrfs_delayed_ref_node *tree_insert(struct rb_root *root,
123                                                   struct rb_node *node)
124 {
125         struct rb_node **p = &root->rb_node;
126         struct rb_node *parent_node = NULL;
127         struct btrfs_delayed_ref_node *entry;
128         struct btrfs_delayed_ref_node *ins;
129         int cmp;
130
131         ins = rb_entry(node, struct btrfs_delayed_ref_node, rb_node);
132         while (*p) {
133                 parent_node = *p;
134                 entry = rb_entry(parent_node, struct btrfs_delayed_ref_node,
135                                  rb_node);
136
137                 cmp = comp_entry(entry, ins);
138                 if (cmp < 0)
139                         p = &(*p)->rb_left;
140                 else if (cmp > 0)
141                         p = &(*p)->rb_right;
142                 else
143                         return entry;
144         }
145
146         rb_link_node(node, parent_node, p);
147         rb_insert_color(node, root);
148         return NULL;
149 }
150
151 /*
152  * find an head entry based on bytenr. This returns the delayed ref
153  * head if it was able to find one, or NULL if nothing was in that spot
154  */
155 static struct btrfs_delayed_ref_node *find_ref_head(struct rb_root *root,
156                                   u64 bytenr,
157                                   struct btrfs_delayed_ref_node **last)
158 {
159         struct rb_node *n = root->rb_node;
160         struct btrfs_delayed_ref_node *entry;
161         int cmp;
162
163         while (n) {
164                 entry = rb_entry(n, struct btrfs_delayed_ref_node, rb_node);
165                 WARN_ON(!entry->in_tree);
166                 if (last)
167                         *last = entry;
168
169                 if (bytenr < entry->bytenr)
170                         cmp = -1;
171                 else if (bytenr > entry->bytenr)
172                         cmp = 1;
173                 else if (!btrfs_delayed_ref_is_head(entry))
174                         cmp = 1;
175                 else
176                         cmp = 0;
177
178                 if (cmp < 0)
179                         n = n->rb_left;
180                 else if (cmp > 0)
181                         n = n->rb_right;
182                 else
183                         return entry;
184         }
185         return NULL;
186 }
187
188 int btrfs_delayed_ref_lock(struct btrfs_trans_handle *trans,
189                            struct btrfs_delayed_ref_head *head)
190 {
191         struct btrfs_delayed_ref_root *delayed_refs;
192
193         delayed_refs = &trans->transaction->delayed_refs;
194         assert_spin_locked(&delayed_refs->lock);
195         if (mutex_trylock(&head->mutex))
196                 return 0;
197
198         atomic_inc(&head->node.refs);
199         spin_unlock(&delayed_refs->lock);
200
201         mutex_lock(&head->mutex);
202         spin_lock(&delayed_refs->lock);
203         if (!head->node.in_tree) {
204                 mutex_unlock(&head->mutex);
205                 btrfs_put_delayed_ref(&head->node);
206                 return -EAGAIN;
207         }
208         btrfs_put_delayed_ref(&head->node);
209         return 0;
210 }
211
212 int btrfs_find_ref_cluster(struct btrfs_trans_handle *trans,
213                            struct list_head *cluster, u64 start)
214 {
215         int count = 0;
216         struct btrfs_delayed_ref_root *delayed_refs;
217         struct rb_node *node;
218         struct btrfs_delayed_ref_node *ref;
219         struct btrfs_delayed_ref_head *head;
220
221         delayed_refs = &trans->transaction->delayed_refs;
222         if (start == 0) {
223                 node = rb_first(&delayed_refs->root);
224         } else {
225                 ref = NULL;
226                 find_ref_head(&delayed_refs->root, start, &ref);
227                 if (ref) {
228                         struct btrfs_delayed_ref_node *tmp;
229
230                         node = rb_prev(&ref->rb_node);
231                         while (node) {
232                                 tmp = rb_entry(node,
233                                                struct btrfs_delayed_ref_node,
234                                                rb_node);
235                                 if (tmp->bytenr < start)
236                                         break;
237                                 ref = tmp;
238                                 node = rb_prev(&ref->rb_node);
239                         }
240                         node = &ref->rb_node;
241                 } else
242                         node = rb_first(&delayed_refs->root);
243         }
244 again:
245         while (node && count < 32) {
246                 ref = rb_entry(node, struct btrfs_delayed_ref_node, rb_node);
247                 if (btrfs_delayed_ref_is_head(ref)) {
248                         head = btrfs_delayed_node_to_head(ref);
249                         if (list_empty(&head->cluster)) {
250                                 list_add_tail(&head->cluster, cluster);
251                                 delayed_refs->run_delayed_start =
252                                         head->node.bytenr;
253                                 count++;
254
255                                 WARN_ON(delayed_refs->num_heads_ready == 0);
256                                 delayed_refs->num_heads_ready--;
257                         } else if (count) {
258                                 /* the goal of the clustering is to find extents
259                                  * that are likely to end up in the same extent
260                                  * leaf on disk.  So, we don't want them spread
261                                  * all over the tree.  Stop now if we've hit
262                                  * a head that was already in use
263                                  */
264                                 break;
265                         }
266                 }
267                 node = rb_next(node);
268         }
269         if (count) {
270                 return 0;
271         } else if (start) {
272                 /*
273                  * we've gone to the end of the rbtree without finding any
274                  * clusters.  start from the beginning and try again
275                  */
276                 start = 0;
277                 node = rb_first(&delayed_refs->root);
278                 goto again;
279         }
280         return 1;
281 }
282
283 /*
284  * helper function to update an extent delayed ref in the
285  * rbtree.  existing and update must both have the same
286  * bytenr and parent
287  *
288  * This may free existing if the update cancels out whatever
289  * operation it was doing.
290  */
291 static noinline void
292 update_existing_ref(struct btrfs_trans_handle *trans,
293                     struct btrfs_delayed_ref_root *delayed_refs,
294                     struct btrfs_delayed_ref_node *existing,
295                     struct btrfs_delayed_ref_node *update)
296 {
297         if (update->action != existing->action) {
298                 /*
299                  * this is effectively undoing either an add or a
300                  * drop.  We decrement the ref_mod, and if it goes
301                  * down to zero we just delete the entry without
302                  * every changing the extent allocation tree.
303                  */
304                 existing->ref_mod--;
305                 if (existing->ref_mod == 0) {
306                         rb_erase(&existing->rb_node,
307                                  &delayed_refs->root);
308                         existing->in_tree = 0;
309                         btrfs_put_delayed_ref(existing);
310                         delayed_refs->num_entries--;
311                         if (trans->delayed_ref_updates)
312                                 trans->delayed_ref_updates--;
313                 } else {
314                         WARN_ON(existing->type == BTRFS_TREE_BLOCK_REF_KEY ||
315                                 existing->type == BTRFS_SHARED_BLOCK_REF_KEY);
316                 }
317         } else {
318                 WARN_ON(existing->type == BTRFS_TREE_BLOCK_REF_KEY ||
319                         existing->type == BTRFS_SHARED_BLOCK_REF_KEY);
320                 /*
321                  * the action on the existing ref matches
322                  * the action on the ref we're trying to add.
323                  * Bump the ref_mod by one so the backref that
324                  * is eventually added/removed has the correct
325                  * reference count
326                  */
327                 existing->ref_mod += update->ref_mod;
328         }
329 }
330
331 /*
332  * helper function to update the accounting in the head ref
333  * existing and update must have the same bytenr
334  */
335 static noinline void
336 update_existing_head_ref(struct btrfs_delayed_ref_node *existing,
337                          struct btrfs_delayed_ref_node *update)
338 {
339         struct btrfs_delayed_ref_head *existing_ref;
340         struct btrfs_delayed_ref_head *ref;
341
342         existing_ref = btrfs_delayed_node_to_head(existing);
343         ref = btrfs_delayed_node_to_head(update);
344         BUG_ON(existing_ref->is_data != ref->is_data);
345
346         if (ref->must_insert_reserved) {
347                 /* if the extent was freed and then
348                  * reallocated before the delayed ref
349                  * entries were processed, we can end up
350                  * with an existing head ref without
351                  * the must_insert_reserved flag set.
352                  * Set it again here
353                  */
354                 existing_ref->must_insert_reserved = ref->must_insert_reserved;
355
356                 /*
357                  * update the num_bytes so we make sure the accounting
358                  * is done correctly
359                  */
360                 existing->num_bytes = update->num_bytes;
361
362         }
363
364         if (ref->extent_op) {
365                 if (!existing_ref->extent_op) {
366                         existing_ref->extent_op = ref->extent_op;
367                 } else {
368                         if (ref->extent_op->update_key) {
369                                 memcpy(&existing_ref->extent_op->key,
370                                        &ref->extent_op->key,
371                                        sizeof(ref->extent_op->key));
372                                 existing_ref->extent_op->update_key = 1;
373                         }
374                         if (ref->extent_op->update_flags) {
375                                 existing_ref->extent_op->flags_to_set |=
376                                         ref->extent_op->flags_to_set;
377                                 existing_ref->extent_op->update_flags = 1;
378                         }
379                         kfree(ref->extent_op);
380                 }
381         }
382         /*
383          * update the reference mod on the head to reflect this new operation
384          */
385         existing->ref_mod += update->ref_mod;
386 }
387
388 /*
389  * helper function to actually insert a head node into the rbtree.
390  * this does all the dirty work in terms of maintaining the correct
391  * overall modification count.
392  */
393 static noinline int add_delayed_ref_head(struct btrfs_trans_handle *trans,
394                                         struct btrfs_delayed_ref_node *ref,
395                                         u64 bytenr, u64 num_bytes,
396                                         int action, int is_data)
397 {
398         struct btrfs_delayed_ref_node *existing;
399         struct btrfs_delayed_ref_head *head_ref = NULL;
400         struct btrfs_delayed_ref_root *delayed_refs;
401         int count_mod = 1;
402         int must_insert_reserved = 0;
403
404         /*
405          * the head node stores the sum of all the mods, so dropping a ref
406          * should drop the sum in the head node by one.
407          */
408         if (action == BTRFS_UPDATE_DELAYED_HEAD)
409                 count_mod = 0;
410         else if (action == BTRFS_DROP_DELAYED_REF)
411                 count_mod = -1;
412
413         /*
414          * BTRFS_ADD_DELAYED_EXTENT means that we need to update
415          * the reserved accounting when the extent is finally added, or
416          * if a later modification deletes the delayed ref without ever
417          * inserting the extent into the extent allocation tree.
418          * ref->must_insert_reserved is the flag used to record
419          * that accounting mods are required.
420          *
421          * Once we record must_insert_reserved, switch the action to
422          * BTRFS_ADD_DELAYED_REF because other special casing is not required.
423          */
424         if (action == BTRFS_ADD_DELAYED_EXTENT)
425                 must_insert_reserved = 1;
426         else
427                 must_insert_reserved = 0;
428
429         delayed_refs = &trans->transaction->delayed_refs;
430
431         /* first set the basic ref node struct up */
432         atomic_set(&ref->refs, 1);
433         ref->bytenr = bytenr;
434         ref->num_bytes = num_bytes;
435         ref->ref_mod = count_mod;
436         ref->type  = 0;
437         ref->action  = 0;
438         ref->is_head = 1;
439         ref->in_tree = 1;
440
441         head_ref = btrfs_delayed_node_to_head(ref);
442         head_ref->must_insert_reserved = must_insert_reserved;
443         head_ref->is_data = is_data;
444
445         INIT_LIST_HEAD(&head_ref->cluster);
446         mutex_init(&head_ref->mutex);
447
448         trace_btrfs_delayed_ref_head(ref, head_ref, action);
449
450         existing = tree_insert(&delayed_refs->root, &ref->rb_node);
451
452         if (existing) {
453                 update_existing_head_ref(existing, ref);
454                 /*
455                  * we've updated the existing ref, free the newly
456                  * allocated ref
457                  */
458                 kfree(ref);
459         } else {
460                 delayed_refs->num_heads++;
461                 delayed_refs->num_heads_ready++;
462                 delayed_refs->num_entries++;
463                 trans->delayed_ref_updates++;
464         }
465         return 0;
466 }
467
468 /*
469  * helper to insert a delayed tree ref into the rbtree.
470  */
471 static noinline int add_delayed_tree_ref(struct btrfs_trans_handle *trans,
472                                          struct btrfs_delayed_ref_node *ref,
473                                          u64 bytenr, u64 num_bytes, u64 parent,
474                                          u64 ref_root, int level, int action)
475 {
476         struct btrfs_delayed_ref_node *existing;
477         struct btrfs_delayed_tree_ref *full_ref;
478         struct btrfs_delayed_ref_root *delayed_refs;
479
480         if (action == BTRFS_ADD_DELAYED_EXTENT)
481                 action = BTRFS_ADD_DELAYED_REF;
482
483         delayed_refs = &trans->transaction->delayed_refs;
484
485         /* first set the basic ref node struct up */
486         atomic_set(&ref->refs, 1);
487         ref->bytenr = bytenr;
488         ref->num_bytes = num_bytes;
489         ref->ref_mod = 1;
490         ref->action = action;
491         ref->is_head = 0;
492         ref->in_tree = 1;
493
494         full_ref = btrfs_delayed_node_to_tree_ref(ref);
495         if (parent) {
496                 full_ref->parent = parent;
497                 ref->type = BTRFS_SHARED_BLOCK_REF_KEY;
498         } else {
499                 full_ref->root = ref_root;
500                 ref->type = BTRFS_TREE_BLOCK_REF_KEY;
501         }
502         full_ref->level = level;
503
504         trace_btrfs_delayed_tree_ref(ref, full_ref, action);
505
506         existing = tree_insert(&delayed_refs->root, &ref->rb_node);
507
508         if (existing) {
509                 update_existing_ref(trans, delayed_refs, existing, ref);
510                 /*
511                  * we've updated the existing ref, free the newly
512                  * allocated ref
513                  */
514                 kfree(ref);
515         } else {
516                 delayed_refs->num_entries++;
517                 trans->delayed_ref_updates++;
518         }
519         return 0;
520 }
521
522 /*
523  * helper to insert a delayed data ref into the rbtree.
524  */
525 static noinline int add_delayed_data_ref(struct btrfs_trans_handle *trans,
526                                          struct btrfs_delayed_ref_node *ref,
527                                          u64 bytenr, u64 num_bytes, u64 parent,
528                                          u64 ref_root, u64 owner, u64 offset,
529                                          int action)
530 {
531         struct btrfs_delayed_ref_node *existing;
532         struct btrfs_delayed_data_ref *full_ref;
533         struct btrfs_delayed_ref_root *delayed_refs;
534
535         if (action == BTRFS_ADD_DELAYED_EXTENT)
536                 action = BTRFS_ADD_DELAYED_REF;
537
538         delayed_refs = &trans->transaction->delayed_refs;
539
540         /* first set the basic ref node struct up */
541         atomic_set(&ref->refs, 1);
542         ref->bytenr = bytenr;
543         ref->num_bytes = num_bytes;
544         ref->ref_mod = 1;
545         ref->action = action;
546         ref->is_head = 0;
547         ref->in_tree = 1;
548
549         full_ref = btrfs_delayed_node_to_data_ref(ref);
550         if (parent) {
551                 full_ref->parent = parent;
552                 ref->type = BTRFS_SHARED_DATA_REF_KEY;
553         } else {
554                 full_ref->root = ref_root;
555                 ref->type = BTRFS_EXTENT_DATA_REF_KEY;
556         }
557         full_ref->objectid = owner;
558         full_ref->offset = offset;
559
560         trace_btrfs_delayed_data_ref(ref, full_ref, action);
561
562         existing = tree_insert(&delayed_refs->root, &ref->rb_node);
563
564         if (existing) {
565                 update_existing_ref(trans, delayed_refs, existing, ref);
566                 /*
567                  * we've updated the existing ref, free the newly
568                  * allocated ref
569                  */
570                 kfree(ref);
571         } else {
572                 delayed_refs->num_entries++;
573                 trans->delayed_ref_updates++;
574         }
575         return 0;
576 }
577
578 /*
579  * add a delayed tree ref.  This does all of the accounting required
580  * to make sure the delayed ref is eventually processed before this
581  * transaction commits.
582  */
583 int btrfs_add_delayed_tree_ref(struct btrfs_trans_handle *trans,
584                                u64 bytenr, u64 num_bytes, u64 parent,
585                                u64 ref_root,  int level, int action,
586                                struct btrfs_delayed_extent_op *extent_op)
587 {
588         struct btrfs_delayed_tree_ref *ref;
589         struct btrfs_delayed_ref_head *head_ref;
590         struct btrfs_delayed_ref_root *delayed_refs;
591         int ret;
592
593         BUG_ON(extent_op && extent_op->is_data);
594         ref = kmalloc(sizeof(*ref), GFP_NOFS);
595         if (!ref)
596                 return -ENOMEM;
597
598         head_ref = kmalloc(sizeof(*head_ref), GFP_NOFS);
599         if (!head_ref) {
600                 kfree(ref);
601                 return -ENOMEM;
602         }
603
604         head_ref->extent_op = extent_op;
605
606         delayed_refs = &trans->transaction->delayed_refs;
607         spin_lock(&delayed_refs->lock);
608
609         /*
610          * insert both the head node and the new ref without dropping
611          * the spin lock
612          */
613         ret = add_delayed_ref_head(trans, &head_ref->node, bytenr, num_bytes,
614                                    action, 0);
615         BUG_ON(ret);
616
617         ret = add_delayed_tree_ref(trans, &ref->node, bytenr, num_bytes,
618                                    parent, ref_root, level, action);
619         BUG_ON(ret);
620         spin_unlock(&delayed_refs->lock);
621         return 0;
622 }
623
624 /*
625  * add a delayed data ref. it's similar to btrfs_add_delayed_tree_ref.
626  */
627 int btrfs_add_delayed_data_ref(struct btrfs_trans_handle *trans,
628                                u64 bytenr, u64 num_bytes,
629                                u64 parent, u64 ref_root,
630                                u64 owner, u64 offset, int action,
631                                struct btrfs_delayed_extent_op *extent_op)
632 {
633         struct btrfs_delayed_data_ref *ref;
634         struct btrfs_delayed_ref_head *head_ref;
635         struct btrfs_delayed_ref_root *delayed_refs;
636         int ret;
637
638         BUG_ON(extent_op && !extent_op->is_data);
639         ref = kmalloc(sizeof(*ref), GFP_NOFS);
640         if (!ref)
641                 return -ENOMEM;
642
643         head_ref = kmalloc(sizeof(*head_ref), GFP_NOFS);
644         if (!head_ref) {
645                 kfree(ref);
646                 return -ENOMEM;
647         }
648
649         head_ref->extent_op = extent_op;
650
651         delayed_refs = &trans->transaction->delayed_refs;
652         spin_lock(&delayed_refs->lock);
653
654         /*
655          * insert both the head node and the new ref without dropping
656          * the spin lock
657          */
658         ret = add_delayed_ref_head(trans, &head_ref->node, bytenr, num_bytes,
659                                    action, 1);
660         BUG_ON(ret);
661
662         ret = add_delayed_data_ref(trans, &ref->node, bytenr, num_bytes,
663                                    parent, ref_root, owner, offset, action);
664         BUG_ON(ret);
665         spin_unlock(&delayed_refs->lock);
666         return 0;
667 }
668
669 int btrfs_add_delayed_extent_op(struct btrfs_trans_handle *trans,
670                                 u64 bytenr, u64 num_bytes,
671                                 struct btrfs_delayed_extent_op *extent_op)
672 {
673         struct btrfs_delayed_ref_head *head_ref;
674         struct btrfs_delayed_ref_root *delayed_refs;
675         int ret;
676
677         head_ref = kmalloc(sizeof(*head_ref), GFP_NOFS);
678         if (!head_ref)
679                 return -ENOMEM;
680
681         head_ref->extent_op = extent_op;
682
683         delayed_refs = &trans->transaction->delayed_refs;
684         spin_lock(&delayed_refs->lock);
685
686         ret = add_delayed_ref_head(trans, &head_ref->node, bytenr,
687                                    num_bytes, BTRFS_UPDATE_DELAYED_HEAD,
688                                    extent_op->is_data);
689         BUG_ON(ret);
690
691         spin_unlock(&delayed_refs->lock);
692         return 0;
693 }
694
695 /*
696  * this does a simple search for the head node for a given extent.
697  * It must be called with the delayed ref spinlock held, and it returns
698  * the head node if any where found, or NULL if not.
699  */
700 struct btrfs_delayed_ref_head *
701 btrfs_find_delayed_ref_head(struct btrfs_trans_handle *trans, u64 bytenr)
702 {
703         struct btrfs_delayed_ref_node *ref;
704         struct btrfs_delayed_ref_root *delayed_refs;
705
706         delayed_refs = &trans->transaction->delayed_refs;
707         ref = find_ref_head(&delayed_refs->root, bytenr, NULL);
708         if (ref)
709                 return btrfs_delayed_node_to_head(ref);
710         return NULL;
711 }