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TOMOYO Linux Cross Reference
Linux/fs/btrfs/reada.c

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  1 /*
  2  * Copyright (C) 2011 STRATO.  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/pagemap.h>
 21 #include <linux/writeback.h>
 22 #include <linux/blkdev.h>
 23 #include <linux/rbtree.h>
 24 #include <linux/slab.h>
 25 #include <linux/workqueue.h>
 26 #include "ctree.h"
 27 #include "volumes.h"
 28 #include "disk-io.h"
 29 #include "transaction.h"
 30 #include "dev-replace.h"
 31 
 32 #undef DEBUG
 33 
 34 /*
 35  * This is the implementation for the generic read ahead framework.
 36  *
 37  * To trigger a readahead, btrfs_reada_add must be called. It will start
 38  * a read ahead for the given range [start, end) on tree root. The returned
 39  * handle can either be used to wait on the readahead to finish
 40  * (btrfs_reada_wait), or to send it to the background (btrfs_reada_detach).
 41  *
 42  * The read ahead works as follows:
 43  * On btrfs_reada_add, the root of the tree is inserted into a radix_tree.
 44  * reada_start_machine will then search for extents to prefetch and trigger
 45  * some reads. When a read finishes for a node, all contained node/leaf
 46  * pointers that lie in the given range will also be enqueued. The reads will
 47  * be triggered in sequential order, thus giving a big win over a naive
 48  * enumeration. It will also make use of multi-device layouts. Each disk
 49  * will have its on read pointer and all disks will by utilized in parallel.
 50  * Also will no two disks read both sides of a mirror simultaneously, as this
 51  * would waste seeking capacity. Instead both disks will read different parts
 52  * of the filesystem.
 53  * Any number of readaheads can be started in parallel. The read order will be
 54  * determined globally, i.e. 2 parallel readaheads will normally finish faster
 55  * than the 2 started one after another.
 56  */
 57 
 58 #define MAX_IN_FLIGHT 6
 59 
 60 struct reada_extctl {
 61         struct list_head        list;
 62         struct reada_control    *rc;
 63         u64                     generation;
 64 };
 65 
 66 struct reada_extent {
 67         u64                     logical;
 68         struct btrfs_key        top;
 69         int                     err;
 70         struct list_head        extctl;
 71         int                     refcnt;
 72         spinlock_t              lock;
 73         struct reada_zone       *zones[BTRFS_MAX_MIRRORS];
 74         int                     nzones;
 75         struct btrfs_device     *scheduled_for;
 76 };
 77 
 78 struct reada_zone {
 79         u64                     start;
 80         u64                     end;
 81         u64                     elems;
 82         struct list_head        list;
 83         spinlock_t              lock;
 84         int                     locked;
 85         struct btrfs_device     *device;
 86         struct btrfs_device     *devs[BTRFS_MAX_MIRRORS]; /* full list, incl
 87                                                            * self */
 88         int                     ndevs;
 89         struct kref             refcnt;
 90 };
 91 
 92 struct reada_machine_work {
 93         struct btrfs_work       work;
 94         struct btrfs_fs_info    *fs_info;
 95 };
 96 
 97 static void reada_extent_put(struct btrfs_fs_info *, struct reada_extent *);
 98 static void reada_control_release(struct kref *kref);
 99 static void reada_zone_release(struct kref *kref);
100 static void reada_start_machine(struct btrfs_fs_info *fs_info);
101 static void __reada_start_machine(struct btrfs_fs_info *fs_info);
102 
103 static int reada_add_block(struct reada_control *rc, u64 logical,
104                            struct btrfs_key *top, int level, u64 generation);
105 
106 /* recurses */
107 /* in case of err, eb might be NULL */
108 static int __readahead_hook(struct btrfs_root *root, struct extent_buffer *eb,
109                             u64 start, int err)
110 {
111         int level = 0;
112         int nritems;
113         int i;
114         u64 bytenr;
115         u64 generation;
116         struct reada_extent *re;
117         struct btrfs_fs_info *fs_info = root->fs_info;
118         struct list_head list;
119         unsigned long index = start >> PAGE_CACHE_SHIFT;
120         struct btrfs_device *for_dev;
121 
122         if (eb)
123                 level = btrfs_header_level(eb);
124 
125         /* find extent */
126         spin_lock(&fs_info->reada_lock);
127         re = radix_tree_lookup(&fs_info->reada_tree, index);
128         if (re)
129                 re->refcnt++;
130         spin_unlock(&fs_info->reada_lock);
131 
132         if (!re)
133                 return -1;
134 
135         spin_lock(&re->lock);
136         /*
137          * just take the full list from the extent. afterwards we
138          * don't need the lock anymore
139          */
140         list_replace_init(&re->extctl, &list);
141         for_dev = re->scheduled_for;
142         re->scheduled_for = NULL;
143         spin_unlock(&re->lock);
144 
145         if (err == 0) {
146                 nritems = level ? btrfs_header_nritems(eb) : 0;
147                 generation = btrfs_header_generation(eb);
148                 /*
149                  * FIXME: currently we just set nritems to 0 if this is a leaf,
150                  * effectively ignoring the content. In a next step we could
151                  * trigger more readahead depending from the content, e.g.
152                  * fetch the checksums for the extents in the leaf.
153                  */
154         } else {
155                 /*
156                  * this is the error case, the extent buffer has not been
157                  * read correctly. We won't access anything from it and
158                  * just cleanup our data structures. Effectively this will
159                  * cut the branch below this node from read ahead.
160                  */
161                 nritems = 0;
162                 generation = 0;
163         }
164 
165         for (i = 0; i < nritems; i++) {
166                 struct reada_extctl *rec;
167                 u64 n_gen;
168                 struct btrfs_key key;
169                 struct btrfs_key next_key;
170 
171                 btrfs_node_key_to_cpu(eb, &key, i);
172                 if (i + 1 < nritems)
173                         btrfs_node_key_to_cpu(eb, &next_key, i + 1);
174                 else
175                         next_key = re->top;
176                 bytenr = btrfs_node_blockptr(eb, i);
177                 n_gen = btrfs_node_ptr_generation(eb, i);
178 
179                 list_for_each_entry(rec, &list, list) {
180                         struct reada_control *rc = rec->rc;
181 
182                         /*
183                          * if the generation doesn't match, just ignore this
184                          * extctl. This will probably cut off a branch from
185                          * prefetch. Alternatively one could start a new (sub-)
186                          * prefetch for this branch, starting again from root.
187                          * FIXME: move the generation check out of this loop
188                          */
189 #ifdef DEBUG
190                         if (rec->generation != generation) {
191                                 btrfs_debug(root->fs_info,
192                                            "generation mismatch for (%llu,%d,%llu) %llu != %llu",
193                                        key.objectid, key.type, key.offset,
194                                        rec->generation, generation);
195                         }
196 #endif
197                         if (rec->generation == generation &&
198                             btrfs_comp_cpu_keys(&key, &rc->key_end) < 0 &&
199                             btrfs_comp_cpu_keys(&next_key, &rc->key_start) > 0)
200                                 reada_add_block(rc, bytenr, &next_key,
201                                                 level - 1, n_gen);
202                 }
203         }
204         /*
205          * free extctl records
206          */
207         while (!list_empty(&list)) {
208                 struct reada_control *rc;
209                 struct reada_extctl *rec;
210 
211                 rec = list_first_entry(&list, struct reada_extctl, list);
212                 list_del(&rec->list);
213                 rc = rec->rc;
214                 kfree(rec);
215 
216                 kref_get(&rc->refcnt);
217                 if (atomic_dec_and_test(&rc->elems)) {
218                         kref_put(&rc->refcnt, reada_control_release);
219                         wake_up(&rc->wait);
220                 }
221                 kref_put(&rc->refcnt, reada_control_release);
222 
223                 reada_extent_put(fs_info, re);  /* one ref for each entry */
224         }
225         reada_extent_put(fs_info, re);  /* our ref */
226         if (for_dev)
227                 atomic_dec(&for_dev->reada_in_flight);
228 
229         return 0;
230 }
231 
232 /*
233  * start is passed separately in case eb in NULL, which may be the case with
234  * failed I/O
235  */
236 int btree_readahead_hook(struct btrfs_root *root, struct extent_buffer *eb,
237                          u64 start, int err)
238 {
239         int ret;
240 
241         ret = __readahead_hook(root, eb, start, err);
242 
243         reada_start_machine(root->fs_info);
244 
245         return ret;
246 }
247 
248 static struct reada_zone *reada_find_zone(struct btrfs_fs_info *fs_info,
249                                           struct btrfs_device *dev, u64 logical,
250                                           struct btrfs_bio *bbio)
251 {
252         int ret;
253         struct reada_zone *zone;
254         struct btrfs_block_group_cache *cache = NULL;
255         u64 start;
256         u64 end;
257         int i;
258 
259         zone = NULL;
260         spin_lock(&fs_info->reada_lock);
261         ret = radix_tree_gang_lookup(&dev->reada_zones, (void **)&zone,
262                                      logical >> PAGE_CACHE_SHIFT, 1);
263         if (ret == 1)
264                 kref_get(&zone->refcnt);
265         spin_unlock(&fs_info->reada_lock);
266 
267         if (ret == 1) {
268                 if (logical >= zone->start && logical < zone->end)
269                         return zone;
270                 spin_lock(&fs_info->reada_lock);
271                 kref_put(&zone->refcnt, reada_zone_release);
272                 spin_unlock(&fs_info->reada_lock);
273         }
274 
275         cache = btrfs_lookup_block_group(fs_info, logical);
276         if (!cache)
277                 return NULL;
278 
279         start = cache->key.objectid;
280         end = start + cache->key.offset - 1;
281         btrfs_put_block_group(cache);
282 
283         zone = kzalloc(sizeof(*zone), GFP_NOFS);
284         if (!zone)
285                 return NULL;
286 
287         zone->start = start;
288         zone->end = end;
289         INIT_LIST_HEAD(&zone->list);
290         spin_lock_init(&zone->lock);
291         zone->locked = 0;
292         kref_init(&zone->refcnt);
293         zone->elems = 0;
294         zone->device = dev; /* our device always sits at index 0 */
295         for (i = 0; i < bbio->num_stripes; ++i) {
296                 /* bounds have already been checked */
297                 zone->devs[i] = bbio->stripes[i].dev;
298         }
299         zone->ndevs = bbio->num_stripes;
300 
301         spin_lock(&fs_info->reada_lock);
302         ret = radix_tree_insert(&dev->reada_zones,
303                                 (unsigned long)(zone->end >> PAGE_CACHE_SHIFT),
304                                 zone);
305 
306         if (ret == -EEXIST) {
307                 kfree(zone);
308                 ret = radix_tree_gang_lookup(&dev->reada_zones, (void **)&zone,
309                                              logical >> PAGE_CACHE_SHIFT, 1);
310                 if (ret == 1)
311                         kref_get(&zone->refcnt);
312         }
313         spin_unlock(&fs_info->reada_lock);
314 
315         return zone;
316 }
317 
318 static struct reada_extent *reada_find_extent(struct btrfs_root *root,
319                                               u64 logical,
320                                               struct btrfs_key *top, int level)
321 {
322         int ret;
323         struct reada_extent *re = NULL;
324         struct reada_extent *re_exist = NULL;
325         struct btrfs_fs_info *fs_info = root->fs_info;
326         struct btrfs_bio *bbio = NULL;
327         struct btrfs_device *dev;
328         struct btrfs_device *prev_dev;
329         u32 blocksize;
330         u64 length;
331         int nzones = 0;
332         int i;
333         unsigned long index = logical >> PAGE_CACHE_SHIFT;
334         int dev_replace_is_ongoing;
335 
336         spin_lock(&fs_info->reada_lock);
337         re = radix_tree_lookup(&fs_info->reada_tree, index);
338         if (re)
339                 re->refcnt++;
340         spin_unlock(&fs_info->reada_lock);
341 
342         if (re)
343                 return re;
344 
345         re = kzalloc(sizeof(*re), GFP_NOFS);
346         if (!re)
347                 return NULL;
348 
349         blocksize = root->nodesize;
350         re->logical = logical;
351         re->top = *top;
352         INIT_LIST_HEAD(&re->extctl);
353         spin_lock_init(&re->lock);
354         re->refcnt = 1;
355 
356         /*
357          * map block
358          */
359         length = blocksize;
360         ret = btrfs_map_block(fs_info, REQ_GET_READ_MIRRORS, logical, &length,
361                               &bbio, 0);
362         if (ret || !bbio || length < blocksize)
363                 goto error;
364 
365         if (bbio->num_stripes > BTRFS_MAX_MIRRORS) {
366                 btrfs_err(root->fs_info,
367                            "readahead: more than %d copies not supported",
368                            BTRFS_MAX_MIRRORS);
369                 goto error;
370         }
371 
372         for (nzones = 0; nzones < bbio->num_stripes; ++nzones) {
373                 struct reada_zone *zone;
374 
375                 dev = bbio->stripes[nzones].dev;
376                 zone = reada_find_zone(fs_info, dev, logical, bbio);
377                 if (!zone)
378                         break;
379 
380                 re->zones[nzones] = zone;
381                 spin_lock(&zone->lock);
382                 if (!zone->elems)
383                         kref_get(&zone->refcnt);
384                 ++zone->elems;
385                 spin_unlock(&zone->lock);
386                 spin_lock(&fs_info->reada_lock);
387                 kref_put(&zone->refcnt, reada_zone_release);
388                 spin_unlock(&fs_info->reada_lock);
389         }
390         re->nzones = nzones;
391         if (nzones == 0) {
392                 /* not a single zone found, error and out */
393                 goto error;
394         }
395 
396         /* insert extent in reada_tree + all per-device trees, all or nothing */
397         btrfs_dev_replace_lock(&fs_info->dev_replace);
398         spin_lock(&fs_info->reada_lock);
399         ret = radix_tree_insert(&fs_info->reada_tree, index, re);
400         if (ret == -EEXIST) {
401                 re_exist = radix_tree_lookup(&fs_info->reada_tree, index);
402                 BUG_ON(!re_exist);
403                 re_exist->refcnt++;
404                 spin_unlock(&fs_info->reada_lock);
405                 btrfs_dev_replace_unlock(&fs_info->dev_replace);
406                 goto error;
407         }
408         if (ret) {
409                 spin_unlock(&fs_info->reada_lock);
410                 btrfs_dev_replace_unlock(&fs_info->dev_replace);
411                 goto error;
412         }
413         prev_dev = NULL;
414         dev_replace_is_ongoing = btrfs_dev_replace_is_ongoing(
415                         &fs_info->dev_replace);
416         for (i = 0; i < nzones; ++i) {
417                 dev = bbio->stripes[i].dev;
418                 if (dev == prev_dev) {
419                         /*
420                          * in case of DUP, just add the first zone. As both
421                          * are on the same device, there's nothing to gain
422                          * from adding both.
423                          * Also, it wouldn't work, as the tree is per device
424                          * and adding would fail with EEXIST
425                          */
426                         continue;
427                 }
428                 if (!dev->bdev) {
429                         /*
430                          * cannot read ahead on missing device, but for RAID5/6,
431                          * REQ_GET_READ_MIRRORS return 1. So don't skip missing
432                          * device for such case.
433                          */
434                         if (nzones > 1)
435                                 continue;
436                 }
437                 if (dev_replace_is_ongoing &&
438                     dev == fs_info->dev_replace.tgtdev) {
439                         /*
440                          * as this device is selected for reading only as
441                          * a last resort, skip it for read ahead.
442                          */
443                         continue;
444                 }
445                 prev_dev = dev;
446                 ret = radix_tree_insert(&dev->reada_extents, index, re);
447                 if (ret) {
448                         while (--i >= 0) {
449                                 dev = bbio->stripes[i].dev;
450                                 BUG_ON(dev == NULL);
451                                 /* ignore whether the entry was inserted */
452                                 radix_tree_delete(&dev->reada_extents, index);
453                         }
454                         BUG_ON(fs_info == NULL);
455                         radix_tree_delete(&fs_info->reada_tree, index);
456                         spin_unlock(&fs_info->reada_lock);
457                         btrfs_dev_replace_unlock(&fs_info->dev_replace);
458                         goto error;
459                 }
460         }
461         spin_unlock(&fs_info->reada_lock);
462         btrfs_dev_replace_unlock(&fs_info->dev_replace);
463 
464         btrfs_put_bbio(bbio);
465         return re;
466 
467 error:
468         while (nzones) {
469                 struct reada_zone *zone;
470 
471                 --nzones;
472                 zone = re->zones[nzones];
473                 kref_get(&zone->refcnt);
474                 spin_lock(&zone->lock);
475                 --zone->elems;
476                 if (zone->elems == 0) {
477                         /*
478                          * no fs_info->reada_lock needed, as this can't be
479                          * the last ref
480                          */
481                         kref_put(&zone->refcnt, reada_zone_release);
482                 }
483                 spin_unlock(&zone->lock);
484 
485                 spin_lock(&fs_info->reada_lock);
486                 kref_put(&zone->refcnt, reada_zone_release);
487                 spin_unlock(&fs_info->reada_lock);
488         }
489         btrfs_put_bbio(bbio);
490         kfree(re);
491         return re_exist;
492 }
493 
494 static void reada_extent_put(struct btrfs_fs_info *fs_info,
495                              struct reada_extent *re)
496 {
497         int i;
498         unsigned long index = re->logical >> PAGE_CACHE_SHIFT;
499 
500         spin_lock(&fs_info->reada_lock);
501         if (--re->refcnt) {
502                 spin_unlock(&fs_info->reada_lock);
503                 return;
504         }
505 
506         radix_tree_delete(&fs_info->reada_tree, index);
507         for (i = 0; i < re->nzones; ++i) {
508                 struct reada_zone *zone = re->zones[i];
509 
510                 radix_tree_delete(&zone->device->reada_extents, index);
511         }
512 
513         spin_unlock(&fs_info->reada_lock);
514 
515         for (i = 0; i < re->nzones; ++i) {
516                 struct reada_zone *zone = re->zones[i];
517 
518                 kref_get(&zone->refcnt);
519                 spin_lock(&zone->lock);
520                 --zone->elems;
521                 if (zone->elems == 0) {
522                         /* no fs_info->reada_lock needed, as this can't be
523                          * the last ref */
524                         kref_put(&zone->refcnt, reada_zone_release);
525                 }
526                 spin_unlock(&zone->lock);
527 
528                 spin_lock(&fs_info->reada_lock);
529                 kref_put(&zone->refcnt, reada_zone_release);
530                 spin_unlock(&fs_info->reada_lock);
531         }
532         if (re->scheduled_for)
533                 atomic_dec(&re->scheduled_for->reada_in_flight);
534 
535         kfree(re);
536 }
537 
538 static void reada_zone_release(struct kref *kref)
539 {
540         struct reada_zone *zone = container_of(kref, struct reada_zone, refcnt);
541 
542         radix_tree_delete(&zone->device->reada_zones,
543                           zone->end >> PAGE_CACHE_SHIFT);
544 
545         kfree(zone);
546 }
547 
548 static void reada_control_release(struct kref *kref)
549 {
550         struct reada_control *rc = container_of(kref, struct reada_control,
551                                                 refcnt);
552 
553         kfree(rc);
554 }
555 
556 static int reada_add_block(struct reada_control *rc, u64 logical,
557                            struct btrfs_key *top, int level, u64 generation)
558 {
559         struct btrfs_root *root = rc->root;
560         struct reada_extent *re;
561         struct reada_extctl *rec;
562 
563         re = reada_find_extent(root, logical, top, level); /* takes one ref */
564         if (!re)
565                 return -1;
566 
567         rec = kzalloc(sizeof(*rec), GFP_NOFS);
568         if (!rec) {
569                 reada_extent_put(root->fs_info, re);
570                 return -1;
571         }
572 
573         rec->rc = rc;
574         rec->generation = generation;
575         atomic_inc(&rc->elems);
576 
577         spin_lock(&re->lock);
578         list_add_tail(&rec->list, &re->extctl);
579         spin_unlock(&re->lock);
580 
581         /* leave the ref on the extent */
582 
583         return 0;
584 }
585 
586 /*
587  * called with fs_info->reada_lock held
588  */
589 static void reada_peer_zones_set_lock(struct reada_zone *zone, int lock)
590 {
591         int i;
592         unsigned long index = zone->end >> PAGE_CACHE_SHIFT;
593 
594         for (i = 0; i < zone->ndevs; ++i) {
595                 struct reada_zone *peer;
596                 peer = radix_tree_lookup(&zone->devs[i]->reada_zones, index);
597                 if (peer && peer->device != zone->device)
598                         peer->locked = lock;
599         }
600 }
601 
602 /*
603  * called with fs_info->reada_lock held
604  */
605 static int reada_pick_zone(struct btrfs_device *dev)
606 {
607         struct reada_zone *top_zone = NULL;
608         struct reada_zone *top_locked_zone = NULL;
609         u64 top_elems = 0;
610         u64 top_locked_elems = 0;
611         unsigned long index = 0;
612         int ret;
613 
614         if (dev->reada_curr_zone) {
615                 reada_peer_zones_set_lock(dev->reada_curr_zone, 0);
616                 kref_put(&dev->reada_curr_zone->refcnt, reada_zone_release);
617                 dev->reada_curr_zone = NULL;
618         }
619         /* pick the zone with the most elements */
620         while (1) {
621                 struct reada_zone *zone;
622 
623                 ret = radix_tree_gang_lookup(&dev->reada_zones,
624                                              (void **)&zone, index, 1);
625                 if (ret == 0)
626                         break;
627                 index = (zone->end >> PAGE_CACHE_SHIFT) + 1;
628                 if (zone->locked) {
629                         if (zone->elems > top_locked_elems) {
630                                 top_locked_elems = zone->elems;
631                                 top_locked_zone = zone;
632                         }
633                 } else {
634                         if (zone->elems > top_elems) {
635                                 top_elems = zone->elems;
636                                 top_zone = zone;
637                         }
638                 }
639         }
640         if (top_zone)
641                 dev->reada_curr_zone = top_zone;
642         else if (top_locked_zone)
643                 dev->reada_curr_zone = top_locked_zone;
644         else
645                 return 0;
646 
647         dev->reada_next = dev->reada_curr_zone->start;
648         kref_get(&dev->reada_curr_zone->refcnt);
649         reada_peer_zones_set_lock(dev->reada_curr_zone, 1);
650 
651         return 1;
652 }
653 
654 static int reada_start_machine_dev(struct btrfs_fs_info *fs_info,
655                                    struct btrfs_device *dev)
656 {
657         struct reada_extent *re = NULL;
658         int mirror_num = 0;
659         struct extent_buffer *eb = NULL;
660         u64 logical;
661         int ret;
662         int i;
663         int need_kick = 0;
664 
665         spin_lock(&fs_info->reada_lock);
666         if (dev->reada_curr_zone == NULL) {
667                 ret = reada_pick_zone(dev);
668                 if (!ret) {
669                         spin_unlock(&fs_info->reada_lock);
670                         return 0;
671                 }
672         }
673         /*
674          * FIXME currently we issue the reads one extent at a time. If we have
675          * a contiguous block of extents, we could also coagulate them or use
676          * plugging to speed things up
677          */
678         ret = radix_tree_gang_lookup(&dev->reada_extents, (void **)&re,
679                                      dev->reada_next >> PAGE_CACHE_SHIFT, 1);
680         if (ret == 0 || re->logical >= dev->reada_curr_zone->end) {
681                 ret = reada_pick_zone(dev);
682                 if (!ret) {
683                         spin_unlock(&fs_info->reada_lock);
684                         return 0;
685                 }
686                 re = NULL;
687                 ret = radix_tree_gang_lookup(&dev->reada_extents, (void **)&re,
688                                         dev->reada_next >> PAGE_CACHE_SHIFT, 1);
689         }
690         if (ret == 0) {
691                 spin_unlock(&fs_info->reada_lock);
692                 return 0;
693         }
694         dev->reada_next = re->logical + fs_info->tree_root->nodesize;
695         re->refcnt++;
696 
697         spin_unlock(&fs_info->reada_lock);
698 
699         /*
700          * find mirror num
701          */
702         for (i = 0; i < re->nzones; ++i) {
703                 if (re->zones[i]->device == dev) {
704                         mirror_num = i + 1;
705                         break;
706                 }
707         }
708         logical = re->logical;
709 
710         spin_lock(&re->lock);
711         if (re->scheduled_for == NULL) {
712                 re->scheduled_for = dev;
713                 need_kick = 1;
714         }
715         spin_unlock(&re->lock);
716 
717         reada_extent_put(fs_info, re);
718 
719         if (!need_kick)
720                 return 0;
721 
722         atomic_inc(&dev->reada_in_flight);
723         ret = reada_tree_block_flagged(fs_info->extent_root, logical,
724                         mirror_num, &eb);
725         if (ret)
726                 __readahead_hook(fs_info->extent_root, NULL, logical, ret);
727         else if (eb)
728                 __readahead_hook(fs_info->extent_root, eb, eb->start, ret);
729 
730         if (eb)
731                 free_extent_buffer(eb);
732 
733         return 1;
734 
735 }
736 
737 static void reada_start_machine_worker(struct btrfs_work *work)
738 {
739         struct reada_machine_work *rmw;
740         struct btrfs_fs_info *fs_info;
741         int old_ioprio;
742 
743         rmw = container_of(work, struct reada_machine_work, work);
744         fs_info = rmw->fs_info;
745 
746         kfree(rmw);
747 
748         old_ioprio = IOPRIO_PRIO_VALUE(task_nice_ioclass(current),
749                                        task_nice_ioprio(current));
750         set_task_ioprio(current, BTRFS_IOPRIO_READA);
751         __reada_start_machine(fs_info);
752         set_task_ioprio(current, old_ioprio);
753 }
754 
755 static void __reada_start_machine(struct btrfs_fs_info *fs_info)
756 {
757         struct btrfs_device *device;
758         struct btrfs_fs_devices *fs_devices = fs_info->fs_devices;
759         u64 enqueued;
760         u64 total = 0;
761         int i;
762 
763         do {
764                 enqueued = 0;
765                 list_for_each_entry(device, &fs_devices->devices, dev_list) {
766                         if (atomic_read(&device->reada_in_flight) <
767                             MAX_IN_FLIGHT)
768                                 enqueued += reada_start_machine_dev(fs_info,
769                                                                     device);
770                 }
771                 total += enqueued;
772         } while (enqueued && total < 10000);
773 
774         if (enqueued == 0)
775                 return;
776 
777         /*
778          * If everything is already in the cache, this is effectively single
779          * threaded. To a) not hold the caller for too long and b) to utilize
780          * more cores, we broke the loop above after 10000 iterations and now
781          * enqueue to workers to finish it. This will distribute the load to
782          * the cores.
783          */
784         for (i = 0; i < 2; ++i)
785                 reada_start_machine(fs_info);
786 }
787 
788 static void reada_start_machine(struct btrfs_fs_info *fs_info)
789 {
790         struct reada_machine_work *rmw;
791 
792         rmw = kzalloc(sizeof(*rmw), GFP_NOFS);
793         if (!rmw) {
794                 /* FIXME we cannot handle this properly right now */
795                 BUG();
796         }
797         btrfs_init_work(&rmw->work, btrfs_readahead_helper,
798                         reada_start_machine_worker, NULL, NULL);
799         rmw->fs_info = fs_info;
800 
801         btrfs_queue_work(fs_info->readahead_workers, &rmw->work);
802 }
803 
804 #ifdef DEBUG
805 static void dump_devs(struct btrfs_fs_info *fs_info, int all)
806 {
807         struct btrfs_device *device;
808         struct btrfs_fs_devices *fs_devices = fs_info->fs_devices;
809         unsigned long index;
810         int ret;
811         int i;
812         int j;
813         int cnt;
814 
815         spin_lock(&fs_info->reada_lock);
816         list_for_each_entry(device, &fs_devices->devices, dev_list) {
817                 printk(KERN_DEBUG "dev %lld has %d in flight\n", device->devid,
818                         atomic_read(&device->reada_in_flight));
819                 index = 0;
820                 while (1) {
821                         struct reada_zone *zone;
822                         ret = radix_tree_gang_lookup(&device->reada_zones,
823                                                      (void **)&zone, index, 1);
824                         if (ret == 0)
825                                 break;
826                         printk(KERN_DEBUG "  zone %llu-%llu elems %llu locked "
827                                 "%d devs", zone->start, zone->end, zone->elems,
828                                 zone->locked);
829                         for (j = 0; j < zone->ndevs; ++j) {
830                                 printk(KERN_CONT " %lld",
831                                         zone->devs[j]->devid);
832                         }
833                         if (device->reada_curr_zone == zone)
834                                 printk(KERN_CONT " curr off %llu",
835                                         device->reada_next - zone->start);
836                         printk(KERN_CONT "\n");
837                         index = (zone->end >> PAGE_CACHE_SHIFT) + 1;
838                 }
839                 cnt = 0;
840                 index = 0;
841                 while (all) {
842                         struct reada_extent *re = NULL;
843 
844                         ret = radix_tree_gang_lookup(&device->reada_extents,
845                                                      (void **)&re, index, 1);
846                         if (ret == 0)
847                                 break;
848                         printk(KERN_DEBUG
849                                 "  re: logical %llu size %u empty %d for %lld",
850                                 re->logical, fs_info->tree_root->nodesize,
851                                 list_empty(&re->extctl), re->scheduled_for ?
852                                 re->scheduled_for->devid : -1);
853 
854                         for (i = 0; i < re->nzones; ++i) {
855                                 printk(KERN_CONT " zone %llu-%llu devs",
856                                         re->zones[i]->start,
857                                         re->zones[i]->end);
858                                 for (j = 0; j < re->zones[i]->ndevs; ++j) {
859                                         printk(KERN_CONT " %lld",
860                                                 re->zones[i]->devs[j]->devid);
861                                 }
862                         }
863                         printk(KERN_CONT "\n");
864                         index = (re->logical >> PAGE_CACHE_SHIFT) + 1;
865                         if (++cnt > 15)
866                                 break;
867                 }
868         }
869 
870         index = 0;
871         cnt = 0;
872         while (all) {
873                 struct reada_extent *re = NULL;
874 
875                 ret = radix_tree_gang_lookup(&fs_info->reada_tree, (void **)&re,
876                                              index, 1);
877                 if (ret == 0)
878                         break;
879                 if (!re->scheduled_for) {
880                         index = (re->logical >> PAGE_CACHE_SHIFT) + 1;
881                         continue;
882                 }
883                 printk(KERN_DEBUG
884                         "re: logical %llu size %u list empty %d for %lld",
885                         re->logical, fs_info->tree_root->nodesize,
886                         list_empty(&re->extctl),
887                         re->scheduled_for ? re->scheduled_for->devid : -1);
888                 for (i = 0; i < re->nzones; ++i) {
889                         printk(KERN_CONT " zone %llu-%llu devs",
890                                 re->zones[i]->start,
891                                 re->zones[i]->end);
892                         for (i = 0; i < re->nzones; ++i) {
893                                 printk(KERN_CONT " zone %llu-%llu devs",
894                                         re->zones[i]->start,
895                                         re->zones[i]->end);
896                                 for (j = 0; j < re->zones[i]->ndevs; ++j) {
897                                         printk(KERN_CONT " %lld",
898                                                 re->zones[i]->devs[j]->devid);
899                                 }
900                         }
901                 }
902                 printk(KERN_CONT "\n");
903                 index = (re->logical >> PAGE_CACHE_SHIFT) + 1;
904         }
905         spin_unlock(&fs_info->reada_lock);
906 }
907 #endif
908 
909 /*
910  * interface
911  */
912 struct reada_control *btrfs_reada_add(struct btrfs_root *root,
913                         struct btrfs_key *key_start, struct btrfs_key *key_end)
914 {
915         struct reada_control *rc;
916         u64 start;
917         u64 generation;
918         int level;
919         struct extent_buffer *node;
920         static struct btrfs_key max_key = {
921                 .objectid = (u64)-1,
922                 .type = (u8)-1,
923                 .offset = (u64)-1
924         };
925 
926         rc = kzalloc(sizeof(*rc), GFP_NOFS);
927         if (!rc)
928                 return ERR_PTR(-ENOMEM);
929 
930         rc->root = root;
931         rc->key_start = *key_start;
932         rc->key_end = *key_end;
933         atomic_set(&rc->elems, 0);
934         init_waitqueue_head(&rc->wait);
935         kref_init(&rc->refcnt);
936         kref_get(&rc->refcnt); /* one ref for having elements */
937 
938         node = btrfs_root_node(root);
939         start = node->start;
940         level = btrfs_header_level(node);
941         generation = btrfs_header_generation(node);
942         free_extent_buffer(node);
943 
944         if (reada_add_block(rc, start, &max_key, level, generation)) {
945                 kfree(rc);
946                 return ERR_PTR(-ENOMEM);
947         }
948 
949         reada_start_machine(root->fs_info);
950 
951         return rc;
952 }
953 
954 #ifdef DEBUG
955 int btrfs_reada_wait(void *handle)
956 {
957         struct reada_control *rc = handle;
958 
959         while (atomic_read(&rc->elems)) {
960                 wait_event_timeout(rc->wait, atomic_read(&rc->elems) == 0,
961                                    5 * HZ);
962                 dump_devs(rc->root->fs_info,
963                           atomic_read(&rc->elems) < 10 ? 1 : 0);
964         }
965 
966         dump_devs(rc->root->fs_info, atomic_read(&rc->elems) < 10 ? 1 : 0);
967 
968         kref_put(&rc->refcnt, reada_control_release);
969 
970         return 0;
971 }
972 #else
973 int btrfs_reada_wait(void *handle)
974 {
975         struct reada_control *rc = handle;
976 
977         while (atomic_read(&rc->elems)) {
978                 wait_event(rc->wait, atomic_read(&rc->elems) == 0);
979         }
980 
981         kref_put(&rc->refcnt, reada_control_release);
982 
983         return 0;
984 }
985 #endif
986 
987 void btrfs_reada_detach(void *handle)
988 {
989         struct reada_control *rc = handle;
990 
991         kref_put(&rc->refcnt, reada_control_release);
992 }
993 

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