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Linux/fs/ubifs/shrinker.c

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  1 /*
  2  * This file is part of UBIFS.
  3  *
  4  * Copyright (C) 2006-2008 Nokia Corporation.
  5  *
  6  * This program is free software; you can redistribute it and/or modify it
  7  * under the terms of the GNU General Public License version 2 as published by
  8  * the Free Software Foundation.
  9  *
 10  * This program is distributed in the hope that it will be useful, but WITHOUT
 11  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 12  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License for
 13  * more details.
 14  *
 15  * You should have received a copy of the GNU General Public License along with
 16  * this program; if not, write to the Free Software Foundation, Inc., 51
 17  * Franklin St, Fifth Floor, Boston, MA 02110-1301 USA
 18  *
 19  * Authors: Artem Bityutskiy (Битюцкий Артём)
 20  *          Adrian Hunter
 21  */
 22 
 23 /*
 24  * This file implements UBIFS shrinker which evicts clean znodes from the TNC
 25  * tree when Linux VM needs more RAM.
 26  *
 27  * We do not implement any LRU lists to find oldest znodes to free because it
 28  * would add additional overhead to the file system fast paths. So the shrinker
 29  * just walks the TNC tree when searching for znodes to free.
 30  *
 31  * If the root of a TNC sub-tree is clean and old enough, then the children are
 32  * also clean and old enough. So the shrinker walks the TNC in level order and
 33  * dumps entire sub-trees.
 34  *
 35  * The age of znodes is just the time-stamp when they were last looked at.
 36  * The current shrinker first tries to evict old znodes, then young ones.
 37  *
 38  * Since the shrinker is global, it has to protect against races with FS
 39  * un-mounts, which is done by the 'ubifs_infos_lock' and 'c->umount_mutex'.
 40  */
 41 
 42 #include "ubifs.h"
 43 
 44 /* List of all UBIFS file-system instances */
 45 LIST_HEAD(ubifs_infos);
 46 
 47 /*
 48  * We number each shrinker run and record the number on the ubifs_info structure
 49  * so that we can easily work out which ubifs_info structures have already been
 50  * done by the current run.
 51  */
 52 static unsigned int shrinker_run_no;
 53 
 54 /* Protects 'ubifs_infos' list */
 55 DEFINE_SPINLOCK(ubifs_infos_lock);
 56 
 57 /* Global clean znode counter (for all mounted UBIFS instances) */
 58 atomic_long_t ubifs_clean_zn_cnt;
 59 
 60 /**
 61  * shrink_tnc - shrink TNC tree.
 62  * @c: UBIFS file-system description object
 63  * @nr: number of znodes to free
 64  * @age: the age of znodes to free
 65  * @contention: if any contention, this is set to %1
 66  *
 67  * This function traverses TNC tree and frees clean znodes. It does not free
 68  * clean znodes which younger then @age. Returns number of freed znodes.
 69  */
 70 static int shrink_tnc(struct ubifs_info *c, int nr, int age, int *contention)
 71 {
 72         int total_freed = 0;
 73         struct ubifs_znode *znode, *zprev;
 74         int time = get_seconds();
 75 
 76         ubifs_assert(mutex_is_locked(&c->umount_mutex));
 77         ubifs_assert(mutex_is_locked(&c->tnc_mutex));
 78 
 79         if (!c->zroot.znode || atomic_long_read(&c->clean_zn_cnt) == 0)
 80                 return 0;
 81 
 82         /*
 83          * Traverse the TNC tree in levelorder manner, so that it is possible
 84          * to destroy large sub-trees. Indeed, if a znode is old, then all its
 85          * children are older or of the same age.
 86          *
 87          * Note, we are holding 'c->tnc_mutex', so we do not have to lock the
 88          * 'c->space_lock' when _reading_ 'c->clean_zn_cnt', because it is
 89          * changed only when the 'c->tnc_mutex' is held.
 90          */
 91         zprev = NULL;
 92         znode = ubifs_tnc_levelorder_next(c->zroot.znode, NULL);
 93         while (znode && total_freed < nr &&
 94                atomic_long_read(&c->clean_zn_cnt) > 0) {
 95                 int freed;
 96 
 97                 /*
 98                  * If the znode is clean, but it is in the 'c->cnext' list, this
 99                  * means that this znode has just been written to flash as a
100                  * part of commit and was marked clean. They will be removed
101                  * from the list at end commit. We cannot change the list,
102                  * because it is not protected by any mutex (design decision to
103                  * make commit really independent and parallel to main I/O). So
104                  * we just skip these znodes.
105                  *
106                  * Note, the 'clean_zn_cnt' counters are not updated until
107                  * after the commit, so the UBIFS shrinker does not report
108                  * the znodes which are in the 'c->cnext' list as freeable.
109                  *
110                  * Also note, if the root of a sub-tree is not in 'c->cnext',
111                  * then the whole sub-tree is not in 'c->cnext' as well, so it
112                  * is safe to dump whole sub-tree.
113                  */
114 
115                 if (znode->cnext) {
116                         /*
117                          * Very soon these znodes will be removed from the list
118                          * and become freeable.
119                          */
120                         *contention = 1;
121                 } else if (!ubifs_zn_dirty(znode) &&
122                            abs(time - znode->time) >= age) {
123                         if (znode->parent)
124                                 znode->parent->zbranch[znode->iip].znode = NULL;
125                         else
126                                 c->zroot.znode = NULL;
127 
128                         freed = ubifs_destroy_tnc_subtree(znode);
129                         atomic_long_sub(freed, &ubifs_clean_zn_cnt);
130                         atomic_long_sub(freed, &c->clean_zn_cnt);
131                         ubifs_assert(atomic_long_read(&c->clean_zn_cnt) >= 0);
132                         total_freed += freed;
133                         znode = zprev;
134                 }
135 
136                 if (unlikely(!c->zroot.znode))
137                         break;
138 
139                 zprev = znode;
140                 znode = ubifs_tnc_levelorder_next(c->zroot.znode, znode);
141                 cond_resched();
142         }
143 
144         return total_freed;
145 }
146 
147 /**
148  * shrink_tnc_trees - shrink UBIFS TNC trees.
149  * @nr: number of znodes to free
150  * @age: the age of znodes to free
151  * @contention: if any contention, this is set to %1
152  *
153  * This function walks the list of mounted UBIFS file-systems and frees clean
154  * znodes which are older than @age, until at least @nr znodes are freed.
155  * Returns the number of freed znodes.
156  */
157 static int shrink_tnc_trees(int nr, int age, int *contention)
158 {
159         struct ubifs_info *c;
160         struct list_head *p;
161         unsigned int run_no;
162         int freed = 0;
163 
164         spin_lock(&ubifs_infos_lock);
165         do {
166                 run_no = ++shrinker_run_no;
167         } while (run_no == 0);
168         /* Iterate over all mounted UBIFS file-systems and try to shrink them */
169         p = ubifs_infos.next;
170         while (p != &ubifs_infos) {
171                 c = list_entry(p, struct ubifs_info, infos_list);
172                 /*
173                  * We move the ones we do to the end of the list, so we stop
174                  * when we see one we have already done.
175                  */
176                 if (c->shrinker_run_no == run_no)
177                         break;
178                 if (!mutex_trylock(&c->umount_mutex)) {
179                         /* Some un-mount is in progress, try next FS */
180                         *contention = 1;
181                         p = p->next;
182                         continue;
183                 }
184                 /*
185                  * We're holding 'c->umount_mutex', so the file-system won't go
186                  * away.
187                  */
188                 if (!mutex_trylock(&c->tnc_mutex)) {
189                         mutex_unlock(&c->umount_mutex);
190                         *contention = 1;
191                         p = p->next;
192                         continue;
193                 }
194                 spin_unlock(&ubifs_infos_lock);
195                 /*
196                  * OK, now we have TNC locked, the file-system cannot go away -
197                  * it is safe to reap the cache.
198                  */
199                 c->shrinker_run_no = run_no;
200                 freed += shrink_tnc(c, nr, age, contention);
201                 mutex_unlock(&c->tnc_mutex);
202                 spin_lock(&ubifs_infos_lock);
203                 /* Get the next list element before we move this one */
204                 p = p->next;
205                 /*
206                  * Move this one to the end of the list to provide some
207                  * fairness.
208                  */
209                 list_move_tail(&c->infos_list, &ubifs_infos);
210                 mutex_unlock(&c->umount_mutex);
211                 if (freed >= nr)
212                         break;
213         }
214         spin_unlock(&ubifs_infos_lock);
215         return freed;
216 }
217 
218 /**
219  * kick_a_thread - kick a background thread to start commit.
220  *
221  * This function kicks a background thread to start background commit. Returns
222  * %-1 if a thread was kicked or there is another reason to assume the memory
223  * will soon be freed or become freeable. If there are no dirty znodes, returns
224  * %0.
225  */
226 static int kick_a_thread(void)
227 {
228         int i;
229         struct ubifs_info *c;
230 
231         /*
232          * Iterate over all mounted UBIFS file-systems and find out if there is
233          * already an ongoing commit operation there. If no, then iterate for
234          * the second time and initiate background commit.
235          */
236         spin_lock(&ubifs_infos_lock);
237         for (i = 0; i < 2; i++) {
238                 list_for_each_entry(c, &ubifs_infos, infos_list) {
239                         long dirty_zn_cnt;
240 
241                         if (!mutex_trylock(&c->umount_mutex)) {
242                                 /*
243                                  * Some un-mount is in progress, it will
244                                  * certainly free memory, so just return.
245                                  */
246                                 spin_unlock(&ubifs_infos_lock);
247                                 return -1;
248                         }
249 
250                         dirty_zn_cnt = atomic_long_read(&c->dirty_zn_cnt);
251 
252                         if (!dirty_zn_cnt || c->cmt_state == COMMIT_BROKEN ||
253                             c->ro_mount || c->ro_error) {
254                                 mutex_unlock(&c->umount_mutex);
255                                 continue;
256                         }
257 
258                         if (c->cmt_state != COMMIT_RESTING) {
259                                 spin_unlock(&ubifs_infos_lock);
260                                 mutex_unlock(&c->umount_mutex);
261                                 return -1;
262                         }
263 
264                         if (i == 1) {
265                                 list_move_tail(&c->infos_list, &ubifs_infos);
266                                 spin_unlock(&ubifs_infos_lock);
267 
268                                 ubifs_request_bg_commit(c);
269                                 mutex_unlock(&c->umount_mutex);
270                                 return -1;
271                         }
272                         mutex_unlock(&c->umount_mutex);
273                 }
274         }
275         spin_unlock(&ubifs_infos_lock);
276 
277         return 0;
278 }
279 
280 int ubifs_shrinker(struct shrinker *shrink, struct shrink_control *sc)
281 {
282         int nr = sc->nr_to_scan;
283         int freed, contention = 0;
284         long clean_zn_cnt = atomic_long_read(&ubifs_clean_zn_cnt);
285 
286         if (nr == 0)
287                 /*
288                  * Due to the way UBIFS updates the clean znode counter it may
289                  * temporarily be negative.
290                  */
291                 return clean_zn_cnt >= 0 ? clean_zn_cnt : 1;
292 
293         if (!clean_zn_cnt) {
294                 /*
295                  * No clean znodes, nothing to reap. All we can do in this case
296                  * is to kick background threads to start commit, which will
297                  * probably make clean znodes which, in turn, will be freeable.
298                  * And we return -1 which means will make VM call us again
299                  * later.
300                  */
301                 dbg_tnc("no clean znodes, kick a thread");
302                 return kick_a_thread();
303         }
304 
305         freed = shrink_tnc_trees(nr, OLD_ZNODE_AGE, &contention);
306         if (freed >= nr)
307                 goto out;
308 
309         dbg_tnc("not enough old znodes, try to free young ones");
310         freed += shrink_tnc_trees(nr - freed, YOUNG_ZNODE_AGE, &contention);
311         if (freed >= nr)
312                 goto out;
313 
314         dbg_tnc("not enough young znodes, free all");
315         freed += shrink_tnc_trees(nr - freed, 0, &contention);
316 
317         if (!freed && contention) {
318                 dbg_tnc("freed nothing, but contention");
319                 return -1;
320         }
321 
322 out:
323         dbg_tnc("%d znodes were freed, requested %d", freed, nr);
324         return freed;
325 }
326 

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