1 | // SPDX-License-Identifier: GPL-2.0-only |
2 | #include <linux/spinlock.h> |
3 | #include <linux/slab.h> |
4 | #include <linux/list.h> |
5 | #include <linux/list_bl.h> |
6 | #include <linux/module.h> |
7 | #include <linux/sched.h> |
8 | #include <linux/workqueue.h> |
9 | #include <linux/mbcache.h> |
10 | |
11 | /* |
12 | * Mbcache is a simple key-value store. Keys need not be unique, however |
13 | * key-value pairs are expected to be unique (we use this fact in |
14 | * mb_cache_entry_delete_or_get()). |
15 | * |
16 | * Ext2 and ext4 use this cache for deduplication of extended attribute blocks. |
17 | * Ext4 also uses it for deduplication of xattr values stored in inodes. |
18 | * They use hash of data as a key and provide a value that may represent a |
19 | * block or inode number. That's why keys need not be unique (hash of different |
20 | * data may be the same). However user provided value always uniquely |
21 | * identifies a cache entry. |
22 | * |
23 | * We provide functions for creation and removal of entries, search by key, |
24 | * and a special "delete entry with given key-value pair" operation. Fixed |
25 | * size hash table is used for fast key lookups. |
26 | */ |
27 | |
28 | struct mb_cache { |
29 | /* Hash table of entries */ |
30 | struct hlist_bl_head *c_hash; |
31 | /* log2 of hash table size */ |
32 | int c_bucket_bits; |
33 | /* Maximum entries in cache to avoid degrading hash too much */ |
34 | unsigned long c_max_entries; |
35 | /* Protects c_list, c_entry_count */ |
36 | spinlock_t c_list_lock; |
37 | struct list_head c_list; |
38 | /* Number of entries in cache */ |
39 | unsigned long c_entry_count; |
40 | struct shrinker *c_shrink; |
41 | /* Work for shrinking when the cache has too many entries */ |
42 | struct work_struct c_shrink_work; |
43 | }; |
44 | |
45 | static struct kmem_cache *mb_entry_cache; |
46 | |
47 | static unsigned long mb_cache_shrink(struct mb_cache *cache, |
48 | unsigned long nr_to_scan); |
49 | |
50 | static inline struct hlist_bl_head *mb_cache_entry_head(struct mb_cache *cache, |
51 | u32 key) |
52 | { |
53 | return &cache->c_hash[hash_32(val: key, bits: cache->c_bucket_bits)]; |
54 | } |
55 | |
56 | /* |
57 | * Number of entries to reclaim synchronously when there are too many entries |
58 | * in cache |
59 | */ |
60 | #define SYNC_SHRINK_BATCH 64 |
61 | |
62 | /* |
63 | * mb_cache_entry_create - create entry in cache |
64 | * @cache - cache where the entry should be created |
65 | * @mask - gfp mask with which the entry should be allocated |
66 | * @key - key of the entry |
67 | * @value - value of the entry |
68 | * @reusable - is the entry reusable by others? |
69 | * |
70 | * Creates entry in @cache with key @key and value @value. The function returns |
71 | * -EBUSY if entry with the same key and value already exists in cache. |
72 | * Otherwise 0 is returned. |
73 | */ |
74 | int mb_cache_entry_create(struct mb_cache *cache, gfp_t mask, u32 key, |
75 | u64 value, bool reusable) |
76 | { |
77 | struct mb_cache_entry *entry, *dup; |
78 | struct hlist_bl_node *dup_node; |
79 | struct hlist_bl_head *head; |
80 | |
81 | /* Schedule background reclaim if there are too many entries */ |
82 | if (cache->c_entry_count >= cache->c_max_entries) |
83 | schedule_work(work: &cache->c_shrink_work); |
84 | /* Do some sync reclaim if background reclaim cannot keep up */ |
85 | if (cache->c_entry_count >= 2*cache->c_max_entries) |
86 | mb_cache_shrink(cache, SYNC_SHRINK_BATCH); |
87 | |
88 | entry = kmem_cache_alloc(cachep: mb_entry_cache, flags: mask); |
89 | if (!entry) |
90 | return -ENOMEM; |
91 | |
92 | INIT_LIST_HEAD(list: &entry->e_list); |
93 | /* |
94 | * We create entry with two references. One reference is kept by the |
95 | * hash table, the other reference is used to protect us from |
96 | * mb_cache_entry_delete_or_get() until the entry is fully setup. This |
97 | * avoids nesting of cache->c_list_lock into hash table bit locks which |
98 | * is problematic for RT. |
99 | */ |
100 | atomic_set(v: &entry->e_refcnt, i: 2); |
101 | entry->e_key = key; |
102 | entry->e_value = value; |
103 | entry->e_flags = 0; |
104 | if (reusable) |
105 | set_bit(nr: MBE_REUSABLE_B, addr: &entry->e_flags); |
106 | head = mb_cache_entry_head(cache, key); |
107 | hlist_bl_lock(b: head); |
108 | hlist_bl_for_each_entry(dup, dup_node, head, e_hash_list) { |
109 | if (dup->e_key == key && dup->e_value == value) { |
110 | hlist_bl_unlock(b: head); |
111 | kmem_cache_free(s: mb_entry_cache, objp: entry); |
112 | return -EBUSY; |
113 | } |
114 | } |
115 | hlist_bl_add_head(n: &entry->e_hash_list, h: head); |
116 | hlist_bl_unlock(b: head); |
117 | spin_lock(lock: &cache->c_list_lock); |
118 | list_add_tail(new: &entry->e_list, head: &cache->c_list); |
119 | cache->c_entry_count++; |
120 | spin_unlock(lock: &cache->c_list_lock); |
121 | mb_cache_entry_put(cache, entry); |
122 | |
123 | return 0; |
124 | } |
125 | EXPORT_SYMBOL(mb_cache_entry_create); |
126 | |
127 | void __mb_cache_entry_free(struct mb_cache *cache, struct mb_cache_entry *entry) |
128 | { |
129 | struct hlist_bl_head *head; |
130 | |
131 | head = mb_cache_entry_head(cache, key: entry->e_key); |
132 | hlist_bl_lock(b: head); |
133 | hlist_bl_del(n: &entry->e_hash_list); |
134 | hlist_bl_unlock(b: head); |
135 | kmem_cache_free(s: mb_entry_cache, objp: entry); |
136 | } |
137 | EXPORT_SYMBOL(__mb_cache_entry_free); |
138 | |
139 | /* |
140 | * mb_cache_entry_wait_unused - wait to be the last user of the entry |
141 | * |
142 | * @entry - entry to work on |
143 | * |
144 | * Wait to be the last user of the entry. |
145 | */ |
146 | void mb_cache_entry_wait_unused(struct mb_cache_entry *entry) |
147 | { |
148 | wait_var_event(&entry->e_refcnt, atomic_read(&entry->e_refcnt) <= 2); |
149 | } |
150 | EXPORT_SYMBOL(mb_cache_entry_wait_unused); |
151 | |
152 | static struct mb_cache_entry *__entry_find(struct mb_cache *cache, |
153 | struct mb_cache_entry *entry, |
154 | u32 key) |
155 | { |
156 | struct mb_cache_entry *old_entry = entry; |
157 | struct hlist_bl_node *node; |
158 | struct hlist_bl_head *head; |
159 | |
160 | head = mb_cache_entry_head(cache, key); |
161 | hlist_bl_lock(b: head); |
162 | if (entry && !hlist_bl_unhashed(h: &entry->e_hash_list)) |
163 | node = entry->e_hash_list.next; |
164 | else |
165 | node = hlist_bl_first(h: head); |
166 | while (node) { |
167 | entry = hlist_bl_entry(node, struct mb_cache_entry, |
168 | e_hash_list); |
169 | if (entry->e_key == key && |
170 | test_bit(MBE_REUSABLE_B, &entry->e_flags) && |
171 | atomic_inc_not_zero(v: &entry->e_refcnt)) |
172 | goto out; |
173 | node = node->next; |
174 | } |
175 | entry = NULL; |
176 | out: |
177 | hlist_bl_unlock(b: head); |
178 | if (old_entry) |
179 | mb_cache_entry_put(cache, entry: old_entry); |
180 | |
181 | return entry; |
182 | } |
183 | |
184 | /* |
185 | * mb_cache_entry_find_first - find the first reusable entry with the given key |
186 | * @cache: cache where we should search |
187 | * @key: key to look for |
188 | * |
189 | * Search in @cache for a reusable entry with key @key. Grabs reference to the |
190 | * first reusable entry found and returns the entry. |
191 | */ |
192 | struct mb_cache_entry *mb_cache_entry_find_first(struct mb_cache *cache, |
193 | u32 key) |
194 | { |
195 | return __entry_find(cache, NULL, key); |
196 | } |
197 | EXPORT_SYMBOL(mb_cache_entry_find_first); |
198 | |
199 | /* |
200 | * mb_cache_entry_find_next - find next reusable entry with the same key |
201 | * @cache: cache where we should search |
202 | * @entry: entry to start search from |
203 | * |
204 | * Finds next reusable entry in the hash chain which has the same key as @entry. |
205 | * If @entry is unhashed (which can happen when deletion of entry races with the |
206 | * search), finds the first reusable entry in the hash chain. The function drops |
207 | * reference to @entry and returns with a reference to the found entry. |
208 | */ |
209 | struct mb_cache_entry *mb_cache_entry_find_next(struct mb_cache *cache, |
210 | struct mb_cache_entry *entry) |
211 | { |
212 | return __entry_find(cache, entry, key: entry->e_key); |
213 | } |
214 | EXPORT_SYMBOL(mb_cache_entry_find_next); |
215 | |
216 | /* |
217 | * mb_cache_entry_get - get a cache entry by value (and key) |
218 | * @cache - cache we work with |
219 | * @key - key |
220 | * @value - value |
221 | */ |
222 | struct mb_cache_entry *mb_cache_entry_get(struct mb_cache *cache, u32 key, |
223 | u64 value) |
224 | { |
225 | struct hlist_bl_node *node; |
226 | struct hlist_bl_head *head; |
227 | struct mb_cache_entry *entry; |
228 | |
229 | head = mb_cache_entry_head(cache, key); |
230 | hlist_bl_lock(b: head); |
231 | hlist_bl_for_each_entry(entry, node, head, e_hash_list) { |
232 | if (entry->e_key == key && entry->e_value == value && |
233 | atomic_inc_not_zero(v: &entry->e_refcnt)) |
234 | goto out; |
235 | } |
236 | entry = NULL; |
237 | out: |
238 | hlist_bl_unlock(b: head); |
239 | return entry; |
240 | } |
241 | EXPORT_SYMBOL(mb_cache_entry_get); |
242 | |
243 | /* mb_cache_entry_delete_or_get - remove a cache entry if it has no users |
244 | * @cache - cache we work with |
245 | * @key - key |
246 | * @value - value |
247 | * |
248 | * Remove entry from cache @cache with key @key and value @value. The removal |
249 | * happens only if the entry is unused. The function returns NULL in case the |
250 | * entry was successfully removed or there's no entry in cache. Otherwise the |
251 | * function grabs reference of the entry that we failed to delete because it |
252 | * still has users and return it. |
253 | */ |
254 | struct mb_cache_entry *mb_cache_entry_delete_or_get(struct mb_cache *cache, |
255 | u32 key, u64 value) |
256 | { |
257 | struct mb_cache_entry *entry; |
258 | |
259 | entry = mb_cache_entry_get(cache, key, value); |
260 | if (!entry) |
261 | return NULL; |
262 | |
263 | /* |
264 | * Drop the ref we got from mb_cache_entry_get() and the initial hash |
265 | * ref if we are the last user |
266 | */ |
267 | if (atomic_cmpxchg(v: &entry->e_refcnt, old: 2, new: 0) != 2) |
268 | return entry; |
269 | |
270 | spin_lock(lock: &cache->c_list_lock); |
271 | if (!list_empty(head: &entry->e_list)) |
272 | list_del_init(entry: &entry->e_list); |
273 | cache->c_entry_count--; |
274 | spin_unlock(lock: &cache->c_list_lock); |
275 | __mb_cache_entry_free(cache, entry); |
276 | return NULL; |
277 | } |
278 | EXPORT_SYMBOL(mb_cache_entry_delete_or_get); |
279 | |
280 | /* mb_cache_entry_touch - cache entry got used |
281 | * @cache - cache the entry belongs to |
282 | * @entry - entry that got used |
283 | * |
284 | * Marks entry as used to give hit higher chances of surviving in cache. |
285 | */ |
286 | void mb_cache_entry_touch(struct mb_cache *cache, |
287 | struct mb_cache_entry *entry) |
288 | { |
289 | set_bit(nr: MBE_REFERENCED_B, addr: &entry->e_flags); |
290 | } |
291 | EXPORT_SYMBOL(mb_cache_entry_touch); |
292 | |
293 | static unsigned long mb_cache_count(struct shrinker *shrink, |
294 | struct shrink_control *sc) |
295 | { |
296 | struct mb_cache *cache = shrink->private_data; |
297 | |
298 | return cache->c_entry_count; |
299 | } |
300 | |
301 | /* Shrink number of entries in cache */ |
302 | static unsigned long mb_cache_shrink(struct mb_cache *cache, |
303 | unsigned long nr_to_scan) |
304 | { |
305 | struct mb_cache_entry *entry; |
306 | unsigned long shrunk = 0; |
307 | |
308 | spin_lock(lock: &cache->c_list_lock); |
309 | while (nr_to_scan-- && !list_empty(head: &cache->c_list)) { |
310 | entry = list_first_entry(&cache->c_list, |
311 | struct mb_cache_entry, e_list); |
312 | /* Drop initial hash reference if there is no user */ |
313 | if (test_bit(MBE_REFERENCED_B, &entry->e_flags) || |
314 | atomic_cmpxchg(v: &entry->e_refcnt, old: 1, new: 0) != 1) { |
315 | clear_bit(nr: MBE_REFERENCED_B, addr: &entry->e_flags); |
316 | list_move_tail(list: &entry->e_list, head: &cache->c_list); |
317 | continue; |
318 | } |
319 | list_del_init(entry: &entry->e_list); |
320 | cache->c_entry_count--; |
321 | spin_unlock(lock: &cache->c_list_lock); |
322 | __mb_cache_entry_free(cache, entry); |
323 | shrunk++; |
324 | cond_resched(); |
325 | spin_lock(lock: &cache->c_list_lock); |
326 | } |
327 | spin_unlock(lock: &cache->c_list_lock); |
328 | |
329 | return shrunk; |
330 | } |
331 | |
332 | static unsigned long mb_cache_scan(struct shrinker *shrink, |
333 | struct shrink_control *sc) |
334 | { |
335 | struct mb_cache *cache = shrink->private_data; |
336 | return mb_cache_shrink(cache, nr_to_scan: sc->nr_to_scan); |
337 | } |
338 | |
339 | /* We shrink 1/X of the cache when we have too many entries in it */ |
340 | #define SHRINK_DIVISOR 16 |
341 | |
342 | static void mb_cache_shrink_worker(struct work_struct *work) |
343 | { |
344 | struct mb_cache *cache = container_of(work, struct mb_cache, |
345 | c_shrink_work); |
346 | mb_cache_shrink(cache, nr_to_scan: cache->c_max_entries / SHRINK_DIVISOR); |
347 | } |
348 | |
349 | /* |
350 | * mb_cache_create - create cache |
351 | * @bucket_bits: log2 of the hash table size |
352 | * |
353 | * Create cache for keys with 2^bucket_bits hash entries. |
354 | */ |
355 | struct mb_cache *mb_cache_create(int bucket_bits) |
356 | { |
357 | struct mb_cache *cache; |
358 | unsigned long bucket_count = 1UL << bucket_bits; |
359 | unsigned long i; |
360 | |
361 | cache = kzalloc(size: sizeof(struct mb_cache), GFP_KERNEL); |
362 | if (!cache) |
363 | goto err_out; |
364 | cache->c_bucket_bits = bucket_bits; |
365 | cache->c_max_entries = bucket_count << 4; |
366 | INIT_LIST_HEAD(list: &cache->c_list); |
367 | spin_lock_init(&cache->c_list_lock); |
368 | cache->c_hash = kmalloc_array(n: bucket_count, |
369 | size: sizeof(struct hlist_bl_head), |
370 | GFP_KERNEL); |
371 | if (!cache->c_hash) { |
372 | kfree(objp: cache); |
373 | goto err_out; |
374 | } |
375 | for (i = 0; i < bucket_count; i++) |
376 | INIT_HLIST_BL_HEAD(&cache->c_hash[i]); |
377 | |
378 | cache->c_shrink = shrinker_alloc(flags: 0, fmt: "mbcache-shrinker" ); |
379 | if (!cache->c_shrink) { |
380 | kfree(objp: cache->c_hash); |
381 | kfree(objp: cache); |
382 | goto err_out; |
383 | } |
384 | |
385 | cache->c_shrink->count_objects = mb_cache_count; |
386 | cache->c_shrink->scan_objects = mb_cache_scan; |
387 | cache->c_shrink->private_data = cache; |
388 | |
389 | shrinker_register(shrinker: cache->c_shrink); |
390 | |
391 | INIT_WORK(&cache->c_shrink_work, mb_cache_shrink_worker); |
392 | |
393 | return cache; |
394 | |
395 | err_out: |
396 | return NULL; |
397 | } |
398 | EXPORT_SYMBOL(mb_cache_create); |
399 | |
400 | /* |
401 | * mb_cache_destroy - destroy cache |
402 | * @cache: the cache to destroy |
403 | * |
404 | * Free all entries in cache and cache itself. Caller must make sure nobody |
405 | * (except shrinker) can reach @cache when calling this. |
406 | */ |
407 | void mb_cache_destroy(struct mb_cache *cache) |
408 | { |
409 | struct mb_cache_entry *entry, *next; |
410 | |
411 | shrinker_free(shrinker: cache->c_shrink); |
412 | |
413 | /* |
414 | * We don't bother with any locking. Cache must not be used at this |
415 | * point. |
416 | */ |
417 | list_for_each_entry_safe(entry, next, &cache->c_list, e_list) { |
418 | list_del(entry: &entry->e_list); |
419 | WARN_ON(atomic_read(&entry->e_refcnt) != 1); |
420 | mb_cache_entry_put(cache, entry); |
421 | } |
422 | kfree(objp: cache->c_hash); |
423 | kfree(objp: cache); |
424 | } |
425 | EXPORT_SYMBOL(mb_cache_destroy); |
426 | |
427 | static int __init mbcache_init(void) |
428 | { |
429 | mb_entry_cache = kmem_cache_create(name: "mbcache" , |
430 | size: sizeof(struct mb_cache_entry), align: 0, |
431 | SLAB_RECLAIM_ACCOUNT|SLAB_MEM_SPREAD, NULL); |
432 | if (!mb_entry_cache) |
433 | return -ENOMEM; |
434 | return 0; |
435 | } |
436 | |
437 | static void __exit mbcache_exit(void) |
438 | { |
439 | kmem_cache_destroy(s: mb_entry_cache); |
440 | } |
441 | |
442 | module_init(mbcache_init) |
443 | module_exit(mbcache_exit) |
444 | |
445 | MODULE_AUTHOR("Jan Kara <jack@suse.cz>" ); |
446 | MODULE_DESCRIPTION("Meta block cache (for extended attributes)" ); |
447 | MODULE_LICENSE("GPL" ); |
448 | |