1 // SPDX-License-Identifier: GPL-2.0-or-later
2 /*
3 lru_cache.c
4
5 This file is part of DRBD by Philipp Reisner and Lars Ellenberg.
6
7 Copyright (C) 2003-2008, LINBIT Information Technologies GmbH.
8 Copyright (C) 2003-2008, Philipp Reisner <philipp.reisner@linbit.com>.
9 Copyright (C) 2003-2008, Lars Ellenberg <lars.ellenberg@linbit.com>.
10
11
12 */
13
14 #include <linux/module.h>
15 #include <linux/bitops.h>
16 #include <linux/slab.h>
17 #include <linux/string.h> /* for memset */
18 #include <linux/seq_file.h> /* for seq_printf */
19 #include <linux/lru_cache.h>
20
21 MODULE_AUTHOR("Philipp Reisner <phil@linbit.com>, "
22 "Lars Ellenberg <lars@linbit.com>");
23 MODULE_DESCRIPTION("lru_cache - Track sets of hot objects");
24 MODULE_LICENSE("GPL");
25
26 /* this is developers aid only.
27 * it catches concurrent access (lack of locking on the users part) */
28 #define PARANOIA_ENTRY() do { \
29 BUG_ON(!lc); \
30 BUG_ON(!lc->nr_elements); \
31 BUG_ON(test_and_set_bit(__LC_PARANOIA, &lc->flags)); \
32 } while (0)
33
34 #define RETURN(x...) do { \
35 clear_bit_unlock(__LC_PARANOIA, &lc->flags); \
36 return x ; } while (0)
37
38 /* BUG() if e is not one of the elements tracked by lc */
39 #define PARANOIA_LC_ELEMENT(lc, e) do { \
40 struct lru_cache *lc_ = (lc); \
41 struct lc_element *e_ = (e); \
42 unsigned i = e_->lc_index; \
43 BUG_ON(i >= lc_->nr_elements); \
44 BUG_ON(lc_->lc_element[i] != e_); } while (0)
45
46
47 /* We need to atomically
48 * - try to grab the lock (set LC_LOCKED)
49 * - only if there is no pending transaction
50 * (neither LC_DIRTY nor LC_STARVING is set)
51 * Because of PARANOIA_ENTRY() above abusing lc->flags as well,
52 * it is not sufficient to just say
53 * return 0 == cmpxchg(&lc->flags, 0, LC_LOCKED);
54 */
55 int lc_try_lock(struct lru_cache *lc)
56 {
57 unsigned long val;
58 do {
59 val = cmpxchg(&lc->flags, 0, LC_LOCKED);
60 } while (unlikely (val == LC_PARANOIA));
61 /* Spin until no-one is inside a PARANOIA_ENTRY()/RETURN() section. */
62 return 0 == val;
63 }
64
65 /**
66 * lc_create - prepares to track objects in an active set
67 * @name: descriptive name only used in lc_seq_printf_stats and lc_seq_dump_details
68 * @cache: cache root pointer
69 * @max_pending_changes: maximum changes to accumulate until a transaction is required
70 * @e_count: number of elements allowed to be active simultaneously
71 * @e_size: size of the tracked objects
72 * @e_off: offset to the &struct lc_element member in a tracked object
73 *
74 * Returns a pointer to a newly initialized struct lru_cache on success,
75 * or NULL on (allocation) failure.
76 */
77 struct lru_cache *lc_create(const char *name, struct kmem_cache *cache,
78 unsigned max_pending_changes,
79 unsigned e_count, size_t e_size, size_t e_off)
80 {
81 struct hlist_head *slot = NULL;
82 struct lc_element **element = NULL;
83 struct lru_cache *lc;
84 struct lc_element *e;
85 unsigned cache_obj_size = kmem_cache_size(cache);
86 unsigned i;
87
88 WARN_ON(cache_obj_size < e_size);
89 if (cache_obj_size < e_size)
90 return NULL;
91
92 /* e_count too big; would probably fail the allocation below anyways.
93 * for typical use cases, e_count should be few thousand at most. */
94 if (e_count > LC_MAX_ACTIVE)
95 return NULL;
96
97 slot = kcalloc(e_count, sizeof(struct hlist_head), GFP_KERNEL);
98 if (!slot)
99 goto out_fail;
100 element = kcalloc(e_count, sizeof(struct lc_element *), GFP_KERNEL);
101 if (!element)
102 goto out_fail;
103
104 lc = kzalloc(sizeof(*lc), GFP_KERNEL);
105 if (!lc)
106 goto out_fail;
107
108 INIT_LIST_HEAD(&lc->in_use);
109 INIT_LIST_HEAD(&lc->lru);
110 INIT_LIST_HEAD(&lc->free);
111 INIT_LIST_HEAD(&lc->to_be_changed);
112
113 lc->name = name;
114 lc->element_size = e_size;
115 lc->element_off = e_off;
116 lc->nr_elements = e_count;
117 lc->max_pending_changes = max_pending_changes;
118 lc->lc_cache = cache;
119 lc->lc_element = element;
120 lc->lc_slot = slot;
121
122 /* preallocate all objects */
123 for (i = 0; i < e_count; i++) {
124 void *p = kmem_cache_alloc(cache, GFP_KERNEL);
125 if (!p)
126 break;
127 memset(p, 0, lc->element_size);
128 e = p + e_off;
129 e->lc_index = i;
130 e->lc_number = LC_FREE;
131 e->lc_new_number = LC_FREE;
132 list_add(&e->list, &lc->free);
133 element[i] = e;
134 }
135 if (i == e_count)
136 return lc;
137
138 /* else: could not allocate all elements, give up */
139 while (i) {
140 void *p = element[--i];
141 kmem_cache_free(cache, p - e_off);
142 }
143 kfree(lc);
144 out_fail:
145 kfree(element);
146 kfree(slot);
147 return NULL;
148 }
149
150 static void lc_free_by_index(struct lru_cache *lc, unsigned i)
151 {
152 void *p = lc->lc_element[i];
153 WARN_ON(!p);
154 if (p) {
155 p -= lc->element_off;
156 kmem_cache_free(lc->lc_cache, p);
157 }
158 }
159
160 /**
161 * lc_destroy - frees memory allocated by lc_create()
162 * @lc: the lru cache to destroy
163 */
164 void lc_destroy(struct lru_cache *lc)
165 {
166 unsigned i;
167 if (!lc)
168 return;
169 for (i = 0; i < lc->nr_elements; i++)
170 lc_free_by_index(lc, i);
171 kfree(lc->lc_element);
172 kfree(lc->lc_slot);
173 kfree(lc);
174 }
175
176 /**
177 * lc_reset - does a full reset for @lc and the hash table slots.
178 * @lc: the lru cache to operate on
179 *
180 * It is roughly the equivalent of re-allocating a fresh lru_cache object,
181 * basically a short cut to lc_destroy(lc); lc = lc_create(...);
182 */
183 void lc_reset(struct lru_cache *lc)
184 {
185 unsigned i;
186
187 INIT_LIST_HEAD(&lc->in_use);
188 INIT_LIST_HEAD(&lc->lru);
189 INIT_LIST_HEAD(&lc->free);
190 INIT_LIST_HEAD(&lc->to_be_changed);
191 lc->used = 0;
192 lc->hits = 0;
193 lc->misses = 0;
194 lc->starving = 0;
195 lc->locked = 0;
196 lc->changed = 0;
197 lc->pending_changes = 0;
198 lc->flags = 0;
199 memset(lc->lc_slot, 0, sizeof(struct hlist_head) * lc->nr_elements);
200
201 for (i = 0; i < lc->nr_elements; i++) {
202 struct lc_element *e = lc->lc_element[i];
203 void *p = e;
204 p -= lc->element_off;
205 memset(p, 0, lc->element_size);
206 /* re-init it */
207 e->lc_index = i;
208 e->lc_number = LC_FREE;
209 e->lc_new_number = LC_FREE;
210 list_add(&e->list, &lc->free);
211 }
212 }
213
214 /**
215 * lc_seq_printf_stats - print stats about @lc into @seq
216 * @seq: the seq_file to print into
217 * @lc: the lru cache to print statistics of
218 */
219 void lc_seq_printf_stats(struct seq_file *seq, struct lru_cache *lc)
220 {
221 /* NOTE:
222 * total calls to lc_get are
223 * (starving + hits + misses)
224 * misses include "locked" count (update from an other thread in
225 * progress) and "changed", when this in fact lead to an successful
226 * update of the cache.
227 */
228 seq_printf(seq, "\t%s: used:%u/%u hits:%lu misses:%lu starving:%lu locked:%lu changed:%lu\n",
229 lc->name, lc->used, lc->nr_elements,
230 lc->hits, lc->misses, lc->starving, lc->locked, lc->changed);
231 }
232
233 static struct hlist_head *lc_hash_slot(struct lru_cache *lc, unsigned int enr)
234 {
235 return lc->lc_slot + (enr % lc->nr_elements);
236 }
237
238
239 static struct lc_element *__lc_find(struct lru_cache *lc, unsigned int enr,
240 bool include_changing)
241 {
242 struct lc_element *e;
243
244 BUG_ON(!lc);
245 BUG_ON(!lc->nr_elements);
246 hlist_for_each_entry(e, lc_hash_slot(lc, enr), colision) {
247 /* "about to be changed" elements, pending transaction commit,
248 * are hashed by their "new number". "Normal" elements have
249 * lc_number == lc_new_number. */
250 if (e->lc_new_number != enr)
251 continue;
252 if (e->lc_new_number == e->lc_number || include_changing)
253 return e;
254 break;
255 }
256 return NULL;
257 }
258
259 /**
260 * lc_find - find element by label, if present in the hash table
261 * @lc: The lru_cache object
262 * @enr: element number
263 *
264 * Returns the pointer to an element, if the element with the requested
265 * "label" or element number is present in the hash table,
266 * or NULL if not found. Does not change the refcnt.
267 * Ignores elements that are "about to be used", i.e. not yet in the active
268 * set, but still pending transaction commit.
269 */
270 struct lc_element *lc_find(struct lru_cache *lc, unsigned int enr)
271 {
272 return __lc_find(lc, enr, 0);
273 }
274
275 /**
276 * lc_is_used - find element by label
277 * @lc: The lru_cache object
278 * @enr: element number
279 *
280 * Returns true, if the element with the requested "label" or element number is
281 * present in the hash table, and is used (refcnt > 0).
282 * Also finds elements that are not _currently_ used but only "about to be
283 * used", i.e. on the "to_be_changed" list, pending transaction commit.
284 */
285 bool lc_is_used(struct lru_cache *lc, unsigned int enr)
286 {
287 struct lc_element *e = __lc_find(lc, enr, 1);
288 return e && e->refcnt;
289 }
290
291 /**
292 * lc_del - removes an element from the cache
293 * @lc: The lru_cache object
294 * @e: The element to remove
295 *
296 * @e must be unused (refcnt == 0). Moves @e from "lru" to "free" list,
297 * sets @e->enr to %LC_FREE.
298 */
299 void lc_del(struct lru_cache *lc, struct lc_element *e)
300 {
301 PARANOIA_ENTRY();
302 PARANOIA_LC_ELEMENT(lc, e);
303 BUG_ON(e->refcnt);
304
305 e->lc_number = e->lc_new_number = LC_FREE;
306 hlist_del_init(&e->colision);
307 list_move(&e->list, &lc->free);
308 RETURN();
309 }
310
311 static struct lc_element *lc_prepare_for_change(struct lru_cache *lc, unsigned new_number)
312 {
313 struct list_head *n;
314 struct lc_element *e;
315
316 if (!list_empty(&lc->free))
317 n = lc->free.next;
318 else if (!list_empty(&lc->lru))
319 n = lc->lru.prev;
320 else
321 return NULL;
322
323 e = list_entry(n, struct lc_element, list);
324 PARANOIA_LC_ELEMENT(lc, e);
325
326 e->lc_new_number = new_number;
327 if (!hlist_unhashed(&e->colision))
328 __hlist_del(&e->colision);
329 hlist_add_head(&e->colision, lc_hash_slot(lc, new_number));
330 list_move(&e->list, &lc->to_be_changed);
331
332 return e;
333 }
334
335 static int lc_unused_element_available(struct lru_cache *lc)
336 {
337 if (!list_empty(&lc->free))
338 return 1; /* something on the free list */
339 if (!list_empty(&lc->lru))
340 return 1; /* something to evict */
341
342 return 0;
343 }
344
345 /* used as internal flags to __lc_get */
346 enum {
347 LC_GET_MAY_CHANGE = 1,
348 LC_GET_MAY_USE_UNCOMMITTED = 2,
349 };
350
351 static struct lc_element *__lc_get(struct lru_cache *lc, unsigned int enr, unsigned int flags)
352 {
353 struct lc_element *e;
354
355 PARANOIA_ENTRY();
356 if (test_bit(__LC_STARVING, &lc->flags)) {
357 ++lc->starving;
358 RETURN(NULL);
359 }
360
361 e = __lc_find(lc, enr, 1);
362 /* if lc_new_number != lc_number,
363 * this enr is currently being pulled in already,
364 * and will be available once the pending transaction
365 * has been committed. */
366 if (e) {
367 if (e->lc_new_number != e->lc_number) {
368 /* It has been found above, but on the "to_be_changed"
369 * list, not yet committed. Don't pull it in twice,
370 * wait for the transaction, then try again...
371 */
372 if (!(flags & LC_GET_MAY_USE_UNCOMMITTED))
373 RETURN(NULL);
374 /* ... unless the caller is aware of the implications,
375 * probably preparing a cumulative transaction. */
376 ++e->refcnt;
377 ++lc->hits;
378 RETURN(e);
379 }
380 /* else: lc_new_number == lc_number; a real hit. */
381 ++lc->hits;
382 if (e->refcnt++ == 0)
383 lc->used++;
384 list_move(&e->list, &lc->in_use); /* Not evictable... */
385 RETURN(e);
386 }
387 /* e == NULL */
388
389 ++lc->misses;
390 if (!(flags & LC_GET_MAY_CHANGE))
391 RETURN(NULL);
392
393 /* To avoid races with lc_try_lock(), first, mark us dirty
394 * (using test_and_set_bit, as it implies memory barriers), ... */
395 test_and_set_bit(__LC_DIRTY, &lc->flags);
396
397 /* ... only then check if it is locked anyways. If lc_unlock clears
398 * the dirty bit again, that's not a problem, we will come here again.
399 */
400 if (test_bit(__LC_LOCKED, &lc->flags)) {
401 ++lc->locked;
402 RETURN(NULL);
403 }
404
405 /* In case there is nothing available and we can not kick out
406 * the LRU element, we have to wait ...
407 */
408 if (!lc_unused_element_available(lc)) {
409 set_bit(__LC_STARVING, &lc->flags);
410 RETURN(NULL);
411 }
412
413 /* It was not present in the active set. We are going to recycle an
414 * unused (or even "free") element, but we won't accumulate more than
415 * max_pending_changes changes. */
416 if (lc->pending_changes >= lc->max_pending_changes)
417 RETURN(NULL);
418
419 e = lc_prepare_for_change(lc, enr);
420 BUG_ON(!e);
421
422 clear_bit(__LC_STARVING, &lc->flags);
423 BUG_ON(++e->refcnt != 1);
424 lc->used++;
425 lc->pending_changes++;
426
427 RETURN(e);
428 }
429
430 /**
431 * lc_get - get element by label, maybe change the active set
432 * @lc: the lru cache to operate on
433 * @enr: the label to look up
434 *
435 * Finds an element in the cache, increases its usage count,
436 * "touches" and returns it.
437 *
438 * In case the requested number is not present, it needs to be added to the
439 * cache. Therefore it is possible that an other element becomes evicted from
440 * the cache. In either case, the user is notified so he is able to e.g. keep
441 * a persistent log of the cache changes, and therefore the objects in use.
442 *
443 * Return values:
444 * NULL
445 * The cache was marked %LC_STARVING,
446 * or the requested label was not in the active set
447 * and a changing transaction is still pending (@lc was marked %LC_DIRTY).
448 * Or no unused or free element could be recycled (@lc will be marked as
449 * %LC_STARVING, blocking further lc_get() operations).
450 *
451 * pointer to the element with the REQUESTED element number.
452 * In this case, it can be used right away
453 *
454 * pointer to an UNUSED element with some different element number,
455 * where that different number may also be %LC_FREE.
456 *
457 * In this case, the cache is marked %LC_DIRTY,
458 * so lc_try_lock() will no longer succeed.
459 * The returned element pointer is moved to the "to_be_changed" list,
460 * and registered with the new element number on the hash collision chains,
461 * so it is possible to pick it up from lc_is_used().
462 * Up to "max_pending_changes" (see lc_create()) can be accumulated.
463 * The user now should do whatever housekeeping is necessary,
464 * typically serialize on lc_try_lock_for_transaction(), then call
465 * lc_committed(lc) and lc_unlock(), to finish the change.
466 *
467 * NOTE: The user needs to check the lc_number on EACH use, so he recognizes
468 * any cache set change.
469 */
470 struct lc_element *lc_get(struct lru_cache *lc, unsigned int enr)
471 {
472 return __lc_get(lc, enr, LC_GET_MAY_CHANGE);
473 }
474
475 /**
476 * lc_get_cumulative - like lc_get; also finds to-be-changed elements
477 * @lc: the lru cache to operate on
478 * @enr: the label to look up
479 *
480 * Unlike lc_get this also returns the element for @enr, if it is belonging to
481 * a pending transaction, so the return values are like for lc_get(),
482 * plus:
483 *
484 * pointer to an element already on the "to_be_changed" list.
485 * In this case, the cache was already marked %LC_DIRTY.
486 *
487 * Caller needs to make sure that the pending transaction is completed,
488 * before proceeding to actually use this element.
489 */
490 struct lc_element *lc_get_cumulative(struct lru_cache *lc, unsigned int enr)
491 {
492 return __lc_get(lc, enr, LC_GET_MAY_CHANGE|LC_GET_MAY_USE_UNCOMMITTED);
493 }
494
495 /**
496 * lc_try_get - get element by label, if present; do not change the active set
497 * @lc: the lru cache to operate on
498 * @enr: the label to look up
499 *
500 * Finds an element in the cache, increases its usage count,
501 * "touches" and returns it.
502 *
503 * Return values:
504 * NULL
505 * The cache was marked %LC_STARVING,
506 * or the requested label was not in the active set
507 *
508 * pointer to the element with the REQUESTED element number.
509 * In this case, it can be used right away
510 */
511 struct lc_element *lc_try_get(struct lru_cache *lc, unsigned int enr)
512 {
513 return __lc_get(lc, enr, 0);
514 }
515
516 /**
517 * lc_committed - tell @lc that pending changes have been recorded
518 * @lc: the lru cache to operate on
519 *
520 * User is expected to serialize on explicit lc_try_lock_for_transaction()
521 * before the transaction is started, and later needs to lc_unlock() explicitly
522 * as well.
523 */
524 void lc_committed(struct lru_cache *lc)
525 {
526 struct lc_element *e, *tmp;
527
528 PARANOIA_ENTRY();
529 list_for_each_entry_safe(e, tmp, &lc->to_be_changed, list) {
530 /* count number of changes, not number of transactions */
531 ++lc->changed;
532 e->lc_number = e->lc_new_number;
533 list_move(&e->list, &lc->in_use);
534 }
535 lc->pending_changes = 0;
536 RETURN();
537 }
538
539
540 /**
541 * lc_put - give up refcnt of @e
542 * @lc: the lru cache to operate on
543 * @e: the element to put
544 *
545 * If refcnt reaches zero, the element is moved to the lru list,
546 * and a %LC_STARVING (if set) is cleared.
547 * Returns the new (post-decrement) refcnt.
548 */
549 unsigned int lc_put(struct lru_cache *lc, struct lc_element *e)
550 {
551 PARANOIA_ENTRY();
552 PARANOIA_LC_ELEMENT(lc, e);
553 BUG_ON(e->refcnt == 0);
554 BUG_ON(e->lc_number != e->lc_new_number);
555 if (--e->refcnt == 0) {
556 /* move it to the front of LRU. */
557 list_move(&e->list, &lc->lru);
558 lc->used--;
559 clear_bit_unlock(__LC_STARVING, &lc->flags);
560 }
561 RETURN(e->refcnt);
562 }
563
564 /**
565 * lc_element_by_index
566 * @lc: the lru cache to operate on
567 * @i: the index of the element to return
568 */
569 struct lc_element *lc_element_by_index(struct lru_cache *lc, unsigned i)
570 {
571 BUG_ON(i >= lc->nr_elements);
572 BUG_ON(lc->lc_element[i] == NULL);
573 BUG_ON(lc->lc_element[i]->lc_index != i);
574 return lc->lc_element[i];
575 }
576
577 /**
578 * lc_seq_dump_details - Dump a complete LRU cache to seq in textual form.
579 * @lc: the lru cache to operate on
580 * @seq: the &struct seq_file pointer to seq_printf into
581 * @utext: user supplied additional "heading" or other info
582 * @detail: function pointer the user may provide to dump further details
583 * of the object the lc_element is embedded in. May be NULL.
584 * Note: a leading space ' ' and trailing newline '\n' is implied.
585 */
586 void lc_seq_dump_details(struct seq_file *seq, struct lru_cache *lc, char *utext,
587 void (*detail) (struct seq_file *, struct lc_element *))
588 {
589 unsigned int nr_elements = lc->nr_elements;
590 struct lc_element *e;
591 int i;
592
593 seq_printf(seq, "\tnn: lc_number (new nr) refcnt %s\n ", utext);
594 for (i = 0; i < nr_elements; i++) {
595 e = lc_element_by_index(lc, i);
596 if (e->lc_number != e->lc_new_number)
597 seq_printf(seq, "\t%5d: %6d %8d %6d ",
598 i, e->lc_number, e->lc_new_number, e->refcnt);
599 else
600 seq_printf(seq, "\t%5d: %6d %-8s %6d ",
601 i, e->lc_number, "-\"-", e->refcnt);
602 if (detail)
603 detail(seq, e);
604 seq_putc(seq, '\n');
605 }
606 }
607
608 EXPORT_SYMBOL(lc_create);
609 EXPORT_SYMBOL(lc_reset);
610 EXPORT_SYMBOL(lc_destroy);
611 EXPORT_SYMBOL(lc_del);
612 EXPORT_SYMBOL(lc_try_get);
613 EXPORT_SYMBOL(lc_find);
614 EXPORT_SYMBOL(lc_get);
615 EXPORT_SYMBOL(lc_put);
616 EXPORT_SYMBOL(lc_committed);
617 EXPORT_SYMBOL(lc_element_by_index);
618 EXPORT_SYMBOL(lc_seq_printf_stats);
619 EXPORT_SYMBOL(lc_seq_dump_details);
620 EXPORT_SYMBOL(lc_try_lock);
621 EXPORT_SYMBOL(lc_is_used);
622 EXPORT_SYMBOL(lc_get_cumulative);
623
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