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
21MODULE_AUTHOR("Philipp Reisner <phil@linbit.com>, "
22 "Lars Ellenberg <lars@linbit.com>");
23MODULE_DESCRIPTION("lru_cache - Track sets of hot objects");
24MODULE_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 */
55int 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#if 0
64 /* Alternative approach, spin in case someone enters or leaves a
65 * PARANOIA_ENTRY()/RETURN() section. */
66 unsigned long old, new, val;
67 do {
68 old = lc->flags & LC_PARANOIA;
69 new = old | LC_LOCKED;
70 val = cmpxchg(&lc->flags, old, new);
71 } while (unlikely (val == (old ^ LC_PARANOIA)));
72 return old == val;
73#endif
74}
75
76/**
77 * lc_create - prepares to track objects in an active set
78 * @name: descriptive name only used in lc_seq_printf_stats and lc_seq_dump_details
79 * @cache: cache root pointer
80 * @max_pending_changes: maximum changes to accumulate until a transaction is required
81 * @e_count: number of elements allowed to be active simultaneously
82 * @e_size: size of the tracked objects
83 * @e_off: offset to the &struct lc_element member in a tracked object
84 *
85 * Returns a pointer to a newly initialized struct lru_cache on success,
86 * or NULL on (allocation) failure.
87 */
88struct lru_cache *lc_create(const char *name, struct kmem_cache *cache,
89 unsigned max_pending_changes,
90 unsigned e_count, size_t e_size, size_t e_off)
91{
92 struct hlist_head *slot = NULL;
93 struct lc_element **element = NULL;
94 struct lru_cache *lc;
95 struct lc_element *e;
96 unsigned cache_obj_size = kmem_cache_size(cache);
97 unsigned i;
98
99 WARN_ON(cache_obj_size < e_size);
100 if (cache_obj_size < e_size)
101 return NULL;
102
103 /* e_count too big; would probably fail the allocation below anyways.
104 * for typical use cases, e_count should be few thousand at most. */
105 if (e_count > LC_MAX_ACTIVE)
106 return NULL;
107
108 slot = kcalloc(e_count, sizeof(struct hlist_head), GFP_KERNEL);
109 if (!slot)
110 goto out_fail;
111 element = kcalloc(e_count, sizeof(struct lc_element *), GFP_KERNEL);
112 if (!element)
113 goto out_fail;
114
115 lc = kzalloc(sizeof(*lc), GFP_KERNEL);
116 if (!lc)
117 goto out_fail;
118
119 INIT_LIST_HEAD(&lc->in_use);
120 INIT_LIST_HEAD(&lc->lru);
121 INIT_LIST_HEAD(&lc->free);
122 INIT_LIST_HEAD(&lc->to_be_changed);
123
124 lc->name = name;
125 lc->element_size = e_size;
126 lc->element_off = e_off;
127 lc->nr_elements = e_count;
128 lc->max_pending_changes = max_pending_changes;
129 lc->lc_cache = cache;
130 lc->lc_element = element;
131 lc->lc_slot = slot;
132
133 /* preallocate all objects */
134 for (i = 0; i < e_count; i++) {
135 void *p = kmem_cache_alloc(cache, GFP_KERNEL);
136 if (!p)
137 break;
138 memset(p, 0, lc->element_size);
139 e = p + e_off;
140 e->lc_index = i;
141 e->lc_number = LC_FREE;
142 e->lc_new_number = LC_FREE;
143 list_add(&e->list, &lc->free);
144 element[i] = e;
145 }
146 if (i == e_count)
147 return lc;
148
149 /* else: could not allocate all elements, give up */
150 while (i) {
151 void *p = element[--i];
152 kmem_cache_free(cache, p - e_off);
153 }
154 kfree(lc);
155out_fail:
156 kfree(element);
157 kfree(slot);
158 return NULL;
159}
160
161static void lc_free_by_index(struct lru_cache *lc, unsigned i)
162{
163 void *p = lc->lc_element[i];
164 WARN_ON(!p);
165 if (p) {
166 p -= lc->element_off;
167 kmem_cache_free(lc->lc_cache, p);
168 }
169}
170
171/**
172 * lc_destroy - frees memory allocated by lc_create()
173 * @lc: the lru cache to destroy
174 */
175void lc_destroy(struct lru_cache *lc)
176{
177 unsigned i;
178 if (!lc)
179 return;
180 for (i = 0; i < lc->nr_elements; i++)
181 lc_free_by_index(lc, i);
182 kfree(lc->lc_element);
183 kfree(lc->lc_slot);
184 kfree(lc);
185}
186
187/**
188 * lc_reset - does a full reset for @lc and the hash table slots.
189 * @lc: the lru cache to operate on
190 *
191 * It is roughly the equivalent of re-allocating a fresh lru_cache object,
192 * basically a short cut to lc_destroy(lc); lc = lc_create(...);
193 */
194void lc_reset(struct lru_cache *lc)
195{
196 unsigned i;
197
198 INIT_LIST_HEAD(&lc->in_use);
199 INIT_LIST_HEAD(&lc->lru);
200 INIT_LIST_HEAD(&lc->free);
201 INIT_LIST_HEAD(&lc->to_be_changed);
202 lc->used = 0;
203 lc->hits = 0;
204 lc->misses = 0;
205 lc->starving = 0;
206 lc->locked = 0;
207 lc->changed = 0;
208 lc->pending_changes = 0;
209 lc->flags = 0;
210 memset(lc->lc_slot, 0, sizeof(struct hlist_head) * lc->nr_elements);
211
212 for (i = 0; i < lc->nr_elements; i++) {
213 struct lc_element *e = lc->lc_element[i];
214 void *p = e;
215 p -= lc->element_off;
216 memset(p, 0, lc->element_size);
217 /* re-init it */
218 e->lc_index = i;
219 e->lc_number = LC_FREE;
220 e->lc_new_number = LC_FREE;
221 list_add(&e->list, &lc->free);
222 }
223}
224
225/**
226 * lc_seq_printf_stats - print stats about @lc into @seq
227 * @seq: the seq_file to print into
228 * @lc: the lru cache to print statistics of
229 */
230void lc_seq_printf_stats(struct seq_file *seq, struct lru_cache *lc)
231{
232 /* NOTE:
233 * total calls to lc_get are
234 * (starving + hits + misses)
235 * misses include "locked" count (update from an other thread in
236 * progress) and "changed", when this in fact lead to an successful
237 * update of the cache.
238 */
239 seq_printf(seq, "\t%s: used:%u/%u hits:%lu misses:%lu starving:%lu locked:%lu changed:%lu\n",
240 lc->name, lc->used, lc->nr_elements,
241 lc->hits, lc->misses, lc->starving, lc->locked, lc->changed);
242}
243
244static struct hlist_head *lc_hash_slot(struct lru_cache *lc, unsigned int enr)
245{
246 return lc->lc_slot + (enr % lc->nr_elements);
247}
248
249
250static struct lc_element *__lc_find(struct lru_cache *lc, unsigned int enr,
251 bool include_changing)
252{
253 struct lc_element *e;
254
255 BUG_ON(!lc);
256 BUG_ON(!lc->nr_elements);
257 hlist_for_each_entry(e, lc_hash_slot(lc, enr), colision) {
258 /* "about to be changed" elements, pending transaction commit,
259 * are hashed by their "new number". "Normal" elements have
260 * lc_number == lc_new_number. */
261 if (e->lc_new_number != enr)
262 continue;
263 if (e->lc_new_number == e->lc_number || include_changing)
264 return e;
265 break;
266 }
267 return NULL;
268}
269
270/**
271 * lc_find - find element by label, if present in the hash table
272 * @lc: The lru_cache object
273 * @enr: element number
274 *
275 * Returns the pointer to an element, if the element with the requested
276 * "label" or element number is present in the hash table,
277 * or NULL if not found. Does not change the refcnt.
278 * Ignores elements that are "about to be used", i.e. not yet in the active
279 * set, but still pending transaction commit.
280 */
281struct lc_element *lc_find(struct lru_cache *lc, unsigned int enr)
282{
283 return __lc_find(lc, enr, 0);
284}
285
286/**
287 * lc_is_used - find element by label
288 * @lc: The lru_cache object
289 * @enr: element number
290 *
291 * Returns true, if the element with the requested "label" or element number is
292 * present in the hash table, and is used (refcnt > 0).
293 * Also finds elements that are not _currently_ used but only "about to be
294 * used", i.e. on the "to_be_changed" list, pending transaction commit.
295 */
296bool lc_is_used(struct lru_cache *lc, unsigned int enr)
297{
298 struct lc_element *e = __lc_find(lc, enr, 1);
299 return e && e->refcnt;
300}
301
302/**
303 * lc_del - removes an element from the cache
304 * @lc: The lru_cache object
305 * @e: The element to remove
306 *
307 * @e must be unused (refcnt == 0). Moves @e from "lru" to "free" list,
308 * sets @e->enr to %LC_FREE.
309 */
310void lc_del(struct lru_cache *lc, struct lc_element *e)
311{
312 PARANOIA_ENTRY();
313 PARANOIA_LC_ELEMENT(lc, e);
314 BUG_ON(e->refcnt);
315
316 e->lc_number = e->lc_new_number = LC_FREE;
317 hlist_del_init(&e->colision);
318 list_move(&e->list, &lc->free);
319 RETURN();
320}
321
322static struct lc_element *lc_prepare_for_change(struct lru_cache *lc, unsigned new_number)
323{
324 struct list_head *n;
325 struct lc_element *e;
326
327 if (!list_empty(&lc->free))
328 n = lc->free.next;
329 else if (!list_empty(&lc->lru))
330 n = lc->lru.prev;
331 else
332 return NULL;
333
334 e = list_entry(n, struct lc_element, list);
335 PARANOIA_LC_ELEMENT(lc, e);
336
337 e->lc_new_number = new_number;
338 if (!hlist_unhashed(&e->colision))
339 __hlist_del(&e->colision);
340 hlist_add_head(&e->colision, lc_hash_slot(lc, new_number));
341 list_move(&e->list, &lc->to_be_changed);
342
343 return e;
344}
345
346static int lc_unused_element_available(struct lru_cache *lc)
347{
348 if (!list_empty(&lc->free))
349 return 1; /* something on the free list */
350 if (!list_empty(&lc->lru))
351 return 1; /* something to evict */
352
353 return 0;
354}
355
356/* used as internal flags to __lc_get */
357enum {
358 LC_GET_MAY_CHANGE = 1,
359 LC_GET_MAY_USE_UNCOMMITTED = 2,
360};
361
362static struct lc_element *__lc_get(struct lru_cache *lc, unsigned int enr, unsigned int flags)
363{
364 struct lc_element *e;
365
366 PARANOIA_ENTRY();
367 if (lc->flags & LC_STARVING) {
368 ++lc->starving;
369 RETURN(NULL);
370 }
371
372 e = __lc_find(lc, enr, 1);
373 /* if lc_new_number != lc_number,
374 * this enr is currently being pulled in already,
375 * and will be available once the pending transaction
376 * has been committed. */
377 if (e) {
378 if (e->lc_new_number != e->lc_number) {
379 /* It has been found above, but on the "to_be_changed"
380 * list, not yet committed. Don't pull it in twice,
381 * wait for the transaction, then try again...
382 */
383 if (!(flags & LC_GET_MAY_USE_UNCOMMITTED))
384 RETURN(NULL);
385 /* ... unless the caller is aware of the implications,
386 * probably preparing a cumulative transaction. */
387 ++e->refcnt;
388 ++lc->hits;
389 RETURN(e);
390 }
391 /* else: lc_new_number == lc_number; a real hit. */
392 ++lc->hits;
393 if (e->refcnt++ == 0)
394 lc->used++;
395 list_move(&e->list, &lc->in_use); /* Not evictable... */
396 RETURN(e);
397 }
398 /* e == NULL */
399
400 ++lc->misses;
401 if (!(flags & LC_GET_MAY_CHANGE))
402 RETURN(NULL);
403
404 /* To avoid races with lc_try_lock(), first, mark us dirty
405 * (using test_and_set_bit, as it implies memory barriers), ... */
406 test_and_set_bit(__LC_DIRTY, &lc->flags);
407
408 /* ... only then check if it is locked anyways. If lc_unlock clears
409 * the dirty bit again, that's not a problem, we will come here again.
410 */
411 if (test_bit(__LC_LOCKED, &lc->flags)) {
412 ++lc->locked;
413 RETURN(NULL);
414 }
415
416 /* In case there is nothing available and we can not kick out
417 * the LRU element, we have to wait ...
418 */
419 if (!lc_unused_element_available(lc)) {
420 __set_bit(__LC_STARVING, &lc->flags);
421 RETURN(NULL);
422 }
423
424 /* It was not present in the active set. We are going to recycle an
425 * unused (or even "free") element, but we won't accumulate more than
426 * max_pending_changes changes. */
427 if (lc->pending_changes >= lc->max_pending_changes)
428 RETURN(NULL);
429
430 e = lc_prepare_for_change(lc, enr);
431 BUG_ON(!e);
432
433 clear_bit(__LC_STARVING, &lc->flags);
434 BUG_ON(++e->refcnt != 1);
435 lc->used++;
436 lc->pending_changes++;
437
438 RETURN(e);
439}
440
441/**
442 * lc_get - get element by label, maybe change the active set
443 * @lc: the lru cache to operate on
444 * @enr: the label to look up
445 *
446 * Finds an element in the cache, increases its usage count,
447 * "touches" and returns it.
448 *
449 * In case the requested number is not present, it needs to be added to the
450 * cache. Therefore it is possible that an other element becomes evicted from
451 * the cache. In either case, the user is notified so he is able to e.g. keep
452 * a persistent log of the cache changes, and therefore the objects in use.
453 *
454 * Return values:
455 * NULL
456 * The cache was marked %LC_STARVING,
457 * or the requested label was not in the active set
458 * and a changing transaction is still pending (@lc was marked %LC_DIRTY).
459 * Or no unused or free element could be recycled (@lc will be marked as
460 * %LC_STARVING, blocking further lc_get() operations).
461 *
462 * pointer to the element with the REQUESTED element number.
463 * In this case, it can be used right away
464 *
465 * pointer to an UNUSED element with some different element number,
466 * where that different number may also be %LC_FREE.
467 *
468 * In this case, the cache is marked %LC_DIRTY,
469 * so lc_try_lock() will no longer succeed.
470 * The returned element pointer is moved to the "to_be_changed" list,
471 * and registered with the new element number on the hash collision chains,
472 * so it is possible to pick it up from lc_is_used().
473 * Up to "max_pending_changes" (see lc_create()) can be accumulated.
474 * The user now should do whatever housekeeping is necessary,
475 * typically serialize on lc_try_lock_for_transaction(), then call
476 * lc_committed(lc) and lc_unlock(), to finish the change.
477 *
478 * NOTE: The user needs to check the lc_number on EACH use, so he recognizes
479 * any cache set change.
480 */
481struct lc_element *lc_get(struct lru_cache *lc, unsigned int enr)
482{
483 return __lc_get(lc, enr, LC_GET_MAY_CHANGE);
484}
485
486/**
487 * lc_get_cumulative - like lc_get; also finds to-be-changed elements
488 * @lc: the lru cache to operate on
489 * @enr: the label to look up
490 *
491 * Unlike lc_get this also returns the element for @enr, if it is belonging to
492 * a pending transaction, so the return values are like for lc_get(),
493 * plus:
494 *
495 * pointer to an element already on the "to_be_changed" list.
496 * In this case, the cache was already marked %LC_DIRTY.
497 *
498 * Caller needs to make sure that the pending transaction is completed,
499 * before proceeding to actually use this element.
500 */
501struct lc_element *lc_get_cumulative(struct lru_cache *lc, unsigned int enr)
502{
503 return __lc_get(lc, enr, LC_GET_MAY_CHANGE|LC_GET_MAY_USE_UNCOMMITTED);
504}
505
506/**
507 * lc_try_get - get element by label, if present; do not change the active set
508 * @lc: the lru cache to operate on
509 * @enr: the label to look up
510 *
511 * Finds an element in the cache, increases its usage count,
512 * "touches" and returns it.
513 *
514 * Return values:
515 * NULL
516 * The cache was marked %LC_STARVING,
517 * or the requested label was not in the active set
518 *
519 * pointer to the element with the REQUESTED element number.
520 * In this case, it can be used right away
521 */
522struct lc_element *lc_try_get(struct lru_cache *lc, unsigned int enr)
523{
524 return __lc_get(lc, enr, 0);
525}
526
527/**
528 * lc_committed - tell @lc that pending changes have been recorded
529 * @lc: the lru cache to operate on
530 *
531 * User is expected to serialize on explicit lc_try_lock_for_transaction()
532 * before the transaction is started, and later needs to lc_unlock() explicitly
533 * as well.
534 */
535void lc_committed(struct lru_cache *lc)
536{
537 struct lc_element *e, *tmp;
538
539 PARANOIA_ENTRY();
540 list_for_each_entry_safe(e, tmp, &lc->to_be_changed, list) {
541 /* count number of changes, not number of transactions */
542 ++lc->changed;
543 e->lc_number = e->lc_new_number;
544 list_move(&e->list, &lc->in_use);
545 }
546 lc->pending_changes = 0;
547 RETURN();
548}
549
550
551/**
552 * lc_put - give up refcnt of @e
553 * @lc: the lru cache to operate on
554 * @e: the element to put
555 *
556 * If refcnt reaches zero, the element is moved to the lru list,
557 * and a %LC_STARVING (if set) is cleared.
558 * Returns the new (post-decrement) refcnt.
559 */
560unsigned int lc_put(struct lru_cache *lc, struct lc_element *e)
561{
562 PARANOIA_ENTRY();
563 PARANOIA_LC_ELEMENT(lc, e);
564 BUG_ON(e->refcnt == 0);
565 BUG_ON(e->lc_number != e->lc_new_number);
566 if (--e->refcnt == 0) {
567 /* move it to the front of LRU. */
568 list_move(&e->list, &lc->lru);
569 lc->used--;
570 clear_bit_unlock(__LC_STARVING, &lc->flags);
571 }
572 RETURN(e->refcnt);
573}
574
575/**
576 * lc_element_by_index
577 * @lc: the lru cache to operate on
578 * @i: the index of the element to return
579 */
580struct lc_element *lc_element_by_index(struct lru_cache *lc, unsigned i)
581{
582 BUG_ON(i >= lc->nr_elements);
583 BUG_ON(lc->lc_element[i] == NULL);
584 BUG_ON(lc->lc_element[i]->lc_index != i);
585 return lc->lc_element[i];
586}
587
588/**
589 * lc_index_of
590 * @lc: the lru cache to operate on
591 * @e: the element to query for its index position in lc->element
592 */
593unsigned int lc_index_of(struct lru_cache *lc, struct lc_element *e)
594{
595 PARANOIA_LC_ELEMENT(lc, e);
596 return e->lc_index;
597}
598
599/**
600 * lc_set - associate index with label
601 * @lc: the lru cache to operate on
602 * @enr: the label to set
603 * @index: the element index to associate label with.
604 *
605 * Used to initialize the active set to some previously recorded state.
606 */
607void lc_set(struct lru_cache *lc, unsigned int enr, int index)
608{
609 struct lc_element *e;
610 struct list_head *lh;
611
612 if (index < 0 || index >= lc->nr_elements)
613 return;
614
615 e = lc_element_by_index(lc, index);
616 BUG_ON(e->lc_number != e->lc_new_number);
617 BUG_ON(e->refcnt != 0);
618
619 e->lc_number = e->lc_new_number = enr;
620 hlist_del_init(&e->colision);
621 if (enr == LC_FREE)
622 lh = &lc->free;
623 else {
624 hlist_add_head(&e->colision, lc_hash_slot(lc, enr));
625 lh = &lc->lru;
626 }
627 list_move(&e->list, lh);
628}
629
630/**
631 * lc_seq_dump_details - Dump a complete LRU cache to seq in textual form.
632 * @lc: the lru cache to operate on
633 * @seq: the &struct seq_file pointer to seq_printf into
634 * @utext: user supplied additional "heading" or other info
635 * @detail: function pointer the user may provide to dump further details
636 * of the object the lc_element is embedded in. May be NULL.
637 * Note: a leading space ' ' and trailing newline '\n' is implied.
638 */
639void lc_seq_dump_details(struct seq_file *seq, struct lru_cache *lc, char *utext,
640 void (*detail) (struct seq_file *, struct lc_element *))
641{
642 unsigned int nr_elements = lc->nr_elements;
643 struct lc_element *e;
644 int i;
645
646 seq_printf(seq, "\tnn: lc_number (new nr) refcnt %s\n ", utext);
647 for (i = 0; i < nr_elements; i++) {
648 e = lc_element_by_index(lc, i);
649 if (e->lc_number != e->lc_new_number)
650 seq_printf(seq, "\t%5d: %6d %8d %6d ",
651 i, e->lc_number, e->lc_new_number, e->refcnt);
652 else
653 seq_printf(seq, "\t%5d: %6d %-8s %6d ",
654 i, e->lc_number, "-\"-", e->refcnt);
655 if (detail)
656 detail(seq, e);
657 seq_putc(seq, '\n');
658 }
659}
660
661EXPORT_SYMBOL(lc_create);
662EXPORT_SYMBOL(lc_reset);
663EXPORT_SYMBOL(lc_destroy);
664EXPORT_SYMBOL(lc_set);
665EXPORT_SYMBOL(lc_del);
666EXPORT_SYMBOL(lc_try_get);
667EXPORT_SYMBOL(lc_find);
668EXPORT_SYMBOL(lc_get);
669EXPORT_SYMBOL(lc_put);
670EXPORT_SYMBOL(lc_committed);
671EXPORT_SYMBOL(lc_element_by_index);
672EXPORT_SYMBOL(lc_index_of);
673EXPORT_SYMBOL(lc_seq_printf_stats);
674EXPORT_SYMBOL(lc_seq_dump_details);
675EXPORT_SYMBOL(lc_try_lock);
676EXPORT_SYMBOL(lc_is_used);
677EXPORT_SYMBOL(lc_get_cumulative);
678

source code of linux/lib/lru_cache.c