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