// SPDX-License-Identifier: GPL-2.0-or-later /* lru_cache.c This file is part of DRBD by Philipp Reisner and Lars Ellenberg. Copyright (C) 2003-2008, LINBIT Information Technologies GmbH. Copyright (C) 2003-2008, Philipp Reisner <philipp.reisner@linbit.com>. Copyright (C) 2003-2008, Lars Ellenberg <lars.ellenberg@linbit.com>. */ #include <linux/module.h> #include <linux/bitops.h> #include <linux/slab.h> #include <linux/string.h> /* for memset */ #include <linux/seq_file.h> /* for seq_printf */ #include <linux/lru_cache.h> MODULE_AUTHOR("Philipp Reisner <phil@linbit.com>, " "Lars Ellenberg <lars@linbit.com>"); MODULE_DESCRIPTION("lru_cache - Track sets of hot objects"); MODULE_LICENSE("GPL"); /* this is developers aid only. * it catches concurrent access (lack of locking on the users part) */ #define PARANOIA_ENTRY() do { \ BUG_ON(!lc); \ BUG_ON(!lc->nr_elements); \ BUG_ON(test_and_set_bit(__LC_PARANOIA, &lc->flags)); \ } while (0) #define RETURN(x...) do { \ clear_bit_unlock(__LC_PARANOIA, &lc->flags); \ return x ; } while (0) /* BUG() if e is not one of the elements tracked by lc */ #define PARANOIA_LC_ELEMENT(lc, e) do { \ struct lru_cache *lc_ = (lc); \ struct lc_element *e_ = (e); \ unsigned i = e_->lc_index; \ BUG_ON(i >= lc_->nr_elements); \ BUG_ON(lc_->lc_element[i] != e_); } while (0) /* We need to atomically * - try to grab the lock (set LC_LOCKED) * - only if there is no pending transaction * (neither LC_DIRTY nor LC_STARVING is set) * Because of PARANOIA_ENTRY() above abusing lc->flags as well, * it is not sufficient to just say * return 0 == cmpxchg(&lc->flags, 0, LC_LOCKED); */ int lc_try_lock(struct lru_cache *lc) { unsigned long val; do { val = cmpxchg(&lc->flags, 0, LC_LOCKED); } while (unlikely (val == LC_PARANOIA)); /* Spin until no-one is inside a PARANOIA_ENTRY()/RETURN() section. */ return 0 == val; } /** * lc_create - prepares to track objects in an active set * @name: descriptive name only used in lc_seq_printf_stats and lc_seq_dump_details * @cache: cache root pointer * @max_pending_changes: maximum changes to accumulate until a transaction is required * @e_count: number of elements allowed to be active simultaneously * @e_size: size of the tracked objects * @e_off: offset to the &struct lc_element member in a tracked object * * Returns a pointer to a newly initialized struct lru_cache on success, * or NULL on (allocation) failure. */ struct lru_cache *lc_create(const char *name, struct kmem_cache *cache, unsigned max_pending_changes, unsigned e_count, size_t e_size, size_t e_off) { struct hlist_head *slot = NULL; struct lc_element **element = NULL; struct lru_cache *lc; struct lc_element *e; unsigned cache_obj_size = kmem_cache_size(cache); unsigned i; WARN_ON(cache_obj_size < e_size); if (cache_obj_size < e_size) return NULL; /* e_count too big; would probably fail the allocation below anyways. * for typical use cases, e_count should be few thousand at most. */ if (e_count > LC_MAX_ACTIVE) return NULL; slot = kzalloc_objs(struct hlist_head, e_count); if (!slot) goto out_fail; element = kzalloc_objs(struct lc_element *, e_count); if (!element) goto out_fail; lc = kzalloc_obj(*lc); if (!lc) goto out_fail; INIT_LIST_HEAD(&lc->in_use); INIT_LIST_HEAD(&lc->lru); INIT_LIST_HEAD(&lc->free); INIT_LIST_HEAD(&lc->to_be_changed); lc->name = name; lc->element_size = e_size; lc->element_off = e_off; lc->nr_elements = e_count; lc->max_pending_changes = max_pending_changes; lc->lc_cache = cache; lc->lc_element = element; lc->lc_slot = slot; /* preallocate all objects */ for (i = 0; i < e_count; i++) { void *p = kmem_cache_alloc(cache, GFP_KERNEL); if (!p) break; memset(p, 0, lc->element_size); e = p + e_off; e->lc_index = i; e->lc_number = LC_FREE; e->lc_new_number = LC_FREE; list_add(&e->list, &lc->free); element[i] = e; } if (i == e_count) return lc; /* else: could not allocate all elements, give up */ while (i) { void *p = element[--i]; kmem_cache_free(cache, p - e_off); } kfree(lc); out_fail: kfree(element); kfree(slot); return NULL; } static void lc_free_by_index(struct lru_cache *lc, unsigned i) { void *p = lc->lc_element[i]; WARN_ON(!p); if (p) { p -= lc->element_off; kmem_cache_free(lc->lc_cache, p); } } /** * lc_destroy - frees memory allocated by lc_create() * @lc: the lru cache to destroy */ void lc_destroy(struct lru_cache *lc) { unsigned i; if (!lc) return; for (i = 0; i < lc->nr_elements; i++) lc_free_by_index(lc, i); kfree(lc->lc_element); kfree(lc->lc_slot); kfree(lc); } /** * lc_reset - does a full reset for @lc and the hash table slots. * @lc: the lru cache to operate on * * It is roughly the equivalent of re-allocating a fresh lru_cache object, * basically a short cut to lc_destroy(lc); lc = lc_create(...); */ void lc_reset(struct lru_cache *lc) { unsigned i; INIT_LIST_HEAD(&lc->in_use); INIT_LIST_HEAD(&lc->lru); INIT_LIST_HEAD(&lc->free); INIT_LIST_HEAD(&lc->to_be_changed); lc->used = 0; lc->hits = 0; lc->misses = 0; lc->starving = 0; lc->locked = 0; lc->changed = 0; lc->pending_changes = 0; lc->flags = 0; memset(lc->lc_slot, 0, sizeof(struct hlist_head) * lc->nr_elements); for (i = 0; i < lc->nr_elements; i++) { struct lc_element *e = lc->lc_element[i]; void *p = e; p -= lc->element_off; memset(p, 0, lc->element_size); /* re-init it */ e->lc_index = i; e->lc_number = LC_FREE; e->lc_new_number = LC_FREE; list_add(&e->list, &lc->free); } } /** * lc_seq_printf_stats - print stats about @lc into @seq * @seq: the seq_file to print into * @lc: the lru cache to print statistics of */ void lc_seq_printf_stats(struct seq_file *seq, struct lru_cache *lc) { /* NOTE: * total calls to lc_get are * (starving + hits + misses) * misses include "locked" count (update from an other thread in * progress) and "changed", when this in fact lead to an successful * update of the cache. */ seq_printf(seq, "\t%s: used:%u/%u hits:%lu misses:%lu starving:%lu locked:%lu changed:%lu\n", lc->name, lc->used, lc->nr_elements, lc->hits, lc->misses, lc->starving, lc->locked, lc->changed); } static struct hlist_head *lc_hash_slot(struct lru_cache *lc, unsigned int enr) { return lc->lc_slot + (enr % lc->nr_elements); } static struct lc_element *__lc_find(struct lru_cache *lc, unsigned int enr, bool include_changing) { struct lc_element *e; BUG_ON(!lc); BUG_ON(!lc->nr_elements); hlist_for_each_entry(e, lc_hash_slot(lc, enr), collision) { /* "about to be changed" elements, pending transaction commit, * are hashed by their "new number". "Normal" elements have * lc_number == lc_new_number. */ if (e->lc_new_number != enr) continue; if (e->lc_new_number == e->lc_number || include_changing) return e; break; } return NULL; } /** * lc_find - find element by label, if present in the hash table * @lc: The lru_cache object * @enr: element number * * Returns the pointer to an element, if the element with the requested * "label" or element number is present in the hash table, * or NULL if not found. Does not change the refcnt. * Ignores elements that are "about to be used", i.e. not yet in the active * set, but still pending transaction commit. */ struct lc_element *lc_find(struct lru_cache *lc, unsigned int enr) { return __lc_find(lc, enr, 0); } /** * lc_is_used - find element by label * @lc: The lru_cache object * @enr: element number * * Returns true, if the element with the requested "label" or element number is * present in the hash table, and is used (refcnt > 0). * Also finds elements that are not _currently_ used but only "about to be * used", i.e. on the "to_be_changed" list, pending transaction commit. */ bool lc_is_used(struct lru_cache *lc, unsigned int enr) { struct lc_element *e = __lc_find(lc, enr, 1); return e && e->refcnt; } /** * lc_del - removes an element from the cache * @lc: The lru_cache object * @e: The element to remove * * @e must be unused (refcnt == 0). Moves @e from "lru" to "free" list, * sets @e->enr to %LC_FREE. */ void lc_del(struct lru_cache *lc, struct lc_element *e) { PARANOIA_ENTRY(); PARANOIA_LC_ELEMENT(lc, e); BUG_ON(e->refcnt); e->lc_number = e->lc_new_number = LC_FREE; hlist_del_init(&e->collision); list_move(&e->list, &lc->free); RETURN(); } static struct lc_element *lc_prepare_for_change(struct lru_cache *lc, unsigned new_number) { struct list_head *n; struct lc_element *e; if (!list_empty(&lc->free)) n = lc->free.next; else if (!list_empty(&lc->lru)) n = lc->lru.prev; else return NULL; e = list_entry(n, struct lc_element, list); PARANOIA_LC_ELEMENT(lc, e); e->lc_new_number = new_number; if (!hlist_unhashed(&e->collision)) __hlist_del(&e->collision); hlist_add_head(&e->collision, lc_hash_slot(lc, new_number)); list_move(&e->list, &lc->to_be_changed); return e; } static int lc_unused_element_available(struct lru_cache *lc) { if (!list_empty(&lc->free)) return 1; /* something on the free list */ if (!list_empty(&lc->lru)) return 1; /* something to evict */ return 0; } /* used as internal flags to __lc_get */ enum { LC_GET_MAY_CHANGE = 1, LC_GET_MAY_USE_UNCOMMITTED = 2, }; static struct lc_element *__lc_get(struct lru_cache *lc, unsigned int enr, unsigned int flags) { struct lc_element *e; PARANOIA_ENTRY(); if (test_bit(__LC_STARVING, &lc->flags)) { ++lc->starving; RETURN(NULL); } e = __lc_find(lc, enr, 1); /* if lc_new_number != lc_number, * this enr is currently being pulled in already, * and will be available once the pending transaction * has been committed. */ if (e) { if (e->lc_new_number != e->lc_number) { /* It has been found above, but on the "to_be_changed" * list, not yet committed. Don't pull it in twice, * wait for the transaction, then try again... */ if (!(flags & LC_GET_MAY_USE_UNCOMMITTED)) RETURN(NULL); /* ... unless the caller is aware of the implications, * probably preparing a cumulative transaction. */ ++e->refcnt; ++lc->hits; RETURN(e); } /* else: lc_new_number == lc_number; a real hit. */ ++lc->hits; if (e->refcnt++ == 0) lc->used++; list_move(&e->list, &lc->in_use); /* Not evictable... */ RETURN(e); } /* e == NULL */ ++lc->misses; if (!(flags & LC_GET_MAY_CHANGE)) RETURN(NULL); /* To avoid races with lc_try_lock(), first, mark us dirty * (using test_and_set_bit, as it implies memory barriers), ... */ test_and_set_bit(__LC_DIRTY, &lc->flags); /* ... only then check if it is locked anyways. If lc_unlock clears * the dirty bit again, that's not a problem, we will come here again. */ if (test_bit(__LC_LOCKED, &lc->flags)) { ++lc->locked; RETURN(NULL); } /* In case there is nothing available and we can not kick out * the LRU element, we have to wait ... */ if (!lc_unused_element_available(lc)) { set_bit(__LC_STARVING, &lc->flags); RETURN(NULL); } /* It was not present in the active set. We are going to recycle an * unused (or even "free") element, but we won't accumulate more than * max_pending_changes changes. */ if (lc->pending_changes >= lc->max_pending_changes) RETURN(NULL); e = lc_prepare_for_change(lc, enr); BUG_ON(!e); clear_bit(__LC_STARVING, &lc->flags); BUG_ON(++e->refcnt != 1); lc->used++; lc->pending_changes++; RETURN(e); } /** * lc_get - get element by label, maybe change the active set * @lc: the lru cache to operate on * @enr: the label to look up * * Finds an element in the cache, increases its usage count, * "touches" and returns it. * * In case the requested number is not present, it needs to be added to the * cache. Therefore it is possible that an other element becomes evicted from * the cache. In either case, the user is notified so he is able to e.g. keep * a persistent log of the cache changes, and therefore the objects in use. * * Return values: * NULL * The cache was marked %LC_STARVING, * or the requested label was not in the active set * and a changing transaction is still pending (@lc was marked %LC_DIRTY). * Or no unused or free element could be recycled (@lc will be marked as * %LC_STARVING, blocking further lc_get() operations). * * pointer to the element with the REQUESTED element number. * In this case, it can be used right away * * pointer to an UNUSED element with some different element number, * where that different number may also be %LC_FREE. * * In this case, the cache is marked %LC_DIRTY, * so lc_try_lock() will no longer succeed. * The returned element pointer is moved to the "to_be_changed" list, * and registered with the new element number on the hash collision chains, * so it is possible to pick it up from lc_is_used(). * Up to "max_pending_changes" (see lc_create()) can be accumulated. * The user now should do whatever housekeeping is necessary, * typically serialize on lc_try_lock_for_transaction(), then call * lc_committed(lc) and lc_unlock(), to finish the change. * * NOTE: The user needs to check the lc_number on EACH use, so he recognizes * any cache set change. */ struct lc_element *lc_get(struct lru_cache *lc, unsigned int enr) { return __lc_get(lc, enr, LC_GET_MAY_CHANGE); } /** * lc_get_cumulative - like lc_get; also finds to-be-changed elements * @lc: the lru cache to operate on * @enr: the label to look up * * Unlike lc_get this also returns the element for @enr, if it is belonging to * a pending transaction, so the return values are like for lc_get(), * plus: * * pointer to an element already on the "to_be_changed" list. * In this case, the cache was already marked %LC_DIRTY. * * Caller needs to make sure that the pending transaction is completed, * before proceeding to actually use this element. */ struct lc_element *lc_get_cumulative(struct lru_cache *lc, unsigned int enr) { return __lc_get(lc, enr, LC_GET_MAY_CHANGE|LC_GET_MAY_USE_UNCOMMITTED); } /** * lc_try_get - get element by label, if present; do not change the active set * @lc: the lru cache to operate on * @enr: the label to look up * * Finds an element in the cache, increases its usage count, * "touches" and returns it. * * Return values: * NULL * The cache was marked %LC_STARVING, * or the requested label was not in the active set * * pointer to the element with the REQUESTED element number. * In this case, it can be used right away */ struct lc_element *lc_try_get(struct lru_cache *lc, unsigned int enr) { return __lc_get(lc, enr, 0); } /** * lc_committed - tell @lc that pending changes have been recorded * @lc: the lru cache to operate on * * User is expected to serialize on explicit lc_try_lock_for_transaction() * before the transaction is started, and later needs to lc_unlock() explicitly * as well. */ void lc_committed(struct lru_cache *lc) { struct lc_element *e, *tmp; PARANOIA_ENTRY(); list_for_each_entry_safe(e, tmp, &lc->to_be_changed, list) { /* count number of changes, not number of transactions */ ++lc->changed; e->lc_number = e->lc_new_number; list_move(&e->list, &lc->in_use); } lc->pending_changes = 0; RETURN(); } /** * lc_put - give up refcnt of @e * @lc: the lru cache to operate on * @e: the element to put * * If refcnt reaches zero, the element is moved to the lru list, * and a %LC_STARVING (if set) is cleared. * Returns the new (post-decrement) refcnt. */ unsigned int lc_put(struct lru_cache *lc, struct lc_element *e) { PARANOIA_ENTRY(); PARANOIA_LC_ELEMENT(lc, e); BUG_ON(e->refcnt == 0); BUG_ON(e->lc_number != e->lc_new_number); if (--e->refcnt == 0) { /* move it to the front of LRU. */ list_move(&e->list, &lc->lru); lc->used--; clear_bit_unlock(__LC_STARVING, &lc->flags); } RETURN(e->refcnt); } /** * lc_element_by_index * @lc: the lru cache to operate on * @i: the index of the element to return */ struct lc_element *lc_element_by_index(struct lru_cache *lc, unsigned i) { BUG_ON(i >= lc->nr_elements); BUG_ON(lc->lc_element[i] == NULL); BUG_ON(lc->lc_element[i]->lc_index != i); return lc->lc_element[i]; } /** * lc_seq_dump_details - Dump a complete LRU cache to seq in textual form. * @lc: the lru cache to operate on * @seq: the &struct seq_file pointer to seq_printf into * @utext: user supplied additional "heading" or other info * @detail: function pointer the user may provide to dump further details * of the object the lc_element is embedded in. May be NULL. * Note: a leading space ' ' and trailing newline '\n' is implied. */ void lc_seq_dump_details(struct seq_file *seq, struct lru_cache *lc, char *utext, void (*detail) (struct seq_file *, struct lc_element *)) { unsigned int nr_elements = lc->nr_elements; struct lc_element *e; int i; seq_printf(seq, "\tnn: lc_number (new nr) refcnt %s\n ", utext); for (i = 0; i < nr_elements; i++) { e = lc_element_by_index(lc, i); if (e->lc_number != e->lc_new_number) seq_printf(seq, "\t%5d: %6d %8d %6d ", i, e->lc_number, e->lc_new_number, e->refcnt); else seq_printf(seq, "\t%5d: %6d %-8s %6d ", i, e->lc_number, "-\"-", e->refcnt); if (detail) detail(seq, e); seq_putc(seq, '\n'); } } EXPORT_SYMBOL(lc_create); EXPORT_SYMBOL(lc_reset); EXPORT_SYMBOL(lc_destroy); EXPORT_SYMBOL(lc_del); EXPORT_SYMBOL(lc_try_get); EXPORT_SYMBOL(lc_find); EXPORT_SYMBOL(lc_get); EXPORT_SYMBOL(lc_put); EXPORT_SYMBOL(lc_committed); EXPORT_SYMBOL(lc_element_by_index); EXPORT_SYMBOL(lc_seq_printf_stats); EXPORT_SYMBOL(lc_seq_dump_details); EXPORT_SYMBOL(lc_try_lock); EXPORT_SYMBOL(lc_is_used); EXPORT_SYMBOL(lc_get_cumulative);
