// 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 <[email protected]>.
Copyright (C) 2003-2008, Lars Ellenberg <[email protected]>.
*/
#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 <[email protected]>, "
"Lars Ellenberg <[email protected]>");
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 = kcalloc(e_count, sizeof(struct hlist_head), GFP_KERNEL);
if (!slot)
goto out_fail;
element = kcalloc(e_count, sizeof(struct lc_element *), GFP_KERNEL);
if (!element)
goto out_fail;
lc = kzalloc(sizeof(*lc), GFP_KERNEL);
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);
Codebase Browser
linux