aufs: documents, from aufs2.2-3.0

[zen-stable.git] / lib / lru_cache.c

/*
   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>.

   drbd is free software; you can redistribute it and/or modify
   it under the terms of the GNU General Public License as published by
   the Free Software Foundation; either version 2, or (at your option)
   any later version.

   drbd is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
   GNU General Public License for more details.

   You should have received a copy of the GNU General Public License
   along with drbd; see the file COPYING.  If not, write to
   the Free Software Foundation, 675 Mass Ave, Cambridge, MA 02139, USA.

 */

#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(__LC_PARANOIA, &lc->flags); \
        smp_mb__after_clear_bit(); 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)

/**
 * 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
 * @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 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 = kzalloc(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);

        lc->name = name;
        lc->element_size = e_size;
        lc->element_off = e_off;
        lc->nr_elements = e_count;
        lc->new_number = LC_FREE;
        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;
                list_add(&e->list, &lc->free);
                element[i] = e;
        }
        if (i == e_count)
                return lc;

        /* else: could not allocate all elements, give up */
        for (i--; i; i--) {
                void *p = element[i];
                kmem_cache_free(cache, p - e_off);
        }
        kfree(lc);
out_fail:
        kfree(element);
        kfree(slot);
        return NULL;
}

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);
        lc->used = 0;
        lc->hits = 0;
        lc->misses = 0;
        lc->starving = 0;
        lc->dirty = 0;
        lc->changed = 0;
        lc->flags = 0;
        lc->changing_element = NULL;
        lc->new_number = LC_FREE;
        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;
                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
 */
size_t lc_seq_printf_stats(struct seq_file *seq, struct lru_cache *lc)
{
        /* NOTE:
         * total calls to lc_get are
         * (starving + hits + misses)
         * misses include "dirty" count (update from an other thread in
         * progress) and "changed", when this in fact lead to an successful
         * update of the cache.
         */
        return seq_printf(seq, "\t%s: used:%u/%u "
                "hits:%lu misses:%lu starving:%lu dirty:%lu changed:%lu\n",
                lc->name, lc->used, lc->nr_elements,
                lc->hits, lc->misses, lc->starving, lc->dirty, lc->changed);
}

static struct hlist_head *lc_hash_slot(struct lru_cache *lc, unsigned int enr)
{
        return  lc->lc_slot + (enr % lc->nr_elements);
}


/**
 * 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.
 */
struct lc_element *lc_find(struct lru_cache *lc, unsigned int enr)
{
        struct hlist_node *n;
        struct lc_element *e;

        BUG_ON(!lc);
        BUG_ON(!lc->nr_elements);
        hlist_for_each_entry(e, n, lc_hash_slot(lc, enr), colision) {
                if (e->lc_number == enr)
                        return e;
        }
        return NULL;
}

/* returned element will be "recycled" immediately */
static struct lc_element *lc_evict(struct lru_cache *lc)
{
        struct list_head  *n;
        struct lc_element *e;

        if (list_empty(&lc->lru))
                return NULL;

        n = lc->lru.prev;
        e = list_entry(n, struct lc_element, list);

        PARANOIA_LC_ELEMENT(lc, e);

        list_del(&e->list);
        hlist_del(&e->colision);
        return e;
}

/**
 * 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 = LC_FREE;
        hlist_del_init(&e->colision);
        list_move(&e->list, &lc->free);
        RETURN();
}

static struct lc_element *lc_get_unused_element(struct lru_cache *lc)
{
        struct list_head *n;

        if (list_empty(&lc->free))
                return lc_evict(lc);

        n = lc->free.next;
        list_del(n);
        return list_entry(n, struct lc_element, list);
}

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;
}


/**
 * 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 (blocking further changes),
 *          and the returned element pointer is removed from the lru list and
 *          hash collision chains.  The user now should do whatever housekeeping
 *          is necessary.
 *          Then he must call lc_changed(lc,element_pointer), 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)
{
        struct lc_element *e;

        PARANOIA_ENTRY();
        if (lc->flags & LC_STARVING) {
                ++lc->starving;
                RETURN(NULL);
        }

        e = lc_find(lc, enr);
        if (e) {
                ++lc->hits;
                if (e->refcnt++ == 0)
                        lc->used++;
                list_move(&e->list, &lc->in_use); /* Not evictable... */
                RETURN(e);
        }

        ++lc->misses;

        /* 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.
         * user may need to commit a transaction to record that change.
         * we serialize on flags & TF_DIRTY */
        if (test_and_set_bit(__LC_DIRTY, &lc->flags)) {
                ++lc->dirty;
                RETURN(NULL);
        }

        e = lc_get_unused_element(lc);
        BUG_ON(!e);

        clear_bit(__LC_STARVING, &lc->flags);
        BUG_ON(++e->refcnt != 1);
        lc->used++;

        lc->changing_element = e;
        lc->new_number = enr;

        RETURN(e);
}

/* similar to lc_get,
 * but only gets a new reference on an existing element.
 * you either get the requested element, or NULL.
 * will be consolidated into one function.
 */
struct lc_element *lc_try_get(struct lru_cache *lc, unsigned int enr)
{
        struct lc_element *e;

        PARANOIA_ENTRY();
        if (lc->flags & LC_STARVING) {
                ++lc->starving;
                RETURN(NULL);
        }

        e = lc_find(lc, enr);
        if (e) {
                ++lc->hits;
                if (e->refcnt++ == 0)
                        lc->used++;
                list_move(&e->list, &lc->in_use); /* Not evictable... */
        }
        RETURN(e);
}

/**
 * lc_changed - tell @lc that the change has been recorded
 * @lc: the lru cache to operate on
 * @e: the element pending label change
 */
void lc_changed(struct lru_cache *lc, struct lc_element *e)
{
        PARANOIA_ENTRY();
        BUG_ON(e != lc->changing_element);
        PARANOIA_LC_ELEMENT(lc, e);
        ++lc->changed;
        e->lc_number = lc->new_number;
        list_add(&e->list, &lc->in_use);
        hlist_add_head(&e->colision, lc_hash_slot(lc, lc->new_number));
        lc->changing_element = NULL;
        lc->new_number = LC_FREE;
        clear_bit(__LC_DIRTY, &lc->flags);
        smp_mb__after_clear_bit();
        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->changing_element);
        if (--e->refcnt == 0) {
                /* move it to the front of LRU. */
                list_move(&e->list, &lc->lru);
                lc->used--;
                clear_bit(__LC_STARVING, &lc->flags);
                smp_mb__after_clear_bit();
        }
        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_index_of
 * @lc: the lru cache to operate on
 * @e: the element to query for its index position in lc->element
 */
unsigned int lc_index_of(struct lru_cache *lc, struct lc_element *e)
{
        PARANOIA_LC_ELEMENT(lc, e);
        return e->lc_index;
}

/**
 * lc_set - associate index with label
 * @lc: the lru cache to operate on
 * @enr: the label to set
 * @index: the element index to associate label with.
 *
 * Used to initialize the active set to some previously recorded state.
 */
void lc_set(struct lru_cache *lc, unsigned int enr, int index)
{
        struct lc_element *e;

        if (index < 0 || index >= lc->nr_elements)
                return;

        e = lc_element_by_index(lc, index);
        e->lc_number = enr;

        hlist_del_init(&e->colision);
        hlist_add_head(&e->colision, lc_hash_slot(lc, enr));
        list_move(&e->list, e->refcnt ? &lc->in_use : &lc->lru);
}

/**
 * lc_dump - 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 "heading" or other info
 * @detail: function pointer the user may provide to dump further details
 * of the object the lc_element is embedded in.
 */
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 refcnt %s\n ", utext);
        for (i = 0; i < nr_elements; i++) {
                e = lc_element_by_index(lc, i);
                if (e->lc_number == LC_FREE) {
                        seq_printf(seq, "\t%2d: FREE\n", i);
                } else {
                        seq_printf(seq, "\t%2d: %4u %4u    ", i,
                                   e->lc_number, e->refcnt);
                        detail(seq, e);
                }
        }
}

EXPORT_SYMBOL(lc_create);
EXPORT_SYMBOL(lc_reset);
EXPORT_SYMBOL(lc_destroy);
EXPORT_SYMBOL(lc_set);
EXPORT_SYMBOL(lc_del);
EXPORT_SYMBOL(lc_try_get);
EXPORT_SYMBOL(lc_find);
EXPORT_SYMBOL(lc_get);
EXPORT_SYMBOL(lc_put);
EXPORT_SYMBOL(lc_changed);
EXPORT_SYMBOL(lc_element_by_index);
EXPORT_SYMBOL(lc_index_of);
EXPORT_SYMBOL(lc_seq_printf_stats);
EXPORT_SYMBOL(lc_seq_dump_details);