mbcache.c

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/*
 * linux/fs/mbcache.c
 * (C) 2001-2002 Andreas Gruenbacher, <[email protected]>
 */

/*
 * Filesystem Meta Information Block Cache (mbcache)
 *
 * The mbcache caches blocks of block devices that need to be located
 * by their device/block number, as well as by other criteria (such
 * as the block's contents).
 *
 * There can only be one cache entry in a cache per device and block number.
 * Additional indexes need not be unique in this sense. The number of
 * additional indexes (=other criteria) can be hardwired at compile time
 * or specified at cache create time.
 *
 * Each cache entry is of fixed size. An entry may be `valid' or `invalid'
 * in the cache. A valid entry is in the main hash tables of the cache,
 * and may also be in the lru list. An invalid entry is not in any hashes
 * or lists.
 *
 * A valid cache entry is only in the lru list if no handles refer to it.
 * Invalid cache entries will be freed when the last handle to the cache
 * entry is released. Entries that cannot be freed immediately are put
 * back on the lru list.
 */

#include <linux/kernel.h>
#include <linux/module.h>

#include <linux/hash.h>
#include <linux/fs.h>
#include <linux/mm.h>
#include <linux/slab.h>
#include <linux/sched.h>
#include <linux/init.h>
#include <linux/mbcache.h>


#ifdef MB_CACHE_DEBUG
# define mb_debug(f...) do { \
        printk(KERN_DEBUG f); \
        printk("\n"); \
    } while (0)
#define mb_assert(c) do { if (!(c)) \
        printk(KERN_ERR "assertion " #c " failed\n"); \
    } while(0)
#else
# define mb_debug(f...) do { } while(0)
# define mb_assert(c) do { } while(0)
#endif
#define mb_error(f...) do { \
        printk(KERN_ERR f); \
        printk("\n"); \
    } while(0)

#define MB_CACHE_WRITER ((unsigned short)~0U >> 1)

static DECLARE_WAIT_QUEUE_HEAD(mb_cache_queue);
        
MODULE_AUTHOR("Andreas Gruenbacher <[email protected]>");
MODULE_DESCRIPTION("Meta block cache (for extended attributes)");
MODULE_LICENSE("GPL");

EXPORT_SYMBOL(mb_cache_create);
EXPORT_SYMBOL(mb_cache_shrink);
EXPORT_SYMBOL(mb_cache_destroy);
EXPORT_SYMBOL(mb_cache_entry_alloc);
EXPORT_SYMBOL(mb_cache_entry_insert);
EXPORT_SYMBOL(mb_cache_entry_release);
EXPORT_SYMBOL(mb_cache_entry_free);
EXPORT_SYMBOL(mb_cache_entry_get);
#if !defined(MB_CACHE_INDEXES_COUNT) || (MB_CACHE_INDEXES_COUNT > 0)
EXPORT_SYMBOL(mb_cache_entry_find_first);
EXPORT_SYMBOL(mb_cache_entry_find_next);
#endif

/*
 * Global data: list of all mbcache's, lru list, and a spinlock for
 * accessing cache data structures on SMP machines. The lru list is
 * global across all mbcaches.
 */

static LIST_HEAD(mb_cache_list);
static LIST_HEAD(mb_cache_lru_list);
static DEFINE_SPINLOCK(mb_cache_spinlock);

/*
 * What the mbcache registers as to get shrunk dynamically.
 */

static int mb_cache_shrink_fn(struct shrinker *shrink,
                  struct shrink_control *sc);

static struct shrinker mb_cache_shrinker = {
    .shrink = mb_cache_shrink_fn,
    .seeks = DEFAULT_SEEKS,
};

static inline int
__mb_cache_entry_is_hashed(struct mb_cache_entry *ce)
{
    return !list_empty(&ce->e_block_list);
}


static void
__mb_cache_entry_unhash(struct mb_cache_entry *ce)
{
    if (__mb_cache_entry_is_hashed(ce)) {
        list_del_init(&ce->e_block_list);
        list_del(&ce->e_index.o_list);
    }
}


static void
__mb_cache_entry_forget(struct mb_cache_entry *ce, gfp_t gfp_mask)
{
    struct mb_cache *cache = ce->e_cache;

    mb_assert(!(ce->e_used || ce->e_queued));
    kmem_cache_free(cache->c_entry_cache, ce);
    atomic_dec(&cache->c_entry_count);
}


static void
__mb_cache_entry_release_unlock(struct mb_cache_entry *ce)
    __releases(mb_cache_spinlock)
{
    /* Wake up all processes queuing for this cache entry. */
    if (ce->e_queued)
        wake_up_all(&mb_cache_queue);
    if (ce->e_used >= MB_CACHE_WRITER)
        ce->e_used -= MB_CACHE_WRITER;
    ce->e_used--;
    if (!(ce->e_used || ce->e_queued)) {
        if (!__mb_cache_entry_is_hashed(ce))
            goto forget;
        mb_assert(list_empty(&ce->e_lru_list));
        list_add_tail(&ce->e_lru_list, &mb_cache_lru_list);
    }
    spin_unlock(&mb_cache_spinlock);
    return;
forget:
    spin_unlock(&mb_cache_spinlock);
    __mb_cache_entry_forget(ce, GFP_KERNEL);
}


/*
 * mb_cache_shrink_fn()  memory pressure callback
 *
 * This function is called by the kernel memory management when memory
 * gets low.
 *
 * @shrink: (ignored)
 * @sc: shrink_control passed from reclaim
 *
 * Returns the number of objects which are present in the cache.
 */
static int
mb_cache_shrink_fn(struct shrinker *shrink, struct shrink_control *sc)
{
    LIST_HEAD(free_list);
    struct mb_cache *cache;
    struct mb_cache_entry *entry, *tmp;
    int count = 0;
    int nr_to_scan = sc->nr_to_scan;
    gfp_t gfp_mask = sc->gfp_mask;

    mb_debug("trying to free %d entries", nr_to_scan);
    spin_lock(&mb_cache_spinlock);
    while (nr_to_scan-- && !list_empty(&mb_cache_lru_list)) {
        struct mb_cache_entry *ce =
            list_entry(mb_cache_lru_list.next,
                   struct mb_cache_entry, e_lru_list);
        list_move_tail(&ce->e_lru_list, &free_list);
        __mb_cache_entry_unhash(ce);
    }
    list_for_each_entry(cache, &mb_cache_list, c_cache_list) {
        mb_debug("cache %s (%d)", cache->c_name,
              atomic_read(&cache->c_entry_count));
        count += atomic_read(&cache->c_entry_count);
    }
    spin_unlock(&mb_cache_spinlock);
    list_for_each_entry_safe(entry, tmp, &free_list, e_lru_list) {
        __mb_cache_entry_forget(entry, gfp_mask);
    }
    return (count / 100) * sysctl_vfs_cache_pressure;
}


/*
 * mb_cache_create()  create a new cache
 *
 * All entries in one cache are equal size. Cache entries may be from
 * multiple devices. If this is the first mbcache created, registers
 * the cache with kernel memory management. Returns NULL if no more
 * memory was available.
 *
 * @name: name of the cache (informal)
 * @bucket_bits: log2(number of hash buckets)
 */
struct mb_cache *
mb_cache_create(const char *name, int bucket_bits)
{
    int n, bucket_count = 1 << bucket_bits;
    struct mb_cache *cache = NULL;

    cache = kmalloc(sizeof(struct mb_cache), GFP_KERNEL);
    if (!cache)
        return NULL;
    cache->c_name = name;
    atomic_set(&cache->c_entry_count, 0);
    cache->c_bucket_bits = bucket_bits;
    cache->c_block_hash = kmalloc(bucket_count * sizeof(struct list_head),
                                  GFP_KERNEL);
    if (!cache->c_block_hash)
        goto fail;
    for (n=0; n<bucket_count; n++)
        INIT_LIST_HEAD(&cache->c_block_hash[n]);
    cache->c_index_hash = kmalloc(bucket_count * sizeof(struct list_head),
                      GFP_KERNEL);
    if (!cache->c_index_hash)
        goto fail;
    for (n=0; n<bucket_count; n++)
        INIT_LIST_HEAD(&cache->c_index_hash[n]);
    cache->c_entry_cache = kmem_cache_create(name,
        sizeof(struct mb_cache_entry), 0,
        SLAB_RECLAIM_ACCOUNT|SLAB_MEM_SPREAD, NULL);
    if (!cache->c_entry_cache)
        goto fail2;

    /*
     * Set an upper limit on the number of cache entries so that the hash
     * chains won't grow too long.
     */
    cache->c_max_entries = bucket_count << 4;

    spin_lock(&mb_cache_spinlock);
    list_add(&cache->c_cache_list, &mb_cache_list);
    spin_unlock(&mb_cache_spinlock);
    return cache;

fail2:
    kfree(cache->c_index_hash);

fail:
    kfree(cache->c_block_hash);
    kfree(cache);
    return NULL;
}


/*
 * mb_cache_shrink()
 *
 * Removes all cache entries of a device from the cache. All cache entries
 * currently in use cannot be freed, and thus remain in the cache. All others
 * are freed.
 *
 * @bdev: which device's cache entries to shrink
 */
void
mb_cache_shrink(struct block_device *bdev)
{
    LIST_HEAD(free_list);
    struct list_head *l, *ltmp;

    spin_lock(&mb_cache_spinlock);
    list_for_each_safe(l, ltmp, &mb_cache_lru_list) {
        struct mb_cache_entry *ce =
            list_entry(l, struct mb_cache_entry, e_lru_list);
        if (ce->e_bdev == bdev) {
            list_move_tail(&ce->e_lru_list, &free_list);
            __mb_cache_entry_unhash(ce);
        }
    }
    spin_unlock(&mb_cache_spinlock);
    list_for_each_safe(l, ltmp, &free_list) {
        __mb_cache_entry_forget(list_entry(l, struct mb_cache_entry,
                           e_lru_list), GFP_KERNEL);
    }
}


/*
 * mb_cache_destroy()
 *
 * Shrinks the cache to its minimum possible size (hopefully 0 entries),
 * and then destroys it. If this was the last mbcache, un-registers the
 * mbcache from kernel memory management.
 */
void
mb_cache_destroy(struct mb_cache *cache)
{
    LIST_HEAD(free_list);
    struct list_head *l, *ltmp;

    spin_lock(&mb_cache_spinlock);
    list_for_each_safe(l, ltmp, &mb_cache_lru_list) {
        struct mb_cache_entry *ce =
            list_entry(l, struct mb_cache_entry, e_lru_list);
        if (ce->e_cache == cache) {
            list_move_tail(&ce->e_lru_list, &free_list);
            __mb_cache_entry_unhash(ce);
        }
    }
    list_del(&cache->c_cache_list);
    spin_unlock(&mb_cache_spinlock);

    list_for_each_safe(l, ltmp, &free_list) {
        __mb_cache_entry_forget(list_entry(l, struct mb_cache_entry,
                           e_lru_list), GFP_KERNEL);
    }

    if (atomic_read(&cache->c_entry_count) > 0) {
        mb_error("cache %s: %d orphaned entries",
              cache->c_name,
              atomic_read(&cache->c_entry_count));
    }

    kmem_cache_destroy(cache->c_entry_cache);

    kfree(cache->c_index_hash);
    kfree(cache->c_block_hash);
    kfree(cache);
}

/*
 * mb_cache_entry_alloc()
 *
 * Allocates a new cache entry. The new entry will not be valid initially,
 * and thus cannot be looked up yet. It should be filled with data, and
 * then inserted into the cache using mb_cache_entry_insert(). Returns NULL
 * if no more memory was available.
 */
struct mb_cache_entry *
mb_cache_entry_alloc(struct mb_cache *cache, gfp_t gfp_flags)
{
    struct mb_cache_entry *ce = NULL;

    if (atomic_read(&cache->c_entry_count) >= cache->c_max_entries) {
        spin_lock(&mb_cache_spinlock);
        if (!list_empty(&mb_cache_lru_list)) {
            ce = list_entry(mb_cache_lru_list.next,
                    struct mb_cache_entry, e_lru_list);
            list_del_init(&ce->e_lru_list);
            __mb_cache_entry_unhash(ce);
        }
        spin_unlock(&mb_cache_spinlock);
    }
    if (!ce) {
        ce = kmem_cache_alloc(cache->c_entry_cache, gfp_flags);
        if (!ce)
            return NULL;
        atomic_inc(&cache->c_entry_count);
        INIT_LIST_HEAD(&ce->e_lru_list);
        INIT_LIST_HEAD(&ce->e_block_list);
        ce->e_cache = cache;
        ce->e_queued = 0;
    }
    ce->e_used = 1 + MB_CACHE_WRITER;
    return ce;
}


/*
 * mb_cache_entry_insert()
 *
 * Inserts an entry that was allocated using mb_cache_entry_alloc() into
 * the cache. After this, the cache entry can be looked up, but is not yet
 * in the lru list as the caller still holds a handle to it. Returns 0 on
 * success, or -EBUSY if a cache entry for that device + inode exists
 * already (this may happen after a failed lookup, but when another process
 * has inserted the same cache entry in the meantime).
 *
 * @bdev: device the cache entry belongs to
 * @block: block number
 * @key: lookup key
 */
int
mb_cache_entry_insert(struct mb_cache_entry *ce, struct block_device *bdev,
              sector_t block, unsigned int key)
{
    struct mb_cache *cache = ce->e_cache;
    unsigned int bucket;
    struct list_head *l;
    int error = -EBUSY;

    bucket = hash_long((unsigned long)bdev + (block & 0xffffffff), 
               cache->c_bucket_bits);
    spin_lock(&mb_cache_spinlock);
    list_for_each_prev(l, &cache->c_block_hash[bucket]) {
        struct mb_cache_entry *ce =
            list_entry(l, struct mb_cache_entry, e_block_list);
        if (ce->e_bdev == bdev && ce->e_block == block)
            goto out;
    }
    __mb_cache_entry_unhash(ce);
    ce->e_bdev = bdev;
    ce->e_block = block;
    list_add(&ce->e_block_list, &cache->c_block_hash[bucket]);
    ce->e_index.o_key = key;
    bucket = hash_long(key, cache->c_bucket_bits);
    list_add(&ce->e_index.o_list, &cache->c_index_hash[bucket]);
    error = 0;
out:
    spin_unlock(&mb_cache_spinlock);
    return error;
}


/*
 * mb_cache_entry_release()
 *
 * Release a handle to a cache entry. When the last handle to a cache entry
 * is released it is either freed (if it is invalid) or otherwise inserted
 * in to the lru list.
 */
void
mb_cache_entry_release(struct mb_cache_entry *ce)
{
    spin_lock(&mb_cache_spinlock);
    __mb_cache_entry_release_unlock(ce);
}


/*
 * mb_cache_entry_free()
 *
 * This is equivalent to the sequence mb_cache_entry_takeout() --
 * mb_cache_entry_release().
 */
void
mb_cache_entry_free(struct mb_cache_entry *ce)
{
    spin_lock(&mb_cache_spinlock);
    mb_assert(list_empty(&ce->e_lru_list));
    __mb_cache_entry_unhash(ce);
    __mb_cache_entry_release_unlock(ce);
}


/*
 * mb_cache_entry_get()
 *
 * Get a cache entry  by device / block number. (There can only be one entry
 * in the cache per device and block.) Returns NULL if no such cache entry
 * exists. The returned cache entry is locked for exclusive access ("single
 * writer").
 */
struct mb_cache_entry *
mb_cache_entry_get(struct mb_cache *cache, struct block_device *bdev,
           sector_t block)
{
    unsigned int bucket;
    struct list_head *l;
    struct mb_cache_entry *ce;

    bucket = hash_long((unsigned long)bdev + (block & 0xffffffff),
               cache->c_bucket_bits);
    spin_lock(&mb_cache_spinlock);
    list_for_each(l, &cache->c_block_hash[bucket]) {
        ce = list_entry(l, struct mb_cache_entry, e_block_list);
        if (ce->e_bdev == bdev && ce->e_block == block) {
            DEFINE_WAIT(wait);

            if (!list_empty(&ce->e_lru_list))
                list_del_init(&ce->e_lru_list);

            while (ce->e_used > 0) {
                ce->e_queued++;
                prepare_to_wait(&mb_cache_queue, &wait,
                        TASK_UNINTERRUPTIBLE);
                spin_unlock(&mb_cache_spinlock);
                schedule();
                spin_lock(&mb_cache_spinlock);
                ce->e_queued--;
            }
            finish_wait(&mb_cache_queue, &wait);
            ce->e_used += 1 + MB_CACHE_WRITER;

            if (!__mb_cache_entry_is_hashed(ce)) {
                __mb_cache_entry_release_unlock(ce);
                return NULL;
            }
            goto cleanup;
        }
    }
    ce = NULL;

cleanup:
    spin_unlock(&mb_cache_spinlock);
    return ce;
}

#if !defined(MB_CACHE_INDEXES_COUNT) || (MB_CACHE_INDEXES_COUNT > 0)

static struct mb_cache_entry *
__mb_cache_entry_find(struct list_head *l, struct list_head *head,
              struct block_device *bdev, unsigned int key)
{
    while (l != head) {
        struct mb_cache_entry *ce =
            list_entry(l, struct mb_cache_entry, e_index.o_list);
        if (ce->e_bdev == bdev && ce->e_index.o_key == key) {
            DEFINE_WAIT(wait);

            if (!list_empty(&ce->e_lru_list))
                list_del_init(&ce->e_lru_list);

            /* Incrementing before holding the lock gives readers
               priority over writers. */
            ce->e_used++;
            while (ce->e_used >= MB_CACHE_WRITER) {
                ce->e_queued++;
                prepare_to_wait(&mb_cache_queue, &wait,
                        TASK_UNINTERRUPTIBLE);
                spin_unlock(&mb_cache_spinlock);
                schedule();
                spin_lock(&mb_cache_spinlock);
                ce->e_queued--;
            }
            finish_wait(&mb_cache_queue, &wait);

            if (!__mb_cache_entry_is_hashed(ce)) {
                __mb_cache_entry_release_unlock(ce);
                spin_lock(&mb_cache_spinlock);
                return ERR_PTR(-EAGAIN);
            }
            return ce;
        }
        l = l->next;
    }
    return NULL;
}


/*
 * mb_cache_entry_find_first()
 *
 * Find the first cache entry on a given device with a certain key in
 * an additional index. Additional matches can be found with
 * mb_cache_entry_find_next(). Returns NULL if no match was found. The
 * returned cache entry is locked for shared access ("multiple readers").
 *
 * @cache: the cache to search
 * @bdev: the device the cache entry should belong to
 * @key: the key in the index
 */
struct mb_cache_entry *
mb_cache_entry_find_first(struct mb_cache *cache, struct block_device *bdev,
              unsigned int key)
{
    unsigned int bucket = hash_long(key, cache->c_bucket_bits);
    struct list_head *l;
    struct mb_cache_entry *ce;

    spin_lock(&mb_cache_spinlock);
    l = cache->c_index_hash[bucket].next;
    ce = __mb_cache_entry_find(l, &cache->c_index_hash[bucket], bdev, key);
    spin_unlock(&mb_cache_spinlock);
    return ce;
}


/*
 * mb_cache_entry_find_next()
 *
 * Find the next cache entry on a given device with a certain key in an
 * additional index. Returns NULL if no match could be found. The previous
 * entry is atomatically released, so that mb_cache_entry_find_next() can
 * be called like this:
 *
 * entry = mb_cache_entry_find_first();
 * while (entry) {
 *  ...
 *  entry = mb_cache_entry_find_next(entry, ...);
 * }
 *
 * @prev: The previous match
 * @bdev: the device the cache entry should belong to
 * @key: the key in the index
 */
struct mb_cache_entry *
mb_cache_entry_find_next(struct mb_cache_entry *prev,
             struct block_device *bdev, unsigned int key)
{
    struct mb_cache *cache = prev->e_cache;
    unsigned int bucket = hash_long(key, cache->c_bucket_bits);
    struct list_head *l;
    struct mb_cache_entry *ce;

    spin_lock(&mb_cache_spinlock);
    l = prev->e_index.o_list.next;
    ce = __mb_cache_entry_find(l, &cache->c_index_hash[bucket], bdev, key);
    __mb_cache_entry_release_unlock(prev);
    return ce;
}

#endif  /* !defined(MB_CACHE_INDEXES_COUNT) || (MB_CACHE_INDEXES_COUNT > 0) */

static int __init init_mbcache(void)
{
    register_shrinker(&mb_cache_shrinker);
    return 0;
}

static void __exit exit_mbcache(void)
{
    unregister_shrinker(&mb_cache_shrinker);
}

module_init(init_mbcache)
module_exit(exit_mbcache)