// SPDX-License-Identifier: GPL-2.0-only
#include <linux/spinlock.h>
#include <linux/slab.h>
#include <linux/list.h>
#include <linux/list_bl.h>
#include <linux/module.h>
#include <linux/sched.h>
#include <linux/workqueue.h>
#include <linux/mbcache.h>

/*
 * Mbcache is a simple key-value store. Keys need not be unique, however
 * key-value pairs are expected to be unique (we use this fact in
 * mb_cache_entry_delete_or_get()).
 *
 * Ext2 and ext4 use this cache for deduplication of extended attribute blocks.
 * Ext4 also uses it for deduplication of xattr values stored in inodes.
 * They use hash of data as a key and provide a value that may represent a
 * block or inode number. That's why keys need not be unique (hash of different
 * data may be the same). However user provided value always uniquely
 * identifies a cache entry.
 *
 * We provide functions for creation and removal of entries, search by key,
 * and a special "delete entry with given key-value pair" operation. Fixed
 * size hash table is used for fast key lookups.
 */

struct mb_cache {
        /* Hash table of entries */
        struct hlist_bl_head    *c_hash;
        /* log2 of hash table size */
        int                     c_bucket_bits;
        /* Maximum entries in cache to avoid degrading hash too much */
        unsigned long           c_max_entries;
        /* Protects c_list, c_entry_count */
        spinlock_t              c_list_lock;
        struct list_head        c_list;
        /* Number of entries in cache */
        unsigned long           c_entry_count;
        struct shrinker         *c_shrink;
        /* Work for shrinking when the cache has too many entries */
        struct work_struct      c_shrink_work;
};

static struct kmem_cache *mb_entry_cache;

static unsigned long mb_cache_shrink(struct mb_cache *cache,
                                     unsigned long nr_to_scan);

static inline struct hlist_bl_head *mb_cache_entry_head(struct mb_cache *cache,
                                                        u32 key)
{
        return &cache->c_hash[hash_32(key, cache->c_bucket_bits)];
}

/*
 * Number of entries to reclaim synchronously when there are too many entries
 * in cache
 */
#define SYNC_SHRINK_BATCH 64

/*
 * mb_cache_entry_create - create entry in cache
 * @cache - cache where the entry should be created
 * @mask - gfp mask with which the entry should be allocated
 * @key - key of the entry
 * @value - value of the entry
 * @reusable - is the entry reusable by others?
 *
 * Creates entry in @cache with key @key and value @value. The function returns
 * -EBUSY if entry with the same key and value already exists in cache.
 * Otherwise 0 is returned.
 */
int mb_cache_entry_create(struct mb_cache *cache, gfp_t mask, u32 key,
                          u64 value, bool reusable)
{
        struct mb_cache_entry *entry, *dup;
        struct hlist_bl_node *dup_node;
        struct hlist_bl_head *head;

        /* Schedule background reclaim if there are too many entries */
        if (cache->c_entry_count >= cache->c_max_entries)
                schedule_work(&cache->c_shrink_work);
        /* Do some sync reclaim if background reclaim cannot keep up */
        if (cache->c_entry_count >= 2*cache->c_max_entries)
                mb_cache_shrink(cache, SYNC_SHRINK_BATCH);

        entry = kmem_cache_alloc(mb_entry_cache, mask);
        if (!entry)
                return -ENOMEM;

        INIT_LIST_HEAD(&entry->e_list);
        /*
         * We create entry with two references. One reference is kept by the
         * hash table, the other reference is used to protect us from
         * mb_cache_entry_delete_or_get() until the entry is fully setup. This
         * avoids nesting of cache->c_list_lock into hash table bit locks which
         * is problematic for RT.
         */
        atomic_set(&entry->e_refcnt, 2);
        entry->e_key = key;
        entry->e_value = value;
        entry->e_flags = 0;
        if (reusable)
                set_bit(MBE_REUSABLE_B, &entry->e_flags);
        head = mb_cache_entry_head(cache, key);
        hlist_bl_lock(head);
        hlist_bl_for_each_entry(dup, dup_node, head, e_hash_list) {
                if (dup->e_key == key && dup->e_value == value) {
                        hlist_bl_unlock(head);
                        kmem_cache_free(mb_entry_cache, entry);
                        return -EBUSY;
                }
        }
        hlist_bl_add_head(&entry->e_hash_list, head);
        hlist_bl_unlock(head);
        spin_lock(&cache->c_list_lock);
        list_add_tail(&entry->e_list, &cache->c_list);
        cache->c_entry_count++;
        spin_unlock(&cache->c_list_lock);
        mb_cache_entry_put(cache, entry);

        return 0;
}
EXPORT_SYMBOL(mb_cache_entry_create);

void __mb_cache_entry_free(struct mb_cache *cache, struct mb_cache_entry *entry)
{
        struct hlist_bl_head *head;

        head = mb_cache_entry_head(cache, entry->e_key);
        hlist_bl_lock(head);
        hlist_bl_del(&entry->e_hash_list);
        hlist_bl_unlock(head);
        kmem_cache_free(mb_entry_cache, entry);
}
EXPORT_SYMBOL(__mb_cache_entry_free);

/*
 * mb_cache_entry_wait_unused - wait to be the last user of the entry
 *
 * @entry - entry to work on
 *
 * Wait to be the last user of the entry.
 */
void mb_cache_entry_wait_unused(struct mb_cache_entry *entry)
{
        wait_var_event(&entry->e_refcnt, atomic_read(&entry->e_refcnt) <= 2);
}
EXPORT_SYMBOL(mb_cache_entry_wait_unused);

static struct mb_cache_entry *__entry_find(struct mb_cache *cache,
                                           struct mb_cache_entry *entry,
                                           u32 key)
{
        struct mb_cache_entry *old_entry = entry;
        struct hlist_bl_node *node;
        struct hlist_bl_head *head;

        head = mb_cache_entry_head(cache, key);
        hlist_bl_lock(head);
        if (entry && !hlist_bl_unhashed(&entry->e_hash_list))
                node = entry->e_hash_list.next;
        else
                node = hlist_bl_first(head);
        while (node) {
                entry = hlist_bl_entry(node, struct mb_cache_entry,
                                       e_hash_list);
                if (entry->e_key == key &&
                    test_bit(MBE_REUSABLE_B, &entry->e_flags) &&
                    atomic_inc_not_zero(&entry->e_refcnt))
                        goto out;
                node = node->next;
        }
        entry = NULL;
out:
        hlist_bl_unlock(head);
        if (old_entry)
                mb_cache_entry_put(cache, old_entry);

        return entry;
}

/*
 * mb_cache_entry_find_first - find the first reusable entry with the given key
 * @cache: cache where we should search
 * @key: key to look for
 *
 * Search in @cache for a reusable entry with key @key. Grabs reference to the
 * first reusable entry found and returns the entry.
 */
struct mb_cache_entry *mb_cache_entry_find_first(struct mb_cache *cache,
                                                 u32 key)
{
        return __entry_find(cache, NULL, key);
}
EXPORT_SYMBOL(mb_cache_entry_find_first);

/*
 * mb_cache_entry_find_next - find next reusable entry with the same key
 * @cache: cache where we should search
 * @entry: entry to start search from
 *
 * Finds next reusable entry in the hash chain which has the same key as @entry.
 * If @entry is unhashed (which can happen when deletion of entry races with the
 * search), finds the first reusable entry in the hash chain. The function drops
 * reference to @entry and returns with a reference to the found entry.
 */
struct mb_cache_entry *mb_cache_entry_find_next(struct mb_cache *cache,
                                                struct mb_cache_entry *entry)
{
        return __entry_find(cache, entry, entry->e_key);
}
EXPORT_SYMBOL(mb_cache_entry_find_next);

/*
 * mb_cache_entry_get - get a cache entry by value (and key)
 * @cache - cache we work with
 * @key - key
 * @value - value
 */
struct mb_cache_entry *mb_cache_entry_get(struct mb_cache *cache, u32 key,
                                          u64 value)
{
        struct hlist_bl_node *node;
        struct hlist_bl_head *head;
        struct mb_cache_entry *entry;

        head = mb_cache_entry_head(cache, key);
        hlist_bl_lock(head);
        hlist_bl_for_each_entry(entry, node, head, e_hash_list) {
                if (entry->e_key == key && entry->e_value == value &&
                    atomic_inc_not_zero(&entry->e_refcnt))
                        goto out;
        }
        entry = NULL;
out:
        hlist_bl_unlock(head);
        return entry;
}
EXPORT_SYMBOL(mb_cache_entry_get);

/* mb_cache_entry_delete_or_get - remove a cache entry if it has no users
 * @cache - cache we work with
 * @key - key
 * @value - value
 *
 * Remove entry from cache @cache with key @key and value @value. The removal
 * happens only if the entry is unused. The function returns NULL in case the
 * entry was successfully removed or there's no entry in cache. Otherwise the
 * function grabs reference of the entry that we failed to delete because it
 * still has users and return it.
 */
struct mb_cache_entry *mb_cache_entry_delete_or_get(struct mb_cache *cache,
                                                    u32 key, u64 value)
{
        struct mb_cache_entry *entry;

        entry = mb_cache_entry_get(cache, key, value);
        if (!entry)
                return NULL;

        /*
         * Drop the ref we got from mb_cache_entry_get() and the initial hash
         * ref if we are the last user
         */
        if (atomic_cmpxchg(&entry->e_refcnt, 2, 0) != 2)
                return entry;

        spin_lock(&cache->c_list_lock);
        if (!list_empty(&entry->e_list))
                list_del_init(&entry->e_list);
        cache->c_entry_count--;
        spin_unlock(&cache->c_list_lock);
        __mb_cache_entry_free(cache, entry);
        return NULL;
}
EXPORT_SYMBOL(mb_cache_entry_delete_or_get);

/* mb_cache_entry_touch - cache entry got used
 * @cache - cache the entry belongs to
 * @entry - entry that got used
 *
 * Marks entry as used to give hit higher chances of surviving in cache.
 */
void mb_cache_entry_touch(struct mb_cache *cache,
                          struct mb_cache_entry *entry)
{
        set_bit(MBE_REFERENCED_B, &entry->e_flags);
}
EXPORT_SYMBOL(mb_cache_entry_touch);

static unsigned long mb_cache_count(struct shrinker *shrink,
                                    struct shrink_control *sc)
{
        struct mb_cache *cache = shrink->private_data;

        return cache->c_entry_count;
}

/* Shrink number of entries in cache */
static unsigned long mb_cache_shrink(struct mb_cache *cache,
                                     unsigned long nr_to_scan)
{
        struct mb_cache_entry *entry;
        unsigned long shrunk = 0;

        spin_lock(&cache->c_list_lock);
        while (nr_to_scan-- && !list_empty(&cache->c_list)) {
                entry = list_first_entry(&cache->c_list,
                                         struct mb_cache_entry, e_list);
                /* Drop initial hash reference if there is no user */
                if (test_bit(MBE_REFERENCED_B, &entry->e_flags) ||
                    atomic_cmpxchg(&entry->e_refcnt, 1, 0) != 1) {
                        clear_bit(MBE_REFERENCED_B, &entry->e_flags);
                        list_move_tail(&entry->e_list, &cache->c_list);
                        continue;
                }
                list_del_init(&entry->e_list);
                cache->c_entry_count--;
                spin_unlock(&cache->c_list_lock);
                __mb_cache_entry_free(cache, entry);
                shrunk++;
                cond_resched();
                spin_lock(&cache->c_list_lock);
        }
        spin_unlock(&cache->c_list_lock);

        return shrunk;
}

static unsigned long mb_cache_scan(struct shrinker *shrink,
                                   struct shrink_control *sc)
{
        struct mb_cache *cache = shrink->private_data;
        return mb_cache_shrink(cache, sc->nr_to_scan);
}

/* We shrink 1/X of the cache when we have too many entries in it */
#define SHRINK_DIVISOR 16

static void mb_cache_shrink_worker(struct work_struct *work)
{
        struct mb_cache *cache = container_of(work, struct mb_cache,
                                              c_shrink_work);
        mb_cache_shrink(cache, cache->c_max_entries / SHRINK_DIVISOR);
}

/*
 * mb_cache_create - create cache
 * @bucket_bits: log2 of the hash table size
 *
 * Create cache for keys with 2^bucket_bits hash entries.
 */
struct mb_cache *mb_cache_create(int bucket_bits)
{
        struct mb_cache *cache;
        unsigned long bucket_count = 1UL << bucket_bits;
        unsigned long i;

        cache = kzalloc_obj(struct mb_cache);
        if (!cache)
                goto err_out;
        cache->c_bucket_bits = bucket_bits;
        cache->c_max_entries = bucket_count << 4;
        INIT_LIST_HEAD(&cache->c_list);
        spin_lock_init(&cache->c_list_lock);
        cache->c_hash = kmalloc_objs(struct hlist_bl_head, bucket_count);
        if (!cache->c_hash) {
                kfree(cache);
                goto err_out;
        }
        for (i = 0; i < bucket_count; i++)
                INIT_HLIST_BL_HEAD(&cache->c_hash[i]);

        cache->c_shrink = shrinker_alloc(0, "mbcache-shrinker");
        if (!cache->c_shrink) {
                kfree(cache->c_hash);
                kfree(cache);
                goto err_out;
        }

        cache->c_shrink->count_objects = mb_cache_count;
        cache->c_shrink->scan_objects = mb_cache_scan;
        cache->c_shrink->private_data = cache;

        shrinker_register(cache->c_shrink);

        INIT_WORK(&cache->c_shrink_work, mb_cache_shrink_worker);

        return cache;

err_out:
        return NULL;
}
EXPORT_SYMBOL(mb_cache_create);

/*
 * mb_cache_destroy - destroy cache
 * @cache: the cache to destroy
 *
 * Free all entries in cache and cache itself. Caller must make sure nobody
 * (except shrinker) can reach @cache when calling this.
 */
void mb_cache_destroy(struct mb_cache *cache)
{
        struct mb_cache_entry *entry, *next;

        shrinker_free(cache->c_shrink);

        /*
         * We don't bother with any locking. Cache must not be used at this
         * point.
         */
        list_for_each_entry_safe(entry, next, &cache->c_list, e_list) {
                list_del(&entry->e_list);
                WARN_ON(atomic_read(&entry->e_refcnt) != 1);
                mb_cache_entry_put(cache, entry);
        }
        kfree(cache->c_hash);
        kfree(cache);
}
EXPORT_SYMBOL(mb_cache_destroy);

static int __init mbcache_init(void)
{
        mb_entry_cache = KMEM_CACHE(mb_cache_entry, SLAB_RECLAIM_ACCOUNT);
        if (!mb_entry_cache)
                return -ENOMEM;
        return 0;
}

static void __exit mbcache_exit(void)
{
        kmem_cache_destroy(mb_entry_cache);
}

module_init(mbcache_init)
module_exit(mbcache_exit)

MODULE_AUTHOR("Jan Kara <jack@suse.cz>");
MODULE_DESCRIPTION("Meta block cache (for extended attributes)");
MODULE_LICENSE("GPL");