// 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()).

   *
   * 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);

  	/* One ref for hash, one ref returned */
  	atomic_set(&entry->e_refcnt, 1);
  	entry->e_key = key;

  	entry->e_value = value;

  	entry->e_reusable = reusable;

  	entry->e_referenced = 0;

  	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);

  	/* Grab ref for LRU list */
  	atomic_inc(&entry->e_refcnt);
  	cache->c_entry_count++;

  	spin_unlock(&cache->c_list_lock);

  
  	return 0;
  }

  EXPORT_SYMBOL(mb_cache_entry_create);

  void __mb_cache_entry_free(struct mb_cache_entry *entry)

  {

  	kmem_cache_free(mb_entry_cache, entry);

  }

  EXPORT_SYMBOL(__mb_cache_entry_free);

  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 && entry->e_reusable) {

  			atomic_inc(&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(&entry->e_refcnt);
  			goto out;
  		}
  	}
  	entry = NULL;
  out:
  	hlist_bl_unlock(head);
  	return entry;
  }
  EXPORT_SYMBOL(mb_cache_entry_get);

  /* mb_cache_entry_delete - remove a cache entry

   * @cache - cache we work with

   * @key - key
   * @value - value

   *

   * Remove entry from cache @cache with key @key and value @value.

   */

  void mb_cache_entry_delete(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) {

  			/* We keep hash list reference to keep entry alive */
  			hlist_bl_del_init(&entry->e_hash_list);
  			hlist_bl_unlock(head);

  			spin_lock(&cache->c_list_lock);
  			if (!list_empty(&entry->e_list)) {
  				list_del_init(&entry->e_list);

  				if (!WARN_ONCE(cache->c_entry_count == 0,
  		"mbcache: attempt to decrement c_entry_count past zero"))
  					cache->c_entry_count--;

  				atomic_dec(&entry->e_refcnt);
  			}

  			spin_unlock(&cache->c_list_lock);

  			mb_cache_entry_put(cache, entry);

  			return;
  		}
  	}
  	hlist_bl_unlock(head);
  }

  EXPORT_SYMBOL(mb_cache_entry_delete);

  /* 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)

  {

  	entry->e_referenced = 1;

  }

  EXPORT_SYMBOL(mb_cache_entry_touch);

  static unsigned long mb_cache_count(struct shrinker *shrink,
  				    struct shrink_control *sc)

  {

  	struct mb_cache *cache = container_of(shrink, struct mb_cache,
  					      c_shrink);

  
  	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;

  	struct hlist_bl_head *head;

  	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);

  		if (entry->e_referenced) {
  			entry->e_referenced = 0;

  			list_move_tail(&entry->e_list, &cache->c_list);

  			continue;
  		}
  		list_del_init(&entry->e_list);

  		cache->c_entry_count--;
  		/*
  		 * We keep LRU list reference so that entry doesn't go away
  		 * from under us.
  		 */

  		spin_unlock(&cache->c_list_lock);

  		head = mb_cache_entry_head(cache, entry->e_key);

  		hlist_bl_lock(head);
  		if (!hlist_bl_unhashed(&entry->e_hash_list)) {
  			hlist_bl_del_init(&entry->e_hash_list);
  			atomic_dec(&entry->e_refcnt);
  		}
  		hlist_bl_unlock(head);

  		if (mb_cache_entry_put(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 = container_of(shrink, struct mb_cache,

  					      c_shrink);

  	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(sizeof(struct mb_cache), GFP_KERNEL);

  	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_array(bucket_count,
  				      sizeof(struct hlist_bl_head),
  				      GFP_KERNEL);

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

  	cache->c_shrink.seeks = DEFAULT_SEEKS;

  	if (register_shrinker(&cache->c_shrink)) {
  		kfree(cache->c_hash);
  		kfree(cache);
  		goto err_out;
  	}

  	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;

  
  	unregister_shrinker(&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) {

  		if (!hlist_bl_unhashed(&entry->e_hash_list)) {
  			hlist_bl_del_init(&entry->e_hash_list);
  			atomic_dec(&entry->e_refcnt);
  		} else
  			WARN_ON(1);

  		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_create("mbcache",
  				sizeof(struct mb_cache_entry), 0,

  				SLAB_RECLAIM_ACCOUNT|SLAB_MEM_SPREAD, NULL);

  	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");