// SPDX-License-Identifier: GPL-2.0-or-later

  /*
   * zswap.c - zswap driver file
   *
   * zswap is a backend for frontswap that takes pages that are in the process
   * of being swapped out and attempts to compress and store them in a
   * RAM-based memory pool.  This can result in a significant I/O reduction on
   * the swap device and, in the case where decompressing from RAM is faster
   * than reading from the swap device, can also improve workload performance.
   *
   * Copyright (C) 2012  Seth Jennings <sjenning@linux.vnet.ibm.com>

  */
  
  #define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
  
  #include <linux/module.h>
  #include <linux/cpu.h>
  #include <linux/highmem.h>
  #include <linux/slab.h>
  #include <linux/spinlock.h>
  #include <linux/types.h>
  #include <linux/atomic.h>
  #include <linux/frontswap.h>
  #include <linux/rbtree.h>
  #include <linux/swap.h>
  #include <linux/crypto.h>
  #include <linux/mempool.h>

  #include <linux/zpool.h>

  
  #include <linux/mm_types.h>
  #include <linux/page-flags.h>
  #include <linux/swapops.h>
  #include <linux/writeback.h>
  #include <linux/pagemap.h>

  #include <linux/workqueue.h>

  
  /*********************************
  * statistics
  **********************************/

  /* Total bytes used by the compressed storage */
  static u64 zswap_pool_total_size;

  /* The number of compressed pages currently stored in zswap */
  static atomic_t zswap_stored_pages = ATOMIC_INIT(0);

  /* The number of same-value filled pages currently stored in zswap */
  static atomic_t zswap_same_filled_pages = ATOMIC_INIT(0);

  
  /*
   * The statistics below are not protected from concurrent access for
   * performance reasons so they may not be a 100% accurate.  However,
   * they do provide useful information on roughly how many times a
   * certain event is occurring.
  */
  
  /* Pool limit was hit (see zswap_max_pool_percent) */
  static u64 zswap_pool_limit_hit;
  /* Pages written back when pool limit was reached */
  static u64 zswap_written_back_pages;
  /* Store failed due to a reclaim failure after pool limit was reached */
  static u64 zswap_reject_reclaim_fail;
  /* Compressed page was too big for the allocator to (optimally) store */
  static u64 zswap_reject_compress_poor;
  /* Store failed because underlying allocator could not get memory */
  static u64 zswap_reject_alloc_fail;
  /* Store failed because the entry metadata could not be allocated (rare) */
  static u64 zswap_reject_kmemcache_fail;
  /* Duplicate store was encountered (rare) */
  static u64 zswap_duplicate_entry;

  /* Shrinker work queue */
  static struct workqueue_struct *shrink_wq;
  /* Pool limit was hit, we need to calm down */
  static bool zswap_pool_reached_full;

  /*********************************
  * tunables
  **********************************/

  #define ZSWAP_PARAM_UNSET ""

  /* Enable/disable zswap */
  static bool zswap_enabled = IS_ENABLED(CONFIG_ZSWAP_DEFAULT_ON);

  static int zswap_enabled_param_set(const char *,
  				   const struct kernel_param *);
  static struct kernel_param_ops zswap_enabled_param_ops = {
  	.set =		zswap_enabled_param_set,
  	.get =		param_get_bool,
  };
  module_param_cb(enabled, &zswap_enabled_param_ops, &zswap_enabled, 0644);

  /* Crypto compressor to use */

  static char *zswap_compressor = CONFIG_ZSWAP_COMPRESSOR_DEFAULT;

  static int zswap_compressor_param_set(const char *,
  				      const struct kernel_param *);
  static struct kernel_param_ops zswap_compressor_param_ops = {
  	.set =		zswap_compressor_param_set,

  	.get =		param_get_charp,
  	.free =		param_free_charp,

  };
  module_param_cb(compressor, &zswap_compressor_param_ops,

  		&zswap_compressor, 0644);

  /* Compressed storage zpool to use */

  static char *zswap_zpool_type = CONFIG_ZSWAP_ZPOOL_DEFAULT;

  static int zswap_zpool_param_set(const char *, const struct kernel_param *);
  static struct kernel_param_ops zswap_zpool_param_ops = {

  	.set =		zswap_zpool_param_set,
  	.get =		param_get_charp,
  	.free =		param_free_charp,

  };

  module_param_cb(zpool, &zswap_zpool_param_ops, &zswap_zpool_type, 0644);

  /* The maximum percentage of memory that the compressed pool can occupy */
  static unsigned int zswap_max_pool_percent = 20;
  module_param_named(max_pool_percent, zswap_max_pool_percent, uint, 0644);

  /* The threshold for accepting new pages after the max_pool_percent was hit */
  static unsigned int zswap_accept_thr_percent = 90; /* of max pool size */
  module_param_named(accept_threshold_percent, zswap_accept_thr_percent,
  		   uint, 0644);

  /* Enable/disable handling same-value filled pages (enabled by default) */
  static bool zswap_same_filled_pages_enabled = true;
  module_param_named(same_filled_pages_enabled, zswap_same_filled_pages_enabled,
  		   bool, 0644);

  /*********************************

  * data structures

  **********************************/

  struct zswap_pool {
  	struct zpool *zpool;
  	struct crypto_comp * __percpu *tfm;
  	struct kref kref;
  	struct list_head list;

  	struct work_struct release_work;
  	struct work_struct shrink_work;

  	struct hlist_node node;

  	char tfm_name[CRYPTO_MAX_ALG_NAME];

  };

  /*
   * struct zswap_entry
   *
   * This structure contains the metadata for tracking a single compressed
   * page within zswap.
   *
   * rbnode - links the entry into red-black tree for the appropriate swap type

   * offset - the swap offset for the entry.  Index into the red-black tree.

   * refcount - the number of outstanding reference to the entry. This is needed
   *            to protect against premature freeing of the entry by code

   *            concurrent calls to load, invalidate, and writeback.  The lock

   *            for the zswap_tree structure that contains the entry must
   *            be held while changing the refcount.  Since the lock must
   *            be held, there is no reason to also make refcount atomic.

   * length - the length in bytes of the compressed page data.  Needed during

   *          decompression. For a same value filled page length is 0.

   * pool - the zswap_pool the entry's data is in
   * handle - zpool allocation handle that stores the compressed page data

   * value - value of the same-value filled pages which have same content

   */
  struct zswap_entry {
  	struct rb_node rbnode;
  	pgoff_t offset;
  	int refcount;
  	unsigned int length;

  	struct zswap_pool *pool;

  	union {
  		unsigned long handle;
  		unsigned long value;
  	};

  };
  
  struct zswap_header {
  	swp_entry_t swpentry;
  };
  
  /*
   * The tree lock in the zswap_tree struct protects a few things:
   * - the rbtree
   * - the refcount field of each entry in the tree
   */
  struct zswap_tree {
  	struct rb_root rbroot;
  	spinlock_t lock;

  };
  
  static struct zswap_tree *zswap_trees[MAX_SWAPFILES];

  /* RCU-protected iteration */
  static LIST_HEAD(zswap_pools);
  /* protects zswap_pools list modification */
  static DEFINE_SPINLOCK(zswap_pools_lock);

  /* pool counter to provide unique names to zpool */
  static atomic_t zswap_pools_count = ATOMIC_INIT(0);

  /* used by param callback function */
  static bool zswap_init_started;

  /* fatal error during init */
  static bool zswap_init_failed;

  /* init completed, but couldn't create the initial pool */
  static bool zswap_has_pool;

  /*********************************
  * helpers and fwd declarations
  **********************************/
  
  #define zswap_pool_debug(msg, p)				\
  	pr_debug("%s pool %s/%s
  ", msg, (p)->tfm_name,		\
  		 zpool_get_type((p)->zpool))
  
  static int zswap_writeback_entry(struct zpool *pool, unsigned long handle);
  static int zswap_pool_get(struct zswap_pool *pool);
  static void zswap_pool_put(struct zswap_pool *pool);
  
  static const struct zpool_ops zswap_zpool_ops = {
  	.evict = zswap_writeback_entry
  };
  
  static bool zswap_is_full(void)
  {

  	return totalram_pages() * zswap_max_pool_percent / 100 <
  			DIV_ROUND_UP(zswap_pool_total_size, PAGE_SIZE);

  }

  static bool zswap_can_accept(void)
  {
  	return totalram_pages() * zswap_accept_thr_percent / 100 *
  				zswap_max_pool_percent / 100 >
  			DIV_ROUND_UP(zswap_pool_total_size, PAGE_SIZE);
  }

  static void zswap_update_total_size(void)
  {
  	struct zswap_pool *pool;
  	u64 total = 0;
  
  	rcu_read_lock();
  
  	list_for_each_entry_rcu(pool, &zswap_pools, list)
  		total += zpool_get_total_size(pool->zpool);
  
  	rcu_read_unlock();
  
  	zswap_pool_total_size = total;
  }

  /*********************************
  * zswap entry functions
  **********************************/
  static struct kmem_cache *zswap_entry_cache;

  static int __init zswap_entry_cache_create(void)

  {
  	zswap_entry_cache = KMEM_CACHE(zswap_entry, 0);

  	return zswap_entry_cache == NULL;

  }

  static void __init zswap_entry_cache_destroy(void)

  {
  	kmem_cache_destroy(zswap_entry_cache);
  }
  
  static struct zswap_entry *zswap_entry_cache_alloc(gfp_t gfp)
  {
  	struct zswap_entry *entry;
  	entry = kmem_cache_alloc(zswap_entry_cache, gfp);
  	if (!entry)
  		return NULL;
  	entry->refcount = 1;

  	RB_CLEAR_NODE(&entry->rbnode);

  	return entry;
  }
  
  static void zswap_entry_cache_free(struct zswap_entry *entry)
  {
  	kmem_cache_free(zswap_entry_cache, entry);
  }

  /*********************************
  * rbtree functions
  **********************************/
  static struct zswap_entry *zswap_rb_search(struct rb_root *root, pgoff_t offset)
  {
  	struct rb_node *node = root->rb_node;
  	struct zswap_entry *entry;
  
  	while (node) {
  		entry = rb_entry(node, struct zswap_entry, rbnode);
  		if (entry->offset > offset)
  			node = node->rb_left;
  		else if (entry->offset < offset)
  			node = node->rb_right;
  		else
  			return entry;
  	}
  	return NULL;
  }
  
  /*
   * In the case that a entry with the same offset is found, a pointer to
   * the existing entry is stored in dupentry and the function returns -EEXIST
   */
  static int zswap_rb_insert(struct rb_root *root, struct zswap_entry *entry,
  			struct zswap_entry **dupentry)
  {
  	struct rb_node **link = &root->rb_node, *parent = NULL;
  	struct zswap_entry *myentry;
  
  	while (*link) {
  		parent = *link;
  		myentry = rb_entry(parent, struct zswap_entry, rbnode);
  		if (myentry->offset > entry->offset)
  			link = &(*link)->rb_left;
  		else if (myentry->offset < entry->offset)
  			link = &(*link)->rb_right;
  		else {
  			*dupentry = myentry;
  			return -EEXIST;
  		}
  	}
  	rb_link_node(&entry->rbnode, parent, link);
  	rb_insert_color(&entry->rbnode, root);
  	return 0;
  }

  static void zswap_rb_erase(struct rb_root *root, struct zswap_entry *entry)
  {
  	if (!RB_EMPTY_NODE(&entry->rbnode)) {
  		rb_erase(&entry->rbnode, root);
  		RB_CLEAR_NODE(&entry->rbnode);
  	}
  }
  
  /*

   * Carries out the common pattern of freeing and entry's zpool allocation,

   * freeing the entry itself, and decrementing the number of stored pages.
   */

  static void zswap_free_entry(struct zswap_entry *entry)

  {

  	if (!entry->length)
  		atomic_dec(&zswap_same_filled_pages);
  	else {
  		zpool_free(entry->pool->zpool, entry->handle);
  		zswap_pool_put(entry->pool);
  	}

  	zswap_entry_cache_free(entry);
  	atomic_dec(&zswap_stored_pages);

  	zswap_update_total_size();

  }
  
  /* caller must hold the tree lock */
  static void zswap_entry_get(struct zswap_entry *entry)
  {
  	entry->refcount++;
  }
  
  /* caller must hold the tree lock
  * remove from the tree and free it, if nobody reference the entry
  */
  static void zswap_entry_put(struct zswap_tree *tree,
  			struct zswap_entry *entry)
  {
  	int refcount = --entry->refcount;
  
  	BUG_ON(refcount < 0);
  	if (refcount == 0) {
  		zswap_rb_erase(&tree->rbroot, entry);

  		zswap_free_entry(entry);

  	}
  }
  
  /* caller must hold the tree lock */
  static struct zswap_entry *zswap_entry_find_get(struct rb_root *root,
  				pgoff_t offset)
  {

  	struct zswap_entry *entry;

  
  	entry = zswap_rb_search(root, offset);
  	if (entry)
  		zswap_entry_get(entry);
  
  	return entry;
  }

  /*********************************
  * per-cpu code
  **********************************/
  static DEFINE_PER_CPU(u8 *, zswap_dstmem);

  static int zswap_dstmem_prepare(unsigned int cpu)

  {

  	u8 *dst;

  	dst = kmalloc_node(PAGE_SIZE * 2, GFP_KERNEL, cpu_to_node(cpu));

  	if (!dst)

  		return -ENOMEM;

  	per_cpu(zswap_dstmem, cpu) = dst;
  	return 0;

  }

  static int zswap_dstmem_dead(unsigned int cpu)

  {

  	u8 *dst;

  	dst = per_cpu(zswap_dstmem, cpu);
  	kfree(dst);
  	per_cpu(zswap_dstmem, cpu) = NULL;

  	return 0;

  }

  static int zswap_cpu_comp_prepare(unsigned int cpu, struct hlist_node *node)

  {

  	struct zswap_pool *pool = hlist_entry(node, struct zswap_pool, node);

  	struct crypto_comp *tfm;

  	if (WARN_ON(*per_cpu_ptr(pool->tfm, cpu)))
  		return 0;

  	tfm = crypto_alloc_comp(pool->tfm_name, 0, 0);
  	if (IS_ERR_OR_NULL(tfm)) {
  		pr_err("could not alloc crypto comp %s : %ld
  ",
  		       pool->tfm_name, PTR_ERR(tfm));
  		return -ENOMEM;
  	}
  	*per_cpu_ptr(pool->tfm, cpu) = tfm;

  	return 0;

  }

  static int zswap_cpu_comp_dead(unsigned int cpu, struct hlist_node *node)

  {

  	struct zswap_pool *pool = hlist_entry(node, struct zswap_pool, node);
  	struct crypto_comp *tfm;

  	tfm = *per_cpu_ptr(pool->tfm, cpu);
  	if (!IS_ERR_OR_NULL(tfm))
  		crypto_free_comp(tfm);
  	*per_cpu_ptr(pool->tfm, cpu) = NULL;
  	return 0;

  }

  /*********************************

  * pool functions

  **********************************/

  
  static struct zswap_pool *__zswap_pool_current(void)

  {

  	struct zswap_pool *pool;
  
  	pool = list_first_or_null_rcu(&zswap_pools, typeof(*pool), list);

  	WARN_ONCE(!pool && zswap_has_pool,
  		  "%s: no page storage pool!
  ", __func__);

  
  	return pool;
  }
  
  static struct zswap_pool *zswap_pool_current(void)
  {
  	assert_spin_locked(&zswap_pools_lock);
  
  	return __zswap_pool_current();
  }
  
  static struct zswap_pool *zswap_pool_current_get(void)
  {
  	struct zswap_pool *pool;
  
  	rcu_read_lock();
  
  	pool = __zswap_pool_current();

  	if (!zswap_pool_get(pool))

  		pool = NULL;
  
  	rcu_read_unlock();
  
  	return pool;
  }
  
  static struct zswap_pool *zswap_pool_last_get(void)
  {
  	struct zswap_pool *pool, *last = NULL;
  
  	rcu_read_lock();
  
  	list_for_each_entry_rcu(pool, &zswap_pools, list)
  		last = pool;

  	WARN_ONCE(!last && zswap_has_pool,
  		  "%s: no page storage pool!
  ", __func__);
  	if (!zswap_pool_get(last))

  		last = NULL;
  
  	rcu_read_unlock();
  
  	return last;
  }

  /* type and compressor must be null-terminated */

  static struct zswap_pool *zswap_pool_find_get(char *type, char *compressor)
  {
  	struct zswap_pool *pool;
  
  	assert_spin_locked(&zswap_pools_lock);
  
  	list_for_each_entry_rcu(pool, &zswap_pools, list) {

  		if (strcmp(pool->tfm_name, compressor))

  			continue;

  		if (strcmp(zpool_get_type(pool->zpool), type))

  			continue;
  		/* if we can't get it, it's about to be destroyed */
  		if (!zswap_pool_get(pool))
  			continue;
  		return pool;
  	}
  
  	return NULL;
  }

  static void shrink_worker(struct work_struct *w)
  {
  	struct zswap_pool *pool = container_of(w, typeof(*pool),
  						shrink_work);
  
  	if (zpool_shrink(pool->zpool, 1, NULL))
  		zswap_reject_reclaim_fail++;
  	zswap_pool_put(pool);
  }

  static struct zswap_pool *zswap_pool_create(char *type, char *compressor)
  {
  	struct zswap_pool *pool;

  	char name[38]; /* 'zswap' + 32 char (max) num + \0 */

  	gfp_t gfp = __GFP_NORETRY | __GFP_NOWARN | __GFP_KSWAPD_RECLAIM;

  	int ret;

  	if (!zswap_has_pool) {
  		/* if either are unset, pool initialization failed, and we
  		 * need both params to be set correctly before trying to
  		 * create a pool.
  		 */
  		if (!strcmp(type, ZSWAP_PARAM_UNSET))
  			return NULL;
  		if (!strcmp(compressor, ZSWAP_PARAM_UNSET))
  			return NULL;
  	}

  	pool = kzalloc(sizeof(*pool), GFP_KERNEL);

  	if (!pool)

  		return NULL;

  	/* unique name for each pool specifically required by zsmalloc */
  	snprintf(name, 38, "zswap%x", atomic_inc_return(&zswap_pools_count));
  
  	pool->zpool = zpool_create_pool(type, name, gfp, &zswap_zpool_ops);

  	if (!pool->zpool) {
  		pr_err("%s zpool not available
  ", type);
  		goto error;
  	}
  	pr_debug("using %s zpool
  ", zpool_get_type(pool->zpool));
  
  	strlcpy(pool->tfm_name, compressor, sizeof(pool->tfm_name));
  	pool->tfm = alloc_percpu(struct crypto_comp *);
  	if (!pool->tfm) {
  		pr_err("percpu alloc failed
  ");
  		goto error;
  	}

  	ret = cpuhp_state_add_instance(CPUHP_MM_ZSWP_POOL_PREPARE,
  				       &pool->node);
  	if (ret)

  		goto error;
  	pr_debug("using %s compressor
  ", pool->tfm_name);
  
  	/* being the current pool takes 1 ref; this func expects the
  	 * caller to always add the new pool as the current pool
  	 */
  	kref_init(&pool->kref);
  	INIT_LIST_HEAD(&pool->list);

  	INIT_WORK(&pool->shrink_work, shrink_worker);

  
  	zswap_pool_debug("created", pool);
  
  	return pool;
  
  error:
  	free_percpu(pool->tfm);
  	if (pool->zpool)
  		zpool_destroy_pool(pool->zpool);
  	kfree(pool);
  	return NULL;
  }

  static __init struct zswap_pool *__zswap_pool_create_fallback(void)

  {

  	bool has_comp, has_zpool;
  
  	has_comp = crypto_has_comp(zswap_compressor, 0, 0);

  	if (!has_comp && strcmp(zswap_compressor,
  				CONFIG_ZSWAP_COMPRESSOR_DEFAULT)) {

  		pr_err("compressor %s not available, using default %s
  ",

  		       zswap_compressor, CONFIG_ZSWAP_COMPRESSOR_DEFAULT);

  		param_free_charp(&zswap_compressor);

  		zswap_compressor = CONFIG_ZSWAP_COMPRESSOR_DEFAULT;

  		has_comp = crypto_has_comp(zswap_compressor, 0, 0);

  	}

  	if (!has_comp) {
  		pr_err("default compressor %s not available
  ",
  		       zswap_compressor);
  		param_free_charp(&zswap_compressor);
  		zswap_compressor = ZSWAP_PARAM_UNSET;
  	}
  
  	has_zpool = zpool_has_pool(zswap_zpool_type);

  	if (!has_zpool && strcmp(zswap_zpool_type,
  				 CONFIG_ZSWAP_ZPOOL_DEFAULT)) {

  		pr_err("zpool %s not available, using default %s
  ",

  		       zswap_zpool_type, CONFIG_ZSWAP_ZPOOL_DEFAULT);

  		param_free_charp(&zswap_zpool_type);

  		zswap_zpool_type = CONFIG_ZSWAP_ZPOOL_DEFAULT;

  		has_zpool = zpool_has_pool(zswap_zpool_type);

  	}

  	if (!has_zpool) {
  		pr_err("default zpool %s not available
  ",
  		       zswap_zpool_type);
  		param_free_charp(&zswap_zpool_type);
  		zswap_zpool_type = ZSWAP_PARAM_UNSET;
  	}
  
  	if (!has_comp || !has_zpool)
  		return NULL;

  
  	return zswap_pool_create(zswap_zpool_type, zswap_compressor);
  }
  
  static void zswap_pool_destroy(struct zswap_pool *pool)
  {
  	zswap_pool_debug("destroying", pool);

  	cpuhp_state_remove_instance(CPUHP_MM_ZSWP_POOL_PREPARE, &pool->node);

  	free_percpu(pool->tfm);
  	zpool_destroy_pool(pool->zpool);
  	kfree(pool);
  }
  
  static int __must_check zswap_pool_get(struct zswap_pool *pool)
  {

  	if (!pool)
  		return 0;

  	return kref_get_unless_zero(&pool->kref);
  }

  static void __zswap_pool_release(struct work_struct *work)

  {

  	struct zswap_pool *pool = container_of(work, typeof(*pool),
  						release_work);

  
  	synchronize_rcu();

  
  	/* nobody should have been able to get a kref... */
  	WARN_ON(kref_get_unless_zero(&pool->kref));
  
  	/* pool is now off zswap_pools list and has no references. */
  	zswap_pool_destroy(pool);
  }
  
  static void __zswap_pool_empty(struct kref *kref)
  {
  	struct zswap_pool *pool;
  
  	pool = container_of(kref, typeof(*pool), kref);
  
  	spin_lock(&zswap_pools_lock);
  
  	WARN_ON(pool == zswap_pool_current());
  
  	list_del_rcu(&pool->list);

  	INIT_WORK(&pool->release_work, __zswap_pool_release);
  	schedule_work(&pool->release_work);

  
  	spin_unlock(&zswap_pools_lock);
  }
  
  static void zswap_pool_put(struct zswap_pool *pool)
  {
  	kref_put(&pool->kref, __zswap_pool_empty);

  }

  /*********************************

  * param callbacks
  **********************************/

  /* val must be a null-terminated string */

  static int __zswap_param_set(const char *val, const struct kernel_param *kp,
  			     char *type, char *compressor)
  {
  	struct zswap_pool *pool, *put_pool = NULL;

  	char *s = strstrip((char *)val);

  	int ret;

  	if (zswap_init_failed) {
  		pr_err("can't set param, initialization failed
  ");
  		return -ENODEV;
  	}

  	/* no change required */

  	if (!strcmp(s, *(char **)kp->arg) && zswap_has_pool)

  		return 0;

  
  	/* if this is load-time (pre-init) param setting,
  	 * don't create a pool; that's done during init.
  	 */
  	if (!zswap_init_started)

  		return param_set_charp(s, kp);

  
  	if (!type) {

  		if (!zpool_has_pool(s)) {
  			pr_err("zpool %s not available
  ", s);

  			return -ENOENT;
  		}

  		type = s;

  	} else if (!compressor) {

  		if (!crypto_has_comp(s, 0, 0)) {
  			pr_err("compressor %s not available
  ", s);

  			return -ENOENT;
  		}

  		compressor = s;
  	} else {
  		WARN_ON(1);
  		return -EINVAL;

  	}
  
  	spin_lock(&zswap_pools_lock);
  
  	pool = zswap_pool_find_get(type, compressor);
  	if (pool) {
  		zswap_pool_debug("using existing", pool);

  		WARN_ON(pool == zswap_pool_current());

  		list_del_rcu(&pool->list);

  	}

  	spin_unlock(&zswap_pools_lock);
  
  	if (!pool)
  		pool = zswap_pool_create(type, compressor);

  	if (pool)

  		ret = param_set_charp(s, kp);

  	else
  		ret = -EINVAL;

  	spin_lock(&zswap_pools_lock);

  	if (!ret) {
  		put_pool = zswap_pool_current();
  		list_add_rcu(&pool->list, &zswap_pools);

  		zswap_has_pool = true;

  	} else if (pool) {
  		/* add the possibly pre-existing pool to the end of the pools
  		 * list; if it's new (and empty) then it'll be removed and
  		 * destroyed by the put after we drop the lock
  		 */
  		list_add_tail_rcu(&pool->list, &zswap_pools);
  		put_pool = pool;

  	}
  
  	spin_unlock(&zswap_pools_lock);
  
  	if (!zswap_has_pool && !pool) {

  		/* if initial pool creation failed, and this pool creation also
  		 * failed, maybe both compressor and zpool params were bad.
  		 * Allow changing this param, so pool creation will succeed
  		 * when the other param is changed. We already verified this
  		 * param is ok in the zpool_has_pool() or crypto_has_comp()
  		 * checks above.
  		 */
  		ret = param_set_charp(s, kp);

  	}

  	/* drop the ref from either the old current pool,
  	 * or the new pool we failed to add
  	 */
  	if (put_pool)
  		zswap_pool_put(put_pool);
  
  	return ret;
  }
  
  static int zswap_compressor_param_set(const char *val,
  				      const struct kernel_param *kp)
  {
  	return __zswap_param_set(val, kp, zswap_zpool_type, NULL);
  }
  
  static int zswap_zpool_param_set(const char *val,
  				 const struct kernel_param *kp)
  {
  	return __zswap_param_set(val, kp, NULL, zswap_compressor);
  }

  static int zswap_enabled_param_set(const char *val,
  				   const struct kernel_param *kp)
  {
  	if (zswap_init_failed) {
  		pr_err("can't enable, initialization failed
  ");
  		return -ENODEV;
  	}

  	if (!zswap_has_pool && zswap_init_started) {
  		pr_err("can't enable, no pool configured
  ");
  		return -ENODEV;
  	}

  
  	return param_set_bool(val, kp);
  }

  /*********************************

  * writeback code
  **********************************/
  /* return enum for zswap_get_swap_cache_page */
  enum zswap_get_swap_ret {
  	ZSWAP_SWAPCACHE_NEW,
  	ZSWAP_SWAPCACHE_EXIST,

  	ZSWAP_SWAPCACHE_FAIL,

  };
  
  /*
   * zswap_get_swap_cache_page
   *
   * This is an adaption of read_swap_cache_async()
   *
   * This function tries to find a page with the given swap entry
   * in the swapper_space address space (the swap cache).  If the page
   * is found, it is returned in retpage.  Otherwise, a page is allocated,
   * added to the swap cache, and returned in retpage.
   *
   * If success, the swap cache page is returned in retpage

   * Returns ZSWAP_SWAPCACHE_EXIST if page was already in the swap cache
   * Returns ZSWAP_SWAPCACHE_NEW if the new page needs to be populated,
   *     the new page is added to swapcache and locked
   * Returns ZSWAP_SWAPCACHE_FAIL on error

   */
  static int zswap_get_swap_cache_page(swp_entry_t entry,
  				struct page **retpage)
  {

  	bool page_was_allocated;

  	*retpage = __read_swap_cache_async(entry, GFP_KERNEL,
  			NULL, 0, &page_was_allocated);
  	if (page_was_allocated)
  		return ZSWAP_SWAPCACHE_NEW;
  	if (!*retpage)

  		return ZSWAP_SWAPCACHE_FAIL;

  	return ZSWAP_SWAPCACHE_EXIST;
  }
  
  /*
   * Attempts to free an entry by adding a page to the swap cache,
   * decompressing the entry data into the page, and issuing a
   * bio write to write the page back to the swap device.
   *
   * This can be thought of as a "resumed writeback" of the page
   * to the swap device.  We are basically resuming the same swap
   * writeback path that was intercepted with the frontswap_store()
   * in the first place.  After the page has been decompressed into
   * the swap cache, the compressed version stored by zswap can be
   * freed.
   */

  static int zswap_writeback_entry(struct zpool *pool, unsigned long handle)

  {
  	struct zswap_header *zhdr;
  	swp_entry_t swpentry;
  	struct zswap_tree *tree;
  	pgoff_t offset;
  	struct zswap_entry *entry;
  	struct page *page;

  	struct crypto_comp *tfm;

  	u8 *src, *dst;
  	unsigned int dlen;

  	int ret;

  	struct writeback_control wbc = {
  		.sync_mode = WB_SYNC_NONE,
  	};
  
  	/* extract swpentry from data */

  	zhdr = zpool_map_handle(pool, handle, ZPOOL_MM_RO);

  	swpentry = zhdr->swpentry; /* here */

  	tree = zswap_trees[swp_type(swpentry)];
  	offset = swp_offset(swpentry);

  
  	/* find and ref zswap entry */
  	spin_lock(&tree->lock);

  	entry = zswap_entry_find_get(&tree->rbroot, offset);

  	if (!entry) {
  		/* entry was invalidated */
  		spin_unlock(&tree->lock);

  		zpool_unmap_handle(pool, handle);

  		return 0;
  	}

  	spin_unlock(&tree->lock);
  	BUG_ON(offset != entry->offset);
  
  	/* try to allocate swap cache page */
  	switch (zswap_get_swap_cache_page(swpentry, &page)) {

  	case ZSWAP_SWAPCACHE_FAIL: /* no memory or invalidate happened */

  		ret = -ENOMEM;
  		goto fail;

  	case ZSWAP_SWAPCACHE_EXIST:

  		/* page is already in the swap cache, ignore for now */

  		put_page(page);

  		ret = -EEXIST;
  		goto fail;
  
  	case ZSWAP_SWAPCACHE_NEW: /* page is locked */
  		/* decompress */
  		dlen = PAGE_SIZE;

  		src = (u8 *)zhdr + sizeof(struct zswap_header);

  		dst = kmap_atomic(page);

  		tfm = *get_cpu_ptr(entry->pool->tfm);
  		ret = crypto_comp_decompress(tfm, src, entry->length,
  					     dst, &dlen);
  		put_cpu_ptr(entry->pool->tfm);

  		kunmap_atomic(dst);

  		BUG_ON(ret);
  		BUG_ON(dlen != PAGE_SIZE);
  
  		/* page is up to date */
  		SetPageUptodate(page);
  	}

  	/* move it to the tail of the inactive list after end_writeback */
  	SetPageReclaim(page);

  	/* start writeback */
  	__swap_writepage(page, &wbc, end_swap_bio_write);

  	put_page(page);

  	zswap_written_back_pages++;
  
  	spin_lock(&tree->lock);

  	/* drop local reference */

  	zswap_entry_put(tree, entry);

  
  	/*

  	* There are two possible situations for entry here:
  	* (1) refcount is 1(normal case),  entry is valid and on the tree
  	* (2) refcount is 0, entry is freed and not on the tree
  	*     because invalidate happened during writeback
  	*  search the tree and free the entry if find entry
  	*/
  	if (entry == zswap_rb_search(&tree->rbroot, offset))
  		zswap_entry_put(tree, entry);

  	spin_unlock(&tree->lock);

  	goto end;
  
  	/*
  	* if we get here due to ZSWAP_SWAPCACHE_EXIST
  	* a load may happening concurrently
  	* it is safe and okay to not free the entry
  	* if we free the entry in the following put
  	* it it either okay to return !0
  	*/

  fail:
  	spin_lock(&tree->lock);

  	zswap_entry_put(tree, entry);

  	spin_unlock(&tree->lock);

  
  end:

  	zpool_unmap_handle(pool, handle);

  	return ret;
  }

  static int zswap_is_page_same_filled(void *ptr, unsigned long *value)
  {
  	unsigned int pos;
  	unsigned long *page;
  
  	page = (unsigned long *)ptr;
  	for (pos = 1; pos < PAGE_SIZE / sizeof(*page); pos++) {
  		if (page[pos] != page[0])
  			return 0;
  	}
  	*value = page[0];
  	return 1;
  }
  
  static void zswap_fill_page(void *ptr, unsigned long value)
  {
  	unsigned long *page;
  
  	page = (unsigned long *)ptr;
  	memset_l(page, value, PAGE_SIZE / sizeof(unsigned long));
  }

  /*********************************
  * frontswap hooks
  **********************************/
  /* attempts to compress and store an single page */
  static int zswap_frontswap_store(unsigned type, pgoff_t offset,
  				struct page *page)
  {
  	struct zswap_tree *tree = zswap_trees[type];
  	struct zswap_entry *entry, *dupentry;

  	struct crypto_comp *tfm;

  	int ret;

  	unsigned int hlen, dlen = PAGE_SIZE;

  	unsigned long handle, value;

  	char *buf;
  	u8 *src, *dst;

  	struct zswap_header zhdr = { .swpentry = swp_entry(type, offset) };

  	gfp_t gfp;

  	/* THP isn't supported */
  	if (PageTransHuge(page)) {
  		ret = -EINVAL;
  		goto reject;
  	}

  	if (!zswap_enabled || !tree) {

  		ret = -ENODEV;
  		goto reject;
  	}
  
  	/* reclaim space if needed */
  	if (zswap_is_full()) {

  		struct zswap_pool *pool;

  		zswap_pool_limit_hit++;

  		zswap_pool_reached_full = true;
  		pool = zswap_pool_last_get();
  		if (pool)
  			queue_work(shrink_wq, &pool->shrink_work);
  		ret = -ENOMEM;
  		goto reject;
  	}

  	if (zswap_pool_reached_full) {
  	       if (!zswap_can_accept()) {

  			ret = -ENOMEM;
  			goto reject;

  		} else
  			zswap_pool_reached_full = false;

  	}
  
  	/* allocate entry */
  	entry = zswap_entry_cache_alloc(GFP_KERNEL);
  	if (!entry) {
  		zswap_reject_kmemcache_fail++;
  		ret = -ENOMEM;
  		goto reject;
  	}

  	if (zswap_same_filled_pages_enabled) {
  		src = kmap_atomic(page);
  		if (zswap_is_page_same_filled(src, &value)) {
  			kunmap_atomic(src);
  			entry->offset = offset;
  			entry->length = 0;
  			entry->value = value;
  			atomic_inc(&zswap_same_filled_pages);
  			goto insert_entry;
  		}
  		kunmap_atomic(src);
  	}

  	/* if entry is successfully added, it keeps the reference */
  	entry->pool = zswap_pool_current_get();
  	if (!entry->pool) {
  		ret = -EINVAL;
  		goto freepage;
  	}

  	/* compress */
  	dst = get_cpu_var(zswap_dstmem);

  	tfm = *get_cpu_ptr(entry->pool->tfm);

  	src = kmap_atomic(page);

  	ret = crypto_comp_compress(tfm, src, PAGE_SIZE, dst, &dlen);

  	kunmap_atomic(src);

  	put_cpu_ptr(entry->pool->tfm);

  	if (ret) {
  		ret = -EINVAL;

  		goto put_dstmem;

  	}
  
  	/* store */

  	hlen = zpool_evictable(entry->pool->zpool) ? sizeof(zhdr) : 0;

  	gfp = __GFP_NORETRY | __GFP_NOWARN | __GFP_KSWAPD_RECLAIM;
  	if (zpool_malloc_support_movable(entry->pool->zpool))
  		gfp |= __GFP_HIGHMEM | __GFP_MOVABLE;
  	ret = zpool_malloc(entry->pool->zpool, hlen + dlen, gfp, &handle);

  	if (ret == -ENOSPC) {
  		zswap_reject_compress_poor++;

  		goto put_dstmem;

  	}
  	if (ret) {
  		zswap_reject_alloc_fail++;

  		goto put_dstmem;

  	}

  	buf = zpool_map_handle(entry->pool->zpool, handle, ZPOOL_MM_RW);
  	memcpy(buf, &zhdr, hlen);
  	memcpy(buf + hlen, dst, dlen);

  	zpool_unmap_handle(entry->pool->zpool, handle);

  	put_cpu_var(zswap_dstmem);
  
  	/* populate entry */
  	entry->offset = offset;
  	entry->handle = handle;
  	entry->length = dlen;

  insert_entry:

  	/* map */
  	spin_lock(&tree->lock);
  	do {
  		ret = zswap_rb_insert(&tree->rbroot, entry, &dupentry);
  		if (ret == -EEXIST) {
  			zswap_duplicate_entry++;
  			/* remove from rbtree */

  			zswap_rb_erase(&tree->rbroot, dupentry);
  			zswap_entry_put(tree, dupentry);

  		}
  	} while (ret == -EEXIST);
  	spin_unlock(&tree->lock);
  
  	/* update stats */
  	atomic_inc(&zswap_stored_pages);

  	zswap_update_total_size();

  
  	return 0;

  put_dstmem:

  	put_cpu_var(zswap_dstmem);

  	zswap_pool_put(entry->pool);
  freepage:

  	zswap_entry_cache_free(entry);
  reject:
  	return ret;
  }
  
  /*
   * returns 0 if the page was successfully decompressed
   * return -1 on entry not found or error
  */
  static int zswap_frontswap_load(unsigned type, pgoff_t offset,
  				struct page *page)
  {
  	struct zswap_tree *tree = zswap_trees[type];
  	struct zswap_entry *entry;

  	struct crypto_comp *tfm;

  	u8 *src, *dst;
  	unsigned int dlen;

  	int ret;

  
  	/* find */
  	spin_lock(&tree->lock);

  	entry = zswap_entry_find_get(&tree->rbroot, offset);

  	if (!entry) {
  		/* entry was written back */
  		spin_unlock(&tree->lock);
  		return -1;
  	}

  	spin_unlock(&tree->lock);

  	if (!entry->length) {
  		dst = kmap_atomic(page);
  		zswap_fill_page(dst, entry->value);
  		kunmap_atomic(dst);
  		goto freeentry;
  	}

  	/* decompress */
  	dlen = PAGE_SIZE;

  	src = zpool_map_handle(entry->pool->zpool, entry->handle, ZPOOL_MM_RO);
  	if (zpool_evictable(entry->pool->zpool))
  		src += sizeof(struct zswap_header);

  	dst = kmap_atomic(page);

  	tfm = *get_cpu_ptr(entry->pool->tfm);
  	ret = crypto_comp_decompress(tfm, src, entry->length, dst, &dlen);
  	put_cpu_ptr(entry->pool->tfm);

  	kunmap_atomic(dst);

  	zpool_unmap_handle(entry->pool->zpool, entry->handle);

  	BUG_ON(ret);

  freeentry:

  	spin_lock(&tree->lock);

  	zswap_entry_put(tree, entry);

  	spin_unlock(&tree->lock);

  	return 0;
  }
  
  /* frees an entry in zswap */
  static void zswap_frontswap_invalidate_page(unsigned type, pgoff_t offset)
  {
  	struct zswap_tree *tree = zswap_trees[type];
  	struct zswap_entry *entry;

  
  	/* find */
  	spin_lock(&tree->lock);
  	entry = zswap_rb_search(&tree->rbroot, offset);
  	if (!entry) {
  		/* entry was written back */
  		spin_unlock(&tree->lock);
  		return;
  	}
  
  	/* remove from rbtree */

  	zswap_rb_erase(&tree->rbroot, entry);

  
  	/* drop the initial reference from entry creation */

  	zswap_entry_put(tree, entry);

  
  	spin_unlock(&tree->lock);

  }
  
  /* frees all zswap entries for the given swap type */
  static void zswap_frontswap_invalidate_area(unsigned type)
  {
  	struct zswap_tree *tree = zswap_trees[type];

  	struct zswap_entry *entry, *n;

  
  	if (!tree)
  		return;
  
  	/* walk the tree and free everything */
  	spin_lock(&tree->lock);

  	rbtree_postorder_for_each_entry_safe(entry, n, &tree->rbroot, rbnode)

  		zswap_free_entry(entry);

  	tree->rbroot = RB_ROOT;
  	spin_unlock(&tree->lock);

  	kfree(tree);
  	zswap_trees[type] = NULL;

  }

  static void zswap_frontswap_init(unsigned type)
  {
  	struct zswap_tree *tree;

  	tree = kzalloc(sizeof(*tree), GFP_KERNEL);

  	if (!tree) {
  		pr_err("alloc failed, zswap disabled for swap type %d
  ", type);
  		return;
  	}

  	tree->rbroot = RB_ROOT;
  	spin_lock_init(&tree->lock);
  	zswap_trees[type] = tree;

  }
  
  static struct frontswap_ops zswap_frontswap_ops = {
  	.store = zswap_frontswap_store,
  	.load = zswap_frontswap_load,
  	.invalidate_page = zswap_frontswap_invalidate_page,
  	.invalidate_area = zswap_frontswap_invalidate_area,
  	.init = zswap_frontswap_init
  };
  
  /*********************************
  * debugfs functions
  **********************************/
  #ifdef CONFIG_DEBUG_FS
  #include <linux/debugfs.h>
  
  static struct dentry *zswap_debugfs_root;
  
  static int __init zswap_debugfs_init(void)
  {
  	if (!debugfs_initialized())
  		return -ENODEV;
  
  	zswap_debugfs_root = debugfs_create_dir("zswap", NULL);

  	debugfs_create_u64("pool_limit_hit", 0444,
  			   zswap_debugfs_root, &zswap_pool_limit_hit);
  	debugfs_create_u64("reject_reclaim_fail", 0444,
  			   zswap_debugfs_root, &zswap_reject_reclaim_fail);
  	debugfs_create_u64("reject_alloc_fail", 0444,
  			   zswap_debugfs_root, &zswap_reject_alloc_fail);
  	debugfs_create_u64("reject_kmemcache_fail", 0444,
  			   zswap_debugfs_root, &zswap_reject_kmemcache_fail);
  	debugfs_create_u64("reject_compress_poor", 0444,
  			   zswap_debugfs_root, &zswap_reject_compress_poor);
  	debugfs_create_u64("written_back_pages", 0444,
  			   zswap_debugfs_root, &zswap_written_back_pages);
  	debugfs_create_u64("duplicate_entry", 0444,
  			   zswap_debugfs_root, &zswap_duplicate_entry);
  	debugfs_create_u64("pool_total_size", 0444,
  			   zswap_debugfs_root, &zswap_pool_total_size);
  	debugfs_create_atomic_t("stored_pages", 0444,
  				zswap_debugfs_root, &zswap_stored_pages);

  	debugfs_create_atomic_t("same_filled_pages", 0444,

  				zswap_debugfs_root, &zswap_same_filled_pages);

  
  	return 0;
  }
  
  static void __exit zswap_debugfs_exit(void)
  {
  	debugfs_remove_recursive(zswap_debugfs_root);
  }
  #else
  static int __init zswap_debugfs_init(void)
  {
  	return 0;
  }
  
  static void __exit zswap_debugfs_exit(void) { }
  #endif
  
  /*********************************
  * module init and exit
  **********************************/
  static int __init init_zswap(void)
  {

  	struct zswap_pool *pool;

  	int ret;

  	zswap_init_started = true;

  	if (zswap_entry_cache_create()) {
  		pr_err("entry cache creation failed
  ");

  		goto cache_fail;

  	}

  	ret = cpuhp_setup_state(CPUHP_MM_ZSWP_MEM_PREPARE, "mm/zswap:prepare",
  				zswap_dstmem_prepare, zswap_dstmem_dead);
  	if (ret) {

  		pr_err("dstmem alloc failed
  ");
  		goto dstmem_fail;

  	}

  	ret = cpuhp_setup_state_multi(CPUHP_MM_ZSWP_POOL_PREPARE,
  				      "mm/zswap_pool:prepare",
  				      zswap_cpu_comp_prepare,
  				      zswap_cpu_comp_dead);
  	if (ret)
  		goto hp_fail;

  	pool = __zswap_pool_create_fallback();

  	if (pool) {
  		pr_info("loaded using pool %s/%s
  ", pool->tfm_name,
  			zpool_get_type(pool->zpool));
  		list_add(&pool->list, &zswap_pools);
  		zswap_has_pool = true;
  	} else {

  		pr_err("pool creation failed
  ");

  		zswap_enabled = false;

  	}

  	shrink_wq = create_workqueue("zswap-shrink");
  	if (!shrink_wq)
  		goto fallback_fail;

  	frontswap_register_ops(&zswap_frontswap_ops);
  	if (zswap_debugfs_init())
  		pr_warn("debugfs initialization failed
  ");
  	return 0;

  fallback_fail:

  	if (pool)
  		zswap_pool_destroy(pool);

  hp_fail:

  	cpuhp_remove_state(CPUHP_MM_ZSWP_MEM_PREPARE);

  dstmem_fail:

  	zswap_entry_cache_destroy();

  cache_fail:

  	/* if built-in, we aren't unloaded on failure; don't allow use */
  	zswap_init_failed = true;
  	zswap_enabled = false;

  	return -ENOMEM;
  }
  /* must be late so crypto has time to come up */
  late_initcall(init_zswap);
  
  MODULE_LICENSE("GPL");

  MODULE_AUTHOR("Seth Jennings <sjennings@variantweb.net>");

  MODULE_DESCRIPTION("Compressed cache for swap pages");