/*
   * 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>
   *
   * This program 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
   * of the License, or (at your option) any later version.
   *
   * This program 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.
  */
  
  #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>
  
  /*********************************
  * 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 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;
  
  /*********************************
  * tunables
  **********************************/
  /* Enable/disable zswap (disabled by default, fixed at boot for now) */
  static bool zswap_enabled __read_mostly;

  module_param_named(enabled, zswap_enabled, bool, 0444);

  
  /* Compressor to be used by zswap (fixed at boot for now) */
  #define ZSWAP_COMPRESSOR_DEFAULT "lzo"
  static char *zswap_compressor = ZSWAP_COMPRESSOR_DEFAULT;

  module_param_named(compressor, zswap_compressor, charp, 0444);

  
  /* 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);

  /* Compressed storage to use */
  #define ZSWAP_ZPOOL_DEFAULT "zbud"
  static char *zswap_zpool_type = ZSWAP_ZPOOL_DEFAULT;
  module_param_named(zpool, zswap_zpool_type, charp, 0444);
  
  /* zpool is shared by all of zswap backend  */
  static struct zpool *zswap_pool;

  /*********************************
  * compression functions
  **********************************/
  /* per-cpu compression transforms */
  static struct crypto_comp * __percpu *zswap_comp_pcpu_tfms;
  
  enum comp_op {
  	ZSWAP_COMPOP_COMPRESS,
  	ZSWAP_COMPOP_DECOMPRESS
  };
  
  static int zswap_comp_op(enum comp_op op, const u8 *src, unsigned int slen,
  				u8 *dst, unsigned int *dlen)
  {
  	struct crypto_comp *tfm;
  	int ret;
  
  	tfm = *per_cpu_ptr(zswap_comp_pcpu_tfms, get_cpu());
  	switch (op) {
  	case ZSWAP_COMPOP_COMPRESS:
  		ret = crypto_comp_compress(tfm, src, slen, dst, dlen);
  		break;
  	case ZSWAP_COMPOP_DECOMPRESS:
  		ret = crypto_comp_decompress(tfm, src, slen, dst, dlen);
  		break;
  	default:
  		ret = -EINVAL;
  	}
  
  	put_cpu();
  	return ret;
  }
  
  static int __init zswap_comp_init(void)
  {
  	if (!crypto_has_comp(zswap_compressor, 0, 0)) {
  		pr_info("%s compressor not available
  ", zswap_compressor);
  		/* fall back to default compressor */
  		zswap_compressor = ZSWAP_COMPRESSOR_DEFAULT;
  		if (!crypto_has_comp(zswap_compressor, 0, 0))
  			/* can't even load the default compressor */
  			return -ENODEV;
  	}
  	pr_info("using %s compressor
  ", zswap_compressor);
  
  	/* alloc percpu transforms */
  	zswap_comp_pcpu_tfms = alloc_percpu(struct crypto_comp *);
  	if (!zswap_comp_pcpu_tfms)
  		return -ENOMEM;
  	return 0;
  }

  static void __init zswap_comp_exit(void)

  {
  	/* free percpu transforms */

  	free_percpu(zswap_comp_pcpu_tfms);

  }
  
  /*********************************
  * data structures
  **********************************/
  /*
   * 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
   * 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.
   * offset - the swap offset for the entry.  Index into the red-black tree.

   * handle - zpool allocation handle that stores the compressed page data

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

   *          decompression

   */
  struct zswap_entry {
  	struct rb_node rbnode;
  	pgoff_t offset;
  	int refcount;
  	unsigned int length;
  	unsigned long handle;
  };
  
  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];
  
  /*********************************
  * 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)

  {

  	zpool_free(zswap_pool, entry->handle);

  	zswap_entry_cache_free(entry);
  	atomic_dec(&zswap_stored_pages);

  	zswap_pool_total_size = zpool_get_total_size(zswap_pool);

  }
  
  /* 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 = NULL;
  
  	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_cpu_notifier(unsigned long action, unsigned long cpu)
  {
  	struct crypto_comp *tfm;
  	u8 *dst;
  
  	switch (action) {
  	case CPU_UP_PREPARE:
  		tfm = crypto_alloc_comp(zswap_compressor, 0, 0);
  		if (IS_ERR(tfm)) {
  			pr_err("can't allocate compressor transform
  ");
  			return NOTIFY_BAD;
  		}
  		*per_cpu_ptr(zswap_comp_pcpu_tfms, cpu) = tfm;

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

  		if (!dst) {
  			pr_err("can't allocate compressor buffer
  ");
  			crypto_free_comp(tfm);
  			*per_cpu_ptr(zswap_comp_pcpu_tfms, cpu) = NULL;
  			return NOTIFY_BAD;
  		}
  		per_cpu(zswap_dstmem, cpu) = dst;
  		break;
  	case CPU_DEAD:
  	case CPU_UP_CANCELED:
  		tfm = *per_cpu_ptr(zswap_comp_pcpu_tfms, cpu);
  		if (tfm) {
  			crypto_free_comp(tfm);
  			*per_cpu_ptr(zswap_comp_pcpu_tfms, cpu) = NULL;
  		}
  		dst = per_cpu(zswap_dstmem, cpu);
  		kfree(dst);
  		per_cpu(zswap_dstmem, cpu) = NULL;
  		break;
  	default:
  		break;
  	}
  	return NOTIFY_OK;
  }
  
  static int zswap_cpu_notifier(struct notifier_block *nb,
  				unsigned long action, void *pcpu)
  {
  	unsigned long cpu = (unsigned long)pcpu;
  	return __zswap_cpu_notifier(action, cpu);
  }
  
  static struct notifier_block zswap_cpu_notifier_block = {
  	.notifier_call = zswap_cpu_notifier
  };

  static int __init zswap_cpu_init(void)

  {
  	unsigned long cpu;

  	cpu_notifier_register_begin();

  	for_each_online_cpu(cpu)
  		if (__zswap_cpu_notifier(CPU_UP_PREPARE, cpu) != NOTIFY_OK)
  			goto cleanup;

  	__register_cpu_notifier(&zswap_cpu_notifier_block);
  	cpu_notifier_register_done();

  	return 0;
  
  cleanup:
  	for_each_online_cpu(cpu)
  		__zswap_cpu_notifier(CPU_UP_CANCELED, cpu);

  	cpu_notifier_register_done();

  	return -ENOMEM;
  }
  
  /*********************************
  * helpers
  **********************************/
  static bool zswap_is_full(void)
  {

  	return totalram_pages * zswap_max_pool_percent / 100 <

  		DIV_ROUND_UP(zswap_pool_total_size, PAGE_SIZE);

  }

  /*********************************
  * 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)
  {
  	struct page *found_page, *new_page = NULL;

  	struct address_space *swapper_space = swap_address_space(entry);

  	int err;
  
  	*retpage = NULL;
  	do {
  		/*
  		 * First check the swap cache.  Since this is normally
  		 * called after lookup_swap_cache() failed, re-calling
  		 * that would confuse statistics.
  		 */
  		found_page = find_get_page(swapper_space, entry.val);
  		if (found_page)
  			break;
  
  		/*
  		 * Get a new page to read into from swap.
  		 */
  		if (!new_page) {
  			new_page = alloc_page(GFP_KERNEL);
  			if (!new_page)
  				break; /* Out of memory */
  		}
  
  		/*
  		 * call radix_tree_preload() while we can wait.
  		 */
  		err = radix_tree_preload(GFP_KERNEL);
  		if (err)
  			break;
  
  		/*
  		 * Swap entry may have been freed since our caller observed it.
  		 */
  		err = swapcache_prepare(entry);
  		if (err == -EEXIST) { /* seems racy */
  			radix_tree_preload_end();
  			continue;
  		}
  		if (err) { /* swp entry is obsolete ? */
  			radix_tree_preload_end();
  			break;
  		}
  
  		/* May fail (-ENOMEM) if radix-tree node allocation failed. */
  		__set_page_locked(new_page);
  		SetPageSwapBacked(new_page);
  		err = __add_to_swap_cache(new_page, entry);
  		if (likely(!err)) {
  			radix_tree_preload_end();
  			lru_cache_add_anon(new_page);
  			*retpage = new_page;
  			return ZSWAP_SWAPCACHE_NEW;
  		}
  		radix_tree_preload_end();
  		ClearPageSwapBacked(new_page);
  		__clear_page_locked(new_page);
  		/*
  		 * add_to_swap_cache() doesn't return -EEXIST, so we can safely
  		 * clear SWAP_HAS_CACHE flag.
  		 */

  		swapcache_free(entry);

  	} while (err != -ENOMEM);
  
  	if (new_page)
  		page_cache_release(new_page);
  	if (!found_page)

  		return ZSWAP_SWAPCACHE_FAIL;

  	*retpage = found_page;
  	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;
  	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 */

  	zpool_unmap_handle(pool, handle);

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

  		src = (u8 *)zpool_map_handle(zswap_pool, entry->handle,
  				ZPOOL_MM_RO) + sizeof(struct zswap_header);

  		dst = kmap_atomic(page);
  		ret = zswap_comp_op(ZSWAP_COMPOP_DECOMPRESS, src,
  				entry->length, dst, &dlen);
  		kunmap_atomic(dst);

  		zpool_unmap_handle(zswap_pool, entry->handle);

  		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);
  	page_cache_release(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:

  	return ret;
  }
  
  /*********************************
  * 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;
  	int ret;
  	unsigned int dlen = PAGE_SIZE, len;
  	unsigned long handle;
  	char *buf;
  	u8 *src, *dst;
  	struct zswap_header *zhdr;
  
  	if (!tree) {
  		ret = -ENODEV;
  		goto reject;
  	}
  
  	/* reclaim space if needed */
  	if (zswap_is_full()) {
  		zswap_pool_limit_hit++;

  		if (zpool_shrink(zswap_pool, 1, NULL)) {

  			zswap_reject_reclaim_fail++;
  			ret = -ENOMEM;
  			goto reject;
  		}
  	}
  
  	/* allocate entry */
  	entry = zswap_entry_cache_alloc(GFP_KERNEL);
  	if (!entry) {
  		zswap_reject_kmemcache_fail++;
  		ret = -ENOMEM;
  		goto reject;
  	}
  
  	/* compress */
  	dst = get_cpu_var(zswap_dstmem);
  	src = kmap_atomic(page);
  	ret = zswap_comp_op(ZSWAP_COMPOP_COMPRESS, src, PAGE_SIZE, dst, &dlen);
  	kunmap_atomic(src);
  	if (ret) {
  		ret = -EINVAL;
  		goto freepage;
  	}
  
  	/* store */
  	len = dlen + sizeof(struct zswap_header);

  	ret = zpool_malloc(zswap_pool, len, __GFP_NORETRY | __GFP_NOWARN,

  		&handle);
  	if (ret == -ENOSPC) {
  		zswap_reject_compress_poor++;
  		goto freepage;
  	}
  	if (ret) {
  		zswap_reject_alloc_fail++;
  		goto freepage;
  	}

  	zhdr = zpool_map_handle(zswap_pool, handle, ZPOOL_MM_RW);

  	zhdr->swpentry = swp_entry(type, offset);
  	buf = (u8 *)(zhdr + 1);
  	memcpy(buf, dst, dlen);

  	zpool_unmap_handle(zswap_pool, handle);

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

  
  	return 0;
  
  freepage:
  	put_cpu_var(zswap_dstmem);
  	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;
  	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);
  
  	/* decompress */
  	dlen = PAGE_SIZE;

  	src = (u8 *)zpool_map_handle(zswap_pool, entry->handle,
  			ZPOOL_MM_RO) + sizeof(struct zswap_header);

  	dst = kmap_atomic(page);
  	ret = zswap_comp_op(ZSWAP_COMPOP_DECOMPRESS, src, entry->length,
  		dst, &dlen);
  	kunmap_atomic(dst);

  	zpool_unmap_handle(zswap_pool, entry->handle);

  	BUG_ON(ret);
  
  	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 struct zpool_ops zswap_zpool_ops = {

  	.evict = zswap_writeback_entry
  };
  
  static void zswap_frontswap_init(unsigned type)
  {
  	struct zswap_tree *tree;
  
  	tree = kzalloc(sizeof(struct zswap_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);
  	if (!zswap_debugfs_root)
  		return -ENOMEM;
  
  	debugfs_create_u64("pool_limit_hit", S_IRUGO,
  			zswap_debugfs_root, &zswap_pool_limit_hit);
  	debugfs_create_u64("reject_reclaim_fail", S_IRUGO,
  			zswap_debugfs_root, &zswap_reject_reclaim_fail);
  	debugfs_create_u64("reject_alloc_fail", S_IRUGO,
  			zswap_debugfs_root, &zswap_reject_alloc_fail);
  	debugfs_create_u64("reject_kmemcache_fail", S_IRUGO,
  			zswap_debugfs_root, &zswap_reject_kmemcache_fail);
  	debugfs_create_u64("reject_compress_poor", S_IRUGO,
  			zswap_debugfs_root, &zswap_reject_compress_poor);
  	debugfs_create_u64("written_back_pages", S_IRUGO,
  			zswap_debugfs_root, &zswap_written_back_pages);
  	debugfs_create_u64("duplicate_entry", S_IRUGO,
  			zswap_debugfs_root, &zswap_duplicate_entry);

  	debugfs_create_u64("pool_total_size", S_IRUGO,
  			zswap_debugfs_root, &zswap_pool_total_size);

  	debugfs_create_atomic_t("stored_pages", S_IRUGO,
  			zswap_debugfs_root, &zswap_stored_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)
  {

  	gfp_t gfp = __GFP_NORETRY | __GFP_NOWARN;

  	if (!zswap_enabled)
  		return 0;
  
  	pr_info("loading zswap
  ");

  	zswap_pool = zpool_create_pool(zswap_zpool_type, "zswap", gfp,
  					&zswap_zpool_ops);

  	if (!zswap_pool && strcmp(zswap_zpool_type, ZSWAP_ZPOOL_DEFAULT)) {
  		pr_info("%s zpool not available
  ", zswap_zpool_type);
  		zswap_zpool_type = ZSWAP_ZPOOL_DEFAULT;

  		zswap_pool = zpool_create_pool(zswap_zpool_type, "zswap", gfp,

  					&zswap_zpool_ops);
  	}

  	if (!zswap_pool) {

  		pr_err("%s zpool not available
  ", zswap_zpool_type);
  		pr_err("zpool creation failed
  ");

  		goto error;
  	}

  	pr_info("using %s pool
  ", zswap_zpool_type);

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

  		goto cachefail;

  	}
  	if (zswap_comp_init()) {
  		pr_err("compressor initialization failed
  ");
  		goto compfail;
  	}
  	if (zswap_cpu_init()) {
  		pr_err("per-cpu initialization failed
  ");
  		goto pcpufail;
  	}

  	frontswap_register_ops(&zswap_frontswap_ops);
  	if (zswap_debugfs_init())
  		pr_warn("debugfs initialization failed
  ");
  	return 0;
  pcpufail:
  	zswap_comp_exit();
  compfail:

  	zswap_entry_cache_destroy();

  cachefail:

  	zpool_destroy_pool(zswap_pool);

  error:
  	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");