// 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>

/*********************************
* 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;

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

#define ZSWAP_PARAM_UNSET ""

/* Enable/disable zswap (disabled by default) */
static bool zswap_enabled;
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 */
#define ZSWAP_COMPRESSOR_DEFAULT "lzo"
static char *zswap_compressor = 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 */
#define ZSWAP_ZPOOL_DEFAULT "zbud"
static char *zswap_zpool_type = 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);

/* 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 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\n", 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 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\n",
                       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!\n", __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!\n", __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 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\n", type);
                goto error;
        }
        pr_debug("using %s zpool\n", 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\n");
                goto error;
        }

        ret = cpuhp_state_add_instance(CPUHP_MM_ZSWP_POOL_PREPARE,
                                       &pool->node);
        if (ret)
                goto error;
        pr_debug("using %s compressor\n", 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);

        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, ZSWAP_COMPRESSOR_DEFAULT)) {
                pr_err("compressor %s not available, using default %s\n",
                       zswap_compressor, ZSWAP_COMPRESSOR_DEFAULT);
                param_free_charp(&zswap_compressor);
                zswap_compressor = ZSWAP_COMPRESSOR_DEFAULT;
                has_comp = crypto_has_comp(zswap_compressor, 0, 0);
        }
        if (!has_comp) {
                pr_err("default compressor %s not available\n",
                       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, ZSWAP_ZPOOL_DEFAULT)) {
                pr_err("zpool %s not available, using default %s\n",
                       zswap_zpool_type, ZSWAP_ZPOOL_DEFAULT);
                param_free_charp(&zswap_zpool_type);
                zswap_zpool_type = ZSWAP_ZPOOL_DEFAULT;
                has_zpool = zpool_has_pool(zswap_zpool_type);
        }
        if (!has_zpool) {
                pr_err("default zpool %s not available\n",
                       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), 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->work, __zswap_pool_release);
        schedule_work(&pool->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\n");
                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\n", s);
                        return -ENOENT;
                }
                type = s;
        } else if (!compressor) {
                if (!crypto_has_comp(s, 0, 0)) {
                        pr_err("compressor %s not available\n", 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\n");
                return -ENODEV;
        }
        if (!zswap_has_pool && zswap_init_started) {
                pr_err("can't enable, no pool configured\n");
                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_shrink(void)
{
        struct zswap_pool *pool;
        int ret;

        pool = zswap_pool_last_get();
        if (!pool)
                return -ENOENT;

        ret = zpool_shrink(pool->zpool, 1, NULL);

        zswap_pool_put(pool);

        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()) {
                zswap_pool_limit_hit++;
                if (zswap_shrink()) {
                        zswap_reject_reclaim_fail++;
                        ret = -ENOMEM;
                        goto reject;
                }

                /* A second zswap_is_full() check after
                 * zswap_shrink() to make sure it's now
                 * under the max_pool_percent
                 */
                if (zswap_is_full()) {
                        ret = -ENOMEM;
                        goto reject;
                }
        }

        /* 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\n", 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\n");
                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\n");
                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\n", pool->tfm_name,
                        zpool_get_type(pool->zpool));
                list_add(&pool->list, &zswap_pools);
                zswap_has_pool = true;
        } else {
                pr_err("pool creation failed\n");
                zswap_enabled = false;
        }

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

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");