/*
 * Copyright (C) 2012 Red Hat. All rights reserved.
 *
 * This file is released under the GPL.
 */

#include "dm.h"
#include "dm-bio-prison.h"
#include "dm-bio-record.h"
#include "dm-cache-metadata.h"

#include <linux/dm-io.h>
#include <linux/dm-kcopyd.h>
#include <linux/init.h>
#include <linux/mempool.h>
#include <linux/module.h>
#include <linux/slab.h>
#include <linux/vmalloc.h>

#define DM_MSG_PREFIX "cache"

DECLARE_DM_KCOPYD_THROTTLE_WITH_MODULE_PARM(cache_copy_throttle,
        "A percentage of time allocated for copying to and/or from cache");

/*----------------------------------------------------------------*/

/*
 * Glossary:
 *
 * oblock: index of an origin block
 * cblock: index of a cache block
 * promotion: movement of a block from origin to cache
 * demotion: movement of a block from cache to origin
 * migration: movement of a block between the origin and cache device,
 *            either direction
 */

/*----------------------------------------------------------------*/

static size_t bitset_size_in_bytes(unsigned nr_entries)
{
        return sizeof(unsigned long) * dm_div_up(nr_entries, BITS_PER_LONG);
}

static unsigned long *alloc_bitset(unsigned nr_entries)
{
        size_t s = bitset_size_in_bytes(nr_entries);
        return vzalloc(s);
}

static void clear_bitset(void *bitset, unsigned nr_entries)
{
        size_t s = bitset_size_in_bytes(nr_entries);
        memset(bitset, 0, s);
}

static void free_bitset(unsigned long *bits)
{
        vfree(bits);
}

/*----------------------------------------------------------------*/

#define PRISON_CELLS 1024
#define MIGRATION_POOL_SIZE 128
#define COMMIT_PERIOD HZ
#define MIGRATION_COUNT_WINDOW 10

/*
 * The block size of the device holding cache data must be >= 32KB
 */
#define DATA_DEV_BLOCK_SIZE_MIN_SECTORS (32 * 1024 >> SECTOR_SHIFT)

/*
 * FIXME: the cache is read/write for the time being.
 */
enum cache_mode {
        CM_WRITE,               /* metadata may be changed */
        CM_READ_ONLY,           /* metadata may not be changed */
};

struct cache_features {
        enum cache_mode mode;
        bool write_through:1;
};

struct cache_stats {
        atomic_t read_hit;
        atomic_t read_miss;
        atomic_t write_hit;
        atomic_t write_miss;
        atomic_t demotion;
        atomic_t promotion;
        atomic_t copies_avoided;
        atomic_t cache_cell_clash;
        atomic_t commit_count;
        atomic_t discard_count;
};

struct cache {
        struct dm_target *ti;
        struct dm_target_callbacks callbacks;

        /*
         * Metadata is written to this device.
         */
        struct dm_dev *metadata_dev;

        /*
         * The slower of the two data devices.  Typically a spindle.
         */
        struct dm_dev *origin_dev;

        /*
         * The faster of the two data devices.  Typically an SSD.
         */
        struct dm_dev *cache_dev;

        /*
         * Cache features such as write-through.
         */
        struct cache_features features;

        /*
         * Size of the origin device in _complete_ blocks and native sectors.
         */
        dm_oblock_t origin_blocks;
        sector_t origin_sectors;

        /*
         * Size of the cache device in blocks.
         */
        dm_cblock_t cache_size;

        /*
         * Fields for converting from sectors to blocks.
         */
        uint32_t sectors_per_block;
        int sectors_per_block_shift;

        struct dm_cache_metadata *cmd;

        spinlock_t lock;
        struct bio_list deferred_bios;
        struct bio_list deferred_flush_bios;
        struct bio_list deferred_writethrough_bios;
        struct list_head quiesced_migrations;
        struct list_head completed_migrations;
        struct list_head need_commit_migrations;
        sector_t migration_threshold;
        atomic_t nr_migrations;
        wait_queue_head_t migration_wait;

        /*
         * cache_size entries, dirty if set
         */
        dm_cblock_t nr_dirty;
        unsigned long *dirty_bitset;

        /*
         * origin_blocks entries, discarded if set.
         */
        uint32_t discard_block_size; /* a power of 2 times sectors per block */
        dm_dblock_t discard_nr_blocks;
        unsigned long *discard_bitset;

        struct dm_kcopyd_client *copier;
        struct workqueue_struct *wq;
        struct work_struct worker;

        struct delayed_work waker;
        unsigned long last_commit_jiffies;

        struct dm_bio_prison *prison;
        struct dm_deferred_set *all_io_ds;

        mempool_t *migration_pool;
        struct dm_cache_migration *next_migration;

        struct dm_cache_policy *policy;
        unsigned policy_nr_args;

        bool need_tick_bio:1;
        bool sized:1;
        bool quiescing:1;
        bool commit_requested:1;
        bool loaded_mappings:1;
        bool loaded_discards:1;

        struct cache_stats stats;

        /*
         * Rather than reconstructing the table line for the status we just
         * save it and regurgitate.
         */
        unsigned nr_ctr_args;
        const char **ctr_args;
};

struct per_bio_data {
        bool tick:1;
        unsigned req_nr:2;
        struct dm_deferred_entry *all_io_entry;

        /*
         * writethrough fields.  These MUST remain at the end of this
         * structure and the 'cache' member must be the first as it
         * is used to determine the offset of the writethrough fields.
         */
        struct cache *cache;
        dm_cblock_t cblock;
        bio_end_io_t *saved_bi_end_io;
        struct dm_bio_details bio_details;
};

struct dm_cache_migration {
        struct list_head list;
        struct cache *cache;

        unsigned long start_jiffies;
        dm_oblock_t old_oblock;
        dm_oblock_t new_oblock;
        dm_cblock_t cblock;

        bool err:1;
        bool writeback:1;
        bool demote:1;
        bool promote:1;

        struct dm_bio_prison_cell *old_ocell;
        struct dm_bio_prison_cell *new_ocell;
};

/*
 * Processing a bio in the worker thread may require these memory
 * allocations.  We prealloc to avoid deadlocks (the same worker thread
 * frees them back to the mempool).
 */
struct prealloc {
        struct dm_cache_migration *mg;
        struct dm_bio_prison_cell *cell1;
        struct dm_bio_prison_cell *cell2;
};

static void wake_worker(struct cache *cache)
{
        queue_work(cache->wq, &cache->worker);
}

/*----------------------------------------------------------------*/

static struct dm_bio_prison_cell *alloc_prison_cell(struct cache *cache)
{
        /* FIXME: change to use a local slab. */
        return dm_bio_prison_alloc_cell(cache->prison, GFP_NOWAIT);
}

static void free_prison_cell(struct cache *cache, struct dm_bio_prison_cell *cell)
{
        dm_bio_prison_free_cell(cache->prison, cell);
}

static int prealloc_data_structs(struct cache *cache, struct prealloc *p)
{
        if (!p->mg) {
                p->mg = mempool_alloc(cache->migration_pool, GFP_NOWAIT);
                if (!p->mg)
                        return -ENOMEM;
        }

        if (!p->cell1) {
                p->cell1 = alloc_prison_cell(cache);
                if (!p->cell1)
                        return -ENOMEM;
        }

        if (!p->cell2) {
                p->cell2 = alloc_prison_cell(cache);
                if (!p->cell2)
                        return -ENOMEM;
        }

        return 0;
}

static void prealloc_free_structs(struct cache *cache, struct prealloc *p)
{
        if (p->cell2)
                free_prison_cell(cache, p->cell2);

        if (p->cell1)
                free_prison_cell(cache, p->cell1);

        if (p->mg)
                mempool_free(p->mg, cache->migration_pool);
}

static struct dm_cache_migration *prealloc_get_migration(struct prealloc *p)
{
        struct dm_cache_migration *mg = p->mg;

        BUG_ON(!mg);
        p->mg = NULL;

        return mg;
}

/*
 * You must have a cell within the prealloc struct to return.  If not this
 * function will BUG() rather than returning NULL.
 */
static struct dm_bio_prison_cell *prealloc_get_cell(struct prealloc *p)
{
        struct dm_bio_prison_cell *r = NULL;

        if (p->cell1) {
                r = p->cell1;
                p->cell1 = NULL;

        } else if (p->cell2) {
                r = p->cell2;
                p->cell2 = NULL;
        } else
                BUG();

        return r;
}

/*
 * You can't have more than two cells in a prealloc struct.  BUG() will be
 * called if you try and overfill.
 */
static void prealloc_put_cell(struct prealloc *p, struct dm_bio_prison_cell *cell)
{
        if (!p->cell2)
                p->cell2 = cell;

        else if (!p->cell1)
                p->cell1 = cell;

        else
                BUG();
}

/*----------------------------------------------------------------*/

static void build_key(dm_oblock_t oblock, struct dm_cell_key *key)
{
        key->virtual = 0;
        key->dev = 0;
        key->block = from_oblock(oblock);
}

/*
 * The caller hands in a preallocated cell, and a free function for it.
 * The cell will be freed if there's an error, or if it wasn't used because
 * a cell with that key already exists.
 */
typedef void (*cell_free_fn)(void *context, struct dm_bio_prison_cell *cell);

static int bio_detain(struct cache *cache, dm_oblock_t oblock,
                      struct bio *bio, struct dm_bio_prison_cell *cell_prealloc,
                      cell_free_fn free_fn, void *free_context,
                      struct dm_bio_prison_cell **cell_result)
{
        int r;
        struct dm_cell_key key;

        build_key(oblock, &key);
        r = dm_bio_detain(cache->prison, &key, bio, cell_prealloc, cell_result);
        if (r)
                free_fn(free_context, cell_prealloc);

        return r;
}

static int get_cell(struct cache *cache,
                    dm_oblock_t oblock,
                    struct prealloc *structs,
                    struct dm_bio_prison_cell **cell_result)
{
        int r;
        struct dm_cell_key key;
        struct dm_bio_prison_cell *cell_prealloc;

        cell_prealloc = prealloc_get_cell(structs);

        build_key(oblock, &key);
        r = dm_get_cell(cache->prison, &key, cell_prealloc, cell_result);
        if (r)
                prealloc_put_cell(structs, cell_prealloc);

        return r;
}

/*----------------------------------------------------------------*/

static bool is_dirty(struct cache *cache, dm_cblock_t b)
{
        return test_bit(from_cblock(b), cache->dirty_bitset);
}

static void set_dirty(struct cache *cache, dm_oblock_t oblock, dm_cblock_t cblock)
{
        if (!test_and_set_bit(from_cblock(cblock), cache->dirty_bitset)) {
                cache->nr_dirty = to_cblock(from_cblock(cache->nr_dirty) + 1);
                policy_set_dirty(cache->policy, oblock);
        }
}

static void clear_dirty(struct cache *cache, dm_oblock_t oblock, dm_cblock_t cblock)
{
        if (test_and_clear_bit(from_cblock(cblock), cache->dirty_bitset)) {
                policy_clear_dirty(cache->policy, oblock);
                cache->nr_dirty = to_cblock(from_cblock(cache->nr_dirty) - 1);
                if (!from_cblock(cache->nr_dirty))
                        dm_table_event(cache->ti->table);
        }
}

/*----------------------------------------------------------------*/

static bool block_size_is_power_of_two(struct cache *cache)
{
        return cache->sectors_per_block_shift >= 0;
}

static dm_block_t block_div(dm_block_t b, uint32_t n)
{
        do_div(b, n);

        return b;
}

static dm_dblock_t oblock_to_dblock(struct cache *cache, dm_oblock_t oblock)
{
        uint32_t discard_blocks = cache->discard_block_size;
        dm_block_t b = from_oblock(oblock);

        if (!block_size_is_power_of_two(cache))
                discard_blocks = discard_blocks / cache->sectors_per_block;
        else
                discard_blocks >>= cache->sectors_per_block_shift;

        b = block_div(b, discard_blocks);

        return to_dblock(b);
}

static void set_discard(struct cache *cache, dm_dblock_t b)
{
        unsigned long flags;

        atomic_inc(&cache->stats.discard_count);

        spin_lock_irqsave(&cache->lock, flags);
        set_bit(from_dblock(b), cache->discard_bitset);
        spin_unlock_irqrestore(&cache->lock, flags);
}

static void clear_discard(struct cache *cache, dm_dblock_t b)
{
        unsigned long flags;

        spin_lock_irqsave(&cache->lock, flags);
        clear_bit(from_dblock(b), cache->discard_bitset);
        spin_unlock_irqrestore(&cache->lock, flags);
}

static bool is_discarded(struct cache *cache, dm_dblock_t b)
{
        int r;
        unsigned long flags;

        spin_lock_irqsave(&cache->lock, flags);
        r = test_bit(from_dblock(b), cache->discard_bitset);
        spin_unlock_irqrestore(&cache->lock, flags);

        return r;
}

static bool is_discarded_oblock(struct cache *cache, dm_oblock_t b)
{
        int r;
        unsigned long flags;

        spin_lock_irqsave(&cache->lock, flags);
        r = test_bit(from_dblock(oblock_to_dblock(cache, b)),
                     cache->discard_bitset);
        spin_unlock_irqrestore(&cache->lock, flags);

        return r;
}

/*----------------------------------------------------------------*/

static void load_stats(struct cache *cache)
{
        struct dm_cache_statistics stats;

        dm_cache_metadata_get_stats(cache->cmd, &stats);
        atomic_set(&cache->stats.read_hit, stats.read_hits);
        atomic_set(&cache->stats.read_miss, stats.read_misses);
        atomic_set(&cache->stats.write_hit, stats.write_hits);
        atomic_set(&cache->stats.write_miss, stats.write_misses);
}

static void save_stats(struct cache *cache)
{
        struct dm_cache_statistics stats;

        stats.read_hits = atomic_read(&cache->stats.read_hit);
        stats.read_misses = atomic_read(&cache->stats.read_miss);
        stats.write_hits = atomic_read(&cache->stats.write_hit);
        stats.write_misses = atomic_read(&cache->stats.write_miss);

        dm_cache_metadata_set_stats(cache->cmd, &stats);
}

/*----------------------------------------------------------------
 * Per bio data
 *--------------------------------------------------------------*/

/*
 * If using writeback, leave out struct per_bio_data's writethrough fields.
 */
#define PB_DATA_SIZE_WB (offsetof(struct per_bio_data, cache))
#define PB_DATA_SIZE_WT (sizeof(struct per_bio_data))

static size_t get_per_bio_data_size(struct cache *cache)
{
        return cache->features.write_through ? PB_DATA_SIZE_WT : PB_DATA_SIZE_WB;
}

static struct per_bio_data *get_per_bio_data(struct bio *bio, size_t data_size)
{
        struct per_bio_data *pb = dm_per_bio_data(bio, data_size);
        BUG_ON(!pb);
        return pb;
}

static struct per_bio_data *init_per_bio_data(struct bio *bio, size_t data_size)
{
        struct per_bio_data *pb = get_per_bio_data(bio, data_size);

        pb->tick = false;
        pb->req_nr = dm_bio_get_target_bio_nr(bio);
        pb->all_io_entry = NULL;

        return pb;
}

/*----------------------------------------------------------------
 * Remapping
 *--------------------------------------------------------------*/
static void remap_to_origin(struct cache *cache, struct bio *bio)
{
        bio->bi_bdev = cache->origin_dev->bdev;
}

static void remap_to_cache(struct cache *cache, struct bio *bio,
                           dm_cblock_t cblock)
{
        sector_t bi_sector = bio->bi_sector;

        bio->bi_bdev = cache->cache_dev->bdev;
        if (!block_size_is_power_of_two(cache))
                bio->bi_sector = (from_cblock(cblock) * cache->sectors_per_block) +
                                sector_div(bi_sector, cache->sectors_per_block);
        else
                bio->bi_sector = (from_cblock(cblock) << cache->sectors_per_block_shift) |
                                (bi_sector & (cache->sectors_per_block - 1));
}

static void check_if_tick_bio_needed(struct cache *cache, struct bio *bio)
{
        unsigned long flags;
        size_t pb_data_size = get_per_bio_data_size(cache);
        struct per_bio_data *pb = get_per_bio_data(bio, pb_data_size);

        spin_lock_irqsave(&cache->lock, flags);
        if (cache->need_tick_bio &&
            !(bio->bi_rw & (REQ_FUA | REQ_FLUSH | REQ_DISCARD))) {
                pb->tick = true;
                cache->need_tick_bio = false;
        }
        spin_unlock_irqrestore(&cache->lock, flags);
}

static void remap_to_origin_clear_discard(struct cache *cache, struct bio *bio,
                                  dm_oblock_t oblock)
{
        check_if_tick_bio_needed(cache, bio);
        remap_to_origin(cache, bio);
        if (bio_data_dir(bio) == WRITE)
                clear_discard(cache, oblock_to_dblock(cache, oblock));
}

static void remap_to_cache_dirty(struct cache *cache, struct bio *bio,
                                 dm_oblock_t oblock, dm_cblock_t cblock)
{
        remap_to_cache(cache, bio, cblock);
        if (bio_data_dir(bio) == WRITE) {
                set_dirty(cache, oblock, cblock);
                clear_discard(cache, oblock_to_dblock(cache, oblock));
        }
}

static dm_oblock_t get_bio_block(struct cache *cache, struct bio *bio)
{
        sector_t block_nr = bio->bi_sector;

        if (!block_size_is_power_of_two(cache))
                (void) sector_div(block_nr, cache->sectors_per_block);
        else
                block_nr >>= cache->sectors_per_block_shift;

        return to_oblock(block_nr);
}

static int bio_triggers_commit(struct cache *cache, struct bio *bio)
{
        return bio->bi_rw & (REQ_FLUSH | REQ_FUA);
}

static void issue(struct cache *cache, struct bio *bio)
{
        unsigned long flags;

        if (!bio_triggers_commit(cache, bio)) {
                generic_make_request(bio);
                return;
        }

        /*
         * Batch together any bios that trigger commits and then issue a
         * single commit for them in do_worker().
         */
        spin_lock_irqsave(&cache->lock, flags);
        cache->commit_requested = true;
        bio_list_add(&cache->deferred_flush_bios, bio);
        spin_unlock_irqrestore(&cache->lock, flags);
}

static void defer_writethrough_bio(struct cache *cache, struct bio *bio)
{
        unsigned long flags;

        spin_lock_irqsave(&cache->lock, flags);
        bio_list_add(&cache->deferred_writethrough_bios, bio);
        spin_unlock_irqrestore(&cache->lock, flags);

        wake_worker(cache);
}

static void writethrough_endio(struct bio *bio, int err)
{
        struct per_bio_data *pb = get_per_bio_data(bio, PB_DATA_SIZE_WT);
        bio->bi_end_io = pb->saved_bi_end_io;

        if (err) {
                bio_endio(bio, err);
                return;
        }

        dm_bio_restore(&pb->bio_details, bio);
        remap_to_cache(pb->cache, bio, pb->cblock);

        /*
         * We can't issue this bio directly, since we're in interrupt
         * context.  So it gets put on a bio list for processing by the
         * worker thread.
         */
        defer_writethrough_bio(pb->cache, bio);
}

/*
 * When running in writethrough mode we need to send writes to clean blocks
 * to both the cache and origin devices.  In future we'd like to clone the
 * bio and send them in parallel, but for now we're doing them in
 * series as this is easier.
 */
static void remap_to_origin_then_cache(struct cache *cache, struct bio *bio,
                                       dm_oblock_t oblock, dm_cblock_t cblock)
{
        struct per_bio_data *pb = get_per_bio_data(bio, PB_DATA_SIZE_WT);

        pb->cache = cache;
        pb->cblock = cblock;
        pb->saved_bi_end_io = bio->bi_end_io;
        dm_bio_record(&pb->bio_details, bio);
        bio->bi_end_io = writethrough_endio;

        remap_to_origin_clear_discard(pb->cache, bio, oblock);
}

/*----------------------------------------------------------------
 * Migration processing
 *
 * Migration covers moving data from the origin device to the cache, or
 * vice versa.
 *--------------------------------------------------------------*/
static void free_migration(struct dm_cache_migration *mg)
{
        mempool_free(mg, mg->cache->migration_pool);
}

static void inc_nr_migrations(struct cache *cache)
{
        atomic_inc(&cache->nr_migrations);
}

static void dec_nr_migrations(struct cache *cache)
{
        atomic_dec(&cache->nr_migrations);

        /*
         * Wake the worker in case we're suspending the target.
         */
        wake_up(&cache->migration_wait);
}

static void __cell_defer(struct cache *cache, struct dm_bio_prison_cell *cell,
                         bool holder)
{
        (holder ? dm_cell_release : dm_cell_release_no_holder)
                (cache->prison, cell, &cache->deferred_bios);
        free_prison_cell(cache, cell);
}

static void cell_defer(struct cache *cache, struct dm_bio_prison_cell *cell,
                       bool holder)
{
        unsigned long flags;

        spin_lock_irqsave(&cache->lock, flags);
        __cell_defer(cache, cell, holder);
        spin_unlock_irqrestore(&cache->lock, flags);

        wake_worker(cache);
}

static void cleanup_migration(struct dm_cache_migration *mg)
{
        dec_nr_migrations(mg->cache);
        free_migration(mg);
}

static void migration_failure(struct dm_cache_migration *mg)
{
        struct cache *cache = mg->cache;

        if (mg->writeback) {
                DMWARN_LIMIT("writeback failed; couldn't copy block");
                set_dirty(cache, mg->old_oblock, mg->cblock);
                cell_defer(cache, mg->old_ocell, false);

        } else if (mg->demote) {
                DMWARN_LIMIT("demotion failed; couldn't copy block");
                policy_force_mapping(cache->policy, mg->new_oblock, mg->old_oblock);

                cell_defer(cache, mg->old_ocell, mg->promote ? 0 : 1);
                if (mg->promote)
                        cell_defer(cache, mg->new_ocell, 1);
        } else {
                DMWARN_LIMIT("promotion failed; couldn't copy block");
                policy_remove_mapping(cache->policy, mg->new_oblock);
                cell_defer(cache, mg->new_ocell, 1);
        }

        cleanup_migration(mg);
}

static void migration_success_pre_commit(struct dm_cache_migration *mg)
{
        unsigned long flags;
        struct cache *cache = mg->cache;

        if (mg->writeback) {
                cell_defer(cache, mg->old_ocell, false);
                clear_dirty(cache, mg->old_oblock, mg->cblock);
                cleanup_migration(mg);
                return;

        } else if (mg->demote) {
                if (dm_cache_remove_mapping(cache->cmd, mg->cblock)) {
                        DMWARN_LIMIT("demotion failed; couldn't update on disk metadata");
                        policy_force_mapping(cache->policy, mg->new_oblock,
                                             mg->old_oblock);
                        if (mg->promote)
                                cell_defer(cache, mg->new_ocell, true);
                        cleanup_migration(mg);
                        return;
                }
        } else {
                if (dm_cache_insert_mapping(cache->cmd, mg->cblock, mg->new_oblock)) {
                        DMWARN_LIMIT("promotion failed; couldn't update on disk metadata");
                        policy_remove_mapping(cache->policy, mg->new_oblock);
                        cleanup_migration(mg);
                        return;
                }
        }

        spin_lock_irqsave(&cache->lock, flags);
        list_add_tail(&mg->list, &cache->need_commit_migrations);
        cache->commit_requested = true;
        spin_unlock_irqrestore(&cache->lock, flags);
}

static void migration_success_post_commit(struct dm_cache_migration *mg)
{
        unsigned long flags;
        struct cache *cache = mg->cache;

        if (mg->writeback) {
                DMWARN("writeback unexpectedly triggered commit");
                return;

        } else if (mg->demote) {
                cell_defer(cache, mg->old_ocell, mg->promote ? 0 : 1);

                if (mg->promote) {
                        mg->demote = false;

                        spin_lock_irqsave(&cache->lock, flags);
                        list_add_tail(&mg->list, &cache->quiesced_migrations);
                        spin_unlock_irqrestore(&cache->lock, flags);

                } else
                        cleanup_migration(mg);

        } else {
                cell_defer(cache, mg->new_ocell, true);
                clear_dirty(cache, mg->new_oblock, mg->cblock);
                cleanup_migration(mg);
        }
}

static void copy_complete(int read_err, unsigned long write_err, void *context)
{
        unsigned long flags;
        struct dm_cache_migration *mg = (struct dm_cache_migration *) context;
        struct cache *cache = mg->cache;

        if (read_err || write_err)
                mg->err = true;

        spin_lock_irqsave(&cache->lock, flags);
        list_add_tail(&mg->list, &cache->completed_migrations);
        spin_unlock_irqrestore(&cache->lock, flags);

        wake_worker(cache);
}

static void issue_copy_real(struct dm_cache_migration *mg)
{
        int r;
        struct dm_io_region o_region, c_region;
        struct cache *cache = mg->cache;

        o_region.bdev = cache->origin_dev->bdev;
        o_region.count = cache->sectors_per_block;

        c_region.bdev = cache->cache_dev->bdev;
        c_region.sector = from_cblock(mg->cblock) * cache->sectors_per_block;
        c_region.count = cache->sectors_per_block;

        if (mg->writeback || mg->demote) {
                /* demote */
                o_region.sector = from_oblock(mg->old_oblock) * cache->sectors_per_block;
                r = dm_kcopyd_copy(cache->copier, &c_region, 1, &o_region, 0, copy_complete, mg);
        } else {
                /* promote */
                o_region.sector = from_oblock(mg->new_oblock) * cache->sectors_per_block;
                r = dm_kcopyd_copy(cache->copier, &o_region, 1, &c_region, 0, copy_complete, mg);
        }

        if (r < 0)
                migration_failure(mg);
}

static void avoid_copy(struct dm_cache_migration *mg)
{
        atomic_inc(&mg->cache->stats.copies_avoided);
        migration_success_pre_commit(mg);
}

static void issue_copy(struct dm_cache_migration *mg)
{
        bool avoid;
        struct cache *cache = mg->cache;

        if (mg->writeback || mg->demote)
                avoid = !is_dirty(cache, mg->cblock) ||
                        is_discarded_oblock(cache, mg->old_oblock);
        else
                avoid = is_discarded_oblock(cache, mg->new_oblock);

        avoid ? avoid_copy(mg) : issue_copy_real(mg);
}

static void complete_migration(struct dm_cache_migration *mg)
{
        if (mg->err)
                migration_failure(mg);
        else
                migration_success_pre_commit(mg);
}

static void process_migrations(struct cache *cache, struct list_head *head,
                               void (*fn)(struct dm_cache_migration *))
{
        unsigned long flags;
        struct list_head list;
        struct dm_cache_migration *mg, *tmp;

        INIT_LIST_HEAD(&list);
        spin_lock_irqsave(&cache->lock, flags);
        list_splice_init(head, &list);
        spin_unlock_irqrestore(&cache->lock, flags);

        list_for_each_entry_safe(mg, tmp, &list, list)
                fn(mg);
}

static void __queue_quiesced_migration(struct dm_cache_migration *mg)
{
        list_add_tail(&mg->list, &mg->cache->quiesced_migrations);
}

static void queue_quiesced_migration(struct dm_cache_migration *mg)
{
        unsigned long flags;
        struct cache *cache = mg->cache;

        spin_lock_irqsave(&cache->lock, flags);
        __queue_quiesced_migration(mg);
        spin_unlock_irqrestore(&cache->lock, flags);

        wake_worker(cache);
}

static void queue_quiesced_migrations(struct cache *cache, struct list_head *work)
{
        unsigned long flags;
        struct dm_cache_migration *mg, *tmp;

        spin_lock_irqsave(&cache->lock, flags);
        list_for_each_entry_safe(mg, tmp, work, list)
                __queue_quiesced_migration(mg);
        spin_unlock_irqrestore(&cache->lock, flags);

        wake_worker(cache);
}

static void check_for_quiesced_migrations(struct cache *cache,
                                          struct per_bio_data *pb)
{
        struct list_head work;

        if (!pb->all_io_entry)
                return;

        INIT_LIST_HEAD(&work);
        if (pb->all_io_entry)
                dm_deferred_entry_dec(pb->all_io_entry, &work);

        if (!list_empty(&work))
                queue_quiesced_migrations(cache, &work);
}

static void quiesce_migration(struct dm_cache_migration *mg)
{
        if (!dm_deferred_set_add_work(mg->cache->all_io_ds, &mg->list))
                queue_quiesced_migration(mg);
}

static void promote(struct cache *cache, struct prealloc *structs,
                    dm_oblock_t oblock, dm_cblock_t cblock,
                    struct dm_bio_prison_cell *cell)
{
        struct dm_cache_migration *mg = prealloc_get_migration(structs);

        mg->err = false;
        mg->writeback = false;
        mg->demote = false;
        mg->promote = true;
        mg->cache = cache;
        mg->new_oblock = oblock;
        mg->cblock = cblock;
        mg->old_ocell = NULL;
        mg->new_ocell = cell;
        mg->start_jiffies = jiffies;

        inc_nr_migrations(cache);
        quiesce_migration(mg);
}

static void writeback(struct cache *cache, struct prealloc *structs,
                      dm_oblock_t oblock, dm_cblock_t cblock,

                      struct dm_bio_prison_cell *cell)
{
        struct dm_cache_migration *mg = prealloc_get_migration(structs);

        mg->err = false;
        mg->writeback = true;
        mg->demote = false;
        mg->promote = false;
        mg->cache = cache;
        mg->old_oblock = oblock;
        mg->cblock = cblock;
        mg->old_ocell = cell;
        mg->new_ocell = NULL;
        mg->start_jiffies = jiffies;

        inc_nr_migrations(cache);
        quiesce_migration(mg);
}

static void demote_then_promote(struct cache *cache, struct prealloc *structs,
                                dm_oblock_t old_oblock, dm_oblock_t new_oblock,
                                dm_cblock_t cblock,
                                struct dm_bio_prison_cell *old_ocell,
                                struct dm_bio_prison_cell *new_ocell)
{
        struct dm_cache_migration *mg = prealloc_get_migration(structs);

        mg->err = false;
        mg->writeback = false;
        mg->demote = true;
        mg->promote = true;
        mg->cache = cache;
        mg->old_oblock = old_oblock;
        mg->new_oblock = new_oblock;
        mg->cblock = cblock;
        mg->old_ocell = old_ocell;
        mg->new_ocell = new_ocell;
        mg->start_jiffies = jiffies;

        inc_nr_migrations(cache);
        quiesce_migration(mg);
}

/*----------------------------------------------------------------
 * bio processing
 *--------------------------------------------------------------*/
static void defer_bio(struct cache *cache, struct bio *bio)
{
        unsigned long flags;

        spin_lock_irqsave(&cache->lock, flags);
        bio_list_add(&cache->deferred_bios, bio);
        spin_unlock_irqrestore(&cache->lock, flags);

        wake_worker(cache);
}

static void process_flush_bio(struct cache *cache, struct bio *bio)
{
        size_t pb_data_size = get_per_bio_data_size(cache);
        struct per_bio_data *pb = get_per_bio_data(bio, pb_data_size);

        BUG_ON(bio->bi_size);
        if (!pb->req_nr)
                remap_to_origin(cache, bio);
        else
                remap_to_cache(cache, bio, 0);

        issue(cache, bio);
}

/*
 * People generally discard large parts of a device, eg, the whole device
 * when formatting.  Splitting these large discards up into cache block
 * sized ios and then quiescing (always neccessary for discard) takes too
 * long.
 *
 * We keep it simple, and allow any size of discard to come in, and just
 * mark off blocks on the discard bitset.  No passdown occurs!
 *
 * To implement passdown we need to change the bio_prison such that a cell
 * can have a key that spans many blocks.
 */
static void process_discard_bio(struct cache *cache, struct bio *bio)
{
        dm_block_t start_block = dm_sector_div_up(bio->bi_sector,
                                                  cache->discard_block_size);
        dm_block_t end_block = bio->bi_sector + bio_sectors(bio);
        dm_block_t b;

        end_block = block_div(end_block, cache->discard_block_size);

        for (b = start_block; b < end_block; b++)
                set_discard(cache, to_dblock(b));

        bio_endio(bio, 0);
}

static bool spare_migration_bandwidth(struct cache *cache)
{
        sector_t current_volume = (atomic_read(&cache->nr_migrations) + 1) *
                cache->sectors_per_block;
        return current_volume < cache->migration_threshold;
}

static bool is_writethrough_io(struct cache *cache, struct bio *bio,
                               dm_cblock_t cblock)
{
        return bio_data_dir(bio) == WRITE &&
                cache->features.write_through && !is_dirty(cache, cblock);
}

static void inc_hit_counter(struct cache *cache, struct bio *bio)
{
        atomic_inc(bio_data_dir(bio) == READ ?
                   &cache->stats.read_hit : &cache->stats.write_hit);
}

static void inc_miss_counter(struct cache *cache, struct bio *bio)
{
        atomic_inc(bio_data_dir(bio) == READ ?
                   &cache->stats.read_miss : &cache->stats.write_miss);
}

static void process_bio(struct cache *cache, struct prealloc *structs,
                        struct bio *bio)
{
        int r;
        bool release_cell = true;
        dm_oblock_t block = get_bio_block(cache, bio);
        struct dm_bio_prison_cell *cell_prealloc, *old_ocell, *new_ocell;
        struct policy_result lookup_result;
        size_t pb_data_size = get_per_bio_data_size(cache);
        struct per_bio_data *pb = get_per_bio_data(bio, pb_data_size);
        bool discarded_block = is_discarded_oblock(cache, block);
        bool can_migrate = discarded_block || spare_migration_bandwidth(cache);

        /*
         * Check to see if that block is currently migrating.
         */
        cell_prealloc = prealloc_get_cell(structs);
        r = bio_detain(cache, block, bio, cell_prealloc,
                       (cell_free_fn) prealloc_put_cell,
                       structs, &new_ocell);
        if (r > 0)
                return;

        r = policy_map(cache->policy, block, true, can_migrate, discarded_block,
                       bio, &lookup_result);

        if (r == -EWOULDBLOCK)
                /* migration has been denied */
                lookup_result.op = POLICY_MISS;

        switch (lookup_result.op) {
        case POLICY_HIT:
                inc_hit_counter(cache, bio);
                pb->all_io_entry = dm_deferred_entry_inc(cache->all_io_ds);

                if (is_writethrough_io(cache, bio, lookup_result.cblock))
                        remap_to_origin_then_cache(cache, bio, block, lookup_result.cblock);
                else
                        remap_to_cache_dirty(cache, bio, block, lookup_result.cblock);

                issue(cache, bio);
                break;

        case POLICY_MISS:
                inc_miss_counter(cache, bio);
                pb->all_io_entry = dm_deferred_entry_inc(cache->all_io_ds);
                remap_to_origin_clear_discard(cache, bio, block);
                issue(cache, bio);
                break;

        case POLICY_NEW:
                atomic_inc(&cache->stats.promotion);
                promote(cache, structs, block, lookup_result.cblock, new_ocell);
                release_cell = false;
                break;

        case POLICY_REPLACE:
                cell_prealloc = prealloc_get_cell(structs);
                r = bio_detain(cache, lookup_result.old_oblock, bio, cell_prealloc,
                               (cell_free_fn) prealloc_put_cell,
                               structs, &old_ocell);
                if (r > 0) {
                        /*
                         * We have to be careful to avoid lock inversion of
                         * the cells.  So we back off, and wait for the
                         * old_ocell to become free.
                         */
                        policy_force_mapping(cache->policy, block,
                                             lookup_result.old_oblock);
                        atomic_inc(&cache->stats.cache_cell_clash);
                        break;
                }
                atomic_inc(&cache->stats.demotion);
                atomic_inc(&cache->stats.promotion);

                demote_then_promote(cache, structs, lookup_result.old_oblock,
                                    block, lookup_result.cblock,
                                    old_ocell, new_ocell);
                release_cell = false;
                break;

        default:
                DMERR_LIMIT("%s: erroring bio, unknown policy op: %u", __func__,
                            (unsigned) lookup_result.op);
                bio_io_error(bio);
        }

        if (release_cell)
                cell_defer(cache, new_ocell, false);
}

static int need_commit_due_to_time(struct cache *cache)
{
        return jiffies < cache->last_commit_jiffies ||
               jiffies > cache->last_commit_jiffies + COMMIT_PERIOD;
}

static int commit_if_needed(struct cache *cache)
{
        if (dm_cache_changed_this_transaction(cache->cmd) &&
            (cache->commit_requested || need_commit_due_to_time(cache))) {
                atomic_inc(&cache->stats.commit_count);
                cache->last_commit_jiffies = jiffies;
                cache->commit_requested = false;
                return dm_cache_commit(cache->cmd, false);
        }

        return 0;
}

static void process_deferred_bios(struct cache *cache)
{
        unsigned long flags;
        struct bio_list bios;
        struct bio *bio;
        struct prealloc structs;

        memset(&structs, 0, sizeof(structs));
        bio_list_init(&bios);

        spin_lock_irqsave(&cache->lock, flags);
        bio_list_merge(&bios, &cache->deferred_bios);
        bio_list_init(&cache->deferred_bios);
        spin_unlock_irqrestore(&cache->lock, flags);

        while (!bio_list_empty(&bios)) {
                /*
                 * If we've got no free migration structs, and processing
                 * this bio might require one, we pause until there are some
                 * prepared mappings to process.
                 */
                if (prealloc_data_structs(cache, &structs)) {
                        spin_lock_irqsave(&cache->lock, flags);
                        bio_list_merge(&cache->deferred_bios, &bios);
                        spin_unlock_irqrestore(&cache->lock, flags);
                        break;
                }

                bio = bio_list_pop(&bios);

                if (bio->bi_rw & REQ_FLUSH)
                        process_flush_bio(cache, bio);
                else if (bio->bi_rw & REQ_DISCARD)
                        process_discard_bio(cache, bio);
                else
                        process_bio(cache, &structs, bio);
        }

        prealloc_free_structs(cache, &structs);
}

static void process_deferred_flush_bios(struct cache *cache, bool submit_bios)
{
        unsigned long flags;
        struct bio_list bios;
        struct bio *bio;

        bio_list_init(&bios);

        spin_lock_irqsave(&cache->lock, flags);
        bio_list_merge(&bios, &cache->deferred_flush_bios);
        bio_list_init(&cache->deferred_flush_bios);
        spin_unlock_irqrestore(&cache->lock, flags);

        while ((bio = bio_list_pop(&bios)))
                submit_bios ? generic_make_request(bio) : bio_io_error(bio);
}

static void process_deferred_writethrough_bios(struct cache *cache)
{
        unsigned long flags;
        struct bio_list bios;
        struct bio *bio;

        bio_list_init(&bios);

        spin_lock_irqsave(&cache->lock, flags);
        bio_list_merge(&bios, &cache->deferred_writethrough_bios);
        bio_list_init(&cache->deferred_writethrough_bios);
        spin_unlock_irqrestore(&cache->lock, flags);

        while ((bio = bio_list_pop(&bios)))
                generic_make_request(bio);
}

static void writeback_some_dirty_blocks(struct cache *cache)
{
        int r = 0;
        dm_oblock_t oblock;
        dm_cblock_t cblock;
        struct prealloc structs;
        struct dm_bio_prison_cell *old_ocell;

        memset(&structs, 0, sizeof(structs));

        while (spare_migration_bandwidth(cache)) {
                if (prealloc_data_structs(cache, &structs))
                        break;

                r = policy_writeback_work(cache->policy, &oblock, &cblock);
                if (r)
                        break;

                r = get_cell(cache, oblock, &structs, &old_ocell);
                if (r) {
                        policy_set_dirty(cache->policy, oblock);
                        break;
                }

                writeback(cache, &structs, oblock, cblock, old_ocell);
        }

        prealloc_free_structs(cache, &structs);
}

/*----------------------------------------------------------------
 * Main worker loop
 *--------------------------------------------------------------*/
static void start_quiescing(struct cache *cache)
{
        unsigned long flags;

        spin_lock_irqsave(&cache->lock, flags);
        cache->quiescing = 1;
        spin_unlock_irqrestore(&cache->lock, flags);
}

static void stop_quiescing(struct cache *cache)
{
        unsigned long flags;

        spin_lock_irqsave(&cache->lock, flags);
        cache->quiescing = 0;
        spin_unlock_irqrestore(&cache->lock, flags);
}

static bool is_quiescing(struct cache *cache)
{
        int r;
        unsigned long flags;

        spin_lock_irqsave(&cache->lock, flags);
        r = cache->quiescing;
        spin_unlock_irqrestore(&cache->lock, flags);

        return r;
}

static void wait_for_migrations(struct cache *cache)
{
        wait_event(cache->migration_wait, !atomic_read(&cache->nr_migrations));
}

static void stop_worker(struct cache *cache)
{
        cancel_delayed_work(&cache->waker);
        flush_workqueue(cache->wq);
}

static void requeue_deferred_io(struct cache *cache)
{
        struct bio *bio;
        struct bio_list bios;

        bio_list_init(&bios);
        bio_list_merge(&bios, &cache->deferred_bios);
        bio_list_init(&cache->deferred_bios);

        while ((bio = bio_list_pop(&bios)))
                bio_endio(bio, DM_ENDIO_REQUEUE);
}

static int more_work(struct cache *cache)
{
        if (is_quiescing(cache))
                return !list_empty(&cache->quiesced_migrations) ||
                        !list_empty(&cache->completed_migrations) ||
                        !list_empty(&cache->need_commit_migrations);
        else
                return !bio_list_empty(&cache->deferred_bios) ||
                        !bio_list_empty(&cache->deferred_flush_bios) ||
                        !bio_list_empty(&cache->deferred_writethrough_bios) ||
                        !list_empty(&cache->quiesced_migrations) ||
                        !list_empty(&cache->completed_migrations) ||
                        !list_empty(&cache->need_commit_migrations);
}

static void do_worker(struct work_struct *ws)
{
        struct cache *cache = container_of(ws, struct cache, worker);

        do {
                if (!is_quiescing(cache))
                        process_deferred_bios(cache);

                process_migrations(cache, &cache->quiesced_migrations, issue_copy);
                process_migrations(cache, &cache->completed_migrations, complete_migration);

                writeback_some_dirty_blocks(cache);

                process_deferred_writethrough_bios(cache);

                if (commit_if_needed(cache)) {
                        process_deferred_flush_bios(cache, false);

                        /*
                         * FIXME: rollback metadata or just go into a
                         * failure mode and error everything
                         */
                } else {
                        process_deferred_flush_bios(cache, true);
                        process_migrations(cache, &cache->need_commit_migrations,
                                           migration_success_post_commit);
                }
        } while (more_work(cache));
}

/*
 * We want to commit periodically so that not too much
 * unwritten metadata builds up.
 */
static void do_waker(struct work_struct *ws)
{
        struct cache *cache = container_of(to_delayed_work(ws), struct cache, waker);
        policy_tick(cache->policy);
        wake_worker(cache);
        queue_delayed_work(cache->wq, &cache->waker, COMMIT_PERIOD);
}

/*----------------------------------------------------------------*/

static int is_congested(struct dm_dev *dev, int bdi_bits)
{
        struct request_queue *q = bdev_get_queue(dev->bdev);
        return bdi_congested(&q->backing_dev_info, bdi_bits);
}

static int cache_is_congested(struct dm_target_callbacks *cb, int bdi_bits)
{
        struct cache *cache = container_of(cb, struct cache, callbacks);

        return is_congested(cache->origin_dev, bdi_bits) ||
                is_congested(cache->cache_dev, bdi_bits);
}

/*----------------------------------------------------------------
 * Target methods
 *--------------------------------------------------------------*/

/*
 * This function gets called on the error paths of the constructor, so we
 * have to cope with a partially initialised struct.
 */
static void destroy(struct cache *cache)
{
        unsigned i;

        if (cache->next_migration)
                mempool_free(cache->next_migration, cache->migration_pool);

        if (cache->migration_pool)
                mempool_destroy(cache->migration_pool);

        if (cache->all_io_ds)
                dm_deferred_set_destroy(cache->all_io_ds);

        if (cache->prison)
                dm_bio_prison_destroy(cache->prison);

        if (cache->wq)
                destroy_workqueue(cache->wq);

        if (cache->dirty_bitset)
                free_bitset(cache->dirty_bitset);

        if (cache->discard_bitset)
                free_bitset(cache->discard_bitset);

        if (cache->copier)
                dm_kcopyd_client_destroy(cache->copier);

        if (cache->cmd)
                dm_cache_metadata_close(cache->cmd);

        if (cache->metadata_dev)
                dm_put_device(cache->ti, cache->metadata_dev);

        if (cache->origin_dev)
                dm_put_device(cache->ti, cache->origin_dev);

        if (cache->cache_dev)
                dm_put_device(cache->ti, cache->cache_dev);

        if (cache->policy)
                dm_cache_policy_destroy(cache->policy);

        for (i = 0; i < cache->nr_ctr_args ; i++)
                kfree(cache->ctr_args[i]);
        kfree(cache->ctr_args);

        kfree(cache);
}

static void cache_dtr(struct dm_target *ti)
{
        struct cache *cache = ti->private;

        destroy(cache);
}

static sector_t get_dev_size(struct dm_dev *dev)
{
        return i_size_read(dev->bdev->bd_inode) >> SECTOR_SHIFT;
}

/*----------------------------------------------------------------*/

/*
 * Construct a cache device mapping.
 *
 * cache <metadata dev> <cache dev> <origin dev> <block size>
 *       <#feature args> [<feature arg>]*
 *       <policy> <#policy args> [<policy arg>]*
 *
 * metadata dev    : fast device holding the persistent metadata
 * cache dev       : fast device holding cached data blocks
 * origin dev      : slow device holding original data blocks
 * block size      : cache unit size in sectors
 *
 * #feature args   : number of feature arguments passed
 * feature args    : writethrough.  (The default is writeback.)
 *
 * policy          : the replacement policy to use
 * #policy args    : an even number of policy arguments corresponding
 *                   to key/value pairs passed to the policy
 * policy args     : key/value pairs passed to the policy
 *                   E.g. 'sequential_threshold 1024'
 *                   See cache-policies.txt for details.
 *
 * Optional feature arguments are:
 *   writethrough  : write through caching that prohibits cache block
 *                   content from being different from origin block content.
 *                   Without this argument, the default behaviour is to write
 *                   back cache block contents later for performance reasons,
 *                   so they may differ from the corresponding origin blocks.
 */
struct cache_args {
        struct dm_target *ti;

        struct dm_dev *metadata_dev;

        struct dm_dev *cache_dev;
        sector_t cache_sectors;

        struct dm_dev *origin_dev;
        sector_t origin_sectors;

        uint32_t block_size;

        const char *policy_name;
        int policy_argc;
        const char **policy_argv;

        struct cache_features features;
};

static void destroy_cache_args(struct cache_args *ca)
{
        if (ca->metadata_dev)
                dm_put_device(ca->ti, ca->metadata_dev);

        if (ca->cache_dev)
                dm_put_device(ca->ti, ca->cache_dev);

        if (ca->origin_dev)
                dm_put_device(ca->ti, ca->origin_dev);

        kfree(ca);
}

static bool at_least_one_arg(struct dm_arg_set *as, char **error)
{
        if (!as->argc) {
                *error = "Insufficient args";
                return false;
        }

        return true;
}

static int parse_metadata_dev(struct cache_args *ca, struct dm_arg_set *as,
                              char **error)
{
        int r;
        sector_t metadata_dev_size;
        char b[BDEVNAME_SIZE];

        if (!at_least_one_arg(as, error))
                return -EINVAL;

        r = dm_get_device(ca->ti, dm_shift_arg(as), FMODE_READ | FMODE_WRITE,
                          &ca->metadata_dev);
        if (r) {
                *error = "Error opening metadata device";
                return r;
        }

        metadata_dev_size = get_dev_size(ca->metadata_dev);
        if (metadata_dev_size > DM_CACHE_METADATA_MAX_SECTORS_WARNING)
                DMWARN("Metadata device %s is larger than %u sectors: excess space will not be used.",
                       bdevname(ca->metadata_dev->bdev, b), THIN_METADATA_MAX_SECTORS);

        return 0;
}

static int parse_cache_dev(struct cache_args *ca, struct dm_arg_set *as,
                           char **error)
{
        int r;

        if (!at_least_one_arg(as, error))
                return -EINVAL;

        r = dm_get_device(ca->ti, dm_shift_arg(as), FMODE_READ | FMODE_WRITE,
                          &ca->cache_dev);
        if (r) {
                *error = "Error opening cache device";
                return r;
        }
        ca->cache_sectors = get_dev_size(ca->cache_dev);

        return 0;
}

static int parse_origin_dev(struct cache_args *ca, struct dm_arg_set *as,
                            char **error)
{
        int r;

        if (!at_least_one_arg(as, error))
                return -EINVAL;

        r = dm_get_device(ca->ti, dm_shift_arg(as), FMODE_READ | FMODE_WRITE,
                          &ca->origin_dev);
        if (r) {
                *error = "Error opening origin device";
                return r;
        }

        ca->origin_sectors = get_dev_size(ca->origin_dev);
        if (ca->ti->len > ca->origin_sectors) {
                *error = "Device size larger than cached device";
                return -EINVAL;
        }

        return 0;
}

static int parse_block_size(struct cache_args *ca, struct dm_arg_set *as,
                            char **error)
{
        unsigned long tmp;

        if (!at_least_one_arg(as, error))
                return -EINVAL;

        if (kstrtoul(dm_shift_arg(as), 10, &tmp) || !tmp ||
            tmp < DATA_DEV_BLOCK_SIZE_MIN_SECTORS ||
            tmp & (DATA_DEV_BLOCK_SIZE_MIN_SECTORS - 1)) {
                *error = "Invalid data block size";
                return -EINVAL;
        }

        if (tmp > ca->cache_sectors) {
                *error = "Data block size is larger than the cache device";
                return -EINVAL;
        }

        ca->block_size = tmp;

        return 0;
}

static void init_features(struct cache_features *cf)
{
        cf->mode = CM_WRITE;
        cf->write_through = false;
}

static int parse_features(struct cache_args *ca, struct dm_arg_set *as,
                          char **error)
{
        static struct dm_arg _args[] = {
                {0, 1, "Invalid number of cache feature arguments"},
        };

        int r;
        unsigned argc;
        const char *arg;
        struct cache_features *cf = &ca->features;

        init_features(cf);

        r = dm_read_arg_group(_args, as, &argc, error);
        if (r)
                return -EINVAL;

        while (argc--) {
                arg = dm_shift_arg(as);

                if (!strcasecmp(arg, "writeback"))
                        cf->write_through = false;

                else if (!strcasecmp(arg, "writethrough"))
                        cf->write_through = true;

                else {
                        *error = "Unrecognised cache feature requested";
                        return -EINVAL;
                }
        }

        return 0;
}

static int parse_policy(struct cache_args *ca, struct dm_arg_set *as,
                        char **error)
{
        static struct dm_arg _args[] = {
                {0, 1024, "Invalid number of policy arguments"},
        };

        int r;

        if (!at_least_one_arg(as, error))
                return -EINVAL;

        ca->policy_name = dm_shift_arg(as);

        r = dm_read_arg_group(_args, as, &ca->policy_argc, error);
        if (r)
                return -EINVAL;

        ca->policy_argv = (const char **)as->argv;
        dm_consume_args(as, ca->policy_argc);

        return 0;
}

static int parse_cache_args(struct cache_args *ca, int argc, char **argv,
                            char **error)
{
        int r;
        struct dm_arg_set as;

        as.argc = argc;
        as.argv = argv;

        r = parse_metadata_dev(ca, &as, error);
        if (r)
                return r;

        r = parse_cache_dev(ca, &as, error);
        if (r)
                return r;

        r = parse_origin_dev(ca, &as, error);
        if (r)
                return r;

        r = parse_block_size(ca, &as, error);
        if (r)
                return r;

        r = parse_features(ca, &as, error);
        if (r)
                return r;

        r = parse_policy(ca, &as, error);
        if (r)
                return r;

        return 0;
}

/*----------------------------------------------------------------*/

static struct kmem_cache *migration_cache;

#define NOT_CORE_OPTION 1

static int process_config_option(struct cache *cache, const char *key, const char *value)
{
        unsigned long tmp;

        if (!strcasecmp(key, "migration_threshold")) {
                if (kstrtoul(value, 10, &tmp))
                        return -EINVAL;

                cache->migration_threshold = tmp;
                return 0;
        }

        return NOT_CORE_OPTION;
}

static int set_config_value(struct cache *cache, const char *key, const char *value)
{
        int r = process_config_option(cache, key, value);

        if (r == NOT_CORE_OPTION)
                r = policy_set_config_value(cache->policy, key, value);

        if (r)
                DMWARN("bad config value for %s: %s", key, value);

        return r;
}

static int set_config_values(struct cache *cache, int argc, const char **argv)
{
        int r = 0;

        if (argc & 1) {
                DMWARN("Odd number of policy arguments given but they should be <key> <value> pairs.");
                return -EINVAL;
        }

        while (argc) {
                r = set_config_value(cache, argv[0], argv[1]);
                if (r)
                        break;

                argc -= 2;
                argv += 2;
        }

        return r;
}

static int create_cache_policy(struct cache *cache, struct cache_args *ca,
                               char **error)
{
        cache->policy = dm_cache_policy_create(ca->policy_name,
                                               cache->cache_size,
                                               cache->origin_sectors,
                                               cache->sectors_per_block);
        if (!cache->policy) {
                *error = "Error creating cache's policy";
                return -ENOMEM;
        }

        return 0;
}

/*
 * We want the discard block size to be a power of two, at least the size
 * of the cache block size, and have no more than 2^14 discard blocks
 * across the origin.
 */
#define MAX_DISCARD_BLOCKS (1 << 14)

static bool too_many_discard_blocks(sector_t discard_block_size,
                                    sector_t origin_size)
{
        (void) sector_div(origin_size, discard_block_size);

        return origin_size > MAX_DISCARD_BLOCKS;
}

static sector_t calculate_discard_block_size(sector_t cache_block_size,
                                             sector_t origin_size)
{
        sector_t discard_block_size;

        discard_block_size = roundup_pow_of_two(cache_block_size);

        if (origin_size)
                while (too_many_discard_blocks(discard_block_size, origin_size))
                        discard_block_size *= 2;

        return discard_block_size;
}

#define DEFAULT_MIGRATION_THRESHOLD 2048

static int cache_create(struct cache_args *ca, struct cache **result)
{
        int r = 0;
        char **error = &ca->ti->error;
        struct cache *cache;
        struct dm_target *ti = ca->ti;
        dm_block_t origin_blocks;
        struct dm_cache_metadata *cmd;
        bool may_format = ca->features.mode == CM_WRITE;

        cache = kzalloc(sizeof(*cache), GFP_KERNEL);
        if (!cache)
                return -ENOMEM;

        cache->ti = ca->ti;
        ti->private = cache;
        ti->num_flush_bios = 2;
        ti->flush_supported = true;

        ti->num_discard_bios = 1;
        ti->discards_supported = true;
        ti->discard_zeroes_data_unsupported = true;

        cache->features = ca->features;
        ti->per_bio_data_size = get_per_bio_data_size(cache);

        cache->callbacks.congested_fn = cache_is_congested;
        dm_table_add_target_callbacks(ti->table, &cache->callbacks);

        cache->metadata_dev = ca->metadata_dev;
        cache->origin_dev = ca->origin_dev;
        cache->cache_dev = ca->cache_dev;

        ca->metadata_dev = ca->origin_dev = ca->cache_dev = NULL;

        /* FIXME: factor out this whole section */
        origin_blocks = cache->origin_sectors = ca->origin_sectors;
        origin_blocks = block_div(origin_blocks, ca->block_size);
        cache->origin_blocks = to_oblock(origin_blocks);

        cache->sectors_per_block = ca->block_size;
        if (dm_set_target_max_io_len(ti, cache->sectors_per_block)) {
                r = -EINVAL;
                goto bad;
        }

        if (ca->block_size & (ca->block_size - 1)) {
                dm_block_t cache_size = ca->cache_sectors;

                cache->sectors_per_block_shift = -1;
                cache_size = block_div(cache_size, ca->block_size);
                cache->cache_size = to_cblock(cache_size);
        } else {
                cache->sectors_per_block_shift = __ffs(ca->block_size);
                cache->cache_size = to_cblock(ca->cache_sectors >> cache->sectors_per_block_shift);
        }

        r = create_cache_policy(cache, ca, error);
        if (r)
                goto bad;

        cache->policy_nr_args = ca->policy_argc;
        cache->migration_threshold = DEFAULT_MIGRATION_THRESHOLD;

        r = set_config_values(cache, ca->policy_argc, ca->policy_argv);
        if (r) {
                *error = "Error setting cache policy's config values";
                goto bad;
        }

        cmd = dm_cache_metadata_open(cache->metadata_dev->bdev,
                                     ca->block_size, may_format,
                                     dm_cache_policy_get_hint_size(cache->policy));
        if (IS_ERR(cmd)) {
                *error = "Error creating metadata object";
                r = PTR_ERR(cmd);
                goto bad;
        }
        cache->cmd = cmd;

        spin_lock_init(&cache->lock);
        bio_list_init(&cache->deferred_bios);
        bio_list_init(&cache->deferred_flush_bios);
        bio_list_init(&cache->deferred_writethrough_bios);
        INIT_LIST_HEAD(&cache->quiesced_migrations);
        INIT_LIST_HEAD(&cache->completed_migrations);
        INIT_LIST_HEAD(&cache->need_commit_migrations);
        atomic_set(&cache->nr_migrations, 0);
        init_waitqueue_head(&cache->migration_wait);

        r = -ENOMEM;
        cache->nr_dirty = 0;
        cache->dirty_bitset = alloc_bitset(from_cblock(cache->cache_size));
        if (!cache->dirty_bitset) {
                *error = "could not allocate dirty bitset";
                goto bad;
        }
        clear_bitset(cache->dirty_bitset, from_cblock(cache->cache_size));

        cache->discard_block_size =
                calculate_discard_block_size(cache->sectors_per_block,
                                             cache->origin_sectors);
        cache->discard_nr_blocks = oblock_to_dblock(cache, cache->origin_blocks);
        cache->discard_bitset = alloc_bitset(from_dblock(cache->discard_nr_blocks));
        if (!cache->discard_bitset) {
                *error = "could not allocate discard bitset";
                goto bad;
        }
        clear_bitset(cache->discard_bitset, from_dblock(cache->discard_nr_blocks));

        cache->copier = dm_kcopyd_client_create(&dm_kcopyd_throttle);
        if (IS_ERR(cache->copier)) {
                *error = "could not create kcopyd client";
                r = PTR_ERR(cache->copier);
                goto bad;
        }

        cache->wq = alloc_ordered_workqueue("dm-" DM_MSG_PREFIX, WQ_MEM_RECLAIM);
        if (!cache->wq) {
                *error = "could not create workqueue for metadata object";
                goto bad;
        }
        INIT_WORK(&cache->worker, do_worker);
        INIT_DELAYED_WORK(&cache->waker, do_waker);
        cache->last_commit_jiffies = jiffies;

        cache->prison = dm_bio_prison_create(PRISON_CELLS);
        if (!cache->prison) {
                *error = "could not create bio prison";
                goto bad;
        }

        cache->all_io_ds = dm_deferred_set_create();
        if (!cache->all_io_ds) {
                *error = "could not create all_io deferred set";
                goto bad;
        }

        cache->migration_pool = mempool_create_slab_pool(MIGRATION_POOL_SIZE,
                                                         migration_cache);
        if (!cache->migration_pool) {
                *error = "Error creating cache's migration mempool";
                goto bad;
        }

        cache->next_migration = NULL;

        cache->need_tick_bio = true;
        cache->sized = false;
        cache->quiescing = false;
        cache->commit_requested = false;
        cache->loaded_mappings = false;
        cache->loaded_discards = false;

        load_stats(cache);

        atomic_set(&cache->stats.demotion, 0);
        atomic_set(&cache->stats.promotion, 0);
        atomic_set(&cache->stats.copies_avoided, 0);
        atomic_set(&cache->stats.cache_cell_clash, 0);
        atomic_set(&cache->stats.commit_count, 0);
        atomic_set(&cache->stats.discard_count, 0);

        *result = cache;
        return 0;

bad:
        destroy(cache);
        return r;
}

static int copy_ctr_args(struct cache *cache, int argc, const char **argv)
{
        unsigned i;
        const char **copy;

        copy = kcalloc(argc, sizeof(*copy), GFP_KERNEL);
        if (!copy)
                return -ENOMEM;
        for (i = 0; i < argc; i++) {
                copy[i] = kstrdup(argv[i], GFP_KERNEL);
                if (!copy[i]) {
                        while (i--)
                                kfree(copy[i]);
                        kfree(copy);
                        return -ENOMEM;
                }
        }

        cache->nr_ctr_args = argc;
        cache->ctr_args = copy;

        return 0;
}

static int cache_ctr(struct dm_target *ti, unsigned argc, char **argv)
{
        int r = -EINVAL;
        struct cache_args *ca;
        struct cache *cache = NULL;

        ca = kzalloc(sizeof(*ca), GFP_KERNEL);
        if (!ca) {
                ti->error = "Error allocating memory for cache";
                return -ENOMEM;
        }
        ca->ti = ti;

        r = parse_cache_args(ca, argc, argv, &ti->error);
        if (r)
                goto out;

        r = cache_create(ca, &cache);
        if (r)
                goto out;

        r = copy_ctr_args(cache, argc - 3, (const char **)argv + 3);
        if (r) {
                destroy(cache);
                goto out;
        }

        ti->private = cache;

out:
        destroy_cache_args(ca);
        return r;
}

static int cache_map(struct dm_target *ti, struct bio *bio)
{
        struct cache *cache = ti->private;

        int r;
        dm_oblock_t block = get_bio_block(cache, bio);
        size_t pb_data_size = get_per_bio_data_size(cache);
        bool can_migrate = false;
        bool discarded_block;
        struct dm_bio_prison_cell *cell;
        struct policy_result lookup_result;
        struct per_bio_data *pb;

        if (from_oblock(block) > from_oblock(cache->origin_blocks)) {
                /*
                 * This can only occur if the io goes to a partial block at
                 * the end of the origin device.  We don't cache these.
                 * Just remap to the origin and carry on.
                 */
                remap_to_origin_clear_discard(cache, bio, block);
                return DM_MAPIO_REMAPPED;
        }

        pb = init_per_bio_data(bio, pb_data_size);

        if (bio->bi_rw & (REQ_FLUSH | REQ_FUA | REQ_DISCARD)) {
                defer_bio(cache, bio);
                return DM_MAPIO_SUBMITTED;
        }

        /*
         * Check to see if that block is currently migrating.
         */
        cell = alloc_prison_cell(cache);
        if (!cell) {
                defer_bio(cache, bio);
                return DM_MAPIO_SUBMITTED;
        }

        r = bio_detain(cache, block, bio, cell,
                       (cell_free_fn) free_prison_cell,
                       cache, &cell);
        if (r) {
                if (r < 0)
                        defer_bio(cache, bio);

                return DM_MAPIO_SUBMITTED;
        }

        discarded_block = is_discarded_oblock(cache, block);

        r = policy_map(cache->policy, block, false, can_migrate, discarded_block,
                       bio, &lookup_result);
        if (r == -EWOULDBLOCK) {
                cell_defer(cache, cell, true);
                return DM_MAPIO_SUBMITTED;

        } else if (r) {
                DMERR_LIMIT("Unexpected return from cache replacement policy: %d", r);
                bio_io_error(bio);
                return DM_MAPIO_SUBMITTED;
        }

        switch (lookup_result.op) {
        case POLICY_HIT:
                inc_hit_counter(cache, bio);
                pb->all_io_entry = dm_deferred_entry_inc(cache->all_io_ds);

                if (is_writethrough_io(cache, bio, lookup_result.cblock))
                        remap_to_origin_then_cache(cache, bio, block, lookup_result.cblock);
                else
                        remap_to_cache_dirty(cache, bio, block, lookup_result.cblock);

                cell_defer(cache, cell, false);
                break;

        case POLICY_MISS:
                inc_miss_counter(cache, bio);
                pb->all_io_entry = dm_deferred_entry_inc(cache->all_io_ds);

                if (pb->req_nr != 0) {
                        /*
                         * This is a duplicate writethrough io that is no
                         * longer needed because the block has been demoted.
                         */
                        bio_endio(bio, 0);
                        cell_defer(cache, cell, false);
                        return DM_MAPIO_SUBMITTED;
                } else {
                        remap_to_origin_clear_discard(cache, bio, block);
                        cell_defer(cache, cell, false);
                }
                break;

        default:
                DMERR_LIMIT("%s: erroring bio: unknown policy op: %u", __func__,
                            (unsigned) lookup_result.op);
                bio_io_error(bio);
                return DM_MAPIO_SUBMITTED;
        }

        return DM_MAPIO_REMAPPED;
}

static int cache_end_io(struct dm_target *ti, struct bio *bio, int error)
{
        struct cache *cache = ti->private;
        unsigned long flags;
        size_t pb_data_size = get_per_bio_data_size(cache);
        struct per_bio_data *pb = get_per_bio_data(bio, pb_data_size);

        if (pb->tick) {
                policy_tick(cache->policy);

                spin_lock_irqsave(&cache->lock, flags);
                cache->need_tick_bio = true;
                spin_unlock_irqrestore(&cache->lock, flags);
        }

        check_for_quiesced_migrations(cache, pb);

        return 0;
}

static int write_dirty_bitset(struct cache *cache)
{
        unsigned i, r;

        for (i = 0; i < from_cblock(cache->cache_size); i++) {
                r = dm_cache_set_dirty(cache->cmd, to_cblock(i),
                                       is_dirty(cache, to_cblock(i)));
                if (r)
                        return r;
        }

        return 0;
}

static int write_discard_bitset(struct cache *cache)
{
        unsigned i, r;

        r = dm_cache_discard_bitset_resize(cache->cmd, cache->discard_block_size,
                                           cache->discard_nr_blocks);
        if (r) {
                DMERR("could not resize on-disk discard bitset");
                return r;
        }

        for (i = 0; i < from_dblock(cache->discard_nr_blocks); i++) {
                r = dm_cache_set_discard(cache->cmd, to_dblock(i),
                                         is_discarded(cache, to_dblock(i)));
                if (r)
                        return r;
        }

        return 0;
}

static int save_hint(void *context, dm_cblock_t cblock, dm_oblock_t oblock,
                     uint32_t hint)
{
        struct cache *cache = context;
        return dm_cache_save_hint(cache->cmd, cblock, hint);
}

static int write_hints(struct cache *cache)
{
        int r;

        r = dm_cache_begin_hints(cache->cmd, cache->policy);
        if (r) {
                DMERR("dm_cache_begin_hints failed");
                return r;
        }

        r = policy_walk_mappings(cache->policy, save_hint, cache);
        if (r)
                DMERR("policy_walk_mappings failed");

        return r;
}

/*
 * returns true on success
 */
static bool sync_metadata(struct cache *cache)
{
        int r1, r2, r3, r4;

        r1 = write_dirty_bitset(cache);
        if (r1)
                DMERR("could not write dirty bitset");

        r2 = write_discard_bitset(cache);
        if (r2)
                DMERR("could not write discard bitset");

        save_stats(cache);

        r3 = write_hints(cache);
        if (r3)
                DMERR("could not write hints");

        /*
         * If writing the above metadata failed, we still commit, but don't
         * set the clean shutdown flag.  This will effectively force every
         * dirty bit to be set on reload.
         */
        r4 = dm_cache_commit(cache->cmd, !r1 && !r2 && !r3);
        if (r4)
                DMERR("could not write cache metadata.  Data loss may occur.");

        return !r1 && !r2 && !r3 && !r4;
}

static void cache_postsuspend(struct dm_target *ti)
{
        struct cache *cache = ti->private;

        start_quiescing(cache);
        wait_for_migrations(cache);
        stop_worker(cache);
        requeue_deferred_io(cache);
        stop_quiescing(cache);

        (void) sync_metadata(cache);
}

static int load_mapping(void *context, dm_oblock_t oblock, dm_cblock_t cblock,
                        bool dirty, uint32_t hint, bool hint_valid)
{
        int r;
        struct cache *cache = context;

        r = policy_load_mapping(cache->policy, oblock, cblock, hint, hint_valid);
        if (r)
                return r;

        if (dirty)
                set_dirty(cache, oblock, cblock);
        else
                clear_dirty(cache, oblock, cblock);

        return 0;
}

static int load_discard(void *context, sector_t discard_block_size,
                        dm_dblock_t dblock, bool discard)
{
        struct cache *cache = context;

        /* FIXME: handle mis-matched block size */

        if (discard)
                set_discard(cache, dblock);
        else
                clear_discard(cache, dblock);

        return 0;
}

static int cache_preresume(struct dm_target *ti)
{
        int r = 0;
        struct cache *cache = ti->private;
        sector_t actual_cache_size = get_dev_size(cache->cache_dev);
        (void) sector_div(actual_cache_size, cache->sectors_per_block);

        /*
         * Check to see if the cache has resized.
         */
        if (from_cblock(cache->cache_size) != actual_cache_size || !cache->sized) {
                cache->cache_size = to_cblock(actual_cache_size);

                r = dm_cache_resize(cache->cmd, cache->cache_size);
                if (r) {
                        DMERR("could not resize cache metadata");
                        return r;
                }

                cache->sized = true;
        }

        if (!cache->loaded_mappings) {
                r = dm_cache_load_mappings(cache->cmd, cache->policy,
                                           load_mapping, cache);
                if (r) {
                        DMERR("could not load cache mappings");
                        return r;
                }

                cache->loaded_mappings = true;
        }

        if (!cache->loaded_discards) {
                r = dm_cache_load_discards(cache->cmd, load_discard, cache);
                if (r) {
                        DMERR("could not load origin discards");
                        return r;
                }

                cache->loaded_discards = true;
        }

        return r;
}

static void cache_resume(struct dm_target *ti)
{
        struct cache *cache = ti->private;

        cache->need_tick_bio = true;
        do_waker(&cache->waker.work);
}

/*
 * Status format:
 *
 * <#used metadata blocks>/<#total metadata blocks>
 * <#read hits> <#read misses> <#write hits> <#write misses>
 * <#demotions> <#promotions> <#blocks in cache> <#dirty>
 * <#features> <features>*
 * <#core args> <core args>
 * <#policy args> <policy args>*
 */
static void cache_status(struct dm_target *ti, status_type_t type,
                         unsigned status_flags, char *result, unsigned maxlen)
{
        int r = 0;
        unsigned i;
        ssize_t sz = 0;
        dm_block_t nr_free_blocks_metadata = 0;
        dm_block_t nr_blocks_metadata = 0;
        char buf[BDEVNAME_SIZE];
        struct cache *cache = ti->private;
        dm_cblock_t residency;

        switch (type) {
        case STATUSTYPE_INFO:
                /* Commit to ensure statistics aren't out-of-date */
                if (!(status_flags & DM_STATUS_NOFLUSH_FLAG) && !dm_suspended(ti)) {
                        r = dm_cache_commit(cache->cmd, false);
                        if (r)
                                DMERR("could not commit metadata for accurate status");
                }

                r = dm_cache_get_free_metadata_block_count(cache->cmd,
                                                           &nr_free_blocks_metadata);
                if (r) {
                        DMERR("could not get metadata free block count");
                        goto err;
                }

                r = dm_cache_get_metadata_dev_size(cache->cmd, &nr_blocks_metadata);
                if (r) {
                        DMERR("could not get metadata device size");
                        goto err;
                }

                residency = policy_residency(cache->policy);

                DMEMIT("%llu/%llu %u %u %u %u %u %u %llu %u ",
                       (unsigned long long)(nr_blocks_metadata - nr_free_blocks_metadata),
                       (unsigned long long)nr_blocks_metadata,
                       (unsigned) atomic_read(&cache->stats.read_hit),
                       (unsigned) atomic_read(&cache->stats.read_miss),
                       (unsigned) atomic_read(&cache->stats.write_hit),
                       (unsigned) atomic_read(&cache->stats.write_miss),
                       (unsigned) atomic_read(&cache->stats.demotion),
                       (unsigned) atomic_read(&cache->stats.promotion),
                       (unsigned long long) from_cblock(residency),
                       cache->nr_dirty);

                if (cache->features.write_through)
                        DMEMIT("1 writethrough ");
                else
                        DMEMIT("0 ");

                DMEMIT("2 migration_threshold %llu ", (unsigned long long) cache->migration_threshold);
                if (sz < maxlen) {
                        r = policy_emit_config_values(cache->policy, result + sz, maxlen - sz);
                        if (r)
                                DMERR("policy_emit_config_values returned %d", r);
                }

                break;

        case STATUSTYPE_TABLE:
                format_dev_t(buf, cache->metadata_dev->bdev->bd_dev);
                DMEMIT("%s ", buf);
                format_dev_t(buf, cache->cache_dev->bdev->bd_dev);
                DMEMIT("%s ", buf);
                format_dev_t(buf, cache->origin_dev->bdev->bd_dev);
                DMEMIT("%s", buf);

                for (i = 0; i < cache->nr_ctr_args - 1; i++)
                        DMEMIT(" %s", cache->ctr_args[i]);
                if (cache->nr_ctr_args)
                        DMEMIT(" %s", cache->ctr_args[cache->nr_ctr_args - 1]);
        }

        return;

err:
        DMEMIT("Error");
}

/*
 * Supports <key> <value>.
 *
 * The key migration_threshold is supported by the cache target core.
 */
static int cache_message(struct dm_target *ti, unsigned argc, char **argv)
{
        struct cache *cache = ti->private;

        if (argc != 2)
                return -EINVAL;

        return set_config_value(cache, argv[0], argv[1]);
}

static int cache_iterate_devices(struct dm_target *ti,
                                 iterate_devices_callout_fn fn, void *data)
{
        int r = 0;
        struct cache *cache = ti->private;

        r = fn(ti, cache->cache_dev, 0, get_dev_size(cache->cache_dev), data);
        if (!r)
                r = fn(ti, cache->origin_dev, 0, ti->len, data);

        return r;
}

/*
 * We assume I/O is going to the origin (which is the volume
 * more likely to have restrictions e.g. by being striped).
 * (Looking up the exact location of the data would be expensive
 * and could always be out of date by the time the bio is submitted.)
 */
static int cache_bvec_merge(struct dm_target *ti,
                            struct bvec_merge_data *bvm,
                            struct bio_vec *biovec, int max_size)
{
        struct cache *cache = ti->private;
        struct request_queue *q = bdev_get_queue(cache->origin_dev->bdev);

        if (!q->merge_bvec_fn)
                return max_size;

        bvm->bi_bdev = cache->origin_dev->bdev;
        return min(max_size, q->merge_bvec_fn(q, bvm, biovec));
}

static void set_discard_limits(struct cache *cache, struct queue_limits *limits)
{
        /*
         * FIXME: these limits may be incompatible with the cache device
         */
        limits->max_discard_sectors = cache->discard_block_size * 1024;
        limits->discard_granularity = cache->discard_block_size << SECTOR_SHIFT;
}

static void cache_io_hints(struct dm_target *ti, struct queue_limits *limits)
{
        struct cache *cache = ti->private;

        blk_limits_io_min(limits, 0);
        blk_limits_io_opt(limits, cache->sectors_per_block << SECTOR_SHIFT);
        set_discard_limits(cache, limits);
}

/*----------------------------------------------------------------*/

static struct target_type cache_target = {
        .name = "cache",
        .version = {1, 1, 1},
        .module = THIS_MODULE,
        .ctr = cache_ctr,
        .dtr = cache_dtr,
        .map = cache_map,
        .end_io = cache_end_io,
        .postsuspend = cache_postsuspend,
        .preresume = cache_preresume,
        .resume = cache_resume,
        .status = cache_status,
        .message = cache_message,
        .iterate_devices = cache_iterate_devices,
        .merge = cache_bvec_merge,
        .io_hints = cache_io_hints,
};

static int __init dm_cache_init(void)
{
        int r;

        r = dm_register_target(&cache_target);
        if (r) {
                DMERR("cache target registration failed: %d", r);
                return r;
        }

        migration_cache = KMEM_CACHE(dm_cache_migration, 0);
        if (!migration_cache) {
                dm_unregister_target(&cache_target);
                return -ENOMEM;
        }

        return 0;
}

static void __exit dm_cache_exit(void)
{
        dm_unregister_target(&cache_target);
        kmem_cache_destroy(migration_cache);
}

module_init(dm_cache_init);
module_exit(dm_cache_exit);

MODULE_DESCRIPTION(DM_NAME " cache target");
MODULE_AUTHOR("Joe Thornber <ejt@redhat.com>");
MODULE_LICENSE("GPL");