dm snapshot: track snapshot reads

[safe/jmp/linux-2.6] / drivers / md / dm-snap.c

/*
 * dm-snapshot.c
 *
 * Copyright (C) 2001-2002 Sistina Software (UK) Limited.
 *
 * This file is released under the GPL.
 */

#include <linux/blkdev.h>
#include <linux/ctype.h>
#include <linux/device-mapper.h>
#include <linux/fs.h>
#include <linux/init.h>
#include <linux/kdev_t.h>
#include <linux/list.h>
#include <linux/mempool.h>
#include <linux/module.h>
#include <linux/slab.h>
#include <linux/vmalloc.h>
#include <linux/log2.h>
#include <linux/dm-kcopyd.h>

#include "dm-snap.h"
#include "dm-bio-list.h"

#define DM_MSG_PREFIX "snapshots"

/*
 * The percentage increment we will wake up users at
 */
#define WAKE_UP_PERCENT 5

/*
 * kcopyd priority of snapshot operations
 */
#define SNAPSHOT_COPY_PRIORITY 2

/*
 * Reserve 1MB for each snapshot initially (with minimum of 1 page).
 */
#define SNAPSHOT_PAGES (((1UL << 20) >> PAGE_SHIFT) ? : 1)

/*
 * The size of the mempool used to track chunks in use.
 */
#define MIN_IOS 256

static struct workqueue_struct *ksnapd;
static void flush_queued_bios(struct work_struct *work);

struct dm_snap_pending_exception {
        struct dm_snap_exception e;

        /*
         * Origin buffers waiting for this to complete are held
         * in a bio list
         */
        struct bio_list origin_bios;
        struct bio_list snapshot_bios;

        /*
         * Short-term queue of pending exceptions prior to submission.
         */
        struct list_head list;

        /*
         * The primary pending_exception is the one that holds
         * the ref_count and the list of origin_bios for a
         * group of pending_exceptions.  It is always last to get freed.
         * These fields get set up when writing to the origin.
         */
        struct dm_snap_pending_exception *primary_pe;

        /*
         * Number of pending_exceptions processing this chunk.
         * When this drops to zero we must complete the origin bios.
         * If incrementing or decrementing this, hold pe->snap->lock for
         * the sibling concerned and not pe->primary_pe->snap->lock unless
         * they are the same.
         */
        atomic_t ref_count;

        /* Pointer back to snapshot context */
        struct dm_snapshot *snap;

        /*
         * 1 indicates the exception has already been sent to
         * kcopyd.
         */
        int started;
};

/*
 * Hash table mapping origin volumes to lists of snapshots and
 * a lock to protect it
 */
static struct kmem_cache *exception_cache;
static struct kmem_cache *pending_cache;
static mempool_t *pending_pool;

struct dm_snap_tracked_chunk {
        struct hlist_node node;
        chunk_t chunk;
};

static struct kmem_cache *tracked_chunk_cache;

static struct dm_snap_tracked_chunk *track_chunk(struct dm_snapshot *s,
                                                 chunk_t chunk)
{
        struct dm_snap_tracked_chunk *c = mempool_alloc(s->tracked_chunk_pool,
                                                        GFP_NOIO);
        unsigned long flags;

        c->chunk = chunk;

        spin_lock_irqsave(&s->tracked_chunk_lock, flags);
        hlist_add_head(&c->node,
                       &s->tracked_chunk_hash[DM_TRACKED_CHUNK_HASH(chunk)]);
        spin_unlock_irqrestore(&s->tracked_chunk_lock, flags);

        return c;
}

static void stop_tracking_chunk(struct dm_snapshot *s,
                                struct dm_snap_tracked_chunk *c)
{
        unsigned long flags;

        spin_lock_irqsave(&s->tracked_chunk_lock, flags);
        hlist_del(&c->node);
        spin_unlock_irqrestore(&s->tracked_chunk_lock, flags);

        mempool_free(c, s->tracked_chunk_pool);
}

/*
 * One of these per registered origin, held in the snapshot_origins hash
 */
struct origin {
        /* The origin device */
        struct block_device *bdev;

        struct list_head hash_list;

        /* List of snapshots for this origin */
        struct list_head snapshots;
};

/*
 * Size of the hash table for origin volumes. If we make this
 * the size of the minors list then it should be nearly perfect
 */
#define ORIGIN_HASH_SIZE 256
#define ORIGIN_MASK      0xFF
static struct list_head *_origins;
static struct rw_semaphore _origins_lock;

static int init_origin_hash(void)
{
        int i;

        _origins = kmalloc(ORIGIN_HASH_SIZE * sizeof(struct list_head),
                           GFP_KERNEL);
        if (!_origins) {
                DMERR("unable to allocate memory");
                return -ENOMEM;
        }

        for (i = 0; i < ORIGIN_HASH_SIZE; i++)
                INIT_LIST_HEAD(_origins + i);
        init_rwsem(&_origins_lock);

        return 0;
}

static void exit_origin_hash(void)
{
        kfree(_origins);
}

static unsigned origin_hash(struct block_device *bdev)
{
        return bdev->bd_dev & ORIGIN_MASK;
}

static struct origin *__lookup_origin(struct block_device *origin)
{
        struct list_head *ol;
        struct origin *o;

        ol = &_origins[origin_hash(origin)];
        list_for_each_entry (o, ol, hash_list)
                if (bdev_equal(o->bdev, origin))
                        return o;

        return NULL;
}

static void __insert_origin(struct origin *o)
{
        struct list_head *sl = &_origins[origin_hash(o->bdev)];
        list_add_tail(&o->hash_list, sl);
}

/*
 * Make a note of the snapshot and its origin so we can look it
 * up when the origin has a write on it.
 */
static int register_snapshot(struct dm_snapshot *snap)
{
        struct origin *o;
        struct block_device *bdev = snap->origin->bdev;

        down_write(&_origins_lock);
        o = __lookup_origin(bdev);

        if (!o) {
                /* New origin */
                o = kmalloc(sizeof(*o), GFP_KERNEL);
                if (!o) {
                        up_write(&_origins_lock);
                        return -ENOMEM;
                }

                /* Initialise the struct */
                INIT_LIST_HEAD(&o->snapshots);
                o->bdev = bdev;

                __insert_origin(o);
        }

        list_add_tail(&snap->list, &o->snapshots);

        up_write(&_origins_lock);
        return 0;
}

static void unregister_snapshot(struct dm_snapshot *s)
{
        struct origin *o;

        down_write(&_origins_lock);
        o = __lookup_origin(s->origin->bdev);

        list_del(&s->list);
        if (list_empty(&o->snapshots)) {
                list_del(&o->hash_list);
                kfree(o);
        }

        up_write(&_origins_lock);
}

/*
 * Implementation of the exception hash tables.
 * The lowest hash_shift bits of the chunk number are ignored, allowing
 * some consecutive chunks to be grouped together.
 */
static int init_exception_table(struct exception_table *et, uint32_t size,
                                unsigned hash_shift)
{
        unsigned int i;

        et->hash_shift = hash_shift;
        et->hash_mask = size - 1;
        et->table = dm_vcalloc(size, sizeof(struct list_head));
        if (!et->table)
                return -ENOMEM;

        for (i = 0; i < size; i++)
                INIT_LIST_HEAD(et->table + i);

        return 0;
}

static void exit_exception_table(struct exception_table *et, struct kmem_cache *mem)
{
        struct list_head *slot;
        struct dm_snap_exception *ex, *next;
        int i, size;

        size = et->hash_mask + 1;
        for (i = 0; i < size; i++) {
                slot = et->table + i;

                list_for_each_entry_safe (ex, next, slot, hash_list)
                        kmem_cache_free(mem, ex);
        }

        vfree(et->table);
}

static uint32_t exception_hash(struct exception_table *et, chunk_t chunk)
{
        return (chunk >> et->hash_shift) & et->hash_mask;
}

static void insert_exception(struct exception_table *eh,
                             struct dm_snap_exception *e)
{
        struct list_head *l = &eh->table[exception_hash(eh, e->old_chunk)];
        list_add(&e->hash_list, l);
}

static void remove_exception(struct dm_snap_exception *e)
{
        list_del(&e->hash_list);
}

/*
 * Return the exception data for a sector, or NULL if not
 * remapped.
 */
static struct dm_snap_exception *lookup_exception(struct exception_table *et,
                                                  chunk_t chunk)
{
        struct list_head *slot;
        struct dm_snap_exception *e;

        slot = &et->table[exception_hash(et, chunk)];
        list_for_each_entry (e, slot, hash_list)
                if (chunk >= e->old_chunk &&
                    chunk <= e->old_chunk + dm_consecutive_chunk_count(e))
                        return e;

        return NULL;
}

static struct dm_snap_exception *alloc_exception(void)
{
        struct dm_snap_exception *e;

        e = kmem_cache_alloc(exception_cache, GFP_NOIO);
        if (!e)
                e = kmem_cache_alloc(exception_cache, GFP_ATOMIC);

        return e;
}

static void free_exception(struct dm_snap_exception *e)
{
        kmem_cache_free(exception_cache, e);
}

static struct dm_snap_pending_exception *alloc_pending_exception(void)
{
        return mempool_alloc(pending_pool, GFP_NOIO);
}

static void free_pending_exception(struct dm_snap_pending_exception *pe)
{
        mempool_free(pe, pending_pool);
}

static void insert_completed_exception(struct dm_snapshot *s,
                                       struct dm_snap_exception *new_e)
{
        struct exception_table *eh = &s->complete;
        struct list_head *l;
        struct dm_snap_exception *e = NULL;

        l = &eh->table[exception_hash(eh, new_e->old_chunk)];

        /* Add immediately if this table doesn't support consecutive chunks */
        if (!eh->hash_shift)
                goto out;

        /* List is ordered by old_chunk */
        list_for_each_entry_reverse(e, l, hash_list) {
                /* Insert after an existing chunk? */
                if (new_e->old_chunk == (e->old_chunk +
                                         dm_consecutive_chunk_count(e) + 1) &&
                    new_e->new_chunk == (dm_chunk_number(e->new_chunk) +
                                         dm_consecutive_chunk_count(e) + 1)) {
                        dm_consecutive_chunk_count_inc(e);
                        free_exception(new_e);
                        return;
                }

                /* Insert before an existing chunk? */
                if (new_e->old_chunk == (e->old_chunk - 1) &&
                    new_e->new_chunk == (dm_chunk_number(e->new_chunk) - 1)) {
                        dm_consecutive_chunk_count_inc(e);
                        e->old_chunk--;
                        e->new_chunk--;
                        free_exception(new_e);
                        return;
                }

                if (new_e->old_chunk > e->old_chunk)
                        break;
        }

out:
        list_add(&new_e->hash_list, e ? &e->hash_list : l);
}

int dm_add_exception(struct dm_snapshot *s, chunk_t old, chunk_t new)
{
        struct dm_snap_exception *e;

        e = alloc_exception();
        if (!e)
                return -ENOMEM;

        e->old_chunk = old;

        /* Consecutive_count is implicitly initialised to zero */
        e->new_chunk = new;

        insert_completed_exception(s, e);

        return 0;
}

/*
 * Hard coded magic.
 */
static int calc_max_buckets(void)
{
        /* use a fixed size of 2MB */
        unsigned long mem = 2 * 1024 * 1024;
        mem /= sizeof(struct list_head);

        return mem;
}

/*
 * Allocate room for a suitable hash table.
 */
static int init_hash_tables(struct dm_snapshot *s)
{
        sector_t hash_size, cow_dev_size, origin_dev_size, max_buckets;

        /*
         * Calculate based on the size of the original volume or
         * the COW volume...
         */
        cow_dev_size = get_dev_size(s->cow->bdev);
        origin_dev_size = get_dev_size(s->origin->bdev);
        max_buckets = calc_max_buckets();

        hash_size = min(origin_dev_size, cow_dev_size) >> s->chunk_shift;
        hash_size = min(hash_size, max_buckets);

        hash_size = rounddown_pow_of_two(hash_size);
        if (init_exception_table(&s->complete, hash_size,
                                 DM_CHUNK_CONSECUTIVE_BITS))
                return -ENOMEM;

        /*
         * Allocate hash table for in-flight exceptions
         * Make this smaller than the real hash table
         */
        hash_size >>= 3;
        if (hash_size < 64)
                hash_size = 64;

        if (init_exception_table(&s->pending, hash_size, 0)) {
                exit_exception_table(&s->complete, exception_cache);
                return -ENOMEM;
        }

        return 0;
}

/*
 * Round a number up to the nearest 'size' boundary.  size must
 * be a power of 2.
 */
static ulong round_up(ulong n, ulong size)
{
        size--;
        return (n + size) & ~size;
}

static int set_chunk_size(struct dm_snapshot *s, const char *chunk_size_arg,
                          char **error)
{
        unsigned long chunk_size;
        char *value;

        chunk_size = simple_strtoul(chunk_size_arg, &value, 10);
        if (*chunk_size_arg == '\0' || *value != '\0') {
                *error = "Invalid chunk size";
                return -EINVAL;
        }

        if (!chunk_size) {
                s->chunk_size = s->chunk_mask = s->chunk_shift = 0;
                return 0;
        }

        /*
         * Chunk size must be multiple of page size.  Silently
         * round up if it's not.
         */
        chunk_size = round_up(chunk_size, PAGE_SIZE >> 9);

        /* Check chunk_size is a power of 2 */
        if (!is_power_of_2(chunk_size)) {
                *error = "Chunk size is not a power of 2";
                return -EINVAL;
        }

        /* Validate the chunk size against the device block size */
        if (chunk_size % (bdev_hardsect_size(s->cow->bdev) >> 9)) {
                *error = "Chunk size is not a multiple of device blocksize";
                return -EINVAL;
        }

        s->chunk_size = chunk_size;
        s->chunk_mask = chunk_size - 1;
        s->chunk_shift = ffs(chunk_size) - 1;

        return 0;
}

/*
 * Construct a snapshot mapping: <origin_dev> <COW-dev> <p/n> <chunk-size>
 */
static int snapshot_ctr(struct dm_target *ti, unsigned int argc, char **argv)
{
        struct dm_snapshot *s;
        int i;
        int r = -EINVAL;
        char persistent;
        char *origin_path;
        char *cow_path;

        if (argc != 4) {
                ti->error = "requires exactly 4 arguments";
                r = -EINVAL;
                goto bad1;
        }

        origin_path = argv[0];
        cow_path = argv[1];
        persistent = toupper(*argv[2]);

        if (persistent != 'P' && persistent != 'N') {
                ti->error = "Persistent flag is not P or N";
                r = -EINVAL;
                goto bad1;
        }

        s = kmalloc(sizeof(*s), GFP_KERNEL);
        if (s == NULL) {
                ti->error = "Cannot allocate snapshot context private "
                    "structure";
                r = -ENOMEM;
                goto bad1;
        }

        r = dm_get_device(ti, origin_path, 0, ti->len, FMODE_READ, &s->origin);
        if (r) {
                ti->error = "Cannot get origin device";
                goto bad2;
        }

        r = dm_get_device(ti, cow_path, 0, 0,
                          FMODE_READ | FMODE_WRITE, &s->cow);
        if (r) {
                dm_put_device(ti, s->origin);
                ti->error = "Cannot get COW device";
                goto bad2;
        }

        r = set_chunk_size(s, argv[3], &ti->error);
        if (r)
                goto bad3;

        s->type = persistent;

        s->valid = 1;
        s->active = 0;
        s->last_percent = 0;
        init_rwsem(&s->lock);
        spin_lock_init(&s->pe_lock);
        s->ti = ti;

        /* Allocate hash table for COW data */
        if (init_hash_tables(s)) {
                ti->error = "Unable to allocate hash table space";
                r = -ENOMEM;
                goto bad3;
        }

        s->store.snap = s;

        if (persistent == 'P')
                r = dm_create_persistent(&s->store);
        else
                r = dm_create_transient(&s->store);

        if (r) {
                ti->error = "Couldn't create exception store";
                r = -EINVAL;
                goto bad4;
        }

        r = dm_kcopyd_client_create(SNAPSHOT_PAGES, &s->kcopyd_client);
        if (r) {
                ti->error = "Could not create kcopyd client";
                goto bad5;
        }

        s->tracked_chunk_pool = mempool_create_slab_pool(MIN_IOS,
                                                         tracked_chunk_cache);
        if (!s->tracked_chunk_pool) {
                ti->error = "Could not allocate tracked_chunk mempool for "
                            "tracking reads";
                goto bad6;
        }

        for (i = 0; i < DM_TRACKED_CHUNK_HASH_SIZE; i++)
                INIT_HLIST_HEAD(&s->tracked_chunk_hash[i]);

        spin_lock_init(&s->tracked_chunk_lock);

        /* Metadata must only be loaded into one table at once */
        r = s->store.read_metadata(&s->store);
        if (r < 0) {
                ti->error = "Failed to read snapshot metadata";
                goto bad_load_and_register;
        } else if (r > 0) {
                s->valid = 0;
                DMWARN("Snapshot is marked invalid.");
        }

        bio_list_init(&s->queued_bios);
        INIT_WORK(&s->queued_bios_work, flush_queued_bios);

        /* Add snapshot to the list of snapshots for this origin */
        /* Exceptions aren't triggered till snapshot_resume() is called */
        if (register_snapshot(s)) {
                r = -EINVAL;
                ti->error = "Cannot register snapshot origin";
                goto bad_load_and_register;
        }

        ti->private = s;
        ti->split_io = s->chunk_size;

        return 0;

 bad_load_and_register:
        mempool_destroy(s->tracked_chunk_pool);

 bad6:
        dm_kcopyd_client_destroy(s->kcopyd_client);

 bad5:
        s->store.destroy(&s->store);

 bad4:
        exit_exception_table(&s->pending, pending_cache);
        exit_exception_table(&s->complete, exception_cache);

 bad3:
        dm_put_device(ti, s->cow);
        dm_put_device(ti, s->origin);

 bad2:
        kfree(s);

 bad1:
        return r;
}

static void __free_exceptions(struct dm_snapshot *s)
{
        dm_kcopyd_client_destroy(s->kcopyd_client);
        s->kcopyd_client = NULL;

        exit_exception_table(&s->pending, pending_cache);
        exit_exception_table(&s->complete, exception_cache);

        s->store.destroy(&s->store);
}

static void snapshot_dtr(struct dm_target *ti)
{
#ifdef CONFIG_DM_DEBUG
        int i;
#endif
        struct dm_snapshot *s = ti->private;

        flush_workqueue(ksnapd);

        /* Prevent further origin writes from using this snapshot. */
        /* After this returns there can be no new kcopyd jobs. */
        unregister_snapshot(s);

#ifdef CONFIG_DM_DEBUG
        for (i = 0; i < DM_TRACKED_CHUNK_HASH_SIZE; i++)
                BUG_ON(!hlist_empty(&s->tracked_chunk_hash[i]));
#endif

        mempool_destroy(s->tracked_chunk_pool);

        __free_exceptions(s);

        dm_put_device(ti, s->origin);
        dm_put_device(ti, s->cow);

        kfree(s);
}

/*
 * Flush a list of buffers.
 */
static void flush_bios(struct bio *bio)
{
        struct bio *n;

        while (bio) {
                n = bio->bi_next;
                bio->bi_next = NULL;
                generic_make_request(bio);
                bio = n;
        }
}

static void flush_queued_bios(struct work_struct *work)
{
        struct dm_snapshot *s =
                container_of(work, struct dm_snapshot, queued_bios_work);
        struct bio *queued_bios;
        unsigned long flags;

        spin_lock_irqsave(&s->pe_lock, flags);
        queued_bios = bio_list_get(&s->queued_bios);
        spin_unlock_irqrestore(&s->pe_lock, flags);

        flush_bios(queued_bios);
}

/*
 * Error a list of buffers.
 */
static void error_bios(struct bio *bio)
{
        struct bio *n;

        while (bio) {
                n = bio->bi_next;
                bio->bi_next = NULL;
                bio_io_error(bio);
                bio = n;
        }
}

static void __invalidate_snapshot(struct dm_snapshot *s, int err)
{
        if (!s->valid)
                return;

        if (err == -EIO)
                DMERR("Invalidating snapshot: Error reading/writing.");
        else if (err == -ENOMEM)
                DMERR("Invalidating snapshot: Unable to allocate exception.");

        if (s->store.drop_snapshot)
                s->store.drop_snapshot(&s->store);

        s->valid = 0;

        dm_table_event(s->ti->table);
}

static void get_pending_exception(struct dm_snap_pending_exception *pe)
{
        atomic_inc(&pe->ref_count);
}

static struct bio *put_pending_exception(struct dm_snap_pending_exception *pe)
{
        struct dm_snap_pending_exception *primary_pe;
        struct bio *origin_bios = NULL;

        primary_pe = pe->primary_pe;

        /*
         * If this pe is involved in a write to the origin and
         * it is the last sibling to complete then release
         * the bios for the original write to the origin.
         */
        if (primary_pe &&
            atomic_dec_and_test(&primary_pe->ref_count))
                origin_bios = bio_list_get(&primary_pe->origin_bios);

        /*
         * Free the pe if it's not linked to an origin write or if
         * it's not itself a primary pe.
         */
        if (!primary_pe || primary_pe != pe)
                free_pending_exception(pe);

        /*
         * Free the primary pe if nothing references it.
         */
        if (primary_pe && !atomic_read(&primary_pe->ref_count))
                free_pending_exception(primary_pe);

        return origin_bios;
}

static void pending_complete(struct dm_snap_pending_exception *pe, int success)
{
        struct dm_snap_exception *e;
        struct dm_snapshot *s = pe->snap;
        struct bio *origin_bios = NULL;
        struct bio *snapshot_bios = NULL;
        int error = 0;

        if (!success) {
                /* Read/write error - snapshot is unusable */
                down_write(&s->lock);
                __invalidate_snapshot(s, -EIO);
                error = 1;
                goto out;
        }

        e = alloc_exception();
        if (!e) {
                down_write(&s->lock);
                __invalidate_snapshot(s, -ENOMEM);
                error = 1;
                goto out;
        }
        *e = pe->e;

        down_write(&s->lock);
        if (!s->valid) {
                free_exception(e);
                error = 1;
                goto out;
        }

        /*
         * Add a proper exception, and remove the
         * in-flight exception from the list.
         */
        insert_completed_exception(s, e);

 out:
        remove_exception(&pe->e);
        snapshot_bios = bio_list_get(&pe->snapshot_bios);
        origin_bios = put_pending_exception(pe);

        up_write(&s->lock);

        /* Submit any pending write bios */
        if (error)
                error_bios(snapshot_bios);
        else
                flush_bios(snapshot_bios);

        flush_bios(origin_bios);
}

static void commit_callback(void *context, int success)
{
        struct dm_snap_pending_exception *pe = context;

        pending_complete(pe, success);
}

/*
 * Called when the copy I/O has finished.  kcopyd actually runs
 * this code so don't block.
 */
static void copy_callback(int read_err, unsigned long write_err, void *context)
{
        struct dm_snap_pending_exception *pe = context;
        struct dm_snapshot *s = pe->snap;

        if (read_err || write_err)
                pending_complete(pe, 0);

        else
                /* Update the metadata if we are persistent */
                s->store.commit_exception(&s->store, &pe->e, commit_callback,
                                          pe);
}

/*
 * Dispatches the copy operation to kcopyd.
 */
static void start_copy(struct dm_snap_pending_exception *pe)
{
        struct dm_snapshot *s = pe->snap;
        struct dm_io_region src, dest;
        struct block_device *bdev = s->origin->bdev;
        sector_t dev_size;

        dev_size = get_dev_size(bdev);

        src.bdev = bdev;
        src.sector = chunk_to_sector(s, pe->e.old_chunk);
        src.count = min(s->chunk_size, dev_size - src.sector);

        dest.bdev = s->cow->bdev;
        dest.sector = chunk_to_sector(s, pe->e.new_chunk);
        dest.count = src.count;

        /* Hand over to kcopyd */
        dm_kcopyd_copy(s->kcopyd_client,
                    &src, 1, &dest, 0, copy_callback, pe);
}

/*
 * Looks to see if this snapshot already has a pending exception
 * for this chunk, otherwise it allocates a new one and inserts
 * it into the pending table.
 *
 * NOTE: a write lock must be held on snap->lock before calling
 * this.
 */
static struct dm_snap_pending_exception *
__find_pending_exception(struct dm_snapshot *s, struct bio *bio)
{
        struct dm_snap_exception *e;
        struct dm_snap_pending_exception *pe;
        chunk_t chunk = sector_to_chunk(s, bio->bi_sector);

        /*
         * Is there a pending exception for this already ?
         */
        e = lookup_exception(&s->pending, chunk);
        if (e) {
                /* cast the exception to a pending exception */
                pe = container_of(e, struct dm_snap_pending_exception, e);
                goto out;
        }

        /*
         * Create a new pending exception, we don't want
         * to hold the lock while we do this.
         */
        up_write(&s->lock);
        pe = alloc_pending_exception();
        down_write(&s->lock);

        if (!s->valid) {
                free_pending_exception(pe);
                return NULL;
        }

        e = lookup_exception(&s->pending, chunk);
        if (e) {
                free_pending_exception(pe);
                pe = container_of(e, struct dm_snap_pending_exception, e);
                goto out;
        }

        pe->e.old_chunk = chunk;
        bio_list_init(&pe->origin_bios);
        bio_list_init(&pe->snapshot_bios);
        pe->primary_pe = NULL;
        atomic_set(&pe->ref_count, 0);
        pe->snap = s;
        pe->started = 0;

        if (s->store.prepare_exception(&s->store, &pe->e)) {
                free_pending_exception(pe);
                return NULL;
        }

        get_pending_exception(pe);
        insert_exception(&s->pending, &pe->e);

 out:
        return pe;
}

static void remap_exception(struct dm_snapshot *s, struct dm_snap_exception *e,
                            struct bio *bio, chunk_t chunk)
{
        bio->bi_bdev = s->cow->bdev;
        bio->bi_sector = chunk_to_sector(s, dm_chunk_number(e->new_chunk) +
                         (chunk - e->old_chunk)) +
                         (bio->bi_sector & s->chunk_mask);
}

static int snapshot_map(struct dm_target *ti, struct bio *bio,
                        union map_info *map_context)
{
        struct dm_snap_exception *e;
        struct dm_snapshot *s = ti->private;
        int r = DM_MAPIO_REMAPPED;
        chunk_t chunk;
        struct dm_snap_pending_exception *pe = NULL;

        chunk = sector_to_chunk(s, bio->bi_sector);

        /* Full snapshots are not usable */
        /* To get here the table must be live so s->active is always set. */
        if (!s->valid)
                return -EIO;

        /* FIXME: should only take write lock if we need
         * to copy an exception */
        down_write(&s->lock);

        if (!s->valid) {
                r = -EIO;
                goto out_unlock;
        }

        /* If the block is already remapped - use that, else remap it */
        e = lookup_exception(&s->complete, chunk);
        if (e) {
                remap_exception(s, e, bio, chunk);
                goto out_unlock;
        }

        /*
         * Write to snapshot - higher level takes care of RW/RO
         * flags so we should only get this if we are
         * writeable.
         */
        if (bio_rw(bio) == WRITE) {
                pe = __find_pending_exception(s, bio);
                if (!pe) {
                        __invalidate_snapshot(s, -ENOMEM);
                        r = -EIO;
                        goto out_unlock;
                }

                remap_exception(s, &pe->e, bio, chunk);
                bio_list_add(&pe->snapshot_bios, bio);

                r = DM_MAPIO_SUBMITTED;

                if (!pe->started) {
                        /* this is protected by snap->lock */
                        pe->started = 1;
                        up_write(&s->lock);
                        start_copy(pe);
                        goto out;
                }
        } else {
                bio->bi_bdev = s->origin->bdev;
                map_context->ptr = track_chunk(s, chunk);
        }

 out_unlock:
        up_write(&s->lock);
 out:
        return r;
}

static int snapshot_end_io(struct dm_target *ti, struct bio *bio,
                           int error, union map_info *map_context)
{
        struct dm_snapshot *s = ti->private;
        struct dm_snap_tracked_chunk *c = map_context->ptr;

        if (c)
                stop_tracking_chunk(s, c);

        return 0;
}

static void snapshot_resume(struct dm_target *ti)
{
        struct dm_snapshot *s = ti->private;

        down_write(&s->lock);
        s->active = 1;
        up_write(&s->lock);
}

static int snapshot_status(struct dm_target *ti, status_type_t type,
                           char *result, unsigned int maxlen)
{
        struct dm_snapshot *snap = ti->private;

        switch (type) {
        case STATUSTYPE_INFO:
                if (!snap->valid)
                        snprintf(result, maxlen, "Invalid");
                else {
                        if (snap->store.fraction_full) {
                                sector_t numerator, denominator;
                                snap->store.fraction_full(&snap->store,
                                                          &numerator,
                                                          &denominator);
                                snprintf(result, maxlen, "%llu/%llu",
                                        (unsigned long long)numerator,
                                        (unsigned long long)denominator);
                        }
                        else
                                snprintf(result, maxlen, "Unknown");
                }
                break;

        case STATUSTYPE_TABLE:
                /*
                 * kdevname returns a static pointer so we need
                 * to make private copies if the output is to
                 * make sense.
                 */
                snprintf(result, maxlen, "%s %s %c %llu",
                         snap->origin->name, snap->cow->name,
                         snap->type,
                         (unsigned long long)snap->chunk_size);
                break;
        }

        return 0;
}

/*-----------------------------------------------------------------
 * Origin methods
 *---------------------------------------------------------------*/
static int __origin_write(struct list_head *snapshots, struct bio *bio)
{
        int r = DM_MAPIO_REMAPPED, first = 0;
        struct dm_snapshot *snap;
        struct dm_snap_exception *e;
        struct dm_snap_pending_exception *pe, *next_pe, *primary_pe = NULL;
        chunk_t chunk;
        LIST_HEAD(pe_queue);

        /* Do all the snapshots on this origin */
        list_for_each_entry (snap, snapshots, list) {

                down_write(&snap->lock);

                /* Only deal with valid and active snapshots */
                if (!snap->valid || !snap->active)
                        goto next_snapshot;

                /* Nothing to do if writing beyond end of snapshot */
                if (bio->bi_sector >= dm_table_get_size(snap->ti->table))
                        goto next_snapshot;

                /*
                 * Remember, different snapshots can have
                 * different chunk sizes.
                 */
                chunk = sector_to_chunk(snap, bio->bi_sector);

                /*
                 * Check exception table to see if block
                 * is already remapped in this snapshot
                 * and trigger an exception if not.
                 *
                 * ref_count is initialised to 1 so pending_complete()
                 * won't destroy the primary_pe while we're inside this loop.
                 */
                e = lookup_exception(&snap->complete, chunk);
                if (e)
                        goto next_snapshot;

                pe = __find_pending_exception(snap, bio);
                if (!pe) {
                        __invalidate_snapshot(snap, -ENOMEM);
                        goto next_snapshot;
                }

                if (!primary_pe) {
                        /*
                         * Either every pe here has same
                         * primary_pe or none has one yet.
                         */
                        if (pe->primary_pe)
                                primary_pe = pe->primary_pe;
                        else {
                                primary_pe = pe;
                                first = 1;
                        }

                        bio_list_add(&primary_pe->origin_bios, bio);

                        r = DM_MAPIO_SUBMITTED;
                }

                if (!pe->primary_pe) {
                        pe->primary_pe = primary_pe;
                        get_pending_exception(primary_pe);
                }

                if (!pe->started) {
                        pe->started = 1;
                        list_add_tail(&pe->list, &pe_queue);
                }

 next_snapshot:
                up_write(&snap->lock);
        }

        if (!primary_pe)
                return r;

        /*
         * If this is the first time we're processing this chunk and
         * ref_count is now 1 it means all the pending exceptions
         * got completed while we were in the loop above, so it falls to
         * us here to remove the primary_pe and submit any origin_bios.
         */

        if (first && atomic_dec_and_test(&primary_pe->ref_count)) {
                flush_bios(bio_list_get(&primary_pe->origin_bios));
                free_pending_exception(primary_pe);
                /* If we got here, pe_queue is necessarily empty. */
                return r;
        }

        /*
         * Now that we have a complete pe list we can start the copying.
         */
        list_for_each_entry_safe(pe, next_pe, &pe_queue, list)
                start_copy(pe);

        return r;
}

/*
 * Called on a write from the origin driver.
 */
static int do_origin(struct dm_dev *origin, struct bio *bio)
{
        struct origin *o;
        int r = DM_MAPIO_REMAPPED;

        down_read(&_origins_lock);
        o = __lookup_origin(origin->bdev);
        if (o)
                r = __origin_write(&o->snapshots, bio);
        up_read(&_origins_lock);

        return r;
}

/*
 * Origin: maps a linear range of a device, with hooks for snapshotting.
 */

/*
 * Construct an origin mapping: <dev_path>
 * The context for an origin is merely a 'struct dm_dev *'
 * pointing to the real device.
 */
static int origin_ctr(struct dm_target *ti, unsigned int argc, char **argv)
{
        int r;
        struct dm_dev *dev;

        if (argc != 1) {
                ti->error = "origin: incorrect number of arguments";
                return -EINVAL;
        }

        r = dm_get_device(ti, argv[0], 0, ti->len,
                          dm_table_get_mode(ti->table), &dev);
        if (r) {
                ti->error = "Cannot get target device";
                return r;
        }

        ti->private = dev;
        return 0;
}

static void origin_dtr(struct dm_target *ti)
{
        struct dm_dev *dev = ti->private;
        dm_put_device(ti, dev);
}

static int origin_map(struct dm_target *ti, struct bio *bio,
                      union map_info *map_context)
{
        struct dm_dev *dev = ti->private;
        bio->bi_bdev = dev->bdev;

        /* Only tell snapshots if this is a write */
        return (bio_rw(bio) == WRITE) ? do_origin(dev, bio) : DM_MAPIO_REMAPPED;
}

#define min_not_zero(l, r) (l == 0) ? r : ((r == 0) ? l : min(l, r))

/*
 * Set the target "split_io" field to the minimum of all the snapshots'
 * chunk sizes.
 */
static void origin_resume(struct dm_target *ti)
{
        struct dm_dev *dev = ti->private;
        struct dm_snapshot *snap;
        struct origin *o;
        chunk_t chunk_size = 0;

        down_read(&_origins_lock);
        o = __lookup_origin(dev->bdev);
        if (o)
                list_for_each_entry (snap, &o->snapshots, list)
                        chunk_size = min_not_zero(chunk_size, snap->chunk_size);
        up_read(&_origins_lock);

        ti->split_io = chunk_size;
}

static int origin_status(struct dm_target *ti, status_type_t type, char *result,
                         unsigned int maxlen)
{
        struct dm_dev *dev = ti->private;

        switch (type) {
        case STATUSTYPE_INFO:
                result[0] = '\0';
                break;

        case STATUSTYPE_TABLE:
                snprintf(result, maxlen, "%s", dev->name);
                break;
        }

        return 0;
}

static struct target_type origin_target = {
        .name    = "snapshot-origin",
        .version = {1, 6, 0},
        .module  = THIS_MODULE,
        .ctr     = origin_ctr,
        .dtr     = origin_dtr,
        .map     = origin_map,
        .resume  = origin_resume,
        .status  = origin_status,
};

static struct target_type snapshot_target = {
        .name    = "snapshot",
        .version = {1, 6, 0},
        .module  = THIS_MODULE,
        .ctr     = snapshot_ctr,
        .dtr     = snapshot_dtr,
        .map     = snapshot_map,
        .end_io  = snapshot_end_io,
        .resume  = snapshot_resume,
        .status  = snapshot_status,
};

static int __init dm_snapshot_init(void)
{
        int r;

        r = dm_register_target(&snapshot_target);
        if (r) {
                DMERR("snapshot target register failed %d", r);
                return r;
        }

        r = dm_register_target(&origin_target);
        if (r < 0) {
                DMERR("Origin target register failed %d", r);
                goto bad1;
        }

        r = init_origin_hash();
        if (r) {
                DMERR("init_origin_hash failed.");
                goto bad2;
        }

        exception_cache = KMEM_CACHE(dm_snap_exception, 0);
        if (!exception_cache) {
                DMERR("Couldn't create exception cache.");
                r = -ENOMEM;
                goto bad3;
        }

        pending_cache = KMEM_CACHE(dm_snap_pending_exception, 0);
        if (!pending_cache) {
                DMERR("Couldn't create pending cache.");
                r = -ENOMEM;
                goto bad4;
        }

        tracked_chunk_cache = KMEM_CACHE(dm_snap_tracked_chunk, 0);
        if (!tracked_chunk_cache) {
                DMERR("Couldn't create cache to track chunks in use.");
                r = -ENOMEM;
                goto bad5;
        }

        pending_pool = mempool_create_slab_pool(128, pending_cache);
        if (!pending_pool) {
                DMERR("Couldn't create pending pool.");
                r = -ENOMEM;
                goto bad_pending_pool;
        }

        ksnapd = create_singlethread_workqueue("ksnapd");
        if (!ksnapd) {
                DMERR("Failed to create ksnapd workqueue.");
                r = -ENOMEM;
                goto bad6;
        }

        return 0;

      bad6:
        mempool_destroy(pending_pool);
      bad_pending_pool:
        kmem_cache_destroy(tracked_chunk_cache);
      bad5:
        kmem_cache_destroy(pending_cache);
      bad4:
        kmem_cache_destroy(exception_cache);
      bad3:
        exit_origin_hash();
      bad2:
        dm_unregister_target(&origin_target);
      bad1:
        dm_unregister_target(&snapshot_target);
        return r;
}

static void __exit dm_snapshot_exit(void)
{
        int r;

        destroy_workqueue(ksnapd);

        r = dm_unregister_target(&snapshot_target);
        if (r)
                DMERR("snapshot unregister failed %d", r);

        r = dm_unregister_target(&origin_target);
        if (r)
                DMERR("origin unregister failed %d", r);

        exit_origin_hash();
        mempool_destroy(pending_pool);
        kmem_cache_destroy(pending_cache);
        kmem_cache_destroy(exception_cache);
        kmem_cache_destroy(tracked_chunk_cache);
}

/* Module hooks */
module_init(dm_snapshot_init);
module_exit(dm_snapshot_exit);

MODULE_DESCRIPTION(DM_NAME " snapshot target");
MODULE_AUTHOR("Joe Thornber");
MODULE_LICENSE("GPL");