[PATCH] device-mapper snapshot: barriers not supported

[linux-2.6.git] / 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/config.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 "dm-snap.h"
#include "dm-bio-list.h"
#include "kcopyd.h"

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

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

/*
 * Each snapshot reserves this many pages for io
 */
#define SNAPSHOT_PAGES 256

struct pending_exception {
        struct 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;

        /*
         * Other pending_exceptions that are processing this
         * chunk.  When this list is empty, we know we can
         * complete the origins.
         */
        struct list_head siblings;

        /* 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 kmem_cache_t *exception_cache;
static kmem_cache_t *pending_cache;
static mempool_t *pending_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("Device mapper: Snapshot: 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 inline unsigned int 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.
 */
static int init_exception_table(struct exception_table *et, uint32_t size)
{
        unsigned int i;

        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, kmem_cache_t *mem)
{
        struct list_head *slot;
        struct 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 inline uint32_t exception_hash(struct exception_table *et, chunk_t chunk)
{
        return chunk & et->hash_mask;
}

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

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

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

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

        return NULL;
}

static inline struct exception *alloc_exception(void)
{
        struct exception *e;

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

        return e;
}

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

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

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

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

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

        e->old_chunk = old;
        e->new_chunk = new;
        insert_exception(&s->complete, 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;
}

/*
 * Rounds a number down to a power of 2.
 */
static inline uint32_t round_down(uint32_t n)
{
        while (n & (n - 1))
                n &= (n - 1);
        return n;
}

/*
 * 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);

        /* Round it down to a power of 2 */
        hash_size = round_down(hash_size);
        if (init_exception_table(&s->complete, hash_size))
                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)) {
                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 inline ulong round_up(ulong n, ulong size)
{
        size--;
        return (n + size) & ~size;
}

static void read_snapshot_metadata(struct dm_snapshot *s)
{
        if (s->store.read_metadata(&s->store)) {
                down_write(&s->lock);
                s->valid = 0;
                up_write(&s->lock);
        }
}

/*
 * 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;
        unsigned long chunk_size;
        int r = -EINVAL;
        char persistent;
        char *origin_path;
        char *cow_path;
        char *value;
        int blocksize;

        if (argc < 4) {
                ti->error = "dm-snapshot: 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;
        }

        chunk_size = simple_strtoul(argv[3], &value, 10);
        if (chunk_size == 0 || value == NULL) {
                ti->error = "Invalid chunk size";
                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;
        }

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

        /* Validate the chunk size against the device block size */
        blocksize = s->cow->bdev->bd_disk->queue->hardsect_size;
        if (chunk_size % (blocksize >> 9)) {
                ti->error = "Chunk size is not a multiple of device blocksize";
                r = -EINVAL;
                goto bad3;
        }

        /* Check chunk_size is a power of 2 */
        if (chunk_size & (chunk_size - 1)) {
                ti->error = "Chunk size is not a power of 2";
                r = -EINVAL;
                goto bad3;
        }

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

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

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

        /*
         * Check the persistent flag - done here because we need the iobuf
         * to check the LV header
         */
        s->store.snap = s;

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

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

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

        /* Metadata must only be loaded into one table at once */
        read_snapshot_metadata(s);

        /* 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 bad6;
        }

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

        return 0;

 bad6:
        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 snapshot_dtr(struct dm_target *ti)
{
        struct dm_snapshot *s = (struct dm_snapshot *) ti->private;

        unregister_snapshot(s);

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

        /* Deallocate memory used */
        s->store.destroy(&s->store);

        dm_put_device(ti, s->origin);
        dm_put_device(ti, s->cow);
        kcopyd_client_destroy(s->kcopyd_client);
        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;
        }
}

/*
 * 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->bi_size);
                bio = n;
        }
}

static struct bio *__flush_bios(struct pending_exception *pe)
{
        struct pending_exception *sibling;

        if (list_empty(&pe->siblings))
                return bio_list_get(&pe->origin_bios);

        sibling = list_entry(pe->siblings.next,
                             struct pending_exception, siblings);

        list_del(&pe->siblings);

        /* This is fine as long as kcopyd is single-threaded. If kcopyd
         * becomes multi-threaded, we'll need some locking here.
         */
        bio_list_merge(&sibling->origin_bios, &pe->origin_bios);

        return NULL;
}

static void pending_complete(struct pending_exception *pe, int success)
{
        struct exception *e;
        struct dm_snapshot *s = pe->snap;
        struct bio *flush = NULL;

        if (success) {
                e = alloc_exception();
                if (!e) {
                        DMWARN("Unable to allocate exception.");
                        down_write(&s->lock);
                        s->store.drop_snapshot(&s->store);
                        s->valid = 0;
                        flush = __flush_bios(pe);
                        up_write(&s->lock);

                        error_bios(bio_list_get(&pe->snapshot_bios));
                        goto out;
                }
                *e = pe->e;

                /*
                 * Add a proper exception, and remove the
                 * in-flight exception from the list.
                 */
                down_write(&s->lock);
                insert_exception(&s->complete, e);
                remove_exception(&pe->e);
                flush = __flush_bios(pe);

                /* Submit any pending write bios */
                up_write(&s->lock);

                flush_bios(bio_list_get(&pe->snapshot_bios));
        } else {
                /* Read/write error - snapshot is unusable */
                down_write(&s->lock);
                if (s->valid)
                        DMERR("Error reading/writing snapshot");
                s->store.drop_snapshot(&s->store);
                s->valid = 0;
                remove_exception(&pe->e);
                flush = __flush_bios(pe);
                up_write(&s->lock);

                error_bios(bio_list_get(&pe->snapshot_bios));

                dm_table_event(s->table);
        }

 out:
        free_pending_exception(pe);

        if (flush)
                flush_bios(flush);
}

static void commit_callback(void *context, int success)
{
        struct pending_exception *pe = (struct pending_exception *) 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 int write_err, void *context)
{
        struct pending_exception *pe = (struct pending_exception *) 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 pending_exception *pe)
{
        struct dm_snapshot *s = pe->snap;
        struct 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 */
        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 pending_exception *
__find_pending_exception(struct dm_snapshot *s, struct bio *bio)
{
        struct exception *e;
        struct 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 pending_exception, e);

        } else {
                /*
                 * 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);

                e = lookup_exception(&s->pending, chunk);
                if (e) {
                        free_pending_exception(pe);
                        pe = container_of(e, struct pending_exception, e);
                } else {
                        pe->e.old_chunk = chunk;
                        bio_list_init(&pe->origin_bios);
                        bio_list_init(&pe->snapshot_bios);
                        INIT_LIST_HEAD(&pe->siblings);
                        pe->snap = s;
                        pe->started = 0;

                        if (s->store.prepare_exception(&s->store, &pe->e)) {
                                free_pending_exception(pe);
                                s->valid = 0;
                                return NULL;
                        }

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

        return pe;
}

static inline void remap_exception(struct dm_snapshot *s, struct exception *e,
                                   struct bio *bio)
{
        bio->bi_bdev = s->cow->bdev;
        bio->bi_sector = chunk_to_sector(s, e->new_chunk) +
                (bio->bi_sector & s->chunk_mask);
}

static int snapshot_map(struct dm_target *ti, struct bio *bio,
                        union map_info *map_context)
{
        struct exception *e;
        struct dm_snapshot *s = (struct dm_snapshot *) ti->private;
        int r = 1;
        chunk_t chunk;
        struct pending_exception *pe;

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

        /* Full snapshots are not usable */
        if (!s->valid)
                return -EIO;

        if (unlikely(bio_barrier(bio)))
                return -EOPNOTSUPP;

        /*
         * 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) {

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

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

                } else {
                        pe = __find_pending_exception(s, bio);

                        if (!pe) {
                                if (s->store.drop_snapshot)
                                        s->store.drop_snapshot(&s->store);
                                s->valid = 0;
                                r = -EIO;
                                up_write(&s->lock);
                        } else {
                                remap_exception(s, &pe->e, bio);
                                bio_list_add(&pe->snapshot_bios, bio);

                                if (!pe->started) {
                                        /* this is protected by snap->lock */
                                        pe->started = 1;
                                        up_write(&s->lock);
                                        start_copy(pe);
                                } else
                                        up_write(&s->lock);
                                r = 0;
                        }
                }

        } else {
                /*
                 * FIXME: this read path scares me because we
                 * always use the origin when we have a pending
                 * exception.  However I can't think of a
                 * situation where this is wrong - ejt.
                 */

                /* Do reads */
                down_read(&s->lock);

                /* See if it it has been remapped */
                e = lookup_exception(&s->complete, chunk);
                if (e)
                        remap_exception(s, e, bio);
                else
                        bio->bi_bdev = s->origin->bdev;

                up_read(&s->lock);
        }

        return r;
}

static void snapshot_resume(struct dm_target *ti)
{
        struct dm_snapshot *s = (struct dm_snapshot *) 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 = (struct dm_snapshot *) 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,
                                         SECTOR_FORMAT "/" SECTOR_FORMAT,
                                         numerator, 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 " SECTOR_FORMAT,
                         snap->origin->name, snap->cow->name,
                         snap->type, snap->chunk_size);
                break;
        }

        return 0;
}

/*-----------------------------------------------------------------
 * Origin methods
 *---------------------------------------------------------------*/
static void list_merge(struct list_head *l1, struct list_head *l2)
{
        struct list_head *l1_n, *l2_p;

        l1_n = l1->next;
        l2_p = l2->prev;

        l1->next = l2;
        l2->prev = l1;

        l2_p->next = l1_n;
        l1_n->prev = l2_p;
}

static int __origin_write(struct list_head *snapshots, struct bio *bio)
{
        int r = 1, first = 1;
        struct dm_snapshot *snap;
        struct exception *e;
        struct pending_exception *pe, *last = NULL;
        chunk_t chunk;

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

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

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

                down_write(&snap->lock);

                /*
                 * 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.
                 */
                e = lookup_exception(&snap->complete, chunk);
                if (!e) {
                        pe = __find_pending_exception(snap, bio);
                        if (!pe) {
                                snap->store.drop_snapshot(&snap->store);
                                snap->valid = 0;

                        } else {
                                if (last)
                                        list_merge(&pe->siblings,
                                                   &last->siblings);

                                last = pe;
                                r = 0;
                        }
                }

                up_write(&snap->lock);
        }

        /*
         * Now that we have a complete pe list we can start the copying.
         */
        if (last) {
                pe = last;
                do {
                        down_write(&pe->snap->lock);
                        if (first)
                                bio_list_add(&pe->origin_bios, bio);
                        if (!pe->started) {
                                pe->started = 1;
                                up_write(&pe->snap->lock);
                                start_copy(pe);
                        } else
                                up_write(&pe->snap->lock);
                        first = 0;
                        pe = list_entry(pe->siblings.next,
                                        struct pending_exception, siblings);

                } while (pe != last);
        }

        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 = 1;

        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 = "dm-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 = (struct dm_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 = (struct dm_dev *) ti->private;
        bio->bi_bdev = dev->bdev;

        if (unlikely(bio_barrier(bio)))
                return -EOPNOTSUPP;

        /* Only tell snapshots if this is a write */
        return (bio_rw(bio) == WRITE) ? do_origin(dev, bio) : 1;
}

#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 = (struct dm_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 = (struct dm_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, 1, 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, 1, 0},
        .module  = THIS_MODULE,
        .ctr     = snapshot_ctr,
        .dtr     = snapshot_dtr,
        .map     = snapshot_map,
        .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("Device mapper: Origin: register failed %d\n", r);
                goto bad1;
        }

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

        exception_cache = kmem_cache_create("dm-snapshot-ex",
                                            sizeof(struct exception),
                                            __alignof__(struct exception),
                                            0, NULL, NULL);
        if (!exception_cache) {
                DMERR("Couldn't create exception cache.");
                r = -ENOMEM;
                goto bad3;
        }

        pending_cache =
            kmem_cache_create("dm-snapshot-in",
                              sizeof(struct pending_exception),
                              __alignof__(struct pending_exception),
                              0, NULL, NULL);
        if (!pending_cache) {
                DMERR("Couldn't create pending cache.");
                r = -ENOMEM;
                goto bad4;
        }

        pending_pool = mempool_create(128, mempool_alloc_slab,
                                      mempool_free_slab, pending_cache);
        if (!pending_pool) {
                DMERR("Couldn't create pending pool.");
                r = -ENOMEM;
                goto bad5;
        }

        return 0;

      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;

        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);
}

/* 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");