Linux 4.2.1

[linux/fpc-iii.git] / drivers / md / dm-snap-persistent.c

/*
 * Copyright (C) 2001-2002 Sistina Software (UK) Limited.
 * Copyright (C) 2006-2008 Red Hat GmbH
 *
 * This file is released under the GPL.
 */

#include "dm-exception-store.h"

#include <linux/mm.h>
#include <linux/pagemap.h>
#include <linux/vmalloc.h>
#include <linux/export.h>
#include <linux/slab.h>
#include <linux/dm-io.h>
#include "dm-bufio.h"

#define DM_MSG_PREFIX "persistent snapshot"
#define DM_CHUNK_SIZE_DEFAULT_SECTORS 32        /* 16KB */

#define DM_PREFETCH_CHUNKS              12

/*-----------------------------------------------------------------
 * Persistent snapshots, by persistent we mean that the snapshot
 * will survive a reboot.
 *---------------------------------------------------------------*/

/*
 * We need to store a record of which parts of the origin have
 * been copied to the snapshot device.  The snapshot code
 * requires that we copy exception chunks to chunk aligned areas
 * of the COW store.  It makes sense therefore, to store the
 * metadata in chunk size blocks.
 *
 * There is no backward or forward compatibility implemented,
 * snapshots with different disk versions than the kernel will
 * not be usable.  It is expected that "lvcreate" will blank out
 * the start of a fresh COW device before calling the snapshot
 * constructor.
 *
 * The first chunk of the COW device just contains the header.
 * After this there is a chunk filled with exception metadata,
 * followed by as many exception chunks as can fit in the
 * metadata areas.
 *
 * All on disk structures are in little-endian format.  The end
 * of the exceptions info is indicated by an exception with a
 * new_chunk of 0, which is invalid since it would point to the
 * header chunk.
 */

/*
 * Magic for persistent snapshots: "SnAp" - Feeble isn't it.
 */
#define SNAP_MAGIC 0x70416e53

/*
 * The on-disk version of the metadata.
 */
#define SNAPSHOT_DISK_VERSION 1

#define NUM_SNAPSHOT_HDR_CHUNKS 1

struct disk_header {
        __le32 magic;

        /*
         * Is this snapshot valid.  There is no way of recovering
         * an invalid snapshot.
         */
        __le32 valid;

        /*
         * Simple, incrementing version. no backward
         * compatibility.
         */
        __le32 version;

        /* In sectors */
        __le32 chunk_size;
} __packed;

struct disk_exception {
        __le64 old_chunk;
        __le64 new_chunk;
} __packed;

struct core_exception {
        uint64_t old_chunk;
        uint64_t new_chunk;
};

struct commit_callback {
        void (*callback)(void *, int success);
        void *context;
};

/*
 * The top level structure for a persistent exception store.
 */
struct pstore {
        struct dm_exception_store *store;
        int version;
        int valid;
        uint32_t exceptions_per_area;

        /*
         * Now that we have an asynchronous kcopyd there is no
         * need for large chunk sizes, so it wont hurt to have a
         * whole chunks worth of metadata in memory at once.
         */
        void *area;

        /*
         * An area of zeros used to clear the next area.
         */
        void *zero_area;

        /*
         * An area used for header. The header can be written
         * concurrently with metadata (when invalidating the snapshot),
         * so it needs a separate buffer.
         */
        void *header_area;

        /*
         * Used to keep track of which metadata area the data in
         * 'chunk' refers to.
         */
        chunk_t current_area;

        /*
         * The next free chunk for an exception.
         *
         * When creating exceptions, all the chunks here and above are
         * free.  It holds the next chunk to be allocated.  On rare
         * occasions (e.g. after a system crash) holes can be left in
         * the exception store because chunks can be committed out of
         * order.
         *
         * When merging exceptions, it does not necessarily mean all the
         * chunks here and above are free.  It holds the value it would
         * have held if all chunks had been committed in order of
         * allocation.  Consequently the value may occasionally be
         * slightly too low, but since it's only used for 'status' and
         * it can never reach its minimum value too early this doesn't
         * matter.
         */

        chunk_t next_free;

        /*
         * The index of next free exception in the current
         * metadata area.
         */
        uint32_t current_committed;

        atomic_t pending_count;
        uint32_t callback_count;
        struct commit_callback *callbacks;
        struct dm_io_client *io_client;

        struct workqueue_struct *metadata_wq;
};

static int alloc_area(struct pstore *ps)
{
        int r = -ENOMEM;
        size_t len;

        len = ps->store->chunk_size << SECTOR_SHIFT;

        /*
         * Allocate the chunk_size block of memory that will hold
         * a single metadata area.
         */
        ps->area = vmalloc(len);
        if (!ps->area)
                goto err_area;

        ps->zero_area = vzalloc(len);
        if (!ps->zero_area)
                goto err_zero_area;

        ps->header_area = vmalloc(len);
        if (!ps->header_area)
                goto err_header_area;

        return 0;

err_header_area:
        vfree(ps->zero_area);

err_zero_area:
        vfree(ps->area);

err_area:
        return r;
}

static void free_area(struct pstore *ps)
{
        vfree(ps->area);
        ps->area = NULL;
        vfree(ps->zero_area);
        ps->zero_area = NULL;
        vfree(ps->header_area);
        ps->header_area = NULL;
}

struct mdata_req {
        struct dm_io_region *where;
        struct dm_io_request *io_req;
        struct work_struct work;
        int result;
};

static void do_metadata(struct work_struct *work)
{
        struct mdata_req *req = container_of(work, struct mdata_req, work);

        req->result = dm_io(req->io_req, 1, req->where, NULL);
}

/*
 * Read or write a chunk aligned and sized block of data from a device.
 */
static int chunk_io(struct pstore *ps, void *area, chunk_t chunk, int rw,
                    int metadata)
{
        struct dm_io_region where = {
                .bdev = dm_snap_cow(ps->store->snap)->bdev,
                .sector = ps->store->chunk_size * chunk,
                .count = ps->store->chunk_size,
        };
        struct dm_io_request io_req = {
                .bi_rw = rw,
                .mem.type = DM_IO_VMA,
                .mem.ptr.vma = area,
                .client = ps->io_client,
                .notify.fn = NULL,
        };
        struct mdata_req req;

        if (!metadata)
                return dm_io(&io_req, 1, &where, NULL);

        req.where = &where;
        req.io_req = &io_req;

        /*
         * Issue the synchronous I/O from a different thread
         * to avoid generic_make_request recursion.
         */
        INIT_WORK_ONSTACK(&req.work, do_metadata);
        queue_work(ps->metadata_wq, &req.work);
        flush_workqueue(ps->metadata_wq);
        destroy_work_on_stack(&req.work);

        return req.result;
}

/*
 * Convert a metadata area index to a chunk index.
 */
static chunk_t area_location(struct pstore *ps, chunk_t area)
{
        return NUM_SNAPSHOT_HDR_CHUNKS + ((ps->exceptions_per_area + 1) * area);
}

static void skip_metadata(struct pstore *ps)
{
        uint32_t stride = ps->exceptions_per_area + 1;
        chunk_t next_free = ps->next_free;
        if (sector_div(next_free, stride) == NUM_SNAPSHOT_HDR_CHUNKS)
                ps->next_free++;
}

/*
 * Read or write a metadata area.  Remembering to skip the first
 * chunk which holds the header.
 */
static int area_io(struct pstore *ps, int rw)
{
        int r;
        chunk_t chunk;

        chunk = area_location(ps, ps->current_area);

        r = chunk_io(ps, ps->area, chunk, rw, 0);
        if (r)
                return r;

        return 0;
}

static void zero_memory_area(struct pstore *ps)
{
        memset(ps->area, 0, ps->store->chunk_size << SECTOR_SHIFT);
}

static int zero_disk_area(struct pstore *ps, chunk_t area)
{
        return chunk_io(ps, ps->zero_area, area_location(ps, area), WRITE, 0);
}

static int read_header(struct pstore *ps, int *new_snapshot)
{
        int r;
        struct disk_header *dh;
        unsigned chunk_size;
        int chunk_size_supplied = 1;
        char *chunk_err;

        /*
         * Use default chunk size (or logical_block_size, if larger)
         * if none supplied
         */
        if (!ps->store->chunk_size) {
                ps->store->chunk_size = max(DM_CHUNK_SIZE_DEFAULT_SECTORS,
                    bdev_logical_block_size(dm_snap_cow(ps->store->snap)->
                                            bdev) >> 9);
                ps->store->chunk_mask = ps->store->chunk_size - 1;
                ps->store->chunk_shift = ffs(ps->store->chunk_size) - 1;
                chunk_size_supplied = 0;
        }

        ps->io_client = dm_io_client_create();
        if (IS_ERR(ps->io_client))
                return PTR_ERR(ps->io_client);

        r = alloc_area(ps);
        if (r)
                return r;

        r = chunk_io(ps, ps->header_area, 0, READ, 1);
        if (r)
                goto bad;

        dh = ps->header_area;

        if (le32_to_cpu(dh->magic) == 0) {
                *new_snapshot = 1;
                return 0;
        }

        if (le32_to_cpu(dh->magic) != SNAP_MAGIC) {
                DMWARN("Invalid or corrupt snapshot");
                r = -ENXIO;
                goto bad;
        }

        *new_snapshot = 0;
        ps->valid = le32_to_cpu(dh->valid);
        ps->version = le32_to_cpu(dh->version);
        chunk_size = le32_to_cpu(dh->chunk_size);

        if (ps->store->chunk_size == chunk_size)
                return 0;

        if (chunk_size_supplied)
                DMWARN("chunk size %u in device metadata overrides "
                       "table chunk size of %u.",
                       chunk_size, ps->store->chunk_size);

        /* We had a bogus chunk_size. Fix stuff up. */
        free_area(ps);

        r = dm_exception_store_set_chunk_size(ps->store, chunk_size,
                                              &chunk_err);
        if (r) {
                DMERR("invalid on-disk chunk size %u: %s.",
                      chunk_size, chunk_err);
                return r;
        }

        r = alloc_area(ps);
        return r;

bad:
        free_area(ps);
        return r;
}

static int write_header(struct pstore *ps)
{
        struct disk_header *dh;

        memset(ps->header_area, 0, ps->store->chunk_size << SECTOR_SHIFT);

        dh = ps->header_area;
        dh->magic = cpu_to_le32(SNAP_MAGIC);
        dh->valid = cpu_to_le32(ps->valid);
        dh->version = cpu_to_le32(ps->version);
        dh->chunk_size = cpu_to_le32(ps->store->chunk_size);

        return chunk_io(ps, ps->header_area, 0, WRITE, 1);
}

/*
 * Access functions for the disk exceptions, these do the endian conversions.
 */
static struct disk_exception *get_exception(struct pstore *ps, void *ps_area,
                                            uint32_t index)
{
        BUG_ON(index >= ps->exceptions_per_area);

        return ((struct disk_exception *) ps_area) + index;
}

static void read_exception(struct pstore *ps, void *ps_area,
                           uint32_t index, struct core_exception *result)
{
        struct disk_exception *de = get_exception(ps, ps_area, index);

        /* copy it */
        result->old_chunk = le64_to_cpu(de->old_chunk);
        result->new_chunk = le64_to_cpu(de->new_chunk);
}

static void write_exception(struct pstore *ps,
                            uint32_t index, struct core_exception *e)
{
        struct disk_exception *de = get_exception(ps, ps->area, index);

        /* copy it */
        de->old_chunk = cpu_to_le64(e->old_chunk);
        de->new_chunk = cpu_to_le64(e->new_chunk);
}

static void clear_exception(struct pstore *ps, uint32_t index)
{
        struct disk_exception *de = get_exception(ps, ps->area, index);

        /* clear it */
        de->old_chunk = 0;
        de->new_chunk = 0;
}

/*
 * Registers the exceptions that are present in the current area.
 * 'full' is filled in to indicate if the area has been
 * filled.
 */
static int insert_exceptions(struct pstore *ps, void *ps_area,
                             int (*callback)(void *callback_context,
                                             chunk_t old, chunk_t new),
                             void *callback_context,
                             int *full)
{
        int r;
        unsigned int i;
        struct core_exception e;

        /* presume the area is full */
        *full = 1;

        for (i = 0; i < ps->exceptions_per_area; i++) {
                read_exception(ps, ps_area, i, &e);

                /*
                 * If the new_chunk is pointing at the start of
                 * the COW device, where the first metadata area
                 * is we know that we've hit the end of the
                 * exceptions.  Therefore the area is not full.
                 */
                if (e.new_chunk == 0LL) {
                        ps->current_committed = i;
                        *full = 0;
                        break;
                }

                /*
                 * Keep track of the start of the free chunks.
                 */
                if (ps->next_free <= e.new_chunk)
                        ps->next_free = e.new_chunk + 1;

                /*
                 * Otherwise we add the exception to the snapshot.
                 */
                r = callback(callback_context, e.old_chunk, e.new_chunk);
                if (r)
                        return r;
        }

        return 0;
}

static int read_exceptions(struct pstore *ps,
                           int (*callback)(void *callback_context, chunk_t old,
                                           chunk_t new),
                           void *callback_context)
{
        int r, full = 1;
        struct dm_bufio_client *client;
        chunk_t prefetch_area = 0;

        client = dm_bufio_client_create(dm_snap_cow(ps->store->snap)->bdev,
                                        ps->store->chunk_size << SECTOR_SHIFT,
                                        1, 0, NULL, NULL);

        if (IS_ERR(client))
                return PTR_ERR(client);

        /*
         * Setup for one current buffer + desired readahead buffers.
         */
        dm_bufio_set_minimum_buffers(client, 1 + DM_PREFETCH_CHUNKS);

        /*
         * Keeping reading chunks and inserting exceptions until
         * we find a partially full area.
         */
        for (ps->current_area = 0; full; ps->current_area++) {
                struct dm_buffer *bp;
                void *area;
                chunk_t chunk;

                if (unlikely(prefetch_area < ps->current_area))
                        prefetch_area = ps->current_area;

                if (DM_PREFETCH_CHUNKS) do {
                        chunk_t pf_chunk = area_location(ps, prefetch_area);
                        if (unlikely(pf_chunk >= dm_bufio_get_device_size(client)))
                                break;
                        dm_bufio_prefetch(client, pf_chunk, 1);
                        prefetch_area++;
                        if (unlikely(!prefetch_area))
                                break;
                } while (prefetch_area <= ps->current_area + DM_PREFETCH_CHUNKS);

                chunk = area_location(ps, ps->current_area);

                area = dm_bufio_read(client, chunk, &bp);
                if (unlikely(IS_ERR(area))) {
                        r = PTR_ERR(area);
                        goto ret_destroy_bufio;
                }

                r = insert_exceptions(ps, area, callback, callback_context,
                                      &full);

                if (!full)
                        memcpy(ps->area, area, ps->store->chunk_size << SECTOR_SHIFT);

                dm_bufio_release(bp);

                dm_bufio_forget(client, chunk);

                if (unlikely(r))
                        goto ret_destroy_bufio;
        }

        ps->current_area--;

        skip_metadata(ps);

        r = 0;

ret_destroy_bufio:
        dm_bufio_client_destroy(client);

        return r;
}

static struct pstore *get_info(struct dm_exception_store *store)
{
        return (struct pstore *) store->context;
}

static void persistent_usage(struct dm_exception_store *store,
                             sector_t *total_sectors,
                             sector_t *sectors_allocated,
                             sector_t *metadata_sectors)
{
        struct pstore *ps = get_info(store);

        *sectors_allocated = ps->next_free * store->chunk_size;
        *total_sectors = get_dev_size(dm_snap_cow(store->snap)->bdev);

        /*
         * First chunk is the fixed header.
         * Then there are (ps->current_area + 1) metadata chunks, each one
         * separated from the next by ps->exceptions_per_area data chunks.
         */
        *metadata_sectors = (ps->current_area + 1 + NUM_SNAPSHOT_HDR_CHUNKS) *
                            store->chunk_size;
}

static void persistent_dtr(struct dm_exception_store *store)
{
        struct pstore *ps = get_info(store);

        destroy_workqueue(ps->metadata_wq);

        /* Created in read_header */
        if (ps->io_client)
                dm_io_client_destroy(ps->io_client);
        free_area(ps);

        /* Allocated in persistent_read_metadata */
        vfree(ps->callbacks);

        kfree(ps);
}

static int persistent_read_metadata(struct dm_exception_store *store,
                                    int (*callback)(void *callback_context,
                                                    chunk_t old, chunk_t new),
                                    void *callback_context)
{
        int r, uninitialized_var(new_snapshot);
        struct pstore *ps = get_info(store);

        /*
         * Read the snapshot header.
         */
        r = read_header(ps, &new_snapshot);
        if (r)
                return r;

        /*
         * Now we know correct chunk_size, complete the initialisation.
         */
        ps->exceptions_per_area = (ps->store->chunk_size << SECTOR_SHIFT) /
                                  sizeof(struct disk_exception);
        ps->callbacks = dm_vcalloc(ps->exceptions_per_area,
                                   sizeof(*ps->callbacks));
        if (!ps->callbacks)
                return -ENOMEM;

        /*
         * Do we need to setup a new snapshot ?
         */
        if (new_snapshot) {
                r = write_header(ps);
                if (r) {
                        DMWARN("write_header failed");
                        return r;
                }

                ps->current_area = 0;
                zero_memory_area(ps);
                r = zero_disk_area(ps, 0);
                if (r)
                        DMWARN("zero_disk_area(0) failed");
                return r;
        }
        /*
         * Sanity checks.
         */
        if (ps->version != SNAPSHOT_DISK_VERSION) {
                DMWARN("unable to handle snapshot disk version %d",
                       ps->version);
                return -EINVAL;
        }

        /*
         * Metadata are valid, but snapshot is invalidated
         */
        if (!ps->valid)
                return 1;

        /*
         * Read the metadata.
         */
        r = read_exceptions(ps, callback, callback_context);

        return r;
}

static int persistent_prepare_exception(struct dm_exception_store *store,
                                        struct dm_exception *e)
{
        struct pstore *ps = get_info(store);
        sector_t size = get_dev_size(dm_snap_cow(store->snap)->bdev);

        /* Is there enough room ? */
        if (size < ((ps->next_free + 1) * store->chunk_size))
                return -ENOSPC;

        e->new_chunk = ps->next_free;

        /*
         * Move onto the next free pending, making sure to take
         * into account the location of the metadata chunks.
         */
        ps->next_free++;
        skip_metadata(ps);

        atomic_inc(&ps->pending_count);
        return 0;
}

static void persistent_commit_exception(struct dm_exception_store *store,
                                        struct dm_exception *e,
                                        void (*callback) (void *, int success),
                                        void *callback_context)
{
        unsigned int i;
        struct pstore *ps = get_info(store);
        struct core_exception ce;
        struct commit_callback *cb;

        ce.old_chunk = e->old_chunk;
        ce.new_chunk = e->new_chunk;
        write_exception(ps, ps->current_committed++, &ce);

        /*
         * Add the callback to the back of the array.  This code
         * is the only place where the callback array is
         * manipulated, and we know that it will never be called
         * multiple times concurrently.
         */
        cb = ps->callbacks + ps->callback_count++;
        cb->callback = callback;
        cb->context = callback_context;

        /*
         * If there are exceptions in flight and we have not yet
         * filled this metadata area there's nothing more to do.
         */
        if (!atomic_dec_and_test(&ps->pending_count) &&
            (ps->current_committed != ps->exceptions_per_area))
                return;

        /*
         * If we completely filled the current area, then wipe the next one.
         */
        if ((ps->current_committed == ps->exceptions_per_area) &&
            zero_disk_area(ps, ps->current_area + 1))
                ps->valid = 0;

        /*
         * Commit exceptions to disk.
         */
        if (ps->valid && area_io(ps, WRITE_FLUSH_FUA))
                ps->valid = 0;

        /*
         * Advance to the next area if this one is full.
         */
        if (ps->current_committed == ps->exceptions_per_area) {
                ps->current_committed = 0;
                ps->current_area++;
                zero_memory_area(ps);
        }

        for (i = 0; i < ps->callback_count; i++) {
                cb = ps->callbacks + i;
                cb->callback(cb->context, ps->valid);
        }

        ps->callback_count = 0;
}

static int persistent_prepare_merge(struct dm_exception_store *store,
                                    chunk_t *last_old_chunk,
                                    chunk_t *last_new_chunk)
{
        struct pstore *ps = get_info(store);
        struct core_exception ce;
        int nr_consecutive;
        int r;

        /*
         * When current area is empty, move back to preceding area.
         */
        if (!ps->current_committed) {
                /*
                 * Have we finished?
                 */
                if (!ps->current_area)
                        return 0;

                ps->current_area--;
                r = area_io(ps, READ);
                if (r < 0)
                        return r;
                ps->current_committed = ps->exceptions_per_area;
        }

        read_exception(ps, ps->area, ps->current_committed - 1, &ce);
        *last_old_chunk = ce.old_chunk;
        *last_new_chunk = ce.new_chunk;

        /*
         * Find number of consecutive chunks within the current area,
         * working backwards.
         */
        for (nr_consecutive = 1; nr_consecutive < ps->current_committed;
             nr_consecutive++) {
                read_exception(ps, ps->area,
                               ps->current_committed - 1 - nr_consecutive, &ce);
                if (ce.old_chunk != *last_old_chunk - nr_consecutive ||
                    ce.new_chunk != *last_new_chunk - nr_consecutive)
                        break;
        }

        return nr_consecutive;
}

static int persistent_commit_merge(struct dm_exception_store *store,
                                   int nr_merged)
{
        int r, i;
        struct pstore *ps = get_info(store);

        BUG_ON(nr_merged > ps->current_committed);

        for (i = 0; i < nr_merged; i++)
                clear_exception(ps, ps->current_committed - 1 - i);

        r = area_io(ps, WRITE_FLUSH_FUA);
        if (r < 0)
                return r;

        ps->current_committed -= nr_merged;

        /*
         * At this stage, only persistent_usage() uses ps->next_free, so
         * we make no attempt to keep ps->next_free strictly accurate
         * as exceptions may have been committed out-of-order originally.
         * Once a snapshot has become merging, we set it to the value it
         * would have held had all the exceptions been committed in order.
         *
         * ps->current_area does not get reduced by prepare_merge() until
         * after commit_merge() has removed the nr_merged previous exceptions.
         */
        ps->next_free = area_location(ps, ps->current_area) +
                        ps->current_committed + 1;

        return 0;
}

static void persistent_drop_snapshot(struct dm_exception_store *store)
{
        struct pstore *ps = get_info(store);

        ps->valid = 0;
        if (write_header(ps))
                DMWARN("write header failed");
}

static int persistent_ctr(struct dm_exception_store *store,
                          unsigned argc, char **argv)
{
        struct pstore *ps;

        /* allocate the pstore */
        ps = kzalloc(sizeof(*ps), GFP_KERNEL);
        if (!ps)
                return -ENOMEM;

        ps->store = store;
        ps->valid = 1;
        ps->version = SNAPSHOT_DISK_VERSION;
        ps->area = NULL;
        ps->zero_area = NULL;
        ps->header_area = NULL;
        ps->next_free = NUM_SNAPSHOT_HDR_CHUNKS + 1; /* header and 1st area */
        ps->current_committed = 0;

        ps->callback_count = 0;
        atomic_set(&ps->pending_count, 0);
        ps->callbacks = NULL;

        ps->metadata_wq = alloc_workqueue("ksnaphd", WQ_MEM_RECLAIM, 0);
        if (!ps->metadata_wq) {
                kfree(ps);
                DMERR("couldn't start header metadata update thread");
                return -ENOMEM;
        }

        store->context = ps;

        return 0;
}

static unsigned persistent_status(struct dm_exception_store *store,
                                  status_type_t status, char *result,
                                  unsigned maxlen)
{
        unsigned sz = 0;

        switch (status) {
        case STATUSTYPE_INFO:
                break;
        case STATUSTYPE_TABLE:
                DMEMIT(" P %llu", (unsigned long long)store->chunk_size);
        }

        return sz;
}

static struct dm_exception_store_type _persistent_type = {
        .name = "persistent",
        .module = THIS_MODULE,
        .ctr = persistent_ctr,
        .dtr = persistent_dtr,
        .read_metadata = persistent_read_metadata,
        .prepare_exception = persistent_prepare_exception,
        .commit_exception = persistent_commit_exception,
        .prepare_merge = persistent_prepare_merge,
        .commit_merge = persistent_commit_merge,
        .drop_snapshot = persistent_drop_snapshot,
        .usage = persistent_usage,
        .status = persistent_status,
};

static struct dm_exception_store_type _persistent_compat_type = {
        .name = "P",
        .module = THIS_MODULE,
        .ctr = persistent_ctr,
        .dtr = persistent_dtr,
        .read_metadata = persistent_read_metadata,
        .prepare_exception = persistent_prepare_exception,
        .commit_exception = persistent_commit_exception,
        .prepare_merge = persistent_prepare_merge,
        .commit_merge = persistent_commit_merge,
        .drop_snapshot = persistent_drop_snapshot,
        .usage = persistent_usage,
        .status = persistent_status,
};

int dm_persistent_snapshot_init(void)
{
        int r;

        r = dm_exception_store_type_register(&_persistent_type);
        if (r) {
                DMERR("Unable to register persistent exception store type");
                return r;
        }

        r = dm_exception_store_type_register(&_persistent_compat_type);
        if (r) {
                DMERR("Unable to register old-style persistent exception "
                      "store type");
                dm_exception_store_type_unregister(&_persistent_type);
                return r;
        }

        return r;
}

void dm_persistent_snapshot_exit(void)
{
        dm_exception_store_type_unregister(&_persistent_type);
        dm_exception_store_type_unregister(&_persistent_compat_type);
}