/*
   * 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>
  
  #define DM_MSG_PREFIX "persistent snapshot"
  #define DM_CHUNK_SIZE_DEFAULT_SECTORS 32	/* 16KB */
  
  /*-----------------------------------------------------------------
   * 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)
  {

  	if (ps->area)
  		vfree(ps->area);

  	ps->area = NULL;

  
  	if (ps->zero_area)
  		vfree(ps->zero_area);

  	ps->zero_area = NULL;

  
  	if (ps->header_area)
  		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_work(&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);

  }
  
  /*
   * 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, uint32_t index)
  {
  	BUG_ON(index >= ps->exceptions_per_area);
  
  	return ((struct disk_exception *) ps->area) + index;
  }
  
  static void read_exception(struct pstore *ps,

  			   uint32_t index, struct core_exception *result)

  {

  	struct disk_exception *de = get_exception(ps, 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, 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, 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,
  			     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, 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;
  
  	/*
  	 * Keeping reading chunks and inserting exceptions until
  	 * we find a partially full area.
  	 */
  	for (ps->current_area = 0; full; ps->current_area++) {
  		r = area_io(ps, READ);
  		if (r)
  			return r;

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

  		if (r)
  			return r;
  	}
  
  	ps->current_area--;
  
  	return 0;
  }
  
  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 */
  	if (ps->callbacks)
  		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);
  	uint32_t stride;
  	chunk_t next_free;

  	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.
  	 */
  	stride = (ps->exceptions_per_area + 1);
  	next_free = ++ps->next_free;
  	if (sector_div(next_free, stride) == 1)
  		ps->next_free++;
  
  	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->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->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);

  }