Commit 3452c2a1 authored by Mikulas Patocka's avatar Mikulas Patocka Committed by Alasdair G Kergon
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dm snapshot: avoid allocating exceptions in merge 



The snapshot-merge target should not allocate new exceptions because the
intent is to merge all of its exceptions as quickly and safely as
possible.

This patch introduces the snapshot-merge mapping function and updates
__origin_write() so that it doesn't allocate exceptions on any snapshots
that are being merged.

If a write request to a merging snapshot device is to be dispatched
directly to the origin (because the chunk is not remapped or was already
merged), snapshot_merge_map() must make exceptions in other snapshots so
calls do_origin().

Signed-off-by: default avatarMikulas Patocka <mpatocka@redhat.com>
Signed-off-by: default avatarMike Snitzer <snitzer@redhat.com>
Signed-off-by: default avatarAlasdair G Kergon <agk@redhat.com>
parent 515ad66c

drivers/md/dm-snap.c

+56 −1
Original line number Diff line number Diff line
@@ -1308,6 +1308,54 @@ static int snapshot_map(struct dm_target *ti, struct bio *bio, 
	return r;

}



/*

 * A snapshot-merge target behaves like a combination of a snapshot

 * target and a snapshot-origin target.  It only generates new

 * exceptions in other snapshots and not in the one that is being

 * merged.

 *

 * For each chunk, if there is an existing exception, it is used to

 * redirect I/O to the cow device.  Otherwise I/O is sent to the origin,

 * which in turn might generate exceptions in other snapshots.

 */

static int snapshot_merge_map(struct dm_target *ti, struct bio *bio,

			      union map_info *map_context)

{

	struct dm_exception *e;

	struct dm_snapshot *s = ti->private;

	int r = DM_MAPIO_REMAPPED;

	chunk_t chunk;



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



	down_read(&s->lock);



	/* Full snapshots are not usable */

	if (!s->valid) {

		r = -EIO;

		goto out_unlock;

	}



	/* If the block is already remapped - use that */

	e = dm_lookup_exception(&s->complete, chunk);

	if (e) {

		remap_exception(s, e, bio, chunk);

		goto out_unlock;

	}



	bio->bi_bdev = s->origin->bdev;



	if (bio_rw(bio) == WRITE) {

		up_read(&s->lock);

		return do_origin(s->origin, bio);

	}



out_unlock:

	up_read(&s->lock);



	return r;

}



static int snapshot_end_io(struct dm_target *ti, struct bio *bio,

			   int error, union map_info *map_context)

{
@@ -1465,6 +1513,13 @@ static int __origin_write(struct list_head *snapshots, sector_t sector, 


	/* Do all the snapshots on this origin */

	list_for_each_entry (snap, snapshots, list) {

		/*

		 * Don't make new exceptions in a merging snapshot

		 * because it has effectively been deleted

		 */

		if (dm_target_is_snapshot_merge(snap->ti))

			continue;



		down_write(&snap->lock);



		/* Only deal with valid and active snapshots */
@@ -1697,7 +1752,7 @@ static struct target_type merge_target = { 
	.module  = THIS_MODULE,

	.ctr     = snapshot_ctr,

	.dtr     = snapshot_dtr,

	.map     = snapshot_map,

	.map     = snapshot_merge_map,

	.end_io  = snapshot_end_io,

	.postsuspend = snapshot_postsuspend,

	.preresume  = snapshot_preresume,

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