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: Mikulas Patocka <mpatocka@redhat.com> Signed-off-by: Mike Snitzer <snitzer@redhat.com> Signed-off-by: Alasdair G Kergon <agk@redhat.com>
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@ -1308,6 +1308,54 @@ static int snapshot_map(struct dm_target *ti, struct bio *bio,
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return r;
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}
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/*
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* A snapshot-merge target behaves like a combination of a snapshot
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* target and a snapshot-origin target. It only generates new
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* exceptions in other snapshots and not in the one that is being
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* merged.
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*
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* For each chunk, if there is an existing exception, it is used to
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* redirect I/O to the cow device. Otherwise I/O is sent to the origin,
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* which in turn might generate exceptions in other snapshots.
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*/
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static int snapshot_merge_map(struct dm_target *ti, struct bio *bio,
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union map_info *map_context)
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{
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struct dm_exception *e;
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struct dm_snapshot *s = ti->private;
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int r = DM_MAPIO_REMAPPED;
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chunk_t chunk;
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chunk = sector_to_chunk(s->store, bio->bi_sector);
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down_read(&s->lock);
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/* Full snapshots are not usable */
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if (!s->valid) {
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r = -EIO;
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goto out_unlock;
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}
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/* If the block is already remapped - use that */
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e = dm_lookup_exception(&s->complete, chunk);
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if (e) {
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remap_exception(s, e, bio, chunk);
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goto out_unlock;
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}
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bio->bi_bdev = s->origin->bdev;
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if (bio_rw(bio) == WRITE) {
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up_read(&s->lock);
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return do_origin(s->origin, bio);
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}
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out_unlock:
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up_read(&s->lock);
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return r;
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}
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static int snapshot_end_io(struct dm_target *ti, struct bio *bio,
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int error, union map_info *map_context)
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{
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@ -1465,6 +1513,13 @@ static int __origin_write(struct list_head *snapshots, sector_t sector,
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/* Do all the snapshots on this origin */
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list_for_each_entry (snap, snapshots, list) {
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/*
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* Don't make new exceptions in a merging snapshot
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* because it has effectively been deleted
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*/
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if (dm_target_is_snapshot_merge(snap->ti))
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continue;
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down_write(&snap->lock);
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/* Only deal with valid and active snapshots */
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@ -1697,7 +1752,7 @@ static struct target_type merge_target = {
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.module = THIS_MODULE,
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.ctr = snapshot_ctr,
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.dtr = snapshot_dtr,
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.map = snapshot_map,
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.map = snapshot_merge_map,
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.end_io = snapshot_end_io,
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.postsuspend = snapshot_postsuspend,
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.preresume = snapshot_preresume,
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