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    Subject[PATCH v4 06/13] dm snapshot: merge target should not allocate new exceptions
    Date
    From: Mikulas Patocka <mpatocka@redhat.com>

    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.

    Introduce new method, snapshot_merge_map(), that won't allocate
    exceptions. Modify __origin_write() so that it doesn't allocate
    exceptions in merging snapshots.

    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.

    Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
    Signed-off-by: Mike Snitzer <snitzer@redhat.com>
    ---
    drivers/md/dm-snap.c | 49 ++++++++++++++++++++++++++++++++++++++++++++++++-
    1 files changed, 48 insertions(+), 1 deletions(-)

    diff --git a/drivers/md/dm-snap.c b/drivers/md/dm-snap.c
    index e8a1107..b8838e7 100644
    --- a/drivers/md/dm-snap.c
    +++ b/drivers/md/dm-snap.c
    @@ -127,6 +127,11 @@ static int bdev_equal(struct block_device *lhs, struct block_device *rhs)
    return lhs == rhs;
    }

    +static int snapshot_merge_map(struct dm_target *ti, struct bio *bio,
    + union map_info *map_context);
    +
    +#define is_merge(ti) ((ti)->type->map == snapshot_merge_map)
    +
    struct dm_snap_pending_exception {
    struct dm_exception e;

    @@ -1292,6 +1297,44 @@ static int snapshot_map(struct dm_target *ti, struct bio *bio,
    return r;
    }

    +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_write(&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)
    {
    @@ -1442,6 +1485,10 @@ static int __origin_write(struct list_head *snapshots,
    /* Do all the snapshots on this origin */
    list_for_each_entry (snap, snapshots, list) {

    + /* Don't make new exceptions in a merging snapshot */
    + if (is_merge(snap->ti))
    + continue;
    +
    down_write(&snap->lock);

    /* Only deal with valid and active snapshots */
    @@ -1670,7 +1717,7 @@ 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,
    --
    1.6.5.2


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