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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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