Merge target shouldn't allocate new exceptions:
- make new method snapshot_merge_map that don't allocate exceptions.
- modify __origin_write so that it doesn't allocate exceptions in merging
  snapshots.
- disallow creating merging snapshots with memory-only exception store

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>

---
 drivers/md/dm-snap.c |   51 ++++++++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 50 insertions(+), 1 deletion(-)

Index: linux-2.6.28-rc5-devel/drivers/md/dm-snap.c
===================================================================
--- linux-2.6.28-rc5-devel.orig/drivers/md/dm-snap.c	2008-11-25 16:11:20.000000000 +0100
+++ linux-2.6.28-rc5-devel/drivers/md/dm-snap.c	2008-11-25 16:11:23.000000000 +0100
@@ -46,6 +46,11 @@
  */
 #define MIN_IOS 256
 
+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_snap_exception e;
 
@@ -574,6 +579,12 @@ static int snapshot_ctr(struct dm_target
 		goto bad1;
 	}
 
+	if (persistent != 'P' && is_merge(ti)) {
+		ti->error = "Merging snapshot must be persistent";
+		r = -EINVAL;
+		goto bad1;
+	}
+
 	s = kmalloc(sizeof(*s), GFP_KERNEL);
 	if (s == NULL) {
 		ti->error = "Cannot allocate snapshot context private "
@@ -1111,6 +1122,40 @@ static int snapshot_map(struct dm_target
 	return r;
 }
 
+static int snapshot_merge_map(struct dm_target *ti, struct bio *bio,
+			       union map_info *map_context)
+{
+	struct dm_snap_exception *e;
+	struct dm_snapshot *s = ti->private;
+	int r = DM_MAPIO_REMAPPED;
+	chunk_t chunk;
+
+	chunk = sector_to_chunk(s, 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 = lookup_exception(&s->complete, chunk);
+	if (e) {
+		remap_exception(s, e, bio, chunk);
+		goto out_unlock;
+	}
+
+	bio->bi_bdev = s->origin->bdev;
+
+ 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)
 {
@@ -1208,6 +1253,10 @@ static int __origin_write(struct list_he
 	/* Do all the snapshots on this origin */
 	list_for_each_entry (snap, snapshots, list) {
 
+		/* If the snapshot is merging, don't make new exceptions in it
+		   - but in this case no one should be writing to the origin */
+		if (is_merge(snap->ti)) continue;
+
 		down_write(&snap->lock);
 
 		/* Only deal with valid and active snapshots */
@@ -1401,7 +1450,7 @@ static struct target_type snapshot_merge
 	.module  = THIS_MODULE,
 	.ctr     = snapshot_ctr,
 	.dtr     = snapshot_dtr,
-	.map     = snapshot_map,
+	.map     = snapshot_merge_map,
 	.end_io  = snapshot_end_io,
 	.resume  = snapshot_resume,
 	.status  = snapshot_status,