From: Mikulas Patocka <[email-masked]>

To support the merging of snapshots back into their origin we need
to trigger exceptions in other snapshots not being merged without
any incoming bio on the origin device.  The bio parameter to
__origin_write() becomes optional and the sector needs supplying
separately.

Signed-off-by: Mikulas Patocka <[email-masked]>
Signed-off-by: Mike Snitzer <[email-masked]>
Signed-off-by: Alasdair G Kergon <[email-masked]>

---
 drivers/md/dm-snap.c |   23 ++++++++++++++++++-----
 1 file changed, 18 insertions(+), 5 deletions(-)

Index: linux/drivers/md/dm-snap.c
===================================================================
--- linux.orig/drivers/md/dm-snap.c
+++ linux/drivers/md/dm-snap.c
@@ -1467,7 +1467,19 @@ static int snapshot_iterate_devices(stru
 /*-----------------------------------------------------------------
  * Origin methods
  *---------------------------------------------------------------*/
-static int __origin_write(struct list_head *snapshots, struct bio *bio)
+
+/*
+ * If no exceptions need creating, DM_MAPIO_REMAPPED is returned and any
+ * supplied bio was ignored.  The caller may submit it immediately.
+ * (No remapping actually occurs as the origin is always a direct linear
+ * map.)
+ *
+ * If further exceptions are required, DM_MAPIO_SUBMITTED is returned
+ * and any supplied bio is added to a list to be submitted once all
+ * the necessary exceptions exist.
+ */
+static int __origin_write(struct list_head *snapshots, sector_t sector,
+			  struct bio *bio)
 {
 	int r = DM_MAPIO_REMAPPED, first = 0;
 	struct dm_snapshot *snap;
@@ -1486,14 +1498,14 @@ static int __origin_write(struct list_he
 			goto next_snapshot;
 
 		/* Nothing to do if writing beyond end of snapshot */
-		if (bio->bi_sector >= dm_table_get_size(snap->ti->table))
+		if (sector >= dm_table_get_size(snap->ti->table))
 			goto next_snapshot;
 
 		/*
 		 * Remember, different snapshots can have
 		 * different chunk sizes.
 		 */
-		chunk = sector_to_chunk(snap->store, bio->bi_sector);
+		chunk = sector_to_chunk(snap->store, sector);
 
 		/*
 		 * Check exception table to see if block
@@ -1543,7 +1555,8 @@ static int __origin_write(struct list_he
 				first = 1;
 			}
 
-			bio_list_add(&primary_pe->origin_bios, bio);
+			if (bio)
+				bio_list_add(&primary_pe->origin_bios, bio);
 
 			r = DM_MAPIO_SUBMITTED;
 		}
@@ -1599,7 +1612,7 @@ static int do_origin(struct dm_dev *orig
 	down_read(&_origins_lock);
 	o = __lookup_origin(origin->bdev);
 	if (o)
-		r = __origin_write(&o->snapshots, bio);
+		r = __origin_write(&o->snapshots, bio->bi_sector, bio);
 	up_read(&_origins_lock);
 
 	return r;