mount.snapshot: mount previous snapshot with upperdir

Instead of stacking 2 last snapshots as 2 lower layers, use current
snapshot as lower and previous layer as upper layer.
This is needed for previous snapshot to follow merge directory
into current snapshot through rename/delete in snapshot mount.

Old snapshots must be mounted after activation of new snapshot,
because the new "last" snapshot needs to use the "last" upper
and the upper holds the in-use exclusive lock bit until the new
snapshot is activated.

Signed-off-by: Amir Goldstein <[email protected]>

scripts/mount.snapshot

@@ -144,17 +144,23 @@ _cleanup()
 mount_old_snapshots()
 {
 	local old_snapshots=$snapmnt
+	local prev=""
 
 	# mount snapshots from newest to oldest
 	tac "$snapshots" | while read old; do
 		[ "$old" != "$id" ] || continue
 		oldsnap="@/$old"
 		oldmnt="$oldsnap/_"
 		[ -d "$oldsnap" ] || continue
-		old_snapshots="$oldsnap/u:$old_snapshots"
 		[ -d "$oldmnt" ] || continue
-		mount -i -t overlay "$dev@$old" "$oldmnt" \
+		[ -n "$prev" ] || \
+			mount -i -t overlay "$dev@$old" "$oldmnt" \
+			-o"ro,upperdir=$oldsnap/u,workdir=$oldsnap/w,lowerdir=$old_snapshots,$snapmntopts"
+		old_snapshots="$oldsnap/u:$old_snapshots"
+		[ -z "$prev" ] || \
+			mount -i -t overlay "$dev@$old" "$oldmnt" \
 			-o"ro,lowerdir=$old_snapshots,$oldsnapmntopts"
+		prev=$old
 	done
 }
 
@@ -177,8 +183,8 @@ mount -i -t snapshot "$dev@" "$mntdir" \
 # Remount current snapshot overlay read-only
 mount -o remount,ro "$snapmnt" || exit 1
 
-# Mount older snapshot overlays
-mount_old_snapshots
-
 # freeze/thaw to activate new snapshot
 freeze_thaw
+
+# Mount older snapshot overlays
+mount_old_snapshots