dm snapshot: permit invalid activation

Allow invalid snapshots to be activated instead of failing.

This allows userspace to reinstate any given snapshot state - for
example after an unscheduled reboot - and clean up the invalid snapshot
at its leisure.

Cc: stable@kernel.org
Signed-off-by: Milan Broz <mbroz@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/md/dm-exception-store.c

diff --git a/drivers/md/dm-exception-store.c b/drivers/md/dm-exception-store.c
index 8c25c2f..3d65917 100644
--- a/drivers/md/dm-exception-store.c
+++ b/drivers/md/dm-exception-store.c
@@ -489,11 +489,6 @@
 		/*
 		 * Sanity checks.
 		 */
-		if (!ps->valid) {
-			DMWARN("snapshot is marked invalid");
-			return -EINVAL;
-		}
-
 		if (ps->version != SNAPSHOT_DISK_VERSION) {
 			DMWARN("unable to handle snapshot disk version %d",
 			       ps->version);
@@ -501,6 +496,12 @@
 		}
 
 		/*
+		 * Metadata are valid, but snapshot is invalidated
+		 */
+		if (!ps->valid)
+			return 1;
+
+		/*
 		 * Read the metadata.
 		 */
 		r = read_exceptions(ps);

drivers/md/dm-snap.c

diff --git a/drivers/md/dm-snap.c b/drivers/md/dm-snap.c
index 1da4122..854891d 100644
--- a/drivers/md/dm-snap.c
+++ b/drivers/md/dm-snap.c
@@ -523,9 +523,12 @@
 
 	/* Metadata must only be loaded into one table at once */
 	r = s->store.read_metadata(&s->store);
-	if (r) {
+	if (r < 0) {
 		ti->error = "Failed to read snapshot metadata";
 		goto bad6;
+	} else if (r > 0) {
+		s->valid = 0;
+		DMWARN("Snapshot is marked invalid.");
 	}
 
 	bio_list_init(&s->queued_bios);