dm exception store: fix misordered writes

We must zero the next chunk on disk *before* writing out the current chunk, not
after.  Otherwise if the machine crashes at the wrong time, the "end of metadata"
marker may be missing.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>
Cc: stable@kernel.org

drivers/md/dm-exception-store.c

@@ -610,18 +610,23 @@ static void persistent_commit(struct exception_store *store,
 	    (ps->current_committed != ps->exceptions_per_area))
 		return;
 
+	/*
+	 * If we completely filled the current area, then wipe the next one.
+	 */
+	if ((ps->current_committed == ps->exceptions_per_area) &&
+	     zero_disk_area(ps, ps->current_area + 1))
+		ps->valid = 0;
+
 	/*
 	 * Commit exceptions to disk.
 	 */
-	if (area_io(ps, WRITE))
+	if (ps->valid && area_io(ps, WRITE))
 		ps->valid = 0;
 
 	/*
 	 * Advance to the next area if this one is full.
 	 */
 	if (ps->current_committed == ps->exceptions_per_area) {
-		if (zero_disk_area(ps, ps->current_area + 1))
-			ps->valid = 0;
 		ps->current_committed = 0;
 		ps->current_area++;
 		zero_memory_area(ps);