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

1 files changed, 8 insertions, 3 deletions

diff --git a/drivers/md/dm-exception-store.c b/drivers/md/dm-exception-store.c
index fe6cef8df203..6179bf70f98a 100644
--- a/drivers/md/dm-exception-store.c
+++ b/drivers/md/dm-exception-store.c
@@ -611,17 +611,22 @@ static void persistent_commit(struct exception_store *store,
                 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);