dm log: fix resume failed log device

This patch removes the possibility of having uninitialized log state if the
log device has failed.

When a mirror resumes operation, it calls 'resume' on the logging module.  If
disk based logging is being used, the log device is read to fill in the log
state.  If the log device has failed, we cannot simply return, because this
would leave the in-memory log state uninitialized.  Instead, we assume all
regions are out-of-sync and reset the log state.  Failure to do this could
result in the logging code reporting a region as in-sync, even though it
isn't; which could result in a corrupted mirror.

Signed-off-by: Jonathan Brassow <jbrassow@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/md/dm-log.c

@@ -478,7 +478,14 @@ static int disk_resume(struct dirty_log *log)
 		DMWARN("%s: Failed to read header on mirror log device",
 		       lc->log_dev->name);
 		fail_log_device(lc);
-		return r;
+		/*
+		 * If the log device cannot be read, we must assume
+		 * all regions are out-of-sync.  If we simply return
+		 * here, the state will be uninitialized and could
+		 * lead us to return 'in-sync' status for regions
+		 * that are actually 'out-of-sync'.
+		 */
+		lc->header.nr_regions = 0;
 	}
 
 	/* set or clear any new bits -- device has grown */