1 files changed, 20 insertions, 1 deletions

diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c
index 2b521ee67dfa..b8a22a2205cf 100644
--- a/drivers/md/raid5.c
+++ b/drivers/md/raid5.c
@@ -4509,7 +4509,26 @@ static int run(mddev_t *mddev)
                            (old_disks-max_degraded));
                 /* here_old is the first stripe that we might need to read
                  * from */
-                if (here_new >= here_old) {
+                if (mddev->delta_disks == 0) {
+                        /* We cannot be sure it is safe to start an in-place
+                         * reshape.  It is only safe if user-space if monitoring
+                         * and taking constant backups.
+                         * mdadm always starts a situation like this in
+                         * readonly mode so it can take control before
+                         * allowing any writes.  So just check for that.
+                         */
+                        if ((here_new * mddev->new_chunk_sectors != 
+                             here_old * mddev->chunk_sectors) ||
+                            mddev->ro == 0) {
+                                printk(KERN_ERR "raid5: in-place reshape must be started"
+                                       " in read-only mode - aborting\n");
+                                return -EINVAL;
+                        }
+                } else if (mddev->delta_disks < 0
+                    ? (here_new * mddev->new_chunk_sectors <=
+                       here_old * mddev->chunk_sectors)
+                    : (here_new * mddev->new_chunk_sectors >=
+                       here_old * mddev->chunk_sectors)) {
                         /* Reading from the same stripe as writing to - bad */
                         printk(KERN_ERR "raid5: reshape_position too early for "
                                "auto-recovery - aborting.\n");