md/raid10: fix the 'new' raid10 layout to work correctly.

In Linux 3.9 we introduce a new 'far' layout for RAID10 which was
supposed to rotate the replicas differently and so provide better
resilience.  In particular it could survive more combinations of 2
drive failures.

Unfortunately. due to a coding error, this some did what was wanted,
sometimes improved less than we hoped, and sometimes - in very
unlikely circumstances - put multiple replicas on the same device so
the redundancy was harmed.

No public user-space tool has created arrays using this layout so it
is very unlikely that zero-redundancy arrays actually exist.  Probably
no arrays using any form of the new layout exist.  But we cannot be
certain.

So use another bit in the 'layout' number and introduce a bug-fixed
version of the layout.
Also when assembling an array, if it has a zero-redundancy layout,
give a warning.

Reported-by: Heinz Mauelshagen <heinzm@redhat.com>
Signed-off-by: NeilBrown <neilb@suse.com>

1 files changed, 20 insertions, 2 deletions

diff --git a/drivers/md/raid10.c b/drivers/md/raid10.c
index 23de2144ee13..96f365968306 100644
--- a/drivers/md/raid10.c
+++ b/drivers/md/raid10.c
@@ -39,6 +39,7 @@
  *    far_copies (stored in second byte of layout)
  *    far_offset (stored in bit 16 of layout )
  *    use_far_sets (stored in bit 17 of layout )
+ *    use_far_sets_bugfixed (stored in bit 18 of layout )
  *
  * The data to be stored is divided into chunks using chunksize.  Each device
  * is divided into far_copies sections.   In each section, chunks are laid out
@@ -1497,6 +1498,8 @@ static void status(struct seq_file *seq, struct mddev *mddev)
                         seq_printf(seq, " %d offset-copies", conf->geo.far_copies);
                 else
                         seq_printf(seq, " %d far-copies", conf->geo.far_copies);
+                if (conf->geo.far_set_size != conf->geo.raid_disks)
+                        seq_printf(seq, " %d devices per set", conf->geo.far_set_size);
         }
         seq_printf(seq, " [%d/%d] [", conf->geo.raid_disks,
                                         conf->geo.raid_disks - mddev->degraded);
@@ -3394,7 +3397,7 @@ static int setup_geo(struct geom *geo, struct mddev *mddev, enum geo_type new)
                 disks = mddev->raid_disks + mddev->delta_disks;
                 break;
         }
-        if (layout >> 18)
+        if (layout >> 19)
                 return -1;
         if (chunk < (PAGE_SIZE >> 9) ||
             !is_power_of_2(chunk))
@@ -3406,7 +3409,22 @@ static int setup_geo(struct geom *geo, struct mddev *mddev, enum geo_type new)
         geo->near_copies = nc;
         geo->far_copies = fc;
         geo->far_offset = fo;
-        geo->far_set_size = (layout & (1<<17)) ? disks / fc : disks;
+        switch (layout >> 17) {
+        case 0: /* original layout.  simple but not always optimal */
+                geo->far_set_size = disks;
+                break;
+        case 1: /* "improved" layout which was buggy.  Hopefully no-one is
+                 * actually using this, but leave code here just in case.*/
+                geo->far_set_size = disks/fc;
+                WARN(geo->far_set_size < fc,
+                     "This RAID10 layout does not provide data safety - please backup and create new array\n");
+                break;
+        case 2: /* "improved" layout fixed to match documentation */
+                geo->far_set_size = fc * nc;
+                break;
+        default: /* Not a valid layout */
+                return -1;
+        }
         geo->chunk_mask = chunk - 1;
         geo->chunk_shift = ffz(~chunk);
         return nc*fc;