md: do not compute parity unless it is on a failed drive

upstream commit: c337869d95011495fa181536786e74aa2d7ff031

If a block is computed (rather than read) then a check/repair operation
may be lead to believe that the data on disk is correct, when infact it
isn't.  So only compute blocks for failed devices.

This issue has been around since at least 2.6.12, but has become harder to
hit in recent kernels since most reads bypass the cache.

echo repair > /sys/block/mdN/md/sync_action will set the parity blocks to the
correct state.

Cc: <stable@kernel.org>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Neil Brown <neilb@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Chris Wright <chrisw@sous-sol.org>

diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c
index 18a1379ed04c59746d3855fc72666982082f8361..3bd71d9ce35247f4a74fae076c074a0fc8191d07 100644 (file)
--- a/drivers/md/raid5.c
+++ b/drivers/md/raid5.c
@@ -1984,6 +1984,7 @@ static int __handle_issuing_new_read_requests5(struct stripe_head *sh,
                 * have quiesced.
                 */
                if ((s->uptodate == disks - 1) &&
+                   (s->failed && disk_idx == s->failed_num) &&
                    !test_bit(STRIPE_OP_CHECK, &sh->ops.pending)) {
                        set_bit(STRIPE_OP_COMPUTE_BLK, &sh->ops.pending);
                        set_bit(R5_Wantcompute, &dev->flags);
@@ -2069,7 +2070,9 @@ static void handle_issuing_new_read_requests6(struct stripe_head *sh,
                        /* we would like to get this block, possibly
                         * by computing it, but we might not be able to
                         */
-                       if (s->uptodate == disks-1) {
+                       if ((s->uptodate == disks - 1) &&
+                           (s->failed && (i == r6s->failed_num[0] ||
+                                          i == r6s->failed_num[1]))) {
                                pr_debug("Computing stripe %llu block %d\n",
                                       (unsigned long long)sh->sector, i);
                                compute_block_1(sh, i, 0);