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

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>

diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c
index f0f0585c107ef4d79315b09efd171946c0201b70..c37e256b117603f8f68aed3de3a40db9328e70f6 100644 (file)
--- a/drivers/md/raid5.c
+++ b/drivers/md/raid5.c
@@ -2002,6 +2002,7 @@ 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);
@@ -2087,7 +2088,9 @@ 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);