[PATCH 002 of 4] md: Fix newly introduced read-corruption with raid6.

[NeilBrown]

chunk_aligned_read and retry_aligned_read assume that
    data_disks == raid_disks - 1
which is not true for raid6.
So when an aligned read request bypasses the cache, we can get the wrong data.

Also change the calculate of raid_disks in compute_blocknr to make it
more obviously correct.

Signed-off-by: Neil Brown <neilb@xxxxxxx>

### Diffstat output
 ./drivers/md/raid5.c |    8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff .prev/drivers/md/raid5.c ./drivers/md/raid5.c
--- .prev/drivers/md/raid5.c	2006-11-14 10:33:41.000000000 +1100
+++ ./drivers/md/raid5.c	2006-11-14 10:34:17.000000000 +1100
@@ -823,7 +823,8 @@ static sector_t raid5_compute_sector(sec
 static sector_t compute_blocknr(struct stripe_head *sh, int i)
 {
 	raid5_conf_t *conf = sh->raid_conf;
-	int raid_disks = sh->disks, data_disks = raid_disks - 1;
+	int raid_disks = sh->disks;
+	int data_disks = raid_disks - conf->max_degraded;
 	sector_t new_sector = sh->sector, check;
 	int sectors_per_chunk = conf->chunk_size >> 9;
 	sector_t stripe;
@@ -859,7 +860,6 @@ static sector_t compute_blocknr(struct s
 		}
 		break;
 	case 6:
-		data_disks = raid_disks - 2;
 		if (i == raid6_next_disk(sh->pd_idx, raid_disks))
 			return 0; /* It is the Q disk */
 		switch (conf->algorithm) {
@@ -2725,7 +2725,7 @@ static int chunk_aligned_read(request_qu
 	mddev_t *mddev = q->queuedata;
 	raid5_conf_t *conf = mddev_to_conf(mddev);
 	const unsigned int raid_disks = conf->raid_disks;
-	const unsigned int data_disks = raid_disks - 1;
+	const unsigned int data_disks = raid_disks - conf->max_degraded;
 	unsigned int dd_idx, pd_idx;
 	struct bio* align_bi;
 	mdk_rdev_t *rdev;
@@ -3145,7 +3145,7 @@ static int  retry_aligned_read(raid5_con
 	logical_sector = raid_bio->bi_sector & ~((sector_t)STRIPE_SECTORS-1);
 	sector = raid5_compute_sector(	logical_sector,
 					conf->raid_disks,
-					conf->raid_disks-1,
+					conf->raid_disks - conf->max_degraded,
 					&dd_idx,
 					&pd_idx,
 					conf);
-
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