[PATCH] scsi: st: Save the visited block for the partition the drive is in

[Laurence Oberman <loberman@redhat.com>]

set_location() saves the current tape location before switching
partitions, so that switch_partition() can return to it later. It reads
the location with READ POSITION but stores it in the st_partstat of
STp->partition, ignoring the partition the drive reports.

The two differ after a device reset. When the reset is recovered with
MTREW, MTSEEK or MTEOM, st_ioctl() sets STp->partition = 0 and calls
switch_partition() to return to STp->new_partition. A drive that was in
partition 1 is still there, so set_location() files the partition 1
position under partition 0. The next switch to partition 0 locates to
that block: if partition 0 holds less data, MTSETPART fails with EIO
(BLANK CHECK), otherwise the tape is silently positioned at the wrong
block.

IBM ULTRIUM-TD5 (LTO-5), LU reset in partition 1 at block 1025:

  st 12:0:0:0: [st0] Visited block 1025 for partition 0 saved.
  ...
  st 12:0:0:0: [st0] Setting block to 1025 and partition to 0.
  st 12:0:0:0: [st0] Error: 402, cmd: 2b 2 0 0 0 4
  st 12:0:0:0: [st0] Sense Key : Blank Check [current]
  st 12:0:0:0: [st0] Add. Sense: End-of-data detected

Store the location in the st_partstat of the partition reported by READ
POSITION. With the fix:

  st 12:0:0:0: [st0] Visited block 1025 for partition 1 saved.

This is a pre-existing issue. It was not caught when testing the
recent st reset series because the partition test was only run on
scsi_debug: on the LTO drive it was skipped, since can-partitions is
off in the test harness's stinit.conf, as is normal for LTO. scsi_debug
hides the problem because its emulated tape returns to the beginning of
partition 0 on an LU reset. It was found while validating a backport
of that series to RHEL 9.8, when the partition tests were run on real
hardware with can-partitions enabled. The same failure then reproduced
on unmodified RHEL 9.8 and on upstream, and the fix makes the test pass
on v7.3-rc3 and RHEL 9.8 with an LTO-5. 

The harness (https://gitlab.com/loberman/tapetest, test X03) now
refuses to silently skip the partition tests on hardware.

Fixes: 7081dc75df79 ("scsi: st: Restore some drive settings after reset")
Assisted-by: Claude
Signed-off-by: Laurence Oberman <loberman@redhat.com>
---
 drivers/scsi/st.c | 14 ++++++++++----
 1 file changed, 10 insertions(+), 4 deletions(-)

diff --git a/drivers/scsi/st.c b/drivers/scsi/st.c
index e089d3ac063d..63ec9619d1b2 100644
--- a/drivers/scsi/st.c
+++ b/drivers/scsi/st.c
@@ -3246,14 +3246,20 @@ static int set_location(struct scsi_tape *STp, unsigned int block, int partition
 	    partition >= ST_NBR_PARTITIONS)
 		return (-EINVAL);
 	if (partition != STp->partition) {
-		if (get_location(STp, &blk, &p, 1))
+		/*
+		 * Save the location for the partition the drive reports,
+		 * not for STp->partition: after a reset STp->partition may
+		 * not match the partition the drive is in.
+		 */
+		if (get_location(STp, &blk, &p, 1) ||
+		    p < 0 || p >= ST_NBR_PARTITIONS)
 			STps->last_block_valid = 0;
 		else {
-			STps->last_block_valid = 1;
-			STps->last_block_visited = blk;
+			STp->ps[p].last_block_valid = 1;
+			STp->ps[p].last_block_visited = blk;
 			DEBC_printk(STp, "Visited block %d for "
 				    "partition %d saved.\n",
-				    blk, STp->partition);
+				    blk, p);
 		}
 	}
 

base-commit: 9eaf6f9c2e50f67b7a3af1ba170a7da98201fbd5
prerequisite-patch-id: 7ff6c2a8facfef94f8d9e55d289671fc752572ec
prerequisite-patch-id: 8fc95d01ad41ddd3d884090c98b70d210ceb6b51
prerequisite-patch-id: 4fcffbe676c887709189baeb202ad2b5b58094ee
prerequisite-patch-id: 7f269b2366e79524769bca811886375d8a3e83f8
-- 
2.55.0