dm: disable DISCARD if the underlying storage no longer supports it [Linux 3.16.72]

This Linux kernel change "dm: disable DISCARD if the underlying storage no longer supports it" is included in the Linux 3.16.72 release. This change is authored by Mike Snitzer <snitzer [at]> on Wed Apr 3 12:23:11 2019 -0400. The commit for this change in Linux stable tree is 33a7884 (patch) which is from upstream commit bcb4443. The same Linux upstream change may have been applied to various maintained Linux releases and you can find all Linux releases containing changes from upstream bcb4443.

dm: disable DISCARD if the underlying storage no longer supports it

commit bcb44433bba5eaff293888ef22ffa07f1f0347d6 upstream.

Storage devices which report supporting discard commands like
WRITE_SAME_16 with unmap, but reject discard commands sent to the
storage device.  This is a clear storage firmware bug but it doesn't
change the fact that should a program cause discards to be sent to a
multipath device layered on this buggy storage, all paths can end up
failed at the same time from the discards, causing possible I/O loss.

The first discard to a path will fail with Illegal Request, Invalid
field in cdb, e.g.:
 kernel: sd 8:0:8:19: [sdfn] tag#0 FAILED Result: hostbyte=DID_OK driverbyte=DRIVER_SENSE
 kernel: sd 8:0:8:19: [sdfn] tag#0 Sense Key : Illegal Request [current]
 kernel: sd 8:0:8:19: [sdfn] tag#0 Add. Sense: Invalid field in cdb
 kernel: sd 8:0:8:19: [sdfn] tag#0 CDB: Write same(16) 93 08 00 00 00 00 00 a0 08 00 00 00 80 00 00 00
 kernel: blk_update_request: critical target error, dev sdfn, sector 10487808

The SCSI layer converts this to the BLK_STS_TARGET error number, the sd
device disables its support for discard on this path, and because of the
BLK_STS_TARGET error multipath fails the discard without failing any
path or retrying down a different path.  But subsequent discards can
cause path failures.  Any discards sent to the path which already failed
a discard ends up failing with EIO from blk_cloned_rq_check_limits with
an "over max size limit" error since the discard limit was set to 0 by
the sd driver for the path.  As the error is EIO, this now fails the
path and multipath tries to send the discard down the next path.  This
cycle continues as discards are sent until all paths fail.

Fix this by training DM core to disable DISCARD if the underlying
storage already did so.

Also, fix branching in dm_done() and clone_endio() to reflect the
mutually exclussive nature of the IO operations in question.

Reported-by: David Jeffery <>
Signed-off-by: Mike Snitzer <>
[bwh: Backported to 3.16:
 - Keep using op & flag to check operation type
 - Keep using bdev_get_queue() to find queue in clone_endio()
 - WRITE_ZEROES is not handled
 - Use queue_flag_clear() instead of blk_queue_flag_clear()
 - Adjust filenames, context
 - Declare disable_discard() static as its only user is in the same
   source file]
Signed-off-by: Ben Hutchings <>

There are 31 lines of Linux source code added/deleted in this change. Code changes to Linux kernel are as follows.

 drivers/md/dm.c | 31 +++++++++++++++++++++++++------
 1 file changed, 25 insertions(+), 6 deletions(-)

diff --git a/drivers/md/dm.c b/drivers/md/dm.c
index 043ad8d..30ae713 100644
--- a/drivers/md/dm.c
+++ b/drivers/md/dm.c
@@ -756,6 +756,15 @@ static void dec_pending(struct dm_io *io, int error)

+static void disable_discard(struct mapped_device *md)
+   struct queue_limits *limits = dm_get_queue_limits(md);
+   /* device doesn't really support DISCARD, disable it */
+   limits->max_discard_sectors = 0;
+   queue_flag_clear(QUEUE_FLAG_DISCARD, md->queue);
 static void disable_write_same(struct mapped_device *md)
    struct queue_limits *limits = dm_get_queue_limits(md);
@@ -792,9 +801,14 @@ static void clone_endio(struct bio *bio, int error)

-   if (unlikely(r == -EREMOTEIO && (bio->bi_rw & REQ_WRITE_SAME) &&
-            !bdev_get_queue(bio->bi_bdev)->limits.max_write_same_sectors))
-       disable_write_same(md);
+   if (unlikely(r == -EREMOTEIO)) {
+       if (bio->bi_rw & REQ_DISCARD &&
+           !bdev_get_queue(bio->bi_bdev)->limits.max_discard_sectors)
+           disable_discard(md);
+       else if (bio->bi_rw & REQ_WRITE_SAME &&
+            !bdev_get_queue(bio->bi_bdev)->limits.max_write_same_sectors)
+           disable_write_same(md);
+   }

    free_tio(md, tio);
    dec_pending(io, error);
@@ -996,9 +1010,14 @@ static void dm_done(struct request *clone, int error, bool mapped)
            r = rq_end_io(tio->ti, clone, error, &tio->info);

-   if (unlikely(r == -EREMOTEIO && (clone->cmd_flags & REQ_WRITE_SAME) &&
-            !clone->q->limits.max_write_same_sectors))
-       disable_write_same(tio->md);
+   if (unlikely(r == -EREMOTEIO)) {
+       if (clone->cmd_flags & REQ_DISCARD &&
+           !clone->q->limits.max_discard_sectors)
+           disable_discard(tio->md);
+       else if (clone->cmd_flags & REQ_WRITE_SAME &&
+            !clone->q->limits.max_write_same_sectors)
+           disable_write_same(tio->md);
+   }

    if (r <= 0)
        /* The target wants to complete the I/O */

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