block: Fix elv_support_iosched()

A BIO based request queue does not have a tag_set, which prevent testing
for the flag BLK_MQ_F_NO_SCHED indicating that the queue does not
require an elevator. This leads to an incorrect initialization of a
default elevator in some cases such as BIO based null_blk
(queue_mode == BIO) with zoned mode enabled as the default elevator in
this case is mq-deadline instead of "none".

Fix this by testing for a NULL queue mq_ops field which indicates that
the queue is BIO based and should not have an elevator.

Reported-by: Shinichiro Kawasaki <shinichiro.kawasaki@wdc.com>
Reviewed-by: Bob Liu <bob.liu@oracle.com>
Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/block/elevator.c b/block/elevator.c
index 5437059..076ba73 100644 (file)
--- a/block/elevator.c
+++ b/block/elevator.c
@@ -616,7 +616,8 @@ out:
 
 static inline bool elv_support_iosched(struct request_queue *q)
 {
-       if (q->tag_set && (q->tag_set->flags & BLK_MQ_F_NO_SCHED))
+       if (!q->mq_ops ||
+           (q->tag_set && (q->tag_set->flags & BLK_MQ_F_NO_SCHED)))
                return false;
        return true;
 }