[PATCH] block: Fix elv_support_iosched()

[PATCH] block: Fix elv_support_iosched()

[Damien Le Moal]

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 nullblk (queue_mode ==
BIO) with zoned mode enabled as the default elevator in this case is
mq-deadline instead of "none".

Fix this by including the absence of a tag_set for a queue as an
indicator that the queue should not have an elevator.

Reported-by: Shinichiro Kawasaki <shinichiro.kawasaki@wdc.com>
Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>
---
 block/elevator.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/block/elevator.c b/block/elevator.c
index 5437059c9261..995d1e67056c 100644
--- a/block/elevator.c
+++ b/block/elevator.c
@@ -616,7 +616,7 @@ int elevator_switch_mq(struct request_queue *q,
 
 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->tag_set || (q->tag_set->flags & BLK_MQ_F_NO_SCHED))
 		return false;
 	return true;
 }
-- 
2.21.0

Re: [PATCH] block: Fix elv_support_iosched()

[Jens Axboe]

On 10/7/19 4:12 PM, Damien Le Moal wrote:
> 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 nullblk (queue_mode ==
> BIO) with zoned mode enabled as the default elevator in this case is
> mq-deadline instead of "none".
> 
> Fix this by including the absence of a tag_set for a queue as an
> indicator that the queue should not have an elevator.

Why not just check for mq_ops?

-- 
Jens Axboe