Re: [PATCH V2] block: Fix elv_support_iosched()

[Bob Liu <bob.liu@oracle.com>]

On 10/9/19 6:39 AM, 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 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>
> Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>
> ---
> Changes from V1:
> * Test if q->mq_ops is NULL to identify BIO based queues
> 
>  block/elevator.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/block/elevator.c b/block/elevator.c
> index 5437059c9261..076ba7308e65 100644
> --- a/block/elevator.c
> +++ b/block/elevator.c
> @@ -616,7 +616,8 @@ 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->mq_ops ||
> +	    (q->tag_set && (q->tag_set->flags & BLK_MQ_F_NO_SCHED)))
>  		return false;
>  	return true;
>  }
> 

Looks good to me.
Reviewed-by: Bob Liu <bob.liu@oracle.com>