Re: [PATCH 1/3] elevator: lookup mq vs non-mq elevators

[Omar Sandoval <osandov@osandov.com>]

On Wed, Oct 25, 2017 at 12:04:45PM -0600, Jens Axboe wrote:
> If an IO scheduler is selected via elevator= and it doesn't match
> the driver in question wrt blk-mq support, then we fail to boot.
> 
> The elevator= parameter is deprecated and only supported for
> non-mq devices. Augment the elevator lookup API so that we
> pass in if we're looking for an mq capable scheduler or not,
> so that we only ever return a valid type for the queue in
> question.
> 
> Fixes: https://bugzilla.kernel.org/show_bug.cgi?id=196695
> Signed-off-by: Jens Axboe <axboe@kernel.dk>
> ---
>  block/elevator.c | 44 +++++++++++++++++++++-----------------------
>  1 file changed, 21 insertions(+), 23 deletions(-)
> 
> diff --git a/block/elevator.c b/block/elevator.c
> index 7ae50eb2732b..b70e69f795db 100644
> --- a/block/elevator.c
> +++ b/block/elevator.c
> @@ -83,12 +83,15 @@ bool elv_bio_merge_ok(struct request *rq, struct bio *bio)
>  }
>  EXPORT_SYMBOL(elv_bio_merge_ok);
>  
> -static struct elevator_type *elevator_find(const char *name)
> +/*
> + * Return scheduler with name 'name' and with matching 'mq capability
> + */
> +static struct elevator_type *elevator_find(const char *name, bool mq)
>  {
>  	struct elevator_type *e;
>  
>  	list_for_each_entry(e, &elv_list, list) {
> -		if (!strcmp(e->elevator_name, name))
> +		if (!strcmp(e->elevator_name, name) && !(mq ^ e->uses_mq))


How about just (mq == e->uses_mq)?

Besides that, Reviewed-by: Omar Sandoval <osandov@fb.com>

>  			return e;
>  	}
>