Re: [PATCH] block: Do not merge if merging is disabled

[Ming Lei <ming.lei@redhat.com>]

On Thu, Jul 06, 2023 at 01:14:33PM -0700, Bart Van Assche wrote:
> While testing the performance impact of zoned write pipelining, I
> noticed that merging happens even if merging has been disabled via
> sysfs. Fix this.
> 
> Cc: Christoph Hellwig <hch@lst.de>
> Cc: Ming Lei <ming.lei@redhat.com>
> Cc: Damien Le Moal <dlemoal@kernel.org>
> Signed-off-by: Bart Van Assche <bvanassche@acm.org>
> ---
>  block/blk-mq-sched.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/block/blk-mq-sched.c b/block/blk-mq-sched.c
> index 67c95f31b15b..8883721f419a 100644
> --- a/block/blk-mq-sched.c
> +++ b/block/blk-mq-sched.c
> @@ -375,7 +375,8 @@ bool blk_mq_sched_bio_merge(struct request_queue *q, struct bio *bio,
>  bool blk_mq_sched_try_insert_merge(struct request_queue *q, struct request *rq,
>  				   struct list_head *free)
>  {
> -	return rq_mergeable(rq) && elv_attempt_insert_merge(q, rq, free);
> +	return !blk_queue_nomerges(q) && rq_mergeable(rq) &&
> +		elv_attempt_insert_merge(q, rq, free);
>  }
>  EXPORT_SYMBOL_GPL(blk_mq_sched_try_insert_merge);

elv_attempt_insert_merge() does check blk_queue_nomerges() at its entry,
so this patch fix nothing.

Given blk_mq_sched_try_insert_merge is only called from bfq and
deadline, it may not matter to apply this optimization.

Thanks,
Ming