Re: [PATCHv3] blk-mq: pop cached request if it is usable

[Christoph Hellwig <hch@lst.de>]

> +static struct request *blk_mq_get_cached_request(struct blk_plug *plug,
>  		struct request_queue *q, blk_opf_t opf)
>  {
>  	enum hctx_type type = blk_mq_get_hctx_type(opf);
> @@ -3093,27 +3093,10 @@ static struct request *blk_mq_peek_cached_request(struct blk_plug *plug,
>  		return NULL;
>  	if (op_is_flush(rq->cmd_flags) != op_is_flush(opf))
>  		return NULL;
> +	rq_list_pop(&plug->cached_rqs);
>  	return rq;
>  }

Please add a comment about not sleeping between the peek and pop as
in my earlier patch here.

> @@ -3257,12 +3243,10 @@ void blk_mq_submit_bio(struct bio *bio)
>  	return;
>  
>  queue_exit:
> -	/*
> -	 * Don't drop the queue reference if we were trying to use a cached
> -	 * request and thus didn't acquire one.
> -	 */
>  	if (!rq)
>  		blk_queue_exit(q);
> +	else
> +		blk_mq_free_request(rq);
>  }

I think keeping a comment here would be nice, a would be avoid
the inversion of the condition for a trivial if/else.

But on a higher level, I really think you should add it back to the
batch list here, otherwise we're doing lots of roundtrips through
blk_mq_free_request for trivially mergable sequential I/O.