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Date: Fri, 22 May 2026 11:03:40 +0200
From: Christoph Hellwig <hch@lst.de>
To: Keith Busch <kbusch@meta.com>
Cc: axboe@kernel.dk, hch@lst.de, linux-block@vger.kernel.org,
	tom.leiming@gmail.com, Keith Busch <kbusch@kernel.org>
Subject: Re: [PATCHv3] blk-mq: pop cached request if it is usable
Message-ID: <20260522090340.GA7331@lst.de>
References: <20260521190253.242065-1-kbusch@meta.com>
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User-Agent: Mutt/1.5.17 (2007-11-01)

> +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.