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Date:   Mon, 17 Aug 2020 08:31:53 +0200
From:   Christoph Hellwig <hch@....de>
To:     Baolin Wang <baolin.wang@...ux.alibaba.com>
Cc:     axboe@...nel.dk, ming.lei@...hat.com, hch@....de,
        baolin.wang7@...il.com, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH RESEND 4/5] block: Remove blk_mq_attempt_merge()
 function

On Mon, Aug 17, 2020 at 12:09:18PM +0800, Baolin Wang wrote:
>  		unsigned int nr_segs)
>  {
> @@ -447,7 +425,16 @@ bool __blk_mq_sched_bio_merge(struct request_queue *q, struct bio *bio,
>  			!list_empty_careful(&ctx->rq_lists[type])) {
>  		/* default per sw-queue merge */
>  		spin_lock(&ctx->lock);
> -		ret = blk_mq_attempt_merge(q, hctx, ctx, bio, nr_segs);
> +		/*
> +		 * Reverse check our software queue for entries that we could
> +		 * potentially merge with. Currently includes a hand-wavy stop
> +		 * count of 8, to not spend too much time checking for merges.
> +		 */
> +		if (blk_mq_bio_list_merge(q, &ctx->rq_lists[type], bio, nr_segs)) {
> +			ctx->rq_merged++;
> +			ret = true;
> +		}
> +
>  		spin_unlock(&ctx->lock);

This adds an overly long line.  That being said the whole thing could
be nicely simplified to:

	...

	if (e && e->type->ops.bio_merge)
		return e->type->ops.bio_merge(hctx, bio, nr_segs);

	if (!(hctx->flags & BLK_MQ_F_SHOULD_MERGE) ||
	    list_empty_careful(&ctx->rq_lists[hctx->type]))
		return false;

	/*
	 * Reverse check our software queue for entries that we could
	 * potentially merge with. Currently includes a hand-wavy stop count of
	 * 8, to not spend too much time checking for merges.
	 */
	spin_lock(&ctx->lock);
	ret = blk_mq_bio_list_merge(q, &ctx->rq_lists[type], bio, nr_segs);
	if (ret)
		ctx->rq_merged++;
	spin_unlock(&ctx->lock);

Also I think it would make sense to move the locking into
blk_mq_bio_list_merge.

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