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Date:   Tue, 22 Jun 2021 08:53:09 +0200
From:   Christoph Hellwig <hch@lst.de>
To:     Ming Lei <ming.lei@redhat.com>
Cc:     Jens Axboe <axboe@kernel.dk>, Christoph Hellwig <hch@lst.de>,
        linux-block@vger.kernel.org,
        Wang Shanker <shankerwangmiao@gmail.com>
Subject: Re: [PATCH 1/2] block: fix discard request merge
Message-ID: <20210622065309.GA29691@lst.de>
References: <20210609004556.46928-1-ming.lei@redhat.com> <20210609004556.46928-2-ming.lei@redhat.com>
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On Wed, Jun 09, 2021 at 08:45:55AM +0800, Ming Lei wrote:
> index 4d97fb6dd226..bcdff1879c34 100644
> --- a/block/blk-merge.c
> +++ b/block/blk-merge.c
> @@ -559,10 +559,14 @@ static inline unsigned int blk_rq_get_max_segments(struct request *rq)
>  static inline int ll_new_hw_segment(struct request *req, struct bio *bio,
>  		unsigned int nr_phys_segs)
>  {
> -	if (req->nr_phys_segments + nr_phys_segs > blk_rq_get_max_segments(req))
> +	if (blk_integrity_merge_bio(req->q, req, bio) == false)
>  		goto no_merge;
>  
> -	if (blk_integrity_merge_bio(req->q, req, bio) == false)
> +	/* discard request merge won't add new segment */
> +	if (req_op(req) == REQ_OP_DISCARD)
> +		return 1;
> +
> +	if (req->nr_phys_segments + nr_phys_segs > blk_rq_get_max_segments(req))

I'd rather handle this by returning UINT_MAX for discard requests from
blk_rq_get_max_segments given that bio_attempt_discard_merge is only used
for the !DISCARD_MERGE case anyway.