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Subject: Re: [PATCH V4] block: fix the DISCARD request merge
To:     Jianchao Wang <jianchao.w.wang@oracle.com>
Cc:     martin.petersen@oracle.com, tom.leiming@gmail.com, hch@lst.de,
        linux-block@vger.kernel.org, linux-kernel@vger.kernel.org
References: <1540546116-1620-1-git-send-email-jianchao.w.wang@oracle.com>
From:   Jens Axboe <axboe@kernel.dk>
Message-ID: <4ee51782-acbb-bcc9-6692-39e9b8df477f@kernel.dk>
Date:   Fri, 26 Oct 2018 08:25:56 -0600
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On 10/26/18 3:28 AM, Jianchao Wang wrote:
> There are two cases when handle DISCARD merge.
> If max_discard_segments == 1, the bios/requests need to be contiguous
> to merge. If max_discard_segments > 1, it takes every bio as a range
> and different range needn't to be contiguous.
> 
> But now, attempt_merge screws this up. It always consider contiguity
> for DISCARD for the case max_discard_segments > 1 and cannot merge
> contiguous DISCARD for the case max_discard_segments == 1, because
> rq_attempt_discard_merge always returns false in this case.
> This patch fixes both of the two cases above.

Looks good to me, but:

> diff --git a/block/blk-merge.c b/block/blk-merge.c
> index 42a4674..cf817c7a 100644
> --- a/block/blk-merge.c
> +++ b/block/blk-merge.c
> @@ -714,6 +714,32 @@ static void blk_account_io_merge(struct request *req)
>  		part_stat_unlock();
>  	}
>  }
> +/*
> + * Two cases of handling DISCARD merge:
> + * If max_discard_segments > 1, the driver takes every bio
> + * as a range and send them to controller together. The ranges
> + * needn't to be contiguous.
> + * Otherwise, the bios/requests will be handled as same as
> + * others which should be contiguous.
> + */
> +static inline bool blk_discard_mergable(struct request *req)
> +{
> +	if (req_op(req) == REQ_OP_DISCARD &&
> +	    queue_max_discard_segments(req->q) > 1)
> +		return true;
> +	else
> +		return false;
> +}

Please get rid of the redundant else.

-- 
Jens Axboe