linux-kernel - Re: [PATCH V2] block: fix the DISCARD request merge

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Date:   Mon, 22 Oct 2018 11:00:58 +0200
From:   Christoph Hellwig <hch@....de>
To:     Jianchao Wang <jianchao.w.wang@...cle.com>
Cc:     axboe@...nel.dk, martin.petersen@...cle.com, tom.leiming@...il.com,
        hch@....de, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH V2] block: fix the DISCARD request merge

On Sat, Oct 20, 2018 at 10:29:37PM +0800, Jianchao Wang wrote:
> There are two cases when handle DISCARD merge
>  - max_discard_segments == 1
>    bios need to be contiguous
>  - max_discard_segments > 1
>    Only nvme right now. 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.
> 
> Signed-off-by: Jianchao Wang <jianchao.w.wang@...cle.com>
> ---
> 
> V2:
>   - Add max_discard_segments > 1 checking in attempt_merge
>   - Change patch title and comment
>   - Add more comment in attempt_merge
> 
>  block/blk-merge.c | 24 +++++++++++++++++++-----
>  1 file changed, 19 insertions(+), 5 deletions(-)
> 
> diff --git a/block/blk-merge.c b/block/blk-merge.c
> index 42a4674..8f22374 100644
> --- a/block/blk-merge.c
> +++ b/block/blk-merge.c
> @@ -734,8 +734,15 @@ static struct request *attempt_merge(struct request_queue *q,
>  	/*
>  	 * not contiguous
>  	 */
> -	if (blk_rq_pos(req) + blk_rq_sectors(req) != blk_rq_pos(next))
> -		return NULL;
> +	if (blk_rq_pos(req) + blk_rq_sectors(req) != blk_rq_pos(next)) {
> +		/*
> +		 * When max_discard_segments is bigger than 1 (only nvme right
> +		 * now), needn't consider the contiguity.
> +		 */
> +		if (!(req_op(req) == REQ_OP_DISCARD &&
> +		      queue_max_discard_segments(q) > 1))
> +			return NULL;

Why not:

		if (req_op(req) != REQ_OP_DISCARD ||
		    queue_max_discard_segments(q) == 1)

which would be a lot more obvious?

> +	 * counts here.
> +	 * Two cases of Handling DISCARD:
> +	 *  - max_discard_segments == 1
> +	 *    The bios need to be contiguous.
> +	 *  - max_discard_segments > 1
> +	 *    Only nvme right now. It takes every bio as a
> +	 *    range and send them to controller together. The ranges
> +	 *    needn't to be contiguous.

The formatting looks odd.  Also I don't think we should mention the
users here, as others might grow (virtio is in the pipe, SCSI could
be supported easily if someone did the work).