linux-kernel - Re: [PATCH V5] block: fix the DISCARD request merge

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Date:   Mon, 29 Oct 2018 17:44:58 +0800
From:   Ming Lei <tom.leiming@...il.com>
To:     "jianchao.wang" <jianchao.w.wang@...cle.com>
Cc:     Jens Axboe <axboe@...nel.dk>,
        "Martin K. Petersen" <martin.petersen@...cle.com>,
        Christoph Hellwig <hch@....de>,
        linux-block <linux-block@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH V5] block: fix the DISCARD request merge

On Sat, Oct 27, 2018 at 7:50 PM Jianchao Wang
<jianchao.w.wang@...cle.com> 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.
>
> Signed-off-by: Jianchao Wang <jianchao.w.wang@...cle.com>
> ---
>
> V5:
>  - get rid of the redundant 'else' in blk_discard_mergable
>
> V4:
>  - introduce blk_try_req_merge as suggestion of Christoph.
>
> V3:
>  - Introduce blk_discard_mergable into attempt_merge and
>    blk_try_merge.
>  - Some comment changes.
>
> 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 | 46 ++++++++++++++++++++++++++++++++++++----------
>  1 file changed, 36 insertions(+), 10 deletions(-)
>
> diff --git a/block/blk-merge.c b/block/blk-merge.c
> index 42a4674..6b5ad27 100644
> --- a/block/blk-merge.c
> +++ b/block/blk-merge.c
> @@ -714,6 +714,31 @@ 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;
> +       return false;
> +}
> +
> +enum elv_merge blk_try_req_merge(struct request *req, struct request *next)
> +{
> +       if (blk_discard_mergable(req))
> +               return ELEVATOR_DISCARD_MERGE;
> +       else if (blk_rq_pos(req) + blk_rq_sectors(req) == blk_rq_pos(next))
> +               return ELEVATOR_BACK_MERGE;
> +
> +       return ELEVATOR_NO_MERGE;
> +}
>
>  /*
>   * For non-mq, this has to be called with the request spinlock acquired.
> @@ -731,12 +756,6 @@ static struct request *attempt_merge(struct request_queue *q,
>         if (req_op(req) != req_op(next))
>                 return NULL;
>
> -       /*
> -        * not contiguous
> -        */
> -       if (blk_rq_pos(req) + blk_rq_sectors(req) != blk_rq_pos(next))
> -               return NULL;
> -
>         if (rq_data_dir(req) != rq_data_dir(next)
>             || req->rq_disk != next->rq_disk
>             || req_no_special_merge(next))
> @@ -760,11 +779,19 @@ static struct request *attempt_merge(struct request_queue *q,
>          * counts here. Handle DISCARDs separately, as they
>          * have separate settings.
>          */
> -       if (req_op(req) == REQ_OP_DISCARD) {
> +
> +       switch (blk_try_req_merge(req, next)) {
> +       case ELEVATOR_DISCARD_MERGE:
>                 if (!req_attempt_discard_merge(q, req, next))
>                         return NULL;
> -       } else if (!ll_merge_requests_fn(q, req, next))
> +               break;
> +       case ELEVATOR_BACK_MERGE:
> +               if (!ll_merge_requests_fn(q, req, next))
> +                       return NULL;
> +               break;
> +       default:
>                 return NULL;
> +       }
>
>         /*
>          * If failfast settings disagree or any of the two is already
> @@ -888,8 +915,7 @@ bool blk_rq_merge_ok(struct request *rq, struct bio *bio)
>
>  enum elv_merge blk_try_merge(struct request *rq, struct bio *bio)
>  {
> -       if (req_op(rq) == REQ_OP_DISCARD &&
> -           queue_max_discard_segments(rq->q) > 1)
> +       if (blk_discard_mergable(rq))
>                 return ELEVATOR_DISCARD_MERGE;
>         else if (blk_rq_pos(rq) + blk_rq_sectors(rq) == bio->bi_iter.bi_sector)
>                 return ELEVATOR_BACK_MERGE;
> --
> 2.7.4
>

Reviewed-by: Ming Lei <ming.lei@...hat.com>

-- 
Ming Lei