[PATCH V5] block: fix the DISCARD request merge

[PATCH V5] block: fix the DISCARD request merge

[Jianchao Wang]

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@oracle.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

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

[Christoph Hellwig]

On Sat, Oct 27, 2018 at 07:52:14PM +0800, Jianchao Wang wrote:
> +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;

The empty line here look a little odd, but if that is the only thing
I can nitpick about it must be great, so:

Reviewed-by: Christoph Hellwig <hch@lst.de>

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

[Ming Lei]

On Sat, Oct 27, 2018 at 7:50 PM Jianchao Wang
<jianchao.w.wang@oracle.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@oracle.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@redhat.com>

-- 
Ming Lei

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

[Jens Axboe]

On 10/27/18 5:52 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.

Applied, thanks.

-- 
Jens Axboe