On 1/31/18 9:56 PM, Keith Busch wrote:
> On Wed, Jan 31, 2018 at 08:07:41PM -0700, Jens Axboe wrote:
>> if (total_phys_segments > queue_max_segments(q))
>> - return 0;
>> + return 0;
>
> This perhaps unintended change happens to point out another problem:
> queue_max_segments is the wrong limit for discards, which require
> queue_max_discard_segments.
>
> It might be easier to merge discard requests special, like how merging
> a discard bio is handled (untested).
Yeah agreed, we should just split it up completely instead instead of
special casing it in the read/write path.
> diff --git a/block/blk-merge.c b/block/blk-merge.c
> index 8452fc7164cc..01671e1373ff 100644
> --- a/block/blk-merge.c
> +++ b/block/blk-merge.c
> @@ -550,6 +550,28 @@ static bool req_no_special_merge(struct request *req)
> return !q->mq_ops && req->special;
> }
>
> +static bool req_attempt_discard_merge(struct request_queue *q, struct
> request *req,
> + struct request *next)
> +{
> + unsigned short segments = blk_rq_nr_discard_segments(req);
> +
> + if (segments >= queue_max_discard_segments(q))
> + goto no_merge;
> + if (blk_rq_sectors(req) + bio_sectors(next->bio) >
> + blk_rq_get_max_sectors(req, blk_rq_pos(req)))
> + goto no_merge;
> +
> + req->biotail->bi_next = next->bio;
> + req->biotail = next->biotail;
> + req->nr_phys_segments = segments + blk_rq_nr_discard_segments(next);
> + next->bio = NULL;
> + return true;
> +
> +no_merge:
> + req_set_nomerge(q, req);
> + return false;
> +}
> +
> static int ll_merge_requests_fn(struct request_queue *q, struct request *req,
> struct request *next)
> {
> @@ -679,6 +701,15 @@ static struct request *attempt_merge(struct
> request_queue *q,
> if (req->write_hint != next->write_hint)
> return NULL;
>
> + /*
> + * Discards are ... special.
> + */
> + if (req_op(req) == REQ_OP_DISCARD) {
> + if (req_attempt_discard_merge(q, req, next))
> + return next;
> + return NULL;
> + }
> +
> /*
> * If we are allowed to merge, then append bio list
> * from next to rq and release next. merge_requests_fn
This looks fine to me, the bio-to-request merge path already looks correct.
Care to send a properly formatted patch?
--
Jens Axboe
