Hi Jens
On 01/31/2018 11:29 PM, Jens Axboe wrote:
> How about something like the below?
>
>
> diff --git a/block/blk-merge.c b/block/blk-merge.c
> index 8452fc7164cc..cee102fb060e 100644
> --- a/block/blk-merge.c
> +++ b/block/blk-merge.c
> @@ -574,8 +574,13 @@ static int ll_merge_requests_fn(struct request_queue *q, struct request *req,
> blk_rq_get_max_sectors(req, blk_rq_pos(req)))
> return 0;
>
> + /*
> + * For DISCARDs, the segment count isn't interesting since
> + * the requests have no data attached.
> + */
> total_phys_segments = req->nr_phys_segments + next->nr_phys_segments;
> - if (blk_phys_contig_segment(q, req->biotail, next->bio)) {
> + if (total_phys_segments &&
> + blk_phys_contig_segment(q, req->biotail, next->bio)) {
> if (req->nr_phys_segments == 1)
> req->bio->bi_seg_front_size = seg_size;
> if (next->nr_phys_segments == 1)
This patch will avoid the nr_phys_segments to be set to 0xffff,
but the merged req will have two bios but zero nr_phys_segments.
We have to align with the DISCARD merging strategy.
Please refer to:
/*
* Number of discard segments (or ranges) the driver needs to fill in.
* Each discard bio merged into a request is counted as one segment.
*/
static inline unsigned short blk_rq_nr_discard_segments(struct request *rq)
{
return max_t(unsigned short, rq->nr_phys_segments, 1);
}
bool bio_attempt_discard_merge(struct request_queue *q, struct request *req,
struct bio *bio)
{
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(bio) >
blk_rq_get_max_sectors(req, blk_rq_pos(req)))
goto no_merge;
req->biotail->bi_next = bio;
req->biotail = bio;
req->__data_len += bio->bi_iter.bi_size;
req->ioprio = ioprio_best(req->ioprio, bio_prio(bio));
req->nr_phys_segments = segments + 1;
blk_account_io_start(req, false);
return true;
no_merge:
req_set_nomerge(q, req);
return false;
}
blk_rq_nr_discard_segments will get a wrong value finally.
Maybe we could change blk_rq_nr_discard_segments to iterate and count the bios in one request
to decide the DISCARD request nr_phy_segment. And discard the nr_phys_segments operations in
the DISCARD merging path, plus your patch here.
Thanks
Jianchao
