Re: WARNING: CPU: 2 PID: 207 at drivers/nvme/host/core.c:527 nvme_setup_cmd+0x3d3

jianchao.wang
2018-02-01T03:33:12Z

Sorry, Jens, I think I didn't get the point.
Do I miss anything ?

On 02/01/2018 11:07 AM, Jens Axboe wrote:
> Yeah I agree, and my last patch missed that we do care about segments for
> discards. Below should be better...
>
> diff --git a/block/blk-merge.c b/block/blk-merge.c
> index 8452fc7164cc..055057bd727f 100644
> --- a/block/blk-merge.c
> +++ b/block/blk-merge.c
> @@ -553,9 +553,8 @@ static bool req_no_special_merge(struct request *req)
> static int ll_merge_requests_fn(struct request_queue *q, struct request *req,
> struct request *next)
> {
> - int total_phys_segments;
> - unsigned int seg_size =
> - req->biotail->bi_seg_back_size + next->bio->bi_seg_front_size;
> + int total_phys_segments = req->nr_phys_segments +
> + next->nr_phys_segments;

For DISCARD reqs, the total_phys_segments is still zero here.

>
> /*
> * First check if the either of the requests are re-queued
> @@ -574,8 +573,15 @@ 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;
>
> - total_phys_segments = req->nr_phys_segments + next->nr_phys_segments;
> - if (blk_phys_contig_segment(q, req->biotail, next->bio)) {
> + /*
> + * If the requests aren't carrying any data payloads, we don't need
> + * to look at the segment count
> + */
> + if (bio_has_data(next->bio) &&
> + blk_phys_contig_segment(q, req->biotail, next->bio)) {
> + unsigned int seg_size = req->biotail->bi_seg_back_size +
> + next->bio->bi_seg_front_size;

Yes, total_phys_segments will not be decreased.

> +
> if (req->nr_phys_segments == 1)
> req->bio->bi_seg_front_size = seg_size;
> if (next->nr_phys_segments == 1)
> @@ -584,7 +590,7 @@ static int ll_merge_requests_fn(struct request_queue *q, struct request *req,
> }
>
> if (total_phys_segments > queue_max_segments(q))
> - return 0;
> + return 0;
>
> if (blk_integrity_merge_rq(q, req, next) == false)
> return 0;

But finally, the merged DISCARD req's nr_phys_segment is still zero.

blk_rq_nr_discard_segments will return 1 but this req has two bios.
blk_rq_nr_discard_segments 's comment says
--
Each discard bio merged into a request is counted as one segment
--

Maybe patch below should be followed with yours.

diff --git a/block/blk-core.c b/block/blk-core.c
index a2005a4..b444fb7 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -1763,7 +1763,6 @@ bool bio_attempt_discard_merge(struct request_queue *q, struct request *req,
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;
diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index 4f3df80..1af2138 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -1312,7 +1312,13 @@ static inline unsigned short blk_rq_nr_phys_segments(struct request *rq)
*/
static inline unsigned short blk_rq_nr_discard_segments(struct request *rq)
{
- return max_t(unsigned short, rq->nr_phys_segments, 1);
+ struct bio *bio;
+ unsigned short count = 0;
+
+ __rq_for_each_bio(bio, req)
+ count ++;
+
+ return count;
}

Thanks
Jianchao