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

Keith Busch
2018-02-01T18:12:09Z

On Thu, Feb 01, 2018 at 10:58:23AM -0700, Jens Axboe wrote:
> I was able to reproduce on a test box, pretty trivially in fact:
>
> # echo mq-deadline > /sys/block/nvme2n1/queue/scheduler
> # mkfs.ext4 /dev/nvme2n1
> # mount /dev/nvme2n1 /data -o discard
> # dd if=/dev/zero of=/data/10g bs=1M count=10k
> # sync
> # rm /data/10g
> # sync <- triggered

Nice! Thanks, this recipe works for me too.

> Your patch still doesn't work, but mainly because we init the segments
> to 0 when setting up a discard. The below works for me, and cleans up
> the merge path a bit, since your patch was missing various adjustments
> on both the merged and freed request.

Yep, your update is very similiar to my real patch, but I'm missing one
thing (elv_merge_requests). If you're already testing successfully with
your patch, I don't mind if you want to move forward with yours.

> diff --git a/block/blk-core.c b/block/blk-core.c
> index a2005a485335..e4561c95fc23 100644
> --- a/block/blk-core.c
> +++ b/block/blk-core.c
> @@ -3282,6 +3282,8 @@ void blk_rq_bio_prep(struct request_queue *q, struct request *rq,
> {
> if (bio_has_data(bio))
> rq->nr_phys_segments = bio_phys_segments(q, bio);
> + else if (bio_op(bio) == REQ_OP_DISCARD)
> + rq->nr_phys_segments = 1;
>
> rq->__data_len = bio->bi_iter.bi_size;
> rq->bio = rq->biotail = bio;
> diff --git a/block/blk-merge.c b/block/blk-merge.c
> index 8452fc7164cc..782940c65d8a 100644
> --- a/block/blk-merge.c
> +++ b/block/blk-merge.c
> @@ -550,6 +550,24 @@ 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->nr_phys_segments = segments + blk_rq_nr_discard_segments(next);
> + 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)
> {
> @@ -683,9 +701,13 @@ static struct request *attempt_merge(struct request_queue *q,
> * If we are allowed to merge, then append bio list
> * from next to rq and release next. merge_requests_fn
> * will have updated segment counts, update sector
> - * counts here.
> + * counts here. Handle DISCARDs separately, as they
> + * have separate settings.
> */
> - if (!ll_merge_requests_fn(q, req, next))
> + if (req_op(req) == REQ_OP_DISCARD) {
> + if (!req_attempt_discard_merge(q, req, next))
> + return NULL;
> + } else if (!ll_merge_requests_fn(q, req, next))
> return NULL;
>
> /*
> @@ -715,7 +737,8 @@ static struct request *attempt_merge(struct request_queue *q,
>
> req->__data_len += blk_rq_bytes(next);
>
> - elv_merge_requests(q, req, next);
> + if (req_op(req) != REQ_OP_DISCARD)
> + elv_merge_requests(q, req, next);
>
> /*
> * 'next' is going away, so update stats accordingly
>
> --
> Jens Axboe
>