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

[Jens Axboe <axboe@kernel.dk>]

On 2/1/18 12:52 PM, Keith Busch wrote:
> 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
>>
>> 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.
> 
> I'm still finding cases not accounted even your patch. I had to use the
> following on top of that, and this pattern looks like it needs to be
> repeated for all schedulers:
> 
> ---
> diff --git a/block/blk-mq-sched.c b/block/blk-mq-sched.c
> index 55c0a745b427..25c14c58385c 100644
> --- a/block/blk-mq-sched.c
> +++ b/block/blk-mq-sched.c
> @@ -259,6 +259,8 @@ bool blk_mq_sched_try_merge(struct request_queue *q, struct bio *bio,
>  		if (!*merged_request)
>  			elv_merged_request(q, rq, ELEVATOR_FRONT_MERGE);
>  		return true;
> +	case ELEVATOR_DISCARD_MERGE:
> +		return bio_attempt_discard_merge(q, rq, bio);

Yes, we need this to enable the bio-to-request merging.

> diff --git a/block/mq-deadline.c b/block/mq-deadline.c
> index c56f211c8440..a0f5752b6858 100644
> --- a/block/mq-deadline.c
> +++ b/block/mq-deadline.c
> @@ -451,7 +451,7 @@ static int dd_request_merge(struct request_queue *q, struct request **rq,
>  
>  		if (elv_bio_merge_ok(__rq, bio)) {
>  			*rq = __rq;
> -			return ELEVATOR_FRONT_MERGE;
> +			return blk_try_merge(__rq, bio);

This isn't needed. We already know it's a front merge at this point,
and it can only be a normal read/write request.

-- 
Jens Axboe