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

[Keith Busch <keith.busch@intel.com>]

On Thu, Feb 01, 2018 at 08:26:11AM -0700, Jens Axboe wrote:
> On 1/31/18 9:56 PM, Keith Busch wrote:
> 
> > 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?

Sending the patch now. It's a little different from the above so that it
doesn't need to duplicate some of the merging accounting.

Full disclosure, I have not found a way to trigger this merge. I'm just
running 'fio' with trim, randtrim, and trimwrite on device with
mq-deadline for the past hour, and haven't seen a merge happen yet.