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

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

On Tue, Jan 30, 2018 at 01:32:25PM -0700, Jens Axboe wrote:
> On 1/30/18 1:30 PM, Keith Busch wrote:
> > On Tue, Jan 30, 2018 at 08:57:49AM -0700, Jens Axboe wrote:
> >>
> >> Looking at the disassembly, 'n' is 2 and 'segments' is 0xffff.
> > 
> > Is this still a problem if you don't use an IO scheduler? With deadline,
> > I'm not finding any path to bio_attempt_discard_merge which is where the
> > nr_phys_segments is supposed to get it set to 2. Not sure how it could
> > becmoe 0xffff, though.
> 
> blk_mq_make_request() -> blk_mq_sched_bio_merge() -> __blk_mq_sched_bio_merge()
> 	-> blk_mq_attempt_merge() -> bio_attempt_discard_merge()

That's the calls only if you don't have an elevator_queue, right? With
deadline, it looks like it goes through this path (ftrace confirms):

  __blk_mq_sched_bio_merge() -> dd_bio_merge() -> blk_mq_sched_try_merge()

Which doesn't have a case for ELEVATOR_DISCARD_MERGE.

Relavant function_graph:

  46)               |                    blk_mq_make_request() {
  46)   0.133 us    |                      blk_queue_bounce();
  46)   0.370 us    |                      blk_queue_split();
  46)   0.314 us    |                      bio_integrity_prep();
  46)   0.081 us    |                      blk_attempt_plug_merge();
  46)               |                      __blk_mq_sched_bio_merge() {
  46)               |                        dd_bio_merge() {
  46)   0.792 us    |                          _raw_spin_lock();
  46)               |                          blk_mq_sched_try_merge() {