Re: [PATCH 1/2] Don't blacklist nvme

[Mike Snitzer <snitzer@redhat.com>]

On Wed, Feb 15 2017 at  9:01pm -0500,
Mike Snitzer <snitzer@redhat.com> wrote:

> On Tue, Feb 14 2017 at  6:00pm -0500,
> Keith Busch <keith.busch@intel.com> wrote:
> 
> > On Tue, Feb 14, 2017 at 01:35:45PM -0800, Bart Van Assche wrote:
> > > On 02/14/2017 01:19 PM, Keith Busch wrote:
> > > > These devices are mulitpath capable, and have been able to stack with
> > > > dm-mpath since kernel 4.2.
> > > > 
> > > > -	str = STRDUP("^(ram|raw|loop|fd|md|dm-|sr|scd|st|dcssblk|nvme)[0-9]");
> > > > +	str = STRDUP("^(ram|raw|loop|fd|md|dm-|sr|scd|st|dcssblk)[0-9]");
> > > 
> > > Have you checked whether dm-mpath works properly with nvme? Last time I
> > > tried that dm-mpath crashed when it called scsi_dh_attach() because that
> > > last function assumes that it is passed a SCSI device queue instead of
> > > checking whether or not the queue passed to that function is a SCSI
> > > device queue.
> > 
> > Good point. I was unknowingly running with CONFIG_SCSI_DH disabled,
> > and blissfully unaware of its existence! After enabling that option,
> > I see what you mean.
> > 
> > If we don't want to mess with the kernel, I can change the multipath-tools
> > to get around that by appending the following to NVMe hwentry in the
> > second patch in this series:
> > 
> > 	.retain_hwhandler = RETAIN_HWHANDLER_OFF,
> > 
> > And the problem goes away.
> 
> That gives me a clue, I'll take a look.

Looks like drivers/scsi/scsi_dh.c:get_sdev_from_queue() needs to first
check if the request_queue is from a SCSI device (rather than assuming
it is).  Not quite sure how to check that though.

By pushing the check down into scsi_dh it implies DM multipath will be
calling into scsi_dh code for other transports (NVMe, etc).

So an alternative would be to have DM multipath guard SCSI specific code
with an appropriate check.  Again, not sure how that check (e.g. some
new is_scsi_request_queue) should be implemented.

Mike