hch's native NVMe multipathing [was: Re: [PATCH 1/2] Don't blacklist nvme]

[Mike Snitzer]

On Fri, Feb 17 2017 at  4:33am -0500,
Christoph Hellwig <hch at infradead.org> wrote:

> On Thu, Feb 16, 2017 at 10:13:37AM -0500, Mike Snitzer wrote:
> > Not following what you're saying Keith did.  Please feel free to
> > clarify.
> 
> Keith demonstrated what it takes to support NVMe with dm.  He also
> gave a couple presentations on it in addition to various ptches on
> the list.

Well I know very well that Keith was an early consumer of DM multipath
for NVMe (back when Intel was looking at doing dual port NVMe drives).
Keith's interest in NVMe support in multipath is what motivated all the
blk-mq request-based DM multipath advances.

But I'll follow-up with Keith to see if he can sweep through whatever
patches he has and make sure I'm aware of them.

> > The middle man is useful if it can support all transports.  If it only
> > supports some then yeah the utility is certainly reduced.
> 
> Again let's look at what multipathing involves:
> 
>  - discovery of multiple paths for a device, and path preferences:
>    Storage protocol specific
> 
>  - handling of path state and grouping changes:
>    Storage protocol specific
> 
>  - handling of path up / down events:
>    Storage protocol / transport specific if provided
> 
>  - keep alive / path checking:
>    Storage protocol specific with possible generic fallback
> 
>  - path selection:
>    Generic, although building heavily on protocol / transport specific
>    information
> 
> So most of the hard work is transport specific anyway.  And I fully
> agree that generic code should be, well generic.  And with generic
> I mean right in the block layer instead of involving a layer block
> driver that relies on lots of low-level driver information and setup
> from user space.

"Lots of low-level driver information?"  You mean like SCSI's pg_init
and such?

> > I'm going to look at removing any scsi_dh code from DM multipath
> > (someone already proposed removing the 'retain_attached_hw_handler'
> > feature).  Not much point having anything in DM multipath now that scsi
> > discovery has the ability to auto-attach the right scsi_dh via scsi_dh's
> > .match hook.
> 
> Great.
> 
> > As a side-effect it will fix Keith's scsi_dh crash (when
> > operating on NVMe request_queue).
> 
> I think we'll need to have a quick fix for that ASAP, though.

Hannes' latest patch looked good to me.  Should take care of it.  I
assume mkp will pick it up for the 4.11 merge window and mark for
stable?