[PATCH for-next 4/4] nvme-multipath: add multipathing for uring-passthrough commands

[Christoph Hellwig]

On Wed, Jul 13, 2022 at 02:00:56PM +0300, Sagi Grimberg wrote:
> I view uring passthru somewhat as a different thing than sending SG_IO
> ioctls to dm-mpath. But it can be argued otherwise.
>
> BTW, the only consumer of it that I'm aware of commented that he
> expects dm-mpath to retry SG_IO when dm-mpath retry for SG_IO submission
> was attempted (https://www.spinics.net/lists/dm-devel/msg46924.html).

Yeah.  But the point is that if we have a path failure, the kernel
will pick a new path next time anyway, both in dm-mpath and nvme-mpath.

> I still think that there is a problem with the existing semantics for
> passthru requests over mpath device nodes.
>
> Again, I think it will actually be cleaner not to expose passthru
> devices for mpath at all if we are not going to support retry/failover.

I think they are very useful here.  Users of passthrough interface
need to be able to retry anyway, even on non-multipath setups.  And
a dumb retry will do the right thing.