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

[Sagi Grimberg]

>> Maybe the solution is to just not expose a /dev/ng for the mpath device
>> node, but only for bottom namespaces. Then it would be completely
>> equivalent to scsi-generic devices.
>>
>> It just creates an unexpected mix of semantics of best-effort
>> multipathing with just path selection, but no requeue/failover...
> 
> Which is exactly the same semanics as SG_IO on the dm-mpath nodes.

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).

 From Paolo:
"The problem is that userspace does not have a way to direct the command 
to a different path in the resubmission. It may not even have permission 
to issue DM_TABLE_STATUS, or to access the /dev nodes for the underlying 
paths, so without Martin's patches SG_IO on dm-mpath is basically 
unreliable by design."

I didn't manage to track down any followup after that email though...

>> If the user needs to do the retry, discover and understand namespace
>> paths, ANA states, controller states, etc. Why does the user need a
>> mpath chardev at all?
> 
> The user needs to do that for all kinds of other resons anyway,
> as we don't do any retries for passthrough at all.

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.