[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...
>>>>
>>> I did; 'twas me who was involved in the initial customer issue 
>>> leading up to that.
>>>
>>> Amongst all the other issue we've found the prime problem with SG_IO 
>>> is that it needs to be directed to the 'active' path.
>>> For the device-mapper has a distinct callout (dm_prepare_ioctl), 
>>> which essentially returns the current active path device. And then 
>>> the device-mapper core issues the command on that active path.
>>>
>>> All nice and good, _unless_ that command triggers an error.
>>> Normally it'd be intercepted by the dm-multipath end_io handler, and 
>>> would set the path to offline.
>>> But as ioctls do not use the normal I/O path the end_io handler is 
>>> never called, and further SG_IO calls are happily routed down the 
>>> failed path.
>>>
>>> And the customer had to use SG_IO (or, in qemu-speak, LUN 
>>> passthrough) as his application/filesystem makes heavy use of 
>>> persistent reservations.
>>
>> How did this conclude Hannes?
> 
> It didn't. The proposed interface got rejected, and now we need to come 
> up with an alternative solution.
> Which we haven't found yet.

Lets assume for the sake of discussion, had dm-mpath set a path to be
offline on ioctl errors, what would qemu do upon this error? blindly
retry? Until When? Or would qemu need to learn about the path tables in
order to know when there is at least one online path in order to retry?

What is the model that a passthru consumer needs to follow when
operating against a mpath device?