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

[Sagi Grimberg]

On 7/13/22 14:49, Hannes Reinecke wrote:
> On 7/13/22 13:00, Sagi Grimberg wrote:
>>
>>>> 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?