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

[Sagi Grimberg]

>> I think the difference from scsi-generic and controller nvme passthru is
>> that this is a mpath device node (or mpath chardev). This is why I think
>> that users would expect that it would have equivalent multipath
>> capabilities (i.e. failover).
> 
> How is that different from /dev/sg?

/dev/sg it is not a multipath device.

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...

>> In general, I think that uring passthru as an alternative I/O interface
>> and as such needs to be able to failover. If this is not expected from
>> the interface, then why are we exposing a chardev for the mpath device
>> node? why not only the bottom namespaces?
> 
> The failover will happen when you retry, but we leave that retry to
> userspace.  There even is the uevent to tell userspace when a new
> path is up.

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 basically needs to understand everything including indirectly
path selection for the I/O. IMO it is cleaner for the user to just have 
the bottom devices and do the path selection directly.