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

[Sagi Grimberg]

>>>> However io_kiocb is less
>>>> constrained, and could be used as a context to hold such a space.
>>>>
>>>> Even if it is undesired to have io_kiocb be passed to uring_cmd(), it
>>>> can still hold a driver specific space paired with a helper to 
>>>> obtain it
>>>> (i.e. something like io_uring_cmd_to_driver_ctx(ioucmd) ). Then if the
>>>> space is pre-allocated it is only a small memory copy for a stable copy
>>>> that would allow a saner failover design.
>>>
>>> I am thinking along the same lines, but it's not about few bytes of
>>> space rather we need 80 (72 to be precise). Will think more, but
>>> these 72 bytes really stand tall in front of my optimism.
>>
>> You don't have to populate this space on every I/O, you can just
>> populate it when there is no usable path and when you failover a
>> request...
> 
> Getting the space and when/how to populate it - related but diferent
> topics in this context.
> 
> It is about the lifetime of SQE which is valid only for the first
> submission. If we don't make the command stable at that point, we don't
> have another chance. And that is exactly what happens for failover.
> Since we know IO is failed only when it fails, but by that time
> original passthrough-command is gone out of hand. I think if we somehow
> get the space (preallocated), it is ok to copy to command for every IO
> in mpath case.

Yea you're right. you need to populate it as soon as you queue the
uring command.