nvme/pcie hot plug results in /dev name change

[Sagi Grimberg]

>>>> Also not sure if this is going to work out, but it looks like a good start to
>>>> me.
>>>>
>>>> Instead of user pinning the virtual device so that it never goes away though, I
>>>> considered a user tunable "device missing delay" parameter to debounce link
>>>> events. New IOs would be deferred to the requeue_list while the timer is
>>>> active, and then EIO'ed if the timer expires without a successful LIVE
>>>> controller attachment. The use cases I'm considering are short bounces from
>>>> transient link resets, so I'd expect timers to be from a few seconds to maybe a
>>>> minute.
>>>
>>> Isn't this equivalent to dm-mpath queue_if_no_path or no_path_timeout ?
>>
>> Similiar, but generic to non-multipath devices.
>>   
>>> We can keep the mpath device around, but if not, what is the desired
>>> behavior from the upper layers?
>>
>> I don't think we're looking for any behavioral changes in the upper layers.
>   
> That also means no matter if the nvme mpath layer is added or not, upper
> layer still has to handle this kind of failure, so what is the
> difference made from the added nvme-mpath?

What do you expect from the upper layer? The device went away...

The suggestion is that when the device comes back the upper layer will
detect it and reconfigure it (restore it to a raid/dm or remount the
FS)?