nvme/pcie hot plug results in /dev name change

[Christoph Hellwig]

On Mon, Feb 13, 2023 at 09:32:35AM -0700, Keith Busch wrote:
> > Isn't this equivalent to dm-mpath queue_if_no_path or no_path_timeout ?
> 
> Similiar, but generic to non-multipath devices.

Exactly!

> > 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.

There's really two different things, which would both a problem:

 - make the device not go away if we expect it to come back by using
   the multipath code
 - tell the upper layer that a devie went away when it actually did

Right now if user has a block device open, it will keep a reference to
it forever.  If we send a notification to the upper layer not only can
it release that refrence, but it also knows the device went away.
This really helps to e.g. rebalance data in a RAID or RAID-like setup.