nvme/pcie hot plug results in /dev name change

[Keith Busch]

On Tue, Jan 31, 2023 at 10:27:24PM -0800, Christoph Hellwig wrote:
> On Wed, Feb 01, 2023 at 10:33:15AM +0800, Ming Lei wrote:
> > > The native nvme multipath looks like it could be leveraged to improving that
> > > user experience if we wanted to make that layer an option for non-multipath
> > > devices.
> > 
> > Can you share the basic idea? Will nvme mulitpath hold the io error and
> > not propagate to upper layer until new device is probed? What if the
> > new device is probed late, and IO has been timed out and err is returned
> > to upper layer?
> 
> Attached is what I did, but I'm really not sure it is the right final
> approach.

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.