[PATCHv4] nvme: allow to re-attach namespaces after all paths are down

[Keith Busch]

On Mon, May 17, 2021 at 10:58:38AM -0700, Sagi Grimberg wrote:
> 
> > > > On Mon, May 17, 2021 at 10:18:09AM +0200, Hannes Reinecke wrote:
> > > > > We should only remove the ns head from the list of heads per
> > > > > subsystem if the reference count drops to zero. Removing it
> > > > > at the start of nvme_ns_remove() will prevent us from reattaching
> > > > > the namespace to the correct ns head once a path becomes available
> > > > > again.
> > > > 
> > > > This looks fine to me
> > > > 
> > > > Reviewed-by: Keith Busch <kbusch at kernel.org>
> > > > 
> > > > There are potential corner cases associated with namespace management
> > > > that can cause problems here, but it would certainly be a user error
> > > > that needs manual intervention anyway. I suspect those scenarios are
> > > > very uncommon too, so I think we can deal with that later if it becomes
> > > > a problem.
> > > > 
> > > How so?
> > > We're keeping a copy of the NS IDs around, so after reconnect the
> > > namespace would have to provide same set of NS IDs.
> > > 
> > > If the namespace is different yet providing the same NS IDs we'll have a
> > > problem even with the current code, as we're using the ND ID to identify
> > > multipathed namespaces.
> > > 
> > > Or do you have a different scenario in mind?
> > 
> > Sorry, I didn't mean to imply this creates a new problem. It's already a
> > problem. :)
> > 
> > This just provides a chance for a recycled ID for a newly created
> > namespace to attach to an in-use head associated with a previously
> > deleted namespace. I am not really concerned about this scenario,
> > though, since it wouldn't work today anyway.
> 
> That is because we keep the device node around even though it doesn't
> have any bottom devices. Which is what I think Hannes tried to fix
> in an earlier patch.
> 
> Also, if an NVM subsystem happens to recycle nsids, it will trigger
> pretty surely. So I'm not sure this is an obscure case...

I'm not very concerned because (1) deleting an in-use namespace is
surely a user error, and (2) it currently doesn't work either, and no
one's complained yet AFAIK.