Re: [PATCHv2] nvme-mpath: delete disk after last connection

[Keith Busch <kbusch@kernel.org>]

On Tue, Apr 20, 2021 at 07:02:32PM +0200, Hannes Reinecke wrote:
> On 4/20/21 4:39 PM, Christoph Hellwig wrote:
> > On Tue, Apr 20, 2021 at 11:14:36PM +0900, Keith Busch wrote:
> > > On Tue, Apr 20, 2021 at 03:19:10PM +0200, Hannes Reinecke wrote:
> > > > On 4/20/21 10:05 AM, Christoph Hellwig wrote:
> > > > > On Fri, Apr 16, 2021 at 08:24:11AM +0200, Hannes Reinecke wrote:
> > > > > > With the proposed patch, the following messages appear:
> > > > > > 
> > > > > >    [  227.516807] md/raid1:md0: Disk failure on nvme3n1, disabling device.
> > > > > >    [  227.516807] md/raid1:md0: Operation continuing on 1 devices.
> > > > > 
> > > > > So how is this going to work for e.g. a case where the device
> > > > > disappear due to resets or fabrics connection problems?  This now
> > > > > directly teards down the device.
> > > > > 
> > > > Yes, that is correct; the nshead will be removed once the last path is
> > > > _removed_.
> > > 
> > > The end result is also how non-multipath nvme behaves, so I think that's
> > > what users have come to expect.
> > 
> > I'm not sure that is what users expect.  At least the SCSI multipath
> > setups I've worked do not expect it and ensure the queue_if_no_path
> > option is set.
> > 
> Yes, sure. And as I said, I'm happy to implement this option for NVMe, too.
> But that is _not_ what this patch is about.
> 
> NVMe since day one has _removed_ the namespace if the controller goes away
> (ie if you do a PCI hotplug). So customer rightly expects this behaviour to
> continue.
> 
> And this is what the patch does; _aligning_ the behaviour from multipathed
> to non-multipathed controllers when the last path is gone.
> 
> Non-multipathed (ie CMIC==0) controllers will remove the namespace once the
> last _reference_ to that namespace drops (ie the PCI hotplug case).
> Multipathed (ie CMIC!=0) controllers will remove the namespace once the last
> _opener_ goes away.
> The refcount is long gone by that time.
> 
> > > > But key point here is that once the system finds itself in that
> > > > situation it's impossible to recover, as the refcounts are messed.
> > > > Even a manual connect call with the same parameter will _not_ restore
> > > > operation, but rather result in a new namespace.
> > > 
> > > I haven't looked at this yet, but is it really not possible to restore
> > > the original namespace upon the reestablished connection?
> > 
> > It is possible, and in fact is what we do.
> > 
> It is _not_ once the namespace is mounted.
> Or MD has claimed the device.
> And the problem is that the refcount already _is_ zero, so we are already in
> teardown. We're just waiting for the reference to the gendisk to drop.
> Which is never will, as we would have to umount (or detach) the device for
> that, but I/O is still pending which cannot be flushed, so that'll fail.
> And if we try to connect the same namespace again, nvme_find_ns_head() will
> not return the existing ns_head as the refcount is zero.
> Causing a new ns_head to be created.
> 
> If you manage to get this working with the current code please show me from
> the testcase in the description what we should have done differently.

I see what you mean. I think we can detangle this, but it's not as
straight forward as I hoped.

As far as *this* patch goes, I think you and I are aligned on the
behavior, and I still think it's good.

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