Re: [REGRESSION] reads and writes hang if NFS server has v3 disabled

[Jeff Layton <jlayton@kernel.org>]

On Fri, 2026-09-04 at 09:09 -0400, Chuck Lever wrote:
> [ Adding correct addresses from MAINTAINERS ]
> 
> On Fri, Sep 4, 2026, at 9:05 AM, Chuck Lever wrote:
> > I'm using a minimal Fedora 45 VM (kvm) for testing.   I export a
> > directory with rw and mount it from my personal computer running Fedora
> > 45 as well. If /etc/nfs. conf has vers3=n, then reads and writes hang in
> > D+ state, and there's a continuous
> > 
> > 
> > I'm using a minimal Fedora 45 VM (kvm) for testing.  I export a 
> > directory with rw and mount it from my personal computer running Fedora 
> > 45 as well.
> > If /etc/nfs.conf has vers3=n, then reads and writes hang in D+ state, 
> > and there's a continuous loop of V3 NULL calls, with the server replying 
> > that it only supports v4.
> > If nfs.conf has vers3=y, then the reads and writes succeed.
> > Introduced in 70b7e3526c53d9dd, still present in the latest 
> > mainline/master (841e384b841a).
> > 
> > The hung state can be rescued by changing the setting and restarting 
> > nfs-server, at which point the read/write will complete.
> > 
> > I can do more testing / get more information as needed.
> > 
> > Here's links to pcap files with it working/bugged:
> > http://grepper.net/lkml-files/nfs-working.pcap
> > http://grepper.net/lkml-files/nfs-bugged.pcap
> > http://grepper.net/lkml-files/nfs-bug-kernel.config
> > 
> > #regzbot introduced:  70b7e3526c53d9dd7caccdbeff5b0485640d8cf1
> 
> It's not immediately obvious to me why 70b7e3526c53d9dd should
> cause this kind of behavior regression.
> 
> Jeff, it's a netlink change. Can you triage?
> 

Sure.

Looking at the captures, it looks like the server is handing out a
flexfiles v3 pNFS layout for the trivial server, and that's (of course)
failing in this situation.

So we have at least one server bug there -- it shouldn't advertise a
flexfiles v3 layout when it's not serving v3! I'll plan to fix that
soon.

That doesn't seem to have anything to do with commit 70b7e3526c53d9dd
though. I'm not sure why a bisect would have landed there.

What does the /etc/exports entry for this export look like? Disabling
pnfs should work around this for now too if you have it explicitly
enabled here.
-- 
Jeff Layton <jlayton@kernel.org>