Re: [PATCH] NFSv4: fix a mount deadlock in NFS v4.1 client

Trond Myklebust
2024-09-10T00:22:55Z

On Mon, 2024-09-09 at 16:06 -0700, Oleksandr Tymoshenko wrote:
> On Mon, Sep 9, 2024 at 10:56 AM Trond Myklebust
> <trondmy@hammerspace.com> wrote:
> >
> > On Mon, 2024-09-09 at 16:36 +0000, Oleksandr Tymoshenko wrote:
> > > > > nfs41_init_clientid does not signal a failure condition from
> > > > > nfs4_proc_exchange_id and nfs4_proc_create_session to a
> > > > > client
> > > > > which
> > > > > may
> > > > > lead to mount syscall indefinitely blocked in the following
> > > > > stack
> > >
> > > > NACK. This will break all sorts of recovery scenarios, because
> > > > it
> > > > doesn't distinguish between an initial 'mount' and a server
> > > > reboot
> > > > recovery situation.
> > > > Even in the case where we are in the initial mount, it also
> > > > doesn't
> > > > distinguish between transient errors such as NFS4ERR_DELAY or
> > > > reboot
> > > > errors such as NFS4ERR_STALE_CLIENTID, etc.
> > >
> > > > Exactly what is the scenario that is causing your hang? Let's
> > > > try
> > > > to
> > > > address that with a more targeted fix.
> > >
> > > The scenario is as follows: there are several NFS servers and
> > > several
> > > production machines with multiple NFS mounts. This is a
> > > containerized
> > > multi-tennant workflow so every tennant gets its own NFS mount to
> > > access their
> > > data. At some point nfs41_init_clientid fails in the initial
> > > mount.nfs call
> > > and all subsequent mount.nfs calls just hang in
> > > nfs_wait_client_init_complete
> > > until the original one, where nfs4_proc_exchange_id has failed,
> > > is
> > > killed.
> > >
> > > The cause of the nfs41_init_clientid failure in the production
> > > case
> > > is a timeout.
> > > The following error message is observed in logs:
> > >   NFS: state manager: lease expired failed on NFSv4 server <ip>
> > > with
> > > error 110
> > >
> >
> > How about something like the following fix then?
> > 8<-----------------------------------------------
> > From eb402b489bb0d0ada1a3dd9101d4d7e193402e46 Mon Sep 17 00:00:00
> > 2001
> > Message-ID:
> > <eb402b489bb0d0ada1a3dd9101d4d7e193402e46.1725904471.git.trond.mykl
> > ebust@hammerspace.com>
> > From: Trond Myklebust <trond.myklebust@hammerspace.com>
> > Date: Mon, 9 Sep 2024 13:47:07 -0400
> > Subject: [PATCH] NFSv4: Fail mounts if the lease setup times out
> >
> > If the server is down when the client is trying to mount, so that
> > the
> > calls to exchange_id or create_session fail, then we should allow
> > the
> > mount system call to fail rather than hang and block other
> > mount/umount
> > calls.
> >
> > Reported-by: Oleksandr Tymoshenko <ovt@google.com>
> > Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
> > ---
> >  fs/nfs/nfs4state.c | 6 ++++++
> >  1 file changed, 6 insertions(+)
> >
> > diff --git a/fs/nfs/nfs4state.c b/fs/nfs/nfs4state.c
> > index 30aba1dedaba..59dcdf9bc7b4 100644
> > --- a/fs/nfs/nfs4state.c
> > +++ b/fs/nfs/nfs4state.c
> > @@ -2024,6 +2024,12 @@ static int
> > nfs4_handle_reclaim_lease_error(struct nfs_client *clp, int status)
> >                 nfs_mark_client_ready(clp, -EPERM);
> >                 clear_bit(NFS4CLNT_LEASE_CONFIRM, &clp->cl_state);
> >                 return -EPERM;
> > +       case -ETIMEDOUT:
> > +               if (clp->cl_cons_state == NFS_CS_SESSION_INITING) {
> > +                       nfs_mark_client_ready(clp, -EIO);
> > +                       return -EIO;
> > +               }
> > +               fallthrough;
> >         case -EACCES:
> >         case -NFS4ERR_DELAY:
> >         case -EAGAIN:
> > --
>
> This patch fixes the issue in my simulated environment. ETIMEDOUT is
> the error code that
> was observed in the production env but I guess it's not the only
> possible one. Would it make
> sense to handle all error conditions in the NFS_CS_SESSION_INITING
> state or are there
> some others that are recoverable?
>

The only other one that I'm thinking might want to be treated similarly
is the above EACCES error. That's because that is only returned if
there is a problem with your RPCSEC_GSS/krb5 credential. I was thinking
of changing that one too in the same patch, but came to the conclusion
it would be better to treat the two issues with separate fixes.

The other error conditions are all supposed to be transient NFS level
errors. They should not be treated as fatal.

--
Trond Myklebust
Linux NFS client maintainer, Hammerspace
trond.myklebust@hammerspace.com