To: linux-nfs@vger.kernel.org
From: John Ratliff <john@bluemarble.net>
Subject: NFS v3 file lock testing
Message-ID: <a34f4eb6-564d-e752-0909-5cb5f9efc796@bluemarble.net>
Date: Sat, 4 Nov 2017 20:22:35 -0400
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Sender: linux-nfs-owner@vger.kernel.org

We recently migrated our email storage to a server sharing the files 
with NFSv4 rather than NFS v3. We ran into a strange problem, so we 
wanted to find out if it was related to the NFS version or not.

When I tried to mount the NFS share as an NFSv3 share, I was unable to 
do any file locking. I was unable to diagnose the problem, so I moved 
the disk back to the old NFS v3 server until I could do further testing.

Is there a reason I might not be able to do a file lock on an NFS client 
with a server that does NFSv4 and NFSv3?

The server and client are on the same subnet. There is no firewall 
between the two systems. hosts.allow was set to ALL: ALL on both client 
and server just to make sure that wasn't the problem.

Reverting to the old NFSv3 server worked fine. Building another NFSv3 
server didn't exhibit the problem either. So I'm not sure how to find 
out what the problem is.

I used this program to test whether a file could be locked.

---------------------
#! /usr/bin/perl

use strict;
use warnings;

use Fcntl ':flock';

my $file = '/mnt/nfs3/test-file.lock';
open (my $fh, '>>', $file) or die;
flock($fh, LOCK_EX) or die 'could not lock file';
close($fh);
--------------------

Normally, it returns immediately. But on the new NFSv4 server, it times 
out and dies with 'could not lock file'. I don't know if that's the 
proper way to do file locking on NFSv3, but I know my mail server 
daemons can't do locking when that program fails, so it looks like a 
reasonable test.

Any advice for how I can diagnose this problem?

NFSv3 client machine:
   Ubuntu 14.04
   Kernel 3.13.0 64-bit
   nfs-common version 1.2.8

/etc/fstab:
   192.168.1.103:/mnt/nfs3 /mnt/nfs3 nfs tcp,soft,intr,nfsvers=3 0 0

NFSv3 server machine (the broken one):
   Debian 9
   Kernel 4.9.0
   nfs-common version 1.3.4

/etc/exports:
   /srv/nfs4 192.168.1.0/24(rw,sync,fsid=root,crossmnt,no_subtree_check)
   /srv/nfs4/nfs3 192.168.1.0/24(rw,sync,no_subtree_check,no_root_squash)

NFSv3 server machine (the old functional one):
   Debian 8
   Kernel 3.16.0
   nfs-common 1.2.8

/etc/exports:
   /mnt/nfs3 192.168.1.0/24(rw,sync,no_subtree_check,no_root_sqaush)

Please let me know if additional information would be helpful.

Thanks.

--John