Date: Sun, 13 Mar 2005 15:04:12 -0500
From: Daniel Jacobowitz <dan@debian.org>
To: Trond Myklebust <trond.myklebust@fys.uio.no>
Cc: Junfeng Yang <yjf@stanford.edu>, nfs@lists.sourceforge.net,
       Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
       ext2-devel@lists.sourceforge.net, mc@cs.Stanford.EDU
Subject: Re: [CHECKER] inconsistent NFS stat cache (NFS on ext3, 2.6.11)
Message-ID: <20050313200412.GA21521@nevyn.them.org>
Mail-Followup-To: Trond Myklebust <trond.myklebust@fys.uio.no>,
	Junfeng Yang <yjf@stanford.edu>, nfs@lists.sourceforge.net,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	ext2-devel@lists.sourceforge.net, mc@cs.Stanford.EDU
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On Sun, Mar 13, 2005 at 12:04:27AM -0500, Trond Myklebust wrote:
> lau den 12.03.2005 Klokka 03:56 (-0800) skreiv Junfeng Yang:
> > Hi,
> > 
> > We checked NFS on top of ext3 using FiSC (our file system model checker)
> > and found a case where NFS stat cache can contain inconsistent entries.
> > 
> > Basically, to trigger this inconsistency, just do the following steps:
> > 1. create a file A1, write a few bytes to it, so A1 is 4 words
> > 2. create a hard link A2, pointing to A1
> > 3. stat on A2. A2's size is 4 words
> > 4. truncate A1 to a larger size, write a few bytes at the end. now it's
> > 1031 words.
> > 5. stat on A2. it's size is still 4 words, which should be 1031 words
> > 
> > We have a test case to re-create this warning.  You can download it at
> > http://fisc.stanford.edu/bug16/crash.c.  It includes some sudo commands
> > to mount nfs partitions, which you might want to change according to your
> > local settings.
> > 
> > cat /etc/exports shows:
> > /mnt/sbd0-export          localhost(rw,sync)
> > /mnt/sbd1-export          localhost(rw,sync)
> > 
> > Let me know if you have any problems reproducing the warning. We'd
> > appreciate any confirmations/clarifications.
> > 
> 
> This is a known problem. Turn off the (default - grrr) subtree checking
> export option on the server, and it will all work properly. The subtree
> checking option violates the NFS standards for filehandle generation in
> so many ways, that it isn't even funny.

I can't find any documentation about this, but it seems like the same
problem that has been causing me headaches lately; when I replace glibc
from the server side of an nfsroot, the client has a couple of
variously wrong reads before it sees the new files.  If it breaks NFS
so badly, why is it the default for the Linux NFS server?

-- 
Daniel Jacobowitz
CodeSourcery, LLC
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