[solved] Re: [PATCH] Short write in nfsd becomes a full write to the client

[Dale Stimson <DaleStimson@gmail.com>]

On Mon, Jun 29, 2009 at 03:49:25PM -0400, J. Bruce Fields wrote:
> On Mon, Jun 29, 2009 at 03:47:11PM -0400, bfields wrote:
> > On Mon, Jun 29, 2009 at 12:29:51PM -0700, Dale Stimson wrote:
> > > On Mon, Jun 29, 2009 at 10:59:52AM -0400, J. Bruce Fields wrote:
> > > > On Fri, Jun 26, 2009 at 09:24:49PM -0700, Dale Stimson wrote:
> > > > > On Thu, 5 Mar 2009 20:16:14 -0500, David Shaw <dshaw@jabberwocky.com> wrote:
> > > > > > If a filesystem being written to via NFS returns a short write count
> > > > > > (as opposed to an error) to nfsd, nfsd treats that as a success for
> > > > > > the entire write, rather than the short count that actually succeeded.
> > > > > > 
> > > > > > For example, given a 8192 byte write, if the underlying filesystem
> > > > > > only writes 4096 bytes, nfsd will ack back to the nfs client that all
> > > > > > 8192 bytes were written.  The nfs client does have retry logic for
> > > > > > short writes, but this is never called as the client is told the
> > > > > > complete write succeeded.
> > > > > ...
> > > > > > Here is a patch to properly return the short write count to the
> > > > > > client.
> > > > > [patch elided]
> > > > > 
> > > > > I bring this to your attention so you may, if you choose, look into
> > > > > this further:
> > > > > 
> > > > > Problem synopsis:
> > > > > An old client (running RHL 9 with kernel "2.4.20-43.9.legacy")
> > > > > attempts to seek on a file mounted over nfs.  The operation fails
> > > > > with "Illegal seek" or "Input/Output error".  The server is running
> > > > > Fedora 11 kernel-PAE-2.6.29.5-191.fc11.i686, which includes the
> > > > > short write patch.  When this kernel is re-built without the short
> > > > > write patch, everything works as before.
> > > > 
> > > > Does that server kernel have some version of
> > > > a0d24b295aed7a9daf4ca36bd4784e4d40f82303 "nfsd: fix hung up of nfs
> > > > client while sync write data to nfs server" applied?
> > > 
> > > No.  As far as I can see that patch has been merged only in 2.6.30 and is not in any of the 2.6.29.? releases.  There is definitely no Fedora-specific version of that patch present.  The only Fedora-specific patch related to NFS applied in kernel-2.6.29.5-191.fc11.src.rpm is linux-2.6-nfsd-report-short-writes.patch.
> > 
> > That patch fixes a problem with the showrt write patch, so it would be
> > worth retesting with it applied.
> 
> After looking at the trace: yes, looks like the same problem.  (The
> (succesful) FILE_SYNC write request returned with a 0 length.)  So
> retest with the new patch applied, then poke the Fedora people to apply
> it as well.
> 
> --b.

Success.

I applied patch a0d24b295aed7a9daf4ca36bd4784e4d40f82303
"nfsd: fix hung up of nfs client while sync write data to nfs server"
to the otherwise unmodified Fedora 11 kernel 2.6.29.5-191.fc11.i686.PAE
and the problem was resolved.

Thank you for your help.  I will update
https://bugzilla.redhat.com/show_bug.cgi?id=508174
and suggest that the patch be applied to future Fedora 2.6.29-based kernels.