Re: [PATCH - nfs-utils] Fix fallback from tcp to udp

["J. Bruce Fields" <bfields@fieldses.org>]

On Fri, Feb 21, 2014 at 02:26:41PM +1100, NeilBrown wrote:
> On Thu, 20 Feb 2014 15:37:02 -0500 "J. Bruce Fields" <bfields@fieldses.org>
> wrote:
> > Any NFS server has to support either tcp or rpcbind.  But it's OK for a
> > server to support only of those two.  So the only way to handle both
> > cases while continuing to retry after ECONNREFUSED is to alternate
> > between trying nfs4/tcp and rpcbind until you can connect to one or the
> > other.
> > 
> > If it's the rpcbind call that succeeds first then I think we want to do
> > one more try of nfs4/tcp just to make sure it didn't just come up,
> > before falling back to v3.
> > 
> > The rpcbind call is done in userspace, if I understand right, so I think
> > this is doable.  Looking at utils/mount/ I don't understand the mount
> > process well enough to understand exactly how to do it.  Maybe
> > everything but the final nfs_sys_mount needs to be moved out of
> > nfs_do_mount_v3v2 into a new nfs_do_probe_v3v2 and nfs_autonegotiate
> > should alternate between nfs_try_mount_v4 and nfs_do_probe_v3v2 as long
> > as both return ECONNREFUSED, calling nfs_try_mount_v3v2 only if
> > nfs_try_mount_v4 has failed after a succesful nfs_do_probe_v3v2?
> > 
> > Except the v3v2 mount logic seems to actually modify the mount_options,
> > so probably that doesn't quite work.
> 
> I had come to much the same conclusion after reading Steve's mail:  when TCP
> fails we need rpcbind to be sure what to do.
> I suspect it should be fairly straight forward to implement (I'm less
> pessimistic than you).  I'll have a go on Monday.

OK, great!

Yeah, the mount code looked like a maze for me but I probably spent less
than an hour trying to trace through it, I'm sure it's not that bad.

--b.