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

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

On Thu, Feb 20, 2014 at 12:50:15PM -0500, Steve Dickson wrote:
> 
> 
> On 02/17/2014 06:43 PM, NeilBrown wrote:
> > 
> > Protocol negotiation in mount.nfs does not correctly negotiate with a
> > server which only support NFSv3 and UDP.
> > 
> > When mount.nfs attempts an NFSv4 mount and fails with ECONNREFUSED
> > it does not fall back to NFSv3, as this is not recognised as a
> > "does not support NFSv4" error.
> > However ECONNREFUSED is a clear indication that the server doesn't
> > support TCP, and ipso facto does not support NFSv4.
> > So ECONNREFUSED should trigger a fallback from v4 to v2/3.
> I'm also pretty this is the error returned when the server is 
> down or more pointy when server is rebooting...

Probably worth checking that.

> Do we really want to fallback at this point?

>From a bz comment (#984901, not sure why it's private):

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.

?

--b.