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From: "J. Bruce Fields" <bfields@fieldses.org>
To: Steve Dickson <SteveD@redhat.com>
Cc: NeilBrown <neilb@suse.de>, Chuck Lever <chuck.lever@oracle.com>,
	NFS <linux-nfs@vger.kernel.org>,
	Carsten Ziepke <kieltux@gmail.com>
Subject: Re: [PATCH - nfs-utils] Fix fallback from tcp to udp
Date: Thu, 20 Feb 2014 15:37:02 -0500	[thread overview]
Message-ID: <20140220203701.GA13433@fieldses.org> (raw)
In-Reply-To: <53064057.70703@RedHat.com>

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.

  reply	other threads:[~2014-02-20 20:37 UTC|newest]

Thread overview: 7+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2014-02-17 23:43 NeilBrown
2014-02-20 17:50 ` Steve Dickson
2014-02-20 20:37   ` J. Bruce Fields [this message]
2014-02-20 20:42     ` J. Bruce Fields
2014-02-21  3:26     ` NeilBrown
2014-02-21 14:59       ` J. Bruce Fields
2014-02-21 15:22         ` Chuck Lever

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