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

[Chuck Lever <chuck.lever@oracle.com>]

On Feb 21, 2014, at 6:59 AM, J. Bruce Fields <bfields@fieldses.org> wrote:

> 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.

Just a general comment.  Mount negotiation is a maze because

  a) we have so many legacy use cases that still MUST work, and

  b) we have no regression test suite that can confirm that mount.nfs is still operating correctly after code changes

Thus it’s very difficult to clean up over time.  It just accretes more and more logic.  We add little bits here and there because it seems safe, but that adds up.

--
Chuck Lever
chuck[dot]lever[at]oracle[dot]com