From: Trond Myklebust <redacted>
Sent: Tuesday, March 14, 2023 2:40 PM

quoted

On Mar 13, 2023, at 11:17, Andrew Klaassen

[off-list ref] wrote:

quoted

We are using applications which hang if any NFS servers fail to
respond.  We would like to be able to control NFS timeouts so that we
can control the maximum time that the applications hang.  We currently
can't do that with TCP NFS mounts, since RPC calls made to an existing
NFS mount are first subject to the default untuneable Sun RPC timeout
of 2 minutes.

(I'll note that the existing NFS manpage seems to not describe current
behaviour correctly, since it says that this two-minute timeout
applies to initial mount operations (which it does not), and does not
say that the two-minute timeout applies to operations on existing
mounts (which it does).)

An existing thread discussing this patch can be found here:

Link:
https://lore.kernel.org/linux-nfs/45e2e7f05a13abab777b3b0868744cdbfc62
3f2d.camel@kernel.org/T/

This patch uses the RPC call timeout to set the xprt timeout.  In that
discussion thread, Jeff Layton has pointed out that this may or may
not be the ideal approach.  I have suggested these alternatives, and
would be happy to get feedback:

- Create system-wide tuneables for xs_[local|udp|tcp]_default_timeout.
In our case that's less-than-ideal, since we want to change the total
timeout for an NFS mount on a per-server or per-mount basis rather
than a system-wide basis, but it would do in a pinch.

- Add a second set of timeout options to NFS so that RPC call and xprt
timeouts can be specified separately.  I'm guessing no-one is
enthusiastic about option bloat, even if this would be the
theoretically cleanest option.  I'm guessing this would also involve
changing the Sun RPC API and everything that calls it in order for it
to accept the second set of timeout options.

- Use timeo and retrans for the RPC call timeout, and retry for the
xprt timeout.  Or do the opposite.  The NFS manpage describes the
current behaviour incorrectly, so this at least wouldn't make the
documentation any worse.  I assume this would also involve changing the

Sun RPC API.

quoted

Use rpc_create_args->timeout to initialize rpc_xprt->timeout

Just because something can be done in the kernel, it doesn’t mean that it
should be done in the kernel. If you’re unhappy with sunrpc timeouts, then it
should be quite possible to do those calls in userspace, and pass the port
number down as part of the mount syscall.

Thanks for the direction, Trond.  I'll spend some time getting familiar with the code and see if I can make that happen.

I'm currently clueless about how to get started, as there doesn't appear to be any way to override sunrpc timeout defaults for any sunrpc call, so I may have some followup questions once I get my head wrapped around the mount code.

Andrew