Re: [PATCH] rpc.nfsd: mount up nfsdfs is it doesn't appear to be mounted yet

[Neil Brown <neilb@suse.de>]

(I might have just sent an empty reply to this - sorry 'bout that).

On Sat, 28 Aug 2010 07:35:14 -0400
Jeff Layton <jlayton@redhat.com> wrote:

> There's a bit of a chicken and egg problem when nfsd is run the first
> time. On Fedora/RHEL at least, /proc/fs/nfsd is mounted up whenever nfsd
> is plugged in via a modprobe.conf "install" directive.
> 
> If someone runs rpc.nfsd without plugging in nfsd.ko first,
> /proc/fs/nfsd won't be mounted and rpc.nfsd will end up using the legacy
> nfsctl interface. After that, nfsd will be plugged in and subsequent
> rpc.nfsd invocations will use that instead.
> 
> This is a problem as some nfsd command-line options are ignored when the
> legacy interface is used. It'll also be a problem for people who want
> IPv6 enabled servers. The upshot is that we really don't want to use the
> legacy interface unless there is no other option.
> 
> To avoid this situation, have rpc.nfsd check to see if the "threads"
> file is already present. If it's not, then make an attempt to mount
> /proc/fs/nfsd.  This is a "best-effort" sort of thing, however so we
> just ignore the return code from the mount attempt and fall back to
> using nfsctl() if it fails.
> 
> Full disclosure: I'm not 100% thrilled with this patch. It seems ugly
> and kludgey, but I don't see a better way to handle this problem.
> Suggestions welcome.

I don't think it is all that bad.  It is a shame you have to use system()
rather than just calling mount() directly but I guess we need that to
update /etc/mtab.

Suggestions:

 - just don't do that.  Use /etc/init.d/nfsserver start (or whatever the
   distro uses).
 - Make /proc/fs/nfsd and auto-mount point.  That sounds like the systemd
   approach.
 - Print a warning message if the kernel is newer than 2.6 and nfsd-fs isn't
   mounted - see "don't do that" above.
 - If nfsdfs isn't mounted, try a no-op via the nfsd syscall, like
   NFSCTL_GETFD with invalid parameters.  This will trigger a mod-probe which
   will trigger the mount.
   Then check for the mounted filesystem again.  Yes, I think I like that one
   the best.

NeilBrown