Re: [PATCH v1 38/38] nfs: add a Kconfig option for NFS reexporting and documentation

[Jeff Layton <jlayton@poochiereds.net>]

On Wed, 18 Nov 2015 14:30:41 -0800
"Frank Filz" <ffilzlnx@mindspring.com> wrote:

> Jeff Layton said:
> > On Wed, 18 Nov 2015 15:22:20 -0500
> > "J. Bruce Fields" <bfields@fieldses.org> wrote:
> > 
> > > On Tue, Nov 17, 2015 at 06:53:00AM -0500, Jeff Layton wrote:
> > > > +Filehandle size:
> > > > +----------------
> > > > +The maximum filehandle size is governed by the NFS version. Version
> > > > +2 used fixed 32 byte filehandles. Version 3 moved to variable
> > > > +length filehandles that can be up to 64 bytes in size. NFSv4
> > > > +increased that maximum to 128 bytes.
> > > > +
> > > > +When reexporting an NFS filesystem, the underlying filehandle from
> > > > +the server must be embedded inside the filehandles presented to
> > clients.
> > > > +Thus if the underlying server presents filehandles that are too
> > > > +big, the reexporting server can fail to encode them. This can lead
> > > > +to NFSERR_OPNOTSUPP errors being returned to clients.
> > > > +
> > > > +This is not a trivial thing to programatically determine ahead of
> > > > +time (and it can vary even within the same server), so some
> > > > +foreknowledge of how the underlying server constructs filehandles,
> > > > +and their maximum size is a must.
> > >
> > > This is the trickiest one, since it depends on an undocumented
> > > implementation detail of the server.
> > >
> > 
> > Yes, indeed...
> > 
> > > Do we even know if this works for all the exportable Linux filesystems?
> > >
> > > If proxying NFSv4.x servers is actually useful, could we add a per-fs
> > > maximum-filesystem-size attribute to the protocol?
> > >
> > 
> > Erm, I think you mean maximum-filehandle-size, but I get your point...
> > 
> > It's tough to do more than a quick survey, but looking at new-style fh:
> > 
> > The max fsid len seems to be 28 bytes (FSID_UUID16_INUM), though you
> > can get that down to 8 bytes if you specify the fsid directly. The fsid
> choice is
> > weird, because it sort of depends on the filehandle sent by the client
> (which
> > is used as a template), so I guess we really do need to assume worst-case.
> > 
> > Once that's done, the encode_fh routines add the fileid part. btrfs has a
> > pretty large maximum one: 40 bytes. That brings the max size up to 68
> bytes,
> > which is already too large for NFSv3, before we ever get to the part where
> > we embed that inside another fh. We require another 12 bytes on top of the
> > "underlying" filehandle for reexporting.
> > 
> > So, no this may very well not work for all exportable Linux filesystems,
> but it
> > sort of depends on the situation (and to some degree, what gets sent by
> the
> > clients). That's what makes this so hard to figure out programmatically.
> > 
> > As far as extending the protocol...that's not a bad idea, though that's
> > obviously a longer-term solution. I don't think we can reasonably rely on
> that
> > anyway. Maybe though...
> 
> I've been thinking about this kind of thing with Ganesha's proxy server, and
> conveniently, you have also provided a good use case for proxy...
> 
> One option I was going to give Ganesha is the ability to in export
> configuration indicate the upstream server is Ganesha, and expect the export
> configuration to be mirrored (easy for a config tool to do across the set of
> servers, primary and proxy) so that Ganesha could just pass handles through.
> Something similar might be possible for knfsd. With a bit more work, we
> could be prepared to deal with other servers (like Ganesha providing for
> knfsd or visa versa) to break apart the upstream handle to an "export"
> component which can be static, and a "filesystem specific" portion that
> needs to be passed through. So Ganesha could break out knfsd's fsid encoding
> and map that to an exportid, and just pass through the payload handle (the
> portion that comes from the exportfs interface).
> 
> Frank
> 

It would be very tough to just pass the filehandles through here. After
all, we're hooking nfsd up to the nfs client code. You could (in
principle) have a mix of regular filesystems and reexported nfs mounts.
What if there are filehandle collisions between the one you passed
through and one of your local exported filesystems?

Breaking up the filehandle is also pretty much impossible to do in a
general way. The problem of course is that filehandles are really
opaque blobs to the client. You'd have to know ahead of time what part
of it refers to the fsid and what part is the fileid part. knfsd can
compose all sorts of filehandles, and it tries hard to mirror the type
of fh that the client is using.

Beyond that, what do you do when you get one of these "reconstituted"
filehandles back from the client where you've stripped off the fsid
info? At some point you have to reconstruct the original filehandle so
you can call back to the underlying server. Where do you store the fsid
info? Note that it has to be persistent across reboots too or you'll
see stale nfs filehandles on the clients.

-- 
Jeff Layton <jlayton@poochiereds.net>