support for pNFS with Linux as Data Servers

Hello!

Currently, FreeBSD supports pNFS with File and Flexible File
layouts. For the Flexible File layout, FreeBSD supports a tightly
coupled locking model that requires FreeBSD servers as Data Servers
(DSs). I'm interested in adding support for a loosely coupled locking
model that would allow Linux machines to be used as Data Servers. To
that end, I'd like to gauge interest and get feedback about that.

AFAICT there are two changes needed to add this support. The first
change is to allow for file handles of variable length and up to the
maximum size allowed by the RFC. This seems relatively
straightforward, and could be done by changing a bunch of uses of
fhandle_t to instead use a data structure that can store larger file
handles. Also, nfsrv_layoutget() and the use of NFSX_V4FLEXLAYOUT
would need to be updated to allow for variable lengths.

The second change is to support a loosely coupled locking
model. Besides trivially setting the ffdv_tightly_coupled flag of
ff_device_versions4 to false, I see two immediate issues. One issue is
the FreeBSD's special 0x555555555555555555555555 state ID (along with
its special seq num 0xffffffff) is, naturally, not understood by a
Linux Data Server. The other issue is clients will use the synthetic
uid/gid given by the MDS (currently 999/999), and this results in
access errors when the clients talk to the DSs.

I've made some of these changes in a rough manner, not in a
production-ready way, and I/O seems to work and some basic tests pass.

Is the FreeBSD community interested in development in this direction,
i.e. pNFS using FreeBSD for MDS and Linux for DSs?

Thanks!