Re: [RFC PATCH 0/6] nfs-utils: Improving NFS re-exports

["J. Bruce Fields" <bfields@fieldses.org>]

On Thu, Feb 17, 2022 at 02:15:25PM +0100, Richard Weinberger wrote:
> This is the second iteration of the NFS re-export improvement series for nfs-utils.
> While the kernel side didn't change at all and is still small,
> the userspace side saw much more changes.
> Please note that this is still an RFC, there is room for improvement.
> 
> The core idea is adding new export option: reeport=
> Using reexport= it is possible to mark an export entry in the exports file
> explicitly as NFS re-export and select a strategy how unique identifiers
> should be provided.
> "remote-devfsid" is the strategy I have proposed in my first patch,
> I understand that this one is dangerous. But I still find it useful in some
> situations.
> "auto-fsidnum" and "predefined-fsidnum" are new and use a SQLite database as
> backend to keep track of generated ids.
> For a more detailed description see patch "exports: Implement new export option reexport=".

Thanks, I'll try to take a look.

Before upstreaming, I would like us to pick just one.  These kind of
options tend to complicate testing and documentation and debugging.

For an RFC, though, I think it makes sense, so I'm fine with keeping
"reexport=" while we're still exploring the different options.  And,
hey, maybe we end up adding more than one after we've upstreamed the
first one.

> I choose SQLite because nfs-utils already uses it and using SQL ids can nicely
> generated and maintained. It will also scale for large setups where the amount
> of subvolumes is high.
> 
> Beside of id generation this series also addresses the reboot problem.
> If the re-exporting NFS server reboots, uncovered NFS subvolumes are not yet
> mounted and file handles become stale.
> Now mountd/exportd keeps track of uncovered subvolumes and makes sure they get
> uncovered while nfsd starts.
> 
> The whole set of features is currently opt-in via --enable-reexport.
> I'm also not sure about the rearrangement of the reexport code,
> currently it is a helper library.
> 
> Please let me know whether you like this approach.
> If so I'd tidy it up and submit it as non-RFC.
> 
> TODOs/Open questions:
> - When re-exporting, fs.nfs.nfs_mountpoint_timeout should be set to 0
>   to make subvolumes not vanish.
>   Is this something exportfs should do automatically when it sees an export entry with a reexport= option?

Setting the timeout to 0 doesn't help with re-export server reboots.
After a reboot is another case where we could end up in a situation
where a client hands us a filehandle for a filesystem that isn't mounted
yet.

I think you want to keep a path with each entry in the database.  When
mountd gets a request for a filesystem it hasn't seen before, it stats
that path, which should trigger the automounts.

And it'd be good to have a test case with a client (Linux client or
pynfs) that, say, opens a file several mounts deep, then reboots the
reexport server, then tries to, say, write to the file descriptor after
the reboot.  (Or maybe there's a way to force the mounts to expire as a
shortcut instead of doing a full reboot.)

> - exportd saw only minimal testing so far, I wasn't aware of it yet. :-S
> - Currently wtere is no way to release the shared memory which contains the database lock.
>   I guess it could be released via exportfs -f, which is the very last exec in nfs-server.service
> - Add a tool to import/export entries from the reexport database which obeys the shared lock.
> - When doing v4->v4 or v3->v4 re-exports very first read access to a file block a few seconds until
>   the client does a retransmit. 
>   v3->v3 works fine. More investigation needed.

Might want to strace mountd and look at the communication over the
/proc/fs/nfsd/*/channel files, maybe mountd is failing to respond to an
upcall.

--b.

> 
> Looking forward for your feedback!
> 
> Thanks,
> //richard
> 
> Richard Weinberger (6):
>   Implement reexport helper library
>   exports: Implement new export option reexport=
>   export: Implement logic behind reexport=
>   export: Record mounted volumes
>   nfsd: statfs() every known subvolume upon start
>   export: Garbage collect orphaned subvolumes upon start
> 
>  configure.ac                 |  12 +
>  support/Makefile.am          |   4 +
>  support/export/Makefile.am   |   2 +
>  support/export/cache.c       | 241 +++++++++++++++++-
>  support/export/export.h      |   3 +
>  support/include/nfslib.h     |   1 +
>  support/nfs/Makefile.am      |   1 +
>  support/nfs/exports.c        |  73 ++++++
>  support/reexport/Makefile.am |   6 +
>  support/reexport/reexport.c  | 477 +++++++++++++++++++++++++++++++++++
>  support/reexport/reexport.h  |  53 ++++
>  utils/exportd/Makefile.am    |   8 +-
>  utils/exportd/exportd.c      |  17 ++
>  utils/exportfs/Makefile.am   |   4 +
>  utils/mount/Makefile.am      |   6 +
>  utils/mountd/Makefile.am     |   6 +
>  utils/mountd/mountd.c        |   1 +
>  utils/mountd/svc_run.c       |  18 ++
>  utils/nfsd/Makefile.am       |   6 +
>  utils/nfsd/nfsd.c            |  10 +
>  20 files changed, 934 insertions(+), 15 deletions(-)
>  create mode 100644 support/reexport/Makefile.am
>  create mode 100644 support/reexport/reexport.c
>  create mode 100644 support/reexport/reexport.h
> 
> -- 
> 2.31.1