unionfs bugs, a partial patch and some comments [Was: Re: 1-BETA3 Panic: __lockmgr_args: downgrade a recursed lockmgr nfs @ /usr/local/share/deploy-tools/RELENG_11/src/sys/fs/unionfs/union_vnops.c:1905]

[Mark Johnston]

On Mon, Aug 08, 2016 at 10:02:00AM +0200, Harry Schmalzbauer wrote:
> Bezüglich Rick Macklem's Nachricht vom 07.08.2016 23:34 (localtime):
> > Harry Schmalzbauer wrote:
> >> Hello,
> >>
> >> I had another crash which I'm quite sure was triggered by mount_unionfs:
> > Just in case you are not already aware, unionfs is always broken. Read
> > the BUGS
> > section at the end of "man mount_unionfs". If it were easy to fix,
> > someone would
> > have done so long ago. Yes, some use it successfully, but if not...
> > 
> > Sorry, but I suspect that is how it will remain, rick
> 
> Thanks for the hint, not happy to hear that, but I was not aware of that
> explicit warning in man 8 mount_unionfs :-(
> 
> This feature is utterly important for me (all my productive machines
> have "/" read-only mounted and "/etc" is an union to a writable, synch
> mounted separate fs), so back in 2012, after a lot of locking redesign
> has been done in 9-current, I got Attilio Raos attention and he gave out
> some test patches for 9.0.
> He was aware of missing locking adjustments, but patches addressing the
> majority of them didn't work.
> Since then I'm draging a minimal patch which prevents at least the
> kernel panics for me.
> Unfortunately I don't have the skills to continue Attilio Raos work.
> 
> Just for anybody else needing unionfs:
> https://people.freebsd.org/~attilio/unionfs_missing_insmntque_lock.patch
> 
> This patch still applies and I'm successfully using this (unmodified) up
> to FreeBSD-10.3 and never had any panic in all these years.

Having spent some time looking at unionfs, I'm a bit skeptical that this
patch will address the panic you reported earlier, though I'd be
interested to know if it does. Reading the code, I think it will just
address an INVARIANTS-only assertion in insmntque1().

Unfortunately, unionfs is quite difficult to fix within the current
constraints of FreeBSD's VFS. unionfs_readdir() is a particularly good
demonstration of this fact: some callers of VOP_READDIR expect the
cookies returned by the FS to be monotonically increasing, but unionfs
has no straightforward way to make this guarantee.

> 
> I will continue using it for FreeBSD-11 and I guess it will also prevent
> my last reported panics.
> But I wanted to take part in the BETA test without local modifications
> at first.
> 
> Another very importend usage scenario of unionfs for me is for my build
> host(s). I'm (nfs4-)sharing a svn-checked out read-only portstree. My
> inofficial "ports/inofficial" directory perfectly shows up by
> unionfs-mounting it below the unaltered portstree :-)
> 
> For me, unionfs is as important as ZFS (and nullfs) is in FreeBSD.
> 
> First thing to do for me, after I won in lottery, was to find someone
> who can be sponsored fixing unionfs ;-) And bringing MNAMELEN into 21st
> century state, matching ZFS needs:
> https://lists.freebsd.org/pipermail/freebsd-hackers/2015-November/048640.html
> This is another patch I'm carrying for a very long time which solves
> tremendous limitations for me. Without that, I couldn't use ZFS
> snapshots in real world, along with a human-friendly dataset naming :-)
> 
> -Harry
> 
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