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]

[Harry Schmalzbauer]

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.

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