Date: Mon, 8 Aug 2016 23:02:13 -0700 From: Mark Johnston <markj@FreeBSD.org> To: Harry Schmalzbauer <freebsd@omnilan.de> Cc: Rick Macklem <rmacklem@uoguelph.ca>, FreeBSD Stable <freebsd-stable@freebsd.org> Subject: Re: 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] Message-ID: <20160809060213.GA67664@raichu> In-Reply-To: <57A83C78.1070403@omnilan.de> References: <57A79E24.8000100@omnilan.de> <YQBPR01MB0401201977AEA8A803F27B23DD1A0@YQBPR01MB0401.CANPRD01.PROD.OUTLOOK.COM> <57A83C78.1070403@omnilan.de>
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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 > > _______________________________________________ > freebsd-stable@freebsd.org mailing list > https://lists.freebsd.org/mailman/listinfo/freebsd-stable > To unsubscribe, send any mail to "freebsd-stable-unsubscribe@freebsd.org"
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