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Date:   Fri, 20 Nov 2020 17:38:31 -0500
From:   "J. Bruce Fields" <bfields@fieldses.org>
To:     Jeff Layton <jlayton@kernel.org>
Cc:     "J. Bruce Fields" <bfields@redhat.com>,
        Daire Byrne <daire@dneg.com>,
        Trond Myklebust <trondmy@hammerspace.com>,
        linux-cachefs <linux-cachefs@redhat.com>,
        linux-nfs <linux-nfs@vger.kernel.org>
Subject: Re: [PATCH 2/4] nfsd: pre/post attr is using wrong change attribute
Message-ID: <20201120223831.GB7705@fieldses.org>
References: <20201117031601.GB10526@fieldses.org>
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On Tue, Nov 17, 2020 at 10:34:57AM -0500, Jeff Layton wrote:
> On Tue, 2020-11-17 at 10:26 -0500, J. Bruce Fields wrote:
> > On Tue, Nov 17, 2020 at 07:34:49AM -0500, Jeff Layton wrote:
> > > I don't think I described what I was thinking well. Let me try again...
> > > 
> > > There should be no need to change the code in iversion.h -- I think we
> > > can do this in a way that's confined to just nfsd/export code.
> > > 
> > > What I would suggest is to have nfsd4_change_attribute call the
> > > fetch_iversion op if it exists, instead of checking IS_I_VERSION and
> > > doing the stuff in that block. If fetch_iversion is NULL, then just use
> > > the ctime.
> > > 
> > > Then, you just need to make sure that the filesystems' export_ops have
> > > an appropriate fetch_iversion vector. xfs, ext4 and btrfs can just call
> > > inode_query_iversion, and NFS and Ceph can call inode_peek_iversion_raw.
> > > The rest of the filesystems can leave fetch_iversion as NULL (since we
> > > don't want to use it on them).
> > 
> > Thanks for your patience, that makes sense, I'll try it.
> > 
> 
> There is one gotcha in here though... ext4 needs to also handle the case
> where SB_I_VERSION is not set. The simple fix might be to just have
> different export ops for ext4 based on whether it was mounted with -o
> iversion or not, but maybe there is some better way to do it?

I was thinking ext4's export op could check for I_VERSION on its own and
vary behavior based on that.

I'll follow up with new patches in a moment.

I think the first one's all that's needed to fix the problem Daire
identified.  I'm a little less sure of the rest.

Lightly tested, just by running them through my usual regression tests
(which don't re-export) and then running connectathon on a 4.2 re-export
of a 4.2 mount.

The latter triggered a crash preceded by a KASAN use-after free warning.
Looks like it might be a problem with blocking lock notifications,
probably not related to these patches.

--b.