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Subject: Re: [PATCH RFC 2/9] timekeeping: new interfaces for multigrain
 timestamp handing
From:   Jeff Layton <jlayton@kernel.org>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Dave Chinner <david@fromorbit.com>,
        Kent Overstreet <kent.overstreet@linux.dev>,
        Christian Brauner <brauner@kernel.org>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        John Stultz <jstultz@google.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Stephen Boyd <sboyd@kernel.org>,
        Chandan Babu R <chandan.babu@oracle.com>,
        "Darrick J. Wong" <djwong@kernel.org>,
        Theodore Ts'o <tytso@mit.edu>,
        Andreas Dilger <adilger.kernel@dilger.ca>,
        Chris Mason <clm@fb.com>, Josef Bacik <josef@toxicpanda.com>,
        David Sterba <dsterba@suse.com>,
        Hugh Dickins <hughd@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Amir Goldstein <amir73il@gmail.com>, Jan Kara <jack@suse.de>,
        David Howells <dhowells@redhat.com>,
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        linux-btrfs@vger.kernel.org, linux-mm@kvack.org,
        linux-nfs@vger.kernel.org
Date:   Tue, 24 Oct 2023 16:19:17 -0400
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On Tue, 2023-10-24 at 09:40 -1000, Linus Torvalds wrote:
> On Tue, 24 Oct 2023 at 09:07, Jeff Layton <jlayton@kernel.org> wrote:
> >=20
> > The new flag idea is a good one. The catch though is that there are no
> > readers of i_version in-kernel other than NFSD and IMA, so there would
> > be no in-kernel users of I_VERSION_QUERIED_STRICT.
>=20
> I actually see that as an absolute positive.
>=20
> I think we should *conceptually* do those two flags, but then realize
> that there are no users of the STRICT version, and just skip it.
>=20
> So practically speaking, we'd end up with just a weaker version of
> I_VERSION_QUERIED that is that "I don't care about atime" case.
>=20

To be clear, this is not kernel-wide behavior. Most filesystems already
don't bump their i_version on atime updates. XFS is the only one that
does. ext4 used to do that too, but we fixed that several months ago.
I did try to just fix XFS in the same way, but the patch was NAK'ed.

> I really can't find any use that would *want* to see i_version updates
> for any atime updates. Ever.
>=20
> We may have had historical user interfaces for i_version, but I can't
> find any currently.
>=20
> But to be very very clear: I've only done some random grepping, and I
> may have missed something. I'm not dismissing Dave's worries, and he
> may well be entirely correct.
>=20
> Somebody would need to do a much more careful check than my "I can't
> find anything".

Exactly. I'm not really an XFS guy, so I took those folks at their word
that this was behavior that they just can't trivially change.

None of the in-kernel callers that look at i_version want it to be
incremented on atime-onlt updates, however. So IIRC, the objection was
due to offline repair/analysis tools that depend this the value being
incremented in a specific way.
--=20
Jeff Layton <jlayton@kernel.org>