linux-ext4 - Re: [PATCH RFC 2/9] timekeeping: new interfaces for multigrain timestamp handing

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Date:   Tue, 24 Oct 2023 16:19:17 -0400
From:   Jeff Layton <jlayton@...nel.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Dave Chinner <david@...morbit.com>,
        Kent Overstreet <kent.overstreet@...ux.dev>,
        Christian Brauner <brauner@...nel.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        John Stultz <jstultz@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Stephen Boyd <sboyd@...nel.org>,
        Chandan Babu R <chandan.babu@...cle.com>,
        "Darrick J. Wong" <djwong@...nel.org>,
        Theodore Ts'o <tytso@....edu>,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        Chris Mason <clm@...com>, Josef Bacik <josef@...icpanda.com>,
        David Sterba <dsterba@...e.com>,
        Hugh Dickins <hughd@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Amir Goldstein <amir73il@...il.com>, Jan Kara <jack@...e.de>,
        David Howells <dhowells@...hat.com>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-xfs@...r.kernel.org, linux-ext4@...r.kernel.org,
        linux-btrfs@...r.kernel.org, linux-mm@...ck.org,
        linux-nfs@...r.kernel.org
Subject: Re: [PATCH RFC 2/9] timekeeping: new interfaces for multigrain
 timestamp handing

On Tue, 2023-10-24 at 09:40 -1000, Linus Torvalds wrote:
> On Tue, 24 Oct 2023 at 09:07, Jeff Layton <jlayton@...nel.org> wrote:
> > 
> > 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.
> 
> I actually see that as an absolute positive.
> 
> 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.
> 
> 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.
> 

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.
> 
> We may have had historical user interfaces for i_version, but I can't
> find any currently.
> 
> 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.
> 
> 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.
-- 
Jeff Layton <jlayton@...nel.org>