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

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Date:   Tue, 31 Oct 2023 15:43:04 -0400
From:   "John Stoffel" <john@...ffel.org>
To:     Jeff Layton <jlayton@...nel.org>
Cc:     Dave Chinner <david@...morbit.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Amir Goldstein <amir73il@...il.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>,
        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

>>>>> "Jeff" == Jeff Layton <jlayton@...nel.org> writes:

> On Tue, 2023-10-31 at 12:42 +1100, Dave Chinner wrote:
>> On Mon, Oct 30, 2023 at 01:11:56PM -1000, Linus Torvalds wrote:
>> > On Mon, 30 Oct 2023 at 12:37, Dave Chinner <david@...morbit.com> wrote:
>> > > 
>> > > If XFS can ignore relatime or lazytime persistent updates for given
>> > > situations, then *we don't need to make periodic on-disk updates of
>> > > atime*. This makes the whole problem of "persistent atime update bumps
>> > > i_version" go away because then we *aren't making persistent atime
>> > > updates* except when some other persistent modification that bumps
>> > > [cm]time occurs.
>> > 
>> > Well, I think this should be split into two independent questions:
>> > 
>> >  (a) are relatime or lazytime atime updates persistent if nothing else changes?
>> 
>> They only become persistent after 24 hours or, in the case of
>> relatime, immediately persistent if mtime < atime (i.e. read after a
>> modification). Those are the only times that the VFS triggers
>> persistent writeback of atime, and it's the latter case (mtime <
>> atime) that is the specific trigger that exposed the problem with
>> atime bumping i_version in the first place.
>> 
>> >  (b) do atime updates _ever_ update i_version *regardless* of relatime
>> > or lazytime?
>> > 
>> > and honestly, I think the best answer to (b) would be that "no,
>> > i_version should simply not change for atime updates". And I think
>> > that answer is what it is because no user of i_version seems to want
>> > it.
>> 
>> As I keep repeating: Repeatedly stating that "atime should not bump
>> i_version" does not address the questions I'm asking *at all*.
>> 
>> > Now, the reason it's a single question for you is that apparently for
>> > XFS, the only thing that matters is "inode was written to disk" and
>> > that "di_changecount" value is thus related to the persistence of
>> > atime updates, but splitting di_changecount out to be a separate thing
>> > from i_version seems to be on the table, so I think those two things
>> > really could be independent issues.
>> 
>> Wrong way around - we'd have to split i_version out from
>> di_changecount. It's i_version that has changed semantics, not
>> di_changecount, and di_changecount behaviour must remain unchanged.
>> 

> I have to take issue with your characterization of this. The
> requirements for NFS's change counter have not changed. Clearly there
> was a breakdown in communications when it was first implemented in Linux
> that caused atime updates to get counted in the i_version value, but
> that was never intentional and never by design.

This has been bugging me, but all the references to NFS really mean
NFSv4.1 or newer, correct?  I can't see how any of this affects NFSv3
at all, and that's probably the still dominant form of NFS, right?  

John