linux-kernel - Re: [PATCH v8 01/11] timekeeping: move multigrain timestamp floor handling into timekeeper

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Message-ID: <871q102904.ffs@tglx>
Date: Mon, 30 Sep 2024 22:15:39 +0200
From: Thomas Gleixner <tglx@...utronix.de>
To: Jeff Layton <jlayton@...nel.org>, John Stultz <jstultz@...gle.com>,
 Stephen Boyd <sboyd@...nel.org>, Alexander Viro <viro@...iv.linux.org.uk>,
 Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>, Steven
 Rostedt <rostedt@...dmis.org>, Masami Hiramatsu <mhiramat@...nel.org>,
 Mathieu Desnoyers <mathieu.desnoyers@...icios.com>, Jonathan Corbet
 <corbet@....net>, 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>, Chuck Lever
 <chuck.lever@...cle.com>, Vadim Fedorenko <vadim.fedorenko@...ux.dev>
Cc: Randy Dunlap <rdunlap@...radead.org>, linux-kernel@...r.kernel.org,
 linux-fsdevel@...r.kernel.org, linux-trace-kernel@...r.kernel.org,
 linux-doc@...r.kernel.org, linux-xfs@...r.kernel.org,
 linux-ext4@...r.kernel.org, linux-btrfs@...r.kernel.org,
 linux-nfs@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v8 01/11] timekeeping: move multigrain timestamp floor
 handling into timekeeper

On Mon, Sep 30 2024 at 15:27, Jeff Layton wrote:
> On Mon, 2024-09-30 at 21:13 +0200, Thomas Gleixner wrote:
>> So if that's the intended behaviour then the changelog is misleading at
>> best.
>
> That is the intended behavior and I'll plan to fix the changelog to
> clarify this point:
>
> If someone jumps the realtime clock backward by a large value, then the
> realtime timestamp _can_ appear to go backward. This is a problem today
> even without this patchset.

Correct.

> If two files get stamped and a realtime clock jump backward happens in
> between them, all bets are off as to which one will appear to have been
> modified first. I don't think that is something we can reasonably
> prevent, since we must stamp files according to the realtime clock.

True. I just was utterly confused about the changelog.

> The main thing I'm trying to prevent is the timestamps being misordered
> in the absence of such a clock jump. Without tracking the floor as I am
> here, that's a possibility.

Correct.