linux-kernel - Re: [PATCH v8 01/12] timekeeping: add interfaces for handling timestamps with a floor value

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Message-ID: <CANDhNCrKFvUYchQ8UStxUEpBmFpN4ZeP4W4DdwJ5WxZ5EbqjMw@mail.gmail.com>
Date: Tue, 1 Oct 2024 09:17:31 -0700
From: John Stultz <jstultz@...gle.com>
To: Jeff Layton <jlayton@...nel.org>
Cc: Thomas Gleixner <tglx@...utronix.de>, 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>, 
	Randy Dunlap <rdunlap@...radead.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>, Chuck Lever <chuck.lever@...cle.com>, 
	Vadim Fedorenko <vadim.fedorenko@...ux.dev>, 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 01/12] timekeeping: add interfaces for handling
 timestamps with a floor value

On Tue, Oct 1, 2024 at 3:59 AM Jeff Layton <jlayton@...nel.org> wrote:
>
> Multigrain timestamps allow the kernel to use fine-grained timestamps
> when an inode's attributes is being actively observed via ->getattr().
> With this support, it's possible for a file to get a fine-grained
> timestamp, and another modified after it to get a coarse-grained stamp
> that is earlier than the fine-grained time.  If this happens then the
> files can appear to have been modified in reverse order, which breaks
> VFS ordering guarantees.
>
> To prevent this, maintain a floor value for multigrain timestamps.
> Whenever a fine-grained timestamp is handed out, record it, and when
> coarse-grained stamps are handed out, ensure they are not earlier than
> that value. If the coarse-grained timestamp is earlier than the
> fine-grained floor, return the floor value instead.
>
> Add a static singleton atomic64_t into timekeeper.c that we can use to
> keep track of the latest fine-grained time ever handed out. This is
> tracked as a monotonic ktime_t value to ensure that it isn't affected by
> clock jumps. Because it is updated at different times than the rest of
> the timekeeper object, the floor value is managed independently of the
> timekeeper via a cmpxchg() operation, and sits on its own cacheline.
>
> This patch also adds two new public interfaces:
>
> - ktime_get_coarse_real_ts64_mg() fills a timespec64 with the later of the
>   coarse-grained clock and the floor time
>
> - ktime_get_real_ts64_mg() gets the fine-grained clock value, and tries
>   to swap it into the floor. A timespec64 is filled with the result.
>
> Since the floor is global, take care to avoid updating it unless it's
> absolutely necessary. If we do the cmpxchg and find that the value has
> been updated since we fetched it, then we discard the fine-grained time
> that was fetched in favor of the recent update.
>
> Note that the VFS ordering guarantees assume that the realtime clock
> does not experience a backward jump. POSIX requires that we stamp files
> using realtime clock values, so if a backward clock jump occurs, then
> files can appear to have been modified in reverse order.
>
> Tested-by: Randy Dunlap <rdunlap@...radead.org> # documentation bits
> Signed-off-by: Jeff Layton <jlayton@...nel.org>

Acked-by: John Stultz <jstultz@...gle.com>