linux-kernel - Re: [PATCH v2 00/11] fs: multigrain timestamp redux

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Message-ID: <20240701135332.GD504479@perftesting>
Date: Mon, 1 Jul 2024 09:53:32 -0400
From: Josef Bacik <josef@...icpanda.com>
To: Jeff Layton <jlayton@...nel.org>
Cc: 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>,
	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>,
	David Sterba <dsterba@...e.com>, Hugh Dickins <hughd@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andi Kleen <ak@...ux.intel.com>, kernel-team@...com,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-trace-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 v2 00/11] fs: multigrain timestamp redux

On Mon, Jul 01, 2024 at 06:26:36AM -0400, Jeff Layton wrote:
> This set is essentially unchanged from the last one, aside from the
> new file in Documentation/. I had a review comment from Andi Kleen
> suggesting that the ctime_floor should be per time_namespace, but I
> think that's incorrect as the realtime clock is not namespaced.
> 
> At LSF/MM this year, we had a discussion about the inode change
> attribute. At the time I mentioned that I thought I could salvage the
> multigrain timestamp work that had to be reverted last year [1].  That
> version had to be reverted because it was possible for a file to get a
> coarse grained timestamp that appeared to be earlier than another file
> that had recently gotten a fine-grained stamp.
> 
> This version corrects the problem by establishing a per-time_namespace
> ctime_floor value that should prevent this from occurring. In the above
> situation that was problematic before, the two files might end up with
> the same timestamp value, but they won't appear to have been modified in
> the wrong order.
> 
> That problem was discovered by the test-stat-time gnulib test. Note that
> that test still fails on multigrain timestamps, but that's because its
> method of determining the minimum delay that will show a timestamp
> change will no longer work with multigrain timestamps. I have a patch to
> change the testcase to use a different method that I've posted to the
> bug-gnulib mailing list.
> 
> The big question with this set is whether the performance will be
> suitable. The testing I've done seems to show performance parity with
> multigrain timestamps enabled, but it's hard to rule this out regressing
> some workload.
> 
> This set is based on top of Christian's vfs.misc branch (which has the
> earlier change to track inode timestamps as discrete integers). If there
> are no major objections, I'd like to let this soak in linux-next for a
> bit to see if any problems shake out.
> 
> [1]: https://lore.kernel.org/linux-fsdevel/20230807-mgctime-v7-0-d1dec143a704@kernel.org/
> 
> Signed-off-by: Jeff Layton <jlayton@...nel.org>

I have a few nits that need to be addressed, but you can add

Reviewed-by: Josef Bacik <josef@...icpanda.com>

to the series once they're addressed.  Thanks,

Josef