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

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Date:   Wed, 1 Nov 2023 12:23:44 -1000
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Trond Myklebust <trondmy@...merspace.com>
Cc:     Jan Kara <jack@...e.cz>, Chris Mason <clm@...com>,
        Josef Bacik <josef@...icpanda.com>,
        John Stultz <jstultz@...gle.com>,
        "Darrick J. Wong" <djwong@...nel.org>,
        Christian Brauner <brauner@...nel.org>,
        Chandan Babu R <chandan.babu@...cle.com>,
        Hugh Dickins <hughd@...gle.com>,
        linux-xfs <linux-xfs@...r.kernel.org>,
        Dave Chinner <david@...morbit.com>,
        Andrew Morton <akpm@...ux-foundation.org>, dsterba@...e.com,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Jeff Layton <jlayton@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Linux-MM <linux-mm@...ck.org>,
        "open list:NFS, SUNRPC, AND..." <linux-nfs@...r.kernel.org>,
        "Theodore Ts'o" <tytso@....edu>, Al Viro <viro@...iv.linux.org.uk>,
        Ext4 Developers List <linux-ext4@...r.kernel.org>,
        Amir Goldstein <amir73il@...il.com>,
        linux-btrfs <linux-btrfs@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        Kent Overstreet <kent.overstreet@...ux.dev>,
        Stephen Boyd <sboyd@...nel.org>,
        David Howells <dhowells@...hat.com>, Jan Kara <jack@...e.de>
Subject: Re: [PATCH RFC 2/9] timekeeping: new interfaces for multigrain
 timestamp handing

On Wed, Nov 1, 2023, 11:35 Trond Myklebust <trondmy@...merspace.com> wrote:
>
> My client writes to the file and immediately reads the ctime. A 3rd
> party client then writes immediately after my ctime read.
> A reboot occurs (maybe minutes later), then I re-read the ctime, and
> get the same value as before the 3rd party write.
>
> Yes, most of the time that is better than the naked ctime, but not
> across a reboot.

Ahh, I knew I was missing something.

But I think it's fixable, with an additional rule:

 - when generating STATX_CHANGE_COOKIE, if the ctime matches the
current time and the ctime counter is zero, set the ctime counter to
1.

That means that you will have *spurious* cache invalidations of such
cached data after a reboot, but only for reads that happened right
after the file was written.

Now, it's obviously not unheard of to finish writing a file, and then
immediately reading the results again.

But at least those caches should be somewhat limited (and the problem
then only happens when the nfs server is rebooted).

I *assume* that the whole thundering herd issue with lots of clients
tends to be for stable files, not files that were just written and
then immediately cached?

I dunno. I'm sure there's still some thinko here.

             Linus