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

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Date:   Fri, 20 Oct 2023 13:06:13 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Jeff Layton <jlayton@...nel.org>
Cc:     Dave Chinner <david@...morbit.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>,
        Amir Goldstein <amir73il@...il.com>, 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

On Fri, 20 Oct 2023 at 05:12, Jeff Layton <jlayton@...nel.org> wrote:.
>
> I'd _really_ like to see a proper change counter added before it's
> merged, or at least space in the on-disk inode reserved for one until we
> can get it plumbed in.

Hmm. Can we not perhaps just do an in-memory change counter, and try
to initialize it to a random value when instantiating an inode? Do we
even *require* on-disk format changes?

So on reboot, the inode would count as "changed" as far any remote
user is concerned. It would flush client caches, but isn't that what
you'd want anyway? I'd hate to waste lots of memory, but maybe people
would be ok with just a 32-bit random value. And if not...

But I actually came into this whole discussion purely through the
inode timestamp side, so I may *entirely* miss what the change counter
requirements for NFSd actually are. If it needs to be stable across
reboots, my idea is clearly complete garbage.

You can now all jump on me and point out my severe intellectual
limitations. Please use small words when you do ;)

              Linus