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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Mon, 23 Oct 2023 14:18:12 -1000
Message-ID: <CAHk-=whphyjjLwDcEthOOFXXfgwGrtrMnW2iyjdQioV6YSMEPw@mail.gmail.com>
Subject: Re: [PATCH RFC 2/9] timekeeping: new interfaces for multigrain
 timestamp handing
To:     Dave Chinner <david@fromorbit.com>
Cc:     Jeff Layton <jlayton@kernel.org>,
        Kent Overstreet <kent.overstreet@linux.dev>,
        Christian Brauner <brauner@kernel.org>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        John Stultz <jstultz@google.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Stephen Boyd <sboyd@kernel.org>,
        Chandan Babu R <chandan.babu@oracle.com>,
        "Darrick J. Wong" <djwong@kernel.org>,
        "Theodore Ts'o" <tytso@mit.edu>,
        Andreas Dilger <adilger.kernel@dilger.ca>,
        Chris Mason <clm@fb.com>, Josef Bacik <josef@toxicpanda.com>,
        David Sterba <dsterba@suse.com>,
        Hugh Dickins <hughd@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Amir Goldstein <amir73il@gmail.com>, Jan Kara <jack@suse.de>,
        David Howells <dhowells@redhat.com>,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-xfs@vger.kernel.org, linux-ext4@vger.kernel.org,
        linux-btrfs@vger.kernel.org, linux-mm@kvack.org,
        linux-nfs@vger.kernel.org
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On Mon, 23 Oct 2023 at 13:26, Dave Chinner <david@fromorbit.com> wrote:
>
> The problem is the first read request after a modification has been
> made. That is causing relatime to see mtime > atime and triggering
> an atime update. XFS sees this, does an atime update, and in
> committing that persistent inode metadata update, it calls
> inode_maybe_inc_iversion(force = false) to check if an iversion
> update is necessary. The VFS sees I_VERSION_QUERIED, and so it bumps
> i_version and tells XFS to persist it.

Could we perhaps just have a mode where we don't increment i_version
for just atime updates?

Maybe we don't even need a mode, and could just decide that atime
updates aren't i_version updates at all?

Yes, yes, it's obviously technically a "inode modification", but does
anybody actually *want* atime updates with no actual other changes to
be version events?

Or maybe i_version can update, but callers of getattr() could have two
bits for that STATX_CHANGE_COOKIE, one for "I care about atime" and
one for others, and we'd pass that down to inode_query_version, and
we'd have a I_VERSION_QUERIED and a I_VERSION_QUERIED_STRICT, and the
"I care about atime" case ould set the strict one.

Then inode_maybe_inc_iversion() could - for atome updates - skip the
version update *unless* it sees that I_VERSION_QUERIED_STRICT bit.

Does that sound sane to people?

Because it does sound completely insane to me to say "inode changed"
and have a cache invalidation just for an atime update.

              Linus