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

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Date:   Tue, 24 Oct 2023 15:06:52 -0400
From:   Jeff Layton <jlayton@...nel.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Dave Chinner <david@...morbit.com>
Cc:     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 Mon, 2023-10-23 at 14:18 -1000, Linus Torvalds wrote:
> On Mon, 23 Oct 2023 at 13:26, Dave Chinner <david@...morbit.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.
> 


The new flag idea is a good one. The catch though is that there are no
readers of i_version in-kernel other than NFSD and IMA, so there would
be no in-kernel users of I_VERSION_QUERIED_STRICT.

In earlier discussions, I was given to believe that the problem with
changing how this works in XFS involved offline filesystem access tools.
That said, I didn't press for enough details at the time, so I may have
misunderstood Dave's reticence to change how this works.
-- 
Jeff Layton <jlayton@...nel.org>