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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Mon, 23 Oct 2023 18:10:15 -1000
Message-ID: <CAHk-=wjUWM8VVJxUYb=XfqvrS38ACS8RxK2ac9qGp9FDT=USkw@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,
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On Mon, 23 Oct 2023 at 17:40, Dave Chinner <david@fromorbit.com> wrote:
> >
> > Maybe we don't even need a mode, and could just decide that atime
> > updates aren't i_version updates at all?
>
> We do that already - in memory atime updates don't bump i_version at
> all. The issue is the rare persistent atime update requests that
> still happen - they are the ones that trigger an i_version bump on
> XFS, and one of the relatime heuristics tickle this specific issue.

Yes, yes, but that's what I kind of allude to - maybe people still
want the on-disk atime updates, but do they actually want the
i_version updates just because they want more aggressive atime
updates?

> > 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.
>
> This makes correct behaviour reliant on the applicaiton using the
> query mechanism correctly. I have my doubts that userspace
> developers will be able to understand the subtle difference between
> the two options and always choose correctly....

I don't think we _have_ a user space interface.

At least the STATX_CHANGE_COOKIE bit isn't exposed to user space at
all. Not in the uapi headers, but not even in xstat():

        /* STATX_CHANGE_COOKIE is kernel-only for now */
        tmp.stx_mask = stat->result_mask & ~STATX_CHANGE_COOKIE;

So the *only* users of STATX_CHANGE_COOKIE seem to be entirely
in-kernel, unless there is something I'm missing where somebody uses
i_version through some other interface (random ioctl?).

End result: splitting STATX_CHANGE_COOKIE into a "I don't care about
atime" and a "give me all change events" shouldn't possibly break
anything that I can see.

The only other uses of inode_query_iversion() seem to be the explicit
directory optimizations (ie the "I'm caching the offset and don't want
to re-check that it's valid unless required, so give me the inode
version for the directory as a way to decide if I need to re-check"
thing).

And those *definitely* don't want i_version updates on any atime updates.

There might be some other use of inode_query_iversion() that I missed,
of course.  But from a quick look, it really looks to me like we can
relax our i_version updates.

              Linus