linux-kernel - Re: [man-pages RFC PATCH v4] statx, inode: document the new STATX_INO

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Message-ID: <20220912135131.GC9304@fieldses.org>
Date:   Mon, 12 Sep 2022 09:51:31 -0400
From:   bfields@...ldses.org (J. Bruce Fields)
To:     Jeff Layton <jlayton@...nel.org>
Cc:     Florian Weimer <fweimer@...hat.com>, Theodore Ts'o <tytso@....edu>,
        Jan Kara <jack@...e.cz>, NeilBrown <neilb@...e.de>,
        adilger.kernel@...ger.ca, djwong@...nel.org, david@...morbit.com,
        trondmy@...merspace.com, viro@...iv.linux.org.uk,
        zohar@...ux.ibm.com, xiubli@...hat.com, chuck.lever@...cle.com,
        lczerner@...hat.com, brauner@...nel.org, linux-man@...r.kernel.org,
        linux-api@...r.kernel.org, linux-btrfs@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        ceph-devel@...r.kernel.org, linux-ext4@...r.kernel.org,
        linux-nfs@...r.kernel.org, linux-xfs@...r.kernel.org
Subject: Re: [man-pages RFC PATCH v4] statx, inode: document the new
 STATX_INO_VERSION field

On Mon, Sep 12, 2022 at 08:55:04AM -0400, Jeff Layton wrote:
> Because of the "seen" flag, we have a 63 bit counter to play with. Could
> we use a similar scheme to the one we use to handle when "jiffies"
> wraps? Assume that we'd never compare two values that were more than
> 2^62 apart? We could add i_version_before/i_version_after macros to make
> it simple to handle this.

As far as I recall the protocol just assumes it can never wrap.  I guess
you could add a new change_attr_type that works the way you describe.
But without some new protocol clients aren't going to know what to do
with a change attribute that wraps.

I think this just needs to be designed so that wrapping is impossible in
any realistic scenario.  I feel like that's doable?

If we feel we have to catch that case, the only 100% correct behavior
would probably be to make the filesystem readonly.

--b.