linux-ext4 - Re: [man-pages RFC PATCH v4] statx, inode: document the new STATX_INO

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Message-ID: <20220912144730.GD9304@fieldses.org>
Date:   Mon, 12 Sep 2022 10:47:30 -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 10:02:27AM -0400, Jeff Layton wrote:
> On Mon, 2022-09-12 at 09:51 -0400, J. Bruce Fields wrote:
> > 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.
> > 
> 
> Right, I think that's the case now, and with contemporary hardware that
> shouldn't ever happen, but in 10 years when we're looking at femtosecond
> latencies, could this be different? I don't know.

That doesn't sound likely.  We probably need not just 2^63 writes to a
single file, but a dependent sequence of 2^63 interspersed writes and
change attribute reads.

Then there's the question of how many crashes and remounts are possible
for a single filesystem in the worst case.

> 
> > 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.
> 
> What would be the recourse at that point? Rebuild the fs from scratch, I
> guess?

I guess.

--b.