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Date:   Fri, 9 Sep 2022 11:45:06 -0400
To:     Jeff Layton <jlayton@kernel.org>
Cc:     Theodore Ts'o <tytso@mit.edu>, Jan Kara <jack@suse.cz>,
        NeilBrown <neilb@suse.de>, adilger.kernel@dilger.ca,
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        ceph-devel@vger.kernel.org, linux-ext4@vger.kernel.org,
        linux-nfs@vger.kernel.org, linux-xfs@vger.kernel.org
Subject: Re: [man-pages RFC PATCH v4] statx, inode: document the new
 STATX_INO_VERSION field
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From:   bfields@fieldses.org (J. Bruce Fields)
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On Thu, Sep 08, 2022 at 03:07:58PM -0400, Jeff Layton wrote:
> On Thu, 2022-09-08 at 14:22 -0400, J. Bruce Fields wrote:
> > On Thu, Sep 08, 2022 at 01:40:11PM -0400, Jeff Layton wrote:
> > > Yeah, ok. That does make some sense. So we would mix this into the
> > > i_version instead of the ctime when it was available. Preferably, we'd
> > > mix that in when we store the i_version rather than adding it afterward.
> > > 
> > > Ted, how would we access this? Maybe we could just add a new (generic)
> > > super_block field for this that ext4 (and other filesystems) could
> > > populate at mount time?
> > 
> > Couldn't the filesystem just return an ino_version that already includes
> > it?
> > 
> 
> Yes. That's simple if we want to just fold it in during getattr. If we
> want to fold that into the values stored on disk, then I'm a little less
> clear on how that will work.
> 
> Maybe I need a concrete example of how that will work:
> 
> Suppose we have an i_version value X with the previous crash counter
> already factored in that makes it to disk. We hand out a newer version
> X+1 to a client, but that value never makes it to disk.
> 
> The machine crashes and comes back up, and we get a query for i_version
> and it comes back as X. Fine, it's an old version. Now there is a write.
> What do we do to ensure that the new value doesn't collide with X+1? 

I was assuming we could partition i_version's 64 bits somehow: e.g., top
16 bits store the crash counter.  You increment the i_version by: 1)
replacing the top bits by the new crash counter, if it has changed, and
2) incrementing.

Do the numbers work out?  2^16 mounts after unclean shutdowns sounds
like a lot for one filesystem, as does 2^48 changes to a single file,
but people do weird things.  Maybe there's a better partitioning, or
some more flexible way of maintaining an i_version that still allows you
to identify whether a given i_version preceded a crash.

--b.