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

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Date:   Thu, 08 Sep 2022 15:07:58 -0400
From:   Jeff Layton <jlayton@...nel.org>
To:     "J. Bruce Fields" <bfields@...ldses.org>
Cc:     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,
        fweimer@...hat.com, 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 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? 
-- 
Jeff Layton <jlayton@...nel.org>