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From:   "NeilBrown" <neilb@suse.de>
To:     "Jeff Layton" <jlayton@kernel.org>
Cc:     "J. Bruce Fields" <bfields@fieldses.org>,
        "Theodore Ts'o" <tytso@mit.edu>, "Jan Kara" <jack@suse.cz>,
        adilger.kernel@dilger.ca, djwong@kernel.org, david@fromorbit.com,
        trondmy@hammerspace.com, viro@zeniv.linux.org.uk,
        zohar@linux.ibm.com, xiubli@redhat.com, chuck.lever@oracle.com,
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        linux-xfs@vger.kernel.org
Subject: Re: [man-pages RFC PATCH v4] statx, inode: document the new
 STATX_INO_VERSION field
In-reply-to: <44efe219dbf511492b21a653905448d43d0f3363.camel@kernel.org>
References: <79aaf122743a295ddab9525d9847ac767a3942aa.camel@kernel.org>,
 <20220907125211.GB17729@fieldses.org>,
 <771650a814ab1ff4dc5473d679936b747d9b6cf5.camel@kernel.org>,
 <20220907135153.qvgibskeuz427abw@quack3>,
 <166259786233.30452.5417306132987966849@noble.neil.brown.name>,
 <20220908083326.3xsanzk7hy3ff4qs@quack3>, <YxoIjV50xXKiLdL9@mit.edu>,
 <02928a8c5718590bea5739b13d6b6ebe66cac577.camel@kernel.org>,
 <20220908155605.GD8951@fieldses.org>,
 <9e06c506fd6b3e3118da0ec24276e85ea3ee45a1.camel@kernel.org>,
 <20220908182252.GA18939@fieldses.org>,
 <44efe219dbf511492b21a653905448d43d0f3363.camel@kernel.org>
Date:   Fri, 09 Sep 2022 09:01:16 +1000
Message-id: <166267807678.30452.18035749642786839300@noble.neil.brown.name>
Precedence: bulk

On Fri, 09 Sep 2022, 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.

As I understand it, the crash counter would NEVER appear in the on-disk
i_version.
The crash counter is stable while a filesystem is mounted so is the same
when loading an inode from disk and when writing it back.

When loading, add crash counter to on-disk i_version to provide
in-memory i_version.
when storing, subtract crash counter from in-memory i_version to provide
on-disk i_version.

"add" and "subtract" could be any reversible hash, and its inverse.  I
would probably shift the crash counter up 16 and add/subtract.

NeilBrown


> 
> 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@kernel.org>
>