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

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Message-id: <166267807678.30452.18035749642786839300@noble.neil.brown.name>
Date:   Fri, 09 Sep 2022 09:01:16 +1000
From:   "NeilBrown" <neilb@...e.de>
To:     "Jeff Layton" <jlayton@...nel.org>
Cc:     "J. Bruce Fields" <bfields@...ldses.org>,
        "Theodore Ts'o" <tytso@....edu>, "Jan Kara" <jack@...e.cz>,
        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 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@...nel.org>
>