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

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Date:   Wed, 7 Sep 2022 15:51:53 +0200
From:   Jan Kara <jack@...e.cz>
To:     Jeff Layton <jlayton@...nel.org>
Cc:     "J. Bruce Fields" <bfields@...ldses.org>,
        NeilBrown <neilb@...e.de>, tytso@....edu,
        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, jack@...e.cz, 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 Wed 07-09-22 09:12:34, Jeff Layton wrote:
> On Wed, 2022-09-07 at 08:52 -0400, J. Bruce Fields wrote:
> > On Wed, Sep 07, 2022 at 08:47:20AM -0400, Jeff Layton wrote:
> > > On Wed, 2022-09-07 at 21:37 +1000, NeilBrown wrote:
> > > > On Wed, 07 Sep 2022, Jeff Layton wrote:
> > > > > +The change to \fIstatx.stx_ino_version\fP is not atomic with respect to the
> > > > > +other changes in the inode. On a write, for instance, the i_version it usually
> > > > > +incremented before the data is copied into the pagecache. Therefore it is
> > > > > +possible to see a new i_version value while a read still shows the old data.
> > > > 
> > > > Doesn't that make the value useless?
> > > > 
> > > 
> > > No, I don't think so. It's only really useful for comparing to an older
> > > sample anyway. If you do "statx; read; statx" and the value hasn't
> > > changed, then you know that things are stable. 
> > 
> > I don't see how that helps.  It's still possible to get:
> > 
> > 		reader		writer
> > 		------		------
> > 				i_version++
> > 		statx
> > 		read
> > 		statx
> > 				update page cache
> > 
> > right?
> > 
> 
> Yeah, I suppose so -- the statx wouldn't necessitate any locking. In
> that case, maybe this is useless then other than for testing purposes
> and userland NFS servers.
> 
> Would it be better to not consume a statx field with this if so? What
> could we use as an alternate interface? ioctl? Some sort of global
> virtual xattr? It does need to be something per-inode.

I was thinking how hard would it be to increment i_version after updating
data but it will be rather hairy. In particular because of stuff like
IOCB_NOWAIT support which needs to bail if i_version update is needed. So
yeah, I don't think there's an easy way how to provide useful i_version for
general purpose use.

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR