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

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Date:   Fri, 9 Sep 2022 01:10:50 +0000
From:   Trond Myklebust <trondmy@...merspace.com>
To:     "neilb@...e.de" <neilb@...e.de>
CC:     "zohar@...ux.ibm.com" <zohar@...ux.ibm.com>,
        "djwong@...nel.org" <djwong@...nel.org>,
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        "brauner@...nel.org" <brauner@...nel.org>,
        "bfields@...ldses.org" <bfields@...ldses.org>,
        "linux-api@...r.kernel.org" <linux-api@...r.kernel.org>,
        "linux-xfs@...r.kernel.org" <linux-xfs@...r.kernel.org>,
        "david@...morbit.com" <david@...morbit.com>,
        "fweimer@...hat.com" <fweimer@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
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        "ceph-devel@...r.kernel.org" <ceph-devel@...r.kernel.org>
Subject: Re: [man-pages RFC PATCH v4] statx, inode: document the new
 STATX_INO_VERSION field

On Fri, 2022-09-09 at 11:07 +1000, NeilBrown wrote:
> On Fri, 09 Sep 2022, NeilBrown wrote:
> > On Fri, 09 Sep 2022, Trond Myklebust wrote:
> > 
> > > 
> > > IOW: the minimal condition needs to be that for all cases below,
> > > the
> > > application reads 'state B' as having occurred if any data was
> > > committed to disk before the crash.
> > > 
> > > Application                             Filesystem
> > > ===========                             =========
> > > read change attr <- 'state A'
> > > read data <- 'state A'
> > >                                         write data -> 'state B'
> > >                                         <crash>+<reboot>
> > > read change attr <- 'state B'
> > 
> > The important thing here is to not see 'state A'.  Seeing 'state C'
> > should be acceptable.  Worst case we could merge in wall-clock time
> > of
> > system boot, but the filesystem should be able to be more helpful
> > than
> > that.
> > 
> 
> Actually, without the crash+reboot it would still be acceptable to
> see
> "state A" at the end there - but preferably not for long.
> From the NFS perspective, the changeid needs to update by the time of
> a
> close or unlock (so it is visible to open or lock), but before that
> it
> is just best-effort.

Nope. That will inevitably lead to data corruption, since the
application might decide to use the data from state A instead of
revalidating it.

-- 
Trond Myklebust
Linux NFS client maintainer, Hammerspace
trond.myklebust@...merspace.com