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

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Date:   Fri, 9 Sep 2022 02:14:19 +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>,
        "xiubli@...hat.com" <xiubli@...hat.com>,
        "brauner@...nel.org" <brauner@...nel.org>,
        "linux-xfs@...r.kernel.org" <linux-xfs@...r.kernel.org>,
        "linux-api@...r.kernel.org" <linux-api@...r.kernel.org>,
        "bfields@...ldses.org" <bfields@...ldses.org>,
        "david@...morbit.com" <david@...morbit.com>,
        "fweimer@...hat.com" <fweimer@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "chuck.lever@...cle.com" <chuck.lever@...cle.com>,
        "jlayton@...nel.org" <jlayton@...nel.org>,
        "linux-man@...r.kernel.org" <linux-man@...r.kernel.org>,
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        "tytso@....edu" <tytso@....edu>,
        "viro@...iv.linux.org.uk" <viro@...iv.linux.org.uk>,
        "jack@...e.cz" <jack@...e.cz>,
        "linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>,
        "linux-btrfs@...r.kernel.org" <linux-btrfs@...r.kernel.org>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "adilger.kernel@...ger.ca" <adilger.kernel@...ger.ca>,
        "lczerner@...hat.com" <lczerner@...hat.com>,
        "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 01:10 +0000, Trond Myklebust wrote:
> 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.
> 

The point is, NFS is not the only potential use case for change
attributes. We wouldn't be bothering to discuss statx() if it was.

I could be using O_DIRECT, and all the tricks in order to ensure that
my stock broker application (to choose one example) has access to the
absolute very latest prices when I'm trying to execute a trade.
When the filesystem then says 'the prices haven't changed since your
last read because the change attribute on the database file is the
same' in response to a statx() request with the AT_STATX_FORCE_SYNC
flag set, then why shouldn't my application be able to assume it can
serve those prices right out of memory instead of having to go to disk?

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