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

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Message-id: <166319552167.15759.17894784385240679495@noble.neil.brown.name>
Date:   Thu, 15 Sep 2022 08:45:21 +1000
From:   "NeilBrown" <neilb@...e.de>
To:     "Jeff Layton" <jlayton@...nel.org>
Cc:     "Dave Chinner" <david@...morbit.com>,
        "Trond Myklebust" <trondmy@...merspace.com>,
        "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>,
        "fweimer@...hat.com" <fweimer@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "chuck.lever@...cle.com" <chuck.lever@...cle.com>,
        "linux-man@...r.kernel.org" <linux-man@...r.kernel.org>,
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        "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 Wed, 14 Sep 2022, Jeff Layton wrote:
> On Wed, 2022-09-14 at 09:24 +1000, NeilBrown wrote:
> > On Wed, 14 Sep 2022, Jeff Layton wrote:
> > > 
> > > At that point, bumping i_version both before and after makes a bit more
> > > sense, since it better ensures that a change will be noticed, whether
> > > the related read op comes before or after the statx.
> > 
> > How does bumping it before make any sense at all?  Maybe it wouldn't
> > hurt much, but how does it help anyone at all?
> > 
> 
> My assumption (maybe wrong) was that timestamp updates were done before
> the actual write by design. Does doing it before the write make increase
> the chances that the inode metadata writeout will get done in the same
> physical I/O as the data write? IDK, just speculating here.

When the code was written, the inode semaphore (before mutexes) was held
over the whole thing, and timestamp resolution was 1 second.  So
ordering didn't really matter.  Since then locking has bee reduced and
precision increased but no-one saw any need to fix the ordering.  I
think that is fine for timestamps.

But i_version is about absolute precision, so we need to think carefully
about what meets our needs.

> 
> If there's no benefit to doing it before then we should just move it
> afterward.

Great!
Thanks,
NeilBrown