linux-kernel - Re: [PATCH] iversion: update comments with info about atime updates

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Message-ID: <20220823232832.GQ3600936@dread.disaster.area>
Date:   Wed, 24 Aug 2022 09:28:32 +1000
From:   Dave Chinner <david@...morbit.com>
To:     NeilBrown <neilb@...e.de>
Cc:     Jeff Layton <jlayton@...nel.org>, Mimi Zohar <zohar@...ux.ibm.com>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-integrity@...r.kernel.org, linux-nfs@...r.kernel.org,
        linux-xfs@...r.kernel.org, linux-ext4@...r.kernel.org,
        Trond Myklebust <trondmy@...merspace.com>
Subject: Re: [PATCH] iversion: update comments with info about atime updates

On Wed, Aug 24, 2022 at 08:24:47AM +1000, NeilBrown wrote:
> On Tue, 23 Aug 2022, Jeff Layton wrote:
> > On Tue, 2022-08-23 at 21:38 +1000, NeilBrown wrote:
> > > On Tue, 23 Aug 2022, Jeff Layton wrote:
> > > > So, we can refer to that and simply say:
> > > > 
> > > > "If the function updates the mtime or ctime on the inode, then the
> > > > i_version should be incremented. If only the atime is being updated,
> > > > then the i_version should not be incremented. The exception to this rule
> > > > is explicit atime updates via utimes() or similar mechanism, which
> > > > should result in the i_version being incremented."
> > > 
> > > Is that exception needed?  utimes() updates ctime.
> > > 
> > > https://man7.org/linux/man-pages/man2/utimes.2.html
> > > 
> > > doesn't say that, but
> > > 
> > > https://pubs.opengroup.org/onlinepubs/007904875/functions/utimes.html
> > > 
> > > does, as does the code.
> > > 
> > 
> > Oh, good point! I think we can leave that out. Even better!
> 
> Further, implicit mtime updates (file_update_time()) also update ctime.
> So all you need is
>    If the function updates the ctime, then i_version should be
>    incremented.
> 
> and I have to ask - why not just use the ctime?  Why have another number
> that is parallel?
> 
> Timestamps are updated at HZ (ktime_get_course) which is at most every
> millisecond.

Kernel time, and therefore timestamps, can go backwards.

-Dave.
-- 
Dave Chinner
david@...morbit.com