linux-kernel - Re: [PATCH] loop: Update mtime when writing using aops

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Date:	Fri, 9 Apr 2010 10:52:32 +0530
From:	Nikanth Karthikesan <knikanth@...e.de>
To:	Jens Axboe <jens.axboe@...cle.com>
Cc:	Tvrtko Ursulin <tvrtko.ursulin@...hos.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] loop: Update mtime when writing using aops

On Friday 09 April 2010 01:08:38 Jens Axboe wrote:
> On Thu, Apr 08 2010, Tvrtko Ursulin wrote:
> > On Thursday 08 April 2010 13:45:39 Jens Axboe wrote:
> > > On Thu, Apr 08 2010, Nikanth Karthikesan wrote:
> > > > Update mtime when writing to backing filesystem using the address
> > > > space operations write_begin and write_end.
> > >
> > > You forgot to include a 'why' :-)
> > >
> > > Not that I disagree with the patch, just curious what made you make the
> > > change.
> >

Sorry, for the bad terse changelog.

> > You mean how the bug was found? I originally reported this problem
> > against openSUSE 11.2 kernel, where when you have a dm-crypt loop
> > filesystem the container file modification time does not get updated with
> > use. Consequence of that is that backup runs would miss the fact
> > container has changed.
> >
> > Maybe one could argue it is a security feature in this scenario :), but
> > as the problem turns out to be unrelated to the crypto case, rather
> > contained in the loop driver itself, it looks like that would not hold.
> >
> :-)
> 
> I suppose you could update the mtime on tear down, since then it would
> also be safe to actually backup. But same difference in the end I
> suppose, I'll add the patch.
> 

If we update mtime only during tear down, we might miss to backup after a 
system crash or scenarios where the loop device is not deleted after use 
during backup. Also when relatime is used, it is better to change mtime on 
time. :-)

Thanks for taking the patch.

Thanks
Nikanth
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