From mboxrd@z Thu Jan  1 00:00:00 1970
From: Chris Mason <clmason@fusionio.com>
Subject: Re: testing stable pages being modified
Date: Fri, 31 May 2013 09:53:29 -0400
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To: Josef Bacik <JBacik@fusionio.com>, Zach Brown <zab@redhat.com>
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Quoting Josef Bacik (2013-05-31 09:29:07)
> On Fri, May 31, 2013 at 12:24:30AM -0600, Zach Brown wrote:
> > > Changing O_DIRECT in flight has always been a deep dark corner case, and
> > > crc errors are the expected result.  Have you found anyone doing this in
> > > real life?
> > 
> > Agreed; and no, I haven't heard of people accidentally modifying stable
> > pages.
> > 
> 
> Windows does this, it also will send down the same page for different offsets
> which is why we have that special check in check_direct_IO for reads because
> that would cause lots of csum errors too.  I tried to fix the modified in flight
> problem by checking the csums of the pages in the io completion handler and
> re-submitting the IO if the pages had changed, but this of course dramatically
> reduced performance for all of those well behaved O_DIRECT applications, so in
> the end I just set nodatasum for that vm image and carried on.  I'm not sure
> what the solution is for this problem.  Thanks,

Ugh, I forgot about the windows case.  KVM should really be copying the
pages for sectors where IO is already in flight.

We could do the copies ourselves: mount -o dio_copies

-chris