Re: testing stable pages being modified

[Josef Bacik <jbacik@fusionio.com>]

On Fri, May 31, 2013 at 07:53:29AM -0600, Chris Mason wrote:
> 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
>

That would work for us, but what about other people that rely on stable pages,
like *fs on iscsi and such?  It might be good to have a generic mount option
that the vfs notices and makes the copying happen before it gets to the file
system and that way we're all save and don't have a different solution/mount
option for each fs.  Thanks,

Josef