Re: [Xen-devel] Regression: x86/mm: new _PTE_SWP_SOFT_DIRTY bit conflicts with existing use

[Cyrill Gorcunov <gorcunov@xxxxxxxxx>]

On Wed, Aug 21, 2013 at 12:07:13PM -0700, Andy Lutomirski wrote:
> On Wed, Aug 21, 2013 at 12:03 PM, Cyrill Gorcunov <gorcunov@xxxxxxxxx> wrote:
> > On Wed, Aug 21, 2013 at 08:50:26PM +0200, H. Peter Anvin wrote:
> >> But is there a manifest bug or not?  What is the deal with Xen?
> >>
> >
> > I personally don't see bug here because
> >
> >  - this swapped page soft dirty bit is set for non-present entries only,
> >    never for present ones, just at moment we form swap pte entry
> >
> >  - i don't find any code which would test for this bit directly without
> >    is_swap_pte call
> >
> > but the use of paw bit itself is confusing, so I'm working on patch which
> > won't use it. Again, if someone knows where exactly access to pse bit when
> > pte keeps swap entry may happen (for any purpose other than dirty page
> > tracking) please share.
> 
> I doubt that there are cacheability issues here, since the swap type
> already overlaps with PWT and PCD.
> 
> What happens if the page being swapped is a THP page?  (I have no idea
> how, or even if, this works, but presumably PSE is important.)

add_to_swap -> split_huge_page_to_list, so thp will be splitted.

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