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 03:53:36PM +0100, Jan Beulich wrote:
> >>> On 21.08.13 at 16:12, Cyrill Gorcunov <gorcunov@xxxxxxxxx> wrote:
> > On Wed, Aug 21, 2013 at 02:48:20PM +0100, David Vrabel wrote:
> >> All,
> >> 
> >> 179ef71c (mm: save soft-dirty bits on swapped pages) introduces a new
> >> PTE bit on x86 _PTE_SWP_SOFT_DIRTY which has the same value as _PTE_PSE
> >> and _PTE_PAT.
> >> 
> >> With a Xen PV guest, the use of the _PTE_PAT will result in the page
> >> having unexpected cachability which will introduce a range of subtle
> >> performance and correctness issues.  Xen programs the entry 4 in the PAT
> >> table with WC so a page that was previously WB will end up as WC.
> >> 
> > 
> > David, could you please explain, Xen keeps and analyze _PTE_PAT bit
> > for ptes which are not present?
> 
> No, the problem isn't with not-present PTEs (i.e. swap entries),
> but with present ones - the same bit (7) is being used for both,
> according to this comment:
> 
> /*
>  * Tracking soft dirty bit when a page goes to a swap is tricky.
>  * We need a bit which can be stored in pte _and_ not conflict
>  * with swap entry format. On x86 bits 6 and 7 are *not* involved
>  * into swap entry computation, but bit 6 is used for nonlinear
>  * file mapping, so we borrow bit 7 for soft dirty tracking.
>  */
> 
> Or are you telling me that the comment is misleading (at least me),
> and this applies only to not-present PTEs? And even then - where
> would the value of the original PAT bit be stored while swapped
> out (or is it impossible - now and forever - for WC pages to get
> swapped)?

Only to non-present ptes, as far as I know.

do_swap_page
        ...
        pte = mk_pte(page, vma->vm_page_prot);

        /* new pte from vm_page_prot generated */
        ...
        set_pte_at(mm, address, page_table, pte);
        /* and assigned to old place */

with soft dirty in swap it is somehow more weirdy

        pte = mk_pte(page, vma->vm_page_prot);
        ...
        if (pte_swp_soft_dirty(orig_pte))
                pte = pte_mksoft_dirty(pte);
        set_pte_at(mm, address, page_table, pte);

orig_pte has pse bit set if page has been soft dirty
when it reached swap.

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