On Wed, Aug 21, 2013 at 03:53:36PM +0100, Jan Beulich wrote:
> >>> On 21.08.13 at 16:12, Cyrill Gorcunov <gorcu...@gmail.com> 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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