On Wed, May 30, 2012 at 08:34:06PM +0200, Andrea Arcangeli wrote:
> Hi Konrad,
>
> On Wed, May 30, 2012 at 02:22:49PM -0400, Konrad Rzeszutek Wilk wrote:
> > Thank you for loking at this from the xen side. The interesting thing
> > is that I believe the _PAGE_PAT (or _PAGE_PSE) is actually used on
> > Xen on PTEs. It is used to mark the pages WC. <sigh>
>
> Oops, I'm using _PAGE_PSE too on the pte, but only when it's unmapped.
>
> static inline int pte_numa(pte_t pte)
> {
> return (pte_flags(pte) &
> (_PAGE_NUMA_PTE|_PAGE_PRESENT)) == _PAGE_NUMA_PTE;
> }
>
> And _PAGE_UNUSED2 (_PAGE_IOMAP) is used for the pmd but _PAGE_IOMAP by
> Xen should only be set on ptes.
<nods>
>
> The only way to use _PAGE_PSE safe on the pte is if the pte is
> non-present, is this what Xen is also doing? (in turn colliding with
> pte_numa)
The only time the _PAGE_PSE (_PAGE_PAT) is set is when
_PAGE_PCD | _PAGE_PWT are set. It is this ugly transformation
of doing:
if (pat_enabled && _PAGE_PWT | _PAGE_PCD)
pte = ~(_PAGE_PWT | _PAGE_PCD) | _PAGE_PAT;
and then writting the pte with the 7th bit set instead of the
2nd and 3rd to mark it as WC. There is a corresponding reverse too
(to read the pte - so the pte_val calls) - so if _PAGE_PAT is
detected it will remove the _PAGE_PAT and return the PTE as
if it had _PAGE_PWT | _PAGE_PCD.
So that little bit of code will need some tweaking - as it does
that even if _PAGE_PRESENT is not set. Meaning it would
transform your _PAGE_PAT to _PAGE_PWT | _PAGE_PCD. Gah!
>
> Now if I shrink the size of the page_autonuma to one entry per pmd
> (instead of per pte) I may as well drop pte_numa entirely and only
> leave pmd_numa. At the moment it's possible to switch between the two
> models at runtime with sysctl (if one wants to do a more expensive
> granular tracking). I'm still uncertain on the best way to shrink
> the page_autonuma size we'll see.
OK. I can whip up a patch to deal with the 'Gah!' case easily if needed.
Thanks!
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