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SubjectRe: [PATCH] x86/mm: fix regression with huge pages on PAE
On Thu, Nov 12, 2015 at 09:54:18AM +0100, Ingo Molnar wrote:
>
> * Kirill A. Shutemov <kirill.shutemov@linux.intel.com> wrote:
>
> > diff --git a/arch/x86/include/asm/page_types.h b/arch/x86/include/asm/page_types.h
> > index c5b7fb2774d0..cc071c6f7d4d 100644
> > --- a/arch/x86/include/asm/page_types.h
> > +++ b/arch/x86/include/asm/page_types.h
> > @@ -9,19 +9,21 @@
> > #define PAGE_SIZE (_AC(1,UL) << PAGE_SHIFT)
> > #define PAGE_MASK (~(PAGE_SIZE-1))
> >
> > +#define PMD_PAGE_SIZE (_AC(1, UL) << PMD_SHIFT)
> > +#define PMD_PAGE_MASK (~(PMD_PAGE_SIZE-1))
> > +
> > +#define PUD_PAGE_SIZE (_AC(1, UL) << PUD_SHIFT)
> > +#define PUD_PAGE_MASK (~(PUD_PAGE_SIZE-1))
> > +
> > #define __PHYSICAL_MASK ((phys_addr_t)((1ULL << __PHYSICAL_MASK_SHIFT) - 1))
> > #define __VIRTUAL_MASK ((1UL << __VIRTUAL_MASK_SHIFT) - 1)
> >
> > -/* Cast PAGE_MASK to a signed type so that it is sign-extended if
> > +/* Cast *PAGE_MASK to a signed type so that it is sign-extended if
> > virtual addresses are 32-bits but physical addresses are larger
> > (ie, 32-bit PAE). */
> > #define PHYSICAL_PAGE_MASK (((signed long)PAGE_MASK) & __PHYSICAL_MASK)
> > -
> > -#define PMD_PAGE_SIZE (_AC(1, UL) << PMD_SHIFT)
> > -#define PMD_PAGE_MASK (~(PMD_PAGE_SIZE-1))
> > -
> > -#define PUD_PAGE_SIZE (_AC(1, UL) << PUD_SHIFT)
> > -#define PUD_PAGE_MASK (~(PUD_PAGE_SIZE-1))
> > +#define PHYSICAL_PMD_PAGE_MASK (((signed long)PMD_PAGE_MASK) & __PHYSICAL_MASK)
> > +#define PHYSICAL_PUD_PAGE_MASK (((signed long)PUD_PAGE_MASK) & __PHYSICAL_MASK)
>
> that's a really odd way of writing it, 'long' is signed by default ...

See the comment above (it was there before the patch). 'signed' can be
considered as documentation -- we want sign-extension here.

> There seems to be 150+ such cases in the kernel source though - weird.
>
> More importantly, how does this improve things on 32-bit PAE kernels? If I follow
> the values correctly then PMD_PAGE_MASK is 'UL' i.e. 32-bit:
>
> > +#define PMD_PAGE_SIZE (_AC(1, UL) << PMD_SHIFT)
> > +#define PMD_PAGE_MASK (~(PMD_PAGE_SIZE-1))
>
> thus PHYSICAL_PMD_PAGE_MASK is 32-bit too:
>
> > +#define PHYSICAL_PMD_PAGE_MASK (((signed long)PMD_PAGE_MASK) & __PHYSICAL_MASK)
>
> so how is the bug fixed?

Again, see the comment.
I've checked that it generates correct value (using kernel/bounds.c).

--
Kirill A. Shutemov


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