Re: [Xen-devel] [Linux PATCH] Fix to hugepages to work around new PWT handling

[Jeremy Fitzhardinge <jeremy@xxxxxxxx>]

On 06/09/2010 07:02 AM, Dave McCracken wrote:
> Recent changes to Linux include code to set new flags in the pte,
> including _PAGE_PAT and _PAGE_PWT.  That change conflicts with hugepage
> using the pte macros to set up its pmd entries.  This patch resolves
> that problem.
>   

Could you explain this a bit more clearly?  Why is using __pmd not
working in this case?  Is it because the kernel is now setting PAT and
PWT on huge pages?  But PAT isn't even the same flag for huge pages...

> An additional fix here is to make sure the _PAGE_PRESENT bit is set
> before hugepages does a mk_pte(), since Xen depends on that bit to
> trigger the pfn->mfn translation.
>   

Why is the kernel creating a non-present mapping?  If it isn't present,
why does it matter whether we do the pfn->mfn conversion?

    J

> Signed-off-by: Dave McCracken <dave.mccracken@xxxxxxxxxx>
>
> --------
>
>
> --- stable-2.6.32.x//arch/x86/include/asm/hugetlb.h   2010-06-04 
> 12:19:31.000000000 -0500
> +++ 2.6.32-hfix//arch/x86/include/asm/hugetlb.h       2010-06-08 
> 12:23:53.000000000 -0500
> @@ -44,7 +44,7 @@ static inline pte_t huge_ptep_get(pte_t
>  static inline void set_huge_pte_at(struct mm_struct *mm, unsigned long addr,
>                                  pte_t *ptep, pte_t pte)
>  {
> -     set_pmd((pmd_t *)ptep, __pmd(pte_val(pte)));
> +     set_pmd((pmd_t *)ptep, native_make_pmd(native_pte_val(pte)));
>  }
>  
>  static inline pte_t huge_ptep_get_and_clear(struct mm_struct *mm,
> --- stable-2.6.32.x//mm/hugetlb.c     2010-06-04 12:19:36.000000000 -0500
> +++ 2.6.32-hfix//mm/hugetlb.c 2010-06-07 07:11:34.000000000 -0500
> @@ -1732,12 +1732,14 @@ static pte_t make_huge_pte(struct vm_are
>                               int writable)
>  {
>       pte_t entry;
> +     pgprot_t pgprot;
>  
> +     pgprot = __pgprot(pgprot_val(vma->vm_page_prot) | _PAGE_PRESENT);
>       if (writable) {
>               entry =
> -                 pte_mkwrite(pte_mkdirty(mk_pte(page, vma->vm_page_prot)));
> +                 pte_mkwrite(pte_mkdirty(mk_pte(page, pgprot)));
>       } else {
> -             entry = huge_pte_wrprotect(mk_pte(page, vma->vm_page_prot));
> +             entry = huge_pte_wrprotect(mk_pte(page, pgprot));
>       }
>       entry = pte_mkyoung(entry);
>       entry = pte_mkhuge(entry);
>
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> http://lists.xensource.com/xen-devel
>
>   


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