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SubjectRe: [PATCH] x86_64: do not assume head_64.S used 4KB pages when !use_pse
On Thu, 23 Jun 2011, Penttilä Mika wrote:
> > From: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
>
> > head_64.S, which sets up the initial page table on x86_64, is not aware
> > of PSE being enabled or disabled and it always allocates the initial
> > mapping using 2MB pages.
>
> > Therefore on x86_64 find_early_table_space shouldn't update the amount
> > of pages needed for pte pages depending on the size of the initial
> > mapping, because we know for sure that no pte pages have been allocated
> > yet.
>
> > Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
> > Reported-by: Ingo Molnar <mingo@elte.hu>
> > ---
> > arch/x86/mm/init.c | 7 +++++++
> > 1 files changed, 7 insertions(+), 0 deletions(-)
>
> > diff --git a/arch/x86/mm/init.c b/arch/x86/mm/init.c
> > index 36bacfe..1e3098b 100644
> > --- a/arch/x86/mm/init.c
> > +++ b/arch/x86/mm/init.c
> > @@ -42,12 +42,19 @@ static void __init find_early_table_space(unsigned long start,
> > (PMD_SIZE * PTRS_PER_PMD));
> > pmd_mapped *= (PMD_SIZE * PTRS_PER_PMD);
> >
> > + /*
> > + * On x86_64 do not limit the size we need to cover with 4KB pages
> > + * depending on the initial allocation because head_64.S always uses
> > + * 2MB pages.
> > + */
> > +#ifdef CONFIG_X86_32
> > if (start < PFN_PHYS(max_pfn_mapped)) {
> > if (PFN_PHYS(max_pfn_mapped) < end)
> > size -= PFN_PHYS(max_pfn_mapped) - start;
> > else
> > size = 0;
> > }
> > +#endif
>
> And arch/x86/mm/init.c also has:
>
> #if defined(CONFIG_DEBUG_PAGEALLOC) || defined(CONFIG_KMEMCHECK)
> /*
> * For CONFIG_DEBUG_PAGEALLOC, identity mapping will use small pages.
> * This will simplify cpa(), which otherwise needs to support splitting
> * large pages into small in interrupt context, etc.
> */
> use_pse = use_gbpages = 0;
> #else
> use_pse = cpu_has_pse;
> use_gbpages = direct_gbpages;
> #endif
>
>
> So big pages are also not used for DEBUG_PAGEALLOC and KMEMCHECK configs even if head_32.S did.

Right, but that is not a problem because head_32.S always uses 4KB
pages.
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