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SubjectRe: [PATCH] x86_64: do not assume head_64.S used 4KB pages when !use_pse
Date
> 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.

--Mika







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