linux-kernel - Re: [PATCH] x86_64: do not assume head_64.S used 4KB pages when !use

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Date:	Thu, 23 Jun 2011 12:38:54 +0000
From:	Penttilä Mika <mika.penttila@...nos.com>
To:	"stefano.stabellini@...citrix.com" <stefano.stabellini@...citrix.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86_64: do not assume head_64.S used 4KB pages when
 !use_pse

> From: Stefano Stabellini <stefano.stabellini@...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@...citrix.com>
> Reported-by: Ingo Molnar <mingo@...e.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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