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 13:56:38 +0100
From:	Stefano Stabellini <stefano.stabellini@...citrix.com>
To:	Penttilä Mika <mika.penttila@...nos.com>
CC:	Stefano Stabellini <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

On Thu, 23 Jun 2011, Penttilä Mika wrote:
> > 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.

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