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 14:50:23 +0000
From:	Penttilä Mika <mika.penttila@...nos.com>
To:	Stefano Stabellini <stefano.stabellini@...citrix.com>
CC:	"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 [mailto:stefano.stabellini@...citrix.com]
> Sent: 23. kesäkuuta 2011 15:57
> To: Penttilä Mika
> Cc: Stefano Stabellini; 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.

We use large pages FOR PAE kernels on x86-32 there

--Mika