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Subject: Re: [PATCH 2/2] powerpc: mm: change pgtable index size for 64K page
From: Benjamin Herrenschmidt <benh@kernel.crashing.org>
To: Liu Ping Fan <kernelfans@gmail.com>
Date: Fri, 22 Nov 2013 10:46:21 +1100
In-Reply-To: <20131121221150.GB26359@iris.ozlabs.ibm.com>
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Cc: Paul Mackerras <paulus@samba.org>, linuxppc-dev@lists.ozlabs.org
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On Fri, 2013-11-22 at 09:11 +1100, Paul Mackerras wrote:
> On Thu, Nov 21, 2013 at 10:17:55AM +0800, Liu Ping Fan wrote:
> > For 64K page, we waste half of the pte_t page. With this patch, after
> > changing PGD_INDEX_SIZE from 12 to 11, PTE_INDEX_SIZE from 8 to 9,
> > we can improve the usage of pte_t page and shrink the continuous phys
> > size for pgd_t.

Also you did you miss that we use the second half to store the
per-subpage hash info when using 64k on top of HW 4k ?

Cheers,
Ben.

> > Signed-off-by: Liu Ping Fan <pingfank@linux.vnet.ibm.com>
> > ---
> >  arch/powerpc/include/asm/pgtable-ppc64-64k.h | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> > 
> > diff --git a/arch/powerpc/include/asm/pgtable-ppc64-64k.h b/arch/powerpc/include/asm/pgtable-ppc64-64k.h
> > index a56b82f..f6955ff 100644
> > --- a/arch/powerpc/include/asm/pgtable-ppc64-64k.h
> > +++ b/arch/powerpc/include/asm/pgtable-ppc64-64k.h
> > @@ -4,10 +4,10 @@
> >  #include <asm-generic/pgtable-nopud.h>
> >  
> >  
> > -#define PTE_INDEX_SIZE  8
> > +#define PTE_INDEX_SIZE  9
> >  #define PMD_INDEX_SIZE  10
> >  #define PUD_INDEX_SIZE	0
> > -#define PGD_INDEX_SIZE  12
> > +#define PGD_INDEX_SIZE  11
> 
> Nack.  Those definitions are the way they are in order to have the PMD
> map 16MB, which is our large page size, so that transparent huge pages
> work.
> 
> Paul.