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Subject: Re: [PATCH 2/2] powerpc: mm: change pgtable index size for 64K page
From: Michael Ellerman <michael@ellerman.id.au>
To: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Date: Fri, 22 Nov 2013 13:06:08 +1100
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Cc: linuxppc-dev@lists.ozlabs.org, Paul Mackerras <paulus@samba.org>,
 Liu Ping Fan <kernelfans@gmail.com>
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On Fri, 2013-11-22 at 10:46 +1100, Benjamin Herrenschmidt wrote:
> 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 ?

Given the subtleties perhaps a nice big block comment is in order ? :)

cheers