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From: Kumar Gala <galak@kernel.crashing.org>
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Subject: using different format for hugetlbfs
Date: Fri, 4 Dec 2009 01:18:51 -0600
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To: Benjamin Herrenschmidt <benh@kernel.crashing.org>,
	David Gibson <dwg@au1.ibm.com>
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Ben, David,

If we want to support true 4G/4G split on ppc32 using the MSB of the  
address to determine of the pgd_t is for hugetlbfs isn't going to  
work.  Since every pointer in the pgd_t -> pud_t -> pmd_t is point to  
at least a 4K page I would think the low order 12-bits should always  
be 0.

Could we use something like:

addr[0:51] || shift [52:59] || flags [60:63]

with the LSB flag being 'normal pointer' vs 'hugetlbfs mangled  
pointer'.  Seems like shift will at most be 64 so 8-bits should cover  
it.

- k