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Date: Mon, 07 Apr 2014 10:37:55 -0700
From: Dave Hansen <dave.hansen@intel.com>
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To: Mel Gorman <mgorman@suse.de>,
        Linus Torvalds <torvalds@linux-foundation.org>
CC: Cyrill Gorcunov <gorcunov@gmail.com>, Peter Anvin <hpa@zytor.com>,
        Ingo Molnar <mingo@kernel.org>, Steven Noonan <steven@uplinklabs.net>,
        Rik van Riel <riel@redhat.com>, David Vrabel <david.vrabel@citrix.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Andrea Arcangeli <aarcange@redhat.com>, Linux-MM <linux-mm@kvack.org>,
        Linux-X86 <x86@kernel.org>, LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 2/3] x86: Define _PAGE_NUMA with unused physical address
 bits PMD and PTE levels
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On 04/07/2014 08:10 AM, Mel Gorman wrote:
> +/*
> + * Software bits ignored by the page table walker
> + * At the time of writing, different levels have bits that are ignored. Due
> + * to physical address limitations, bits 52:62 should be ignored for the PMD
> + * and PTE levels and are available for use by software. Be aware that this
> + * may change if the physical address space expands.
> + */
> +#define _PAGE_BIT_NUMA		62

Doesn't moving it up to the high bits break pte_modify()'s assumptions?
 I was thinking of this nugget from change_pte_range():

	ptent = ptep_modify_prot_start(mm, addr, pte);
        if (pte_numa(ptent))
        	ptent = pte_mknonnuma(ptent);
	ptent = pte_modify(ptent, newprot);

pte_modify() pulls off all the high bits out of 'ptent' and only adds
them back if they're in newprot (which as far as I can tell comes from
the VMA).  So I _think_ it'll axe the _PAGE_NUMA out of 'ptent'.