linux-kernel - Re: [PATCH 08/19] mm: numa: Create basic numa page hinting infrastructure

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Message-ID: <509A3C7E.9040809@redhat.com>
Date:	Wed, 07 Nov 2012 05:48:30 -0500
From:	Rik van Riel <riel@...hat.com>
To:	Mel Gorman <mgorman@...e.de>
CC:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Ingo Molnar <mingo@...nel.org>,
	Johannes Weiner <hannes@...xchg.org>,
	Hugh Dickins <hughd@...gle.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux-MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 08/19] mm: numa: Create basic numa page hinting infrastructure

On 11/07/2012 05:38 AM, Mel Gorman wrote:
> On Tue, Nov 06, 2012 at 01:58:26PM -0500, Rik van Riel wrote:
>> On 11/06/2012 04:14 AM, Mel Gorman wrote:
>>> Note: This patch started as "mm/mpol: Create special PROT_NONE
>>> 	infrastructure" and preserves the basic idea but steals *very*
>>> 	heavily from "autonuma: numa hinting page faults entry points" for
>>> 	the actual fault handlers without the migration parts.	The end
>>> 	result is barely recognisable as either patch so all Signed-off
>>> 	and Reviewed-bys are dropped. If Peter, Ingo and Andrea are ok with
>>> 	this version, I will re-add the signed-offs-by to reflect the history.
>>>
>>> In order to facilitate a lazy -- fault driven -- migration of pages, create
>>> a special transient PAGE_NUMA variant, we can then use the 'spurious'
>>> protection faults to drive our migrations from.
>>>
>>> Pages that already had an effective PROT_NONE mapping will not be detected
>>
>> The patch itself is good, but the changelog needs a little
>> fix. While you are defining _PAGE_NUMA to _PAGE_PROTNONE on
>> x86, this may be different on other architectures.
>>
>> Therefore, the changelog should refer to PAGE_NUMA, not
>> PROT_NONE.
>>
>
> Fair point. I still want to record the point that PROT_NONE will not
> generate the faults though. How about this?
>
>      In order to facilitate a lazy -- fault driven -- migration of pages, create
>      a special transient PAGE_NUMA variant, we can then use the 'spurious'
>      protection faults to drive our migrations from.
>
>      The meaning of PAGE_NUMA depends on the architecture but on x86 it is
>      effectively PROT_NONE. In this case, PROT_NONE mappings will not be detected
>      to generate these 'spurious' faults for the simple reason that we cannot
>      distinguish them on their protection bits, see pte_numa(). This isn't
>      a problem since PROT_NONE (and possible PROT_WRITE with dirty tracking)
>      aren't used or are rare enough for us to not care about their placement.

Actual PROT_NONE mappings will not generate these NUMA faults
for the reason that the page fault code checks the permission
on the VMA (and will throw a segmentation fault on actual
PROT_NONE mappings), before it ever calls handle_mm_fault.

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