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

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Date:	Wed, 7 Nov 2012 11:00:09 +0000
From:	Mel Gorman <mgorman@...e.de>
To:	Rik van Riel <riel@...hat.com>
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 Wed, Nov 07, 2012 at 05:48:30AM -0500, Rik van Riel wrote:
> 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.
> 

Updated. Thanks.

-- 
Mel Gorman
SUSE Labs
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