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

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Date:	Wed, 7 Nov 2012 10:38:39 +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 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.

> >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.
> >
> >Signed-off-by: Mel Gorman <mgorman@...e.de>
> 
> Other than the changelog ...
> 
> Reviewed-by: Rik van Riel <riel@...hat.com>

Thanks.

-- 
Mel Gorman
SUSE Labs
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