linux-kernel - Re: [PATCH 04/19] mm: numa: define _PAGE

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Date:	Tue, 06 Nov 2012 13:35:23 -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 04/19] mm: numa: define _PAGE_NUMA

On 11/06/2012 04:14 AM, Mel Gorman wrote:
> From: Andrea Arcangeli <aarcange@...hat.com>
>
> The objective of _PAGE_NUMA is to be able to trigger NUMA hinting page
> faults to identify the per NUMA node working set of the thread at
> runtime.
>
> Arming the NUMA hinting page fault mechanism works similarly to
> setting up a mprotect(PROT_NONE) virtual range: the present bit is
> cleared at the same time that _PAGE_NUMA is set, so when the fault
> triggers we can identify it as a NUMA hinting page fault.
>
> _PAGE_NUMA on x86 shares the same bit number of _PAGE_PROTNONE (but it
> could also use a different bitflag, it's up to the architecture to
> decide).
>
> It would be confusing to call the "NUMA hinting page faults" as
> "do_prot_none faults". They're different events and _PAGE_NUMA doesn't
> alter the semantics of mprotect(PROT_NONE) in any way.
>
> Sharing the same bitflag with _PAGE_PROTNONE in fact complicates
> things: it requires us to ensure the code paths executed by
> _PAGE_PROTNONE remains mutually exclusive to the code paths executed
> by _PAGE_NUMA at all times, to avoid _PAGE_NUMA and _PAGE_PROTNONE to
> step into each other toes.
>
> Because we want to be able to set this bitflag in any established pte
> or pmd (while clearing the present bit at the same time) without
> losing information, this bitflag must never be set when the pte and
> pmd are present, so the bitflag picked for _PAGE_NUMA usage, must not
> be used by the swap entry format.
>
> Signed-off-by: Andrea Arcangeli <aarcange@...hat.com>
> Signed-off-by: Mel Gorman <mgorman@...e.de>

Reviewed-by: Rik van Riel <riel@...hat.com>

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