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Date: Tue, 06 Nov 2012 13:35:23 -0500
From: Rik van Riel <riel@redhat.com>
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To: Mel Gorman <mgorman@suse.de>
CC: Peter Zijlstra <a.p.zijlstra@chello.nl>,
        Andrea Arcangeli <aarcange@redhat.com>, Ingo Molnar <mingo@kernel.org>,
        Johannes Weiner <hannes@cmpxchg.org>, Hugh Dickins <hughd@google.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Linux-MM <linux-mm@kvack.org>, LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 04/19] mm: numa: define _PAGE_NUMA
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On 11/06/2012 04:14 AM, Mel Gorman wrote:
> From: Andrea Arcangeli <aarcange@redhat.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@redhat.com>
> Signed-off-by: Mel Gorman <mgorman@suse.de>

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