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Date: Wed, 30 May 2012 20:34:06 +0200
From: Andrea Arcangeli <aarcange@redhat.com>
To: Konrad Rzeszutek Wilk <konrad@darnok.org>
Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        Hillf Danton <dhillf@gmail.com>, Dan Smith <danms@us.ibm.com>,
        Peter Zijlstra <a.p.zijlstra@chello.nl>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@elte.hu>,
        Paul Turner <pjt@google.com>,
        Suresh Siddha <suresh.b.siddha@intel.com>,
        Mike Galbraith <efault@gmx.de>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Lai Jiangshan <laijs@cn.fujitsu.com>,
        Bharata B Rao <bharata.rao@gmail.com>,
        Lee Schermerhorn <Lee.Schermerhorn@hp.com>,
        Rik van Riel <riel@redhat.com>, Johannes Weiner <hannes@cmpxchg.org>,
        Srivatsa Vaddagiri <vatsa@linux.vnet.ibm.com>,
        Christoph Lameter <cl@linux.com>
Subject: Re: [PATCH 04/35] autonuma: define _PAGE_NUMA_PTE and _PAGE_NUMA_PMD
Message-ID: <20120530183406.GH21339@redhat.com>
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Hi Konrad,

On Wed, May 30, 2012 at 02:22:49PM -0400, Konrad Rzeszutek Wilk wrote:
> Thank you for loking at this from the xen side. The interesting thing
> is that I believe the _PAGE_PAT (or _PAGE_PSE) is actually used on
> Xen on PTEs. It is used to mark the pages WC. <sigh>

Oops, I'm using _PAGE_PSE too on the pte, but only when it's unmapped.

static inline int pte_numa(pte_t pte)
{
	return (pte_flags(pte) &
		(_PAGE_NUMA_PTE|_PAGE_PRESENT)) == _PAGE_NUMA_PTE;
}

And _PAGE_UNUSED2 (_PAGE_IOMAP) is used for the pmd but _PAGE_IOMAP by
Xen should only be set on ptes.

The only way to use _PAGE_PSE safe on the pte is if the pte is
non-present, is this what Xen is also doing? (in turn colliding with
pte_numa)

Now if I shrink the size of the page_autonuma to one entry per pmd
(instead of per pte) I may as well drop pte_numa entirely and only
leave pmd_numa. At the moment it's possible to switch between the two
models at runtime with sysctl (if one wants to do a more expensive
granular tracking). I'm still uncertain on the best way to shrink
the page_autonuma size we'll see.
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