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Date: Wed, 30 May 2012 14:22:49 -0400
From: Konrad Rzeszutek Wilk <konrad@darnok.org>
To: Andrea Arcangeli <aarcange@redhat.com>
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: <20120530182247.GA28341@localhost.localdomain>
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On Fri, May 25, 2012 at 07:02:08PM +0200, Andrea Arcangeli wrote:
> We will set these bitflags only when the pmd and pte is non present.
> 
> They work like PROT_NONE but they identify a request for the numa
> hinting page fault to trigger.
> 
> Because we want to be able to set these bitflag in any established pte
> or pmd (while clearing the present bit at the same time) without
> losing information, these bitflags must never be set when the pte and
> pmd are present.
> 
> For _PAGE_NUMA_PTE the pte bitflag used is _PAGE_PSE, which cannot be
> set on ptes and it also fits in between _PAGE_FILE and _PAGE_PROTNONE
> which avoids having to alter the swp entries format.
> 
> For _PAGE_NUMA_PMD, we use a reserved bitflag. pmds never contain
> swap_entries but if in the future we'll swap transparent hugepages, we
> must keep in mind not to use the _PAGE_UNUSED2 bitflag in the swap
> entry format and to start the swap entry offset above it.
> 
> PAGE_UNUSED2 is used by Xen but only on ptes established by ioremap,
> but it's never used on pmds so there's no risk of collision with Xen.

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>



> 
> Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
> ---
>  arch/x86/include/asm/pgtable_types.h |   11 +++++++++++
>  1 files changed, 11 insertions(+), 0 deletions(-)
> 
> diff --git a/arch/x86/include/asm/pgtable_types.h b/arch/x86/include/asm/pgtable_types.h
> index b74cac9..6e2d954 100644
> --- a/arch/x86/include/asm/pgtable_types.h
> +++ b/arch/x86/include/asm/pgtable_types.h
> @@ -71,6 +71,17 @@
>  #define _PAGE_FILE	(_AT(pteval_t, 1) << _PAGE_BIT_FILE)
>  #define _PAGE_PROTNONE	(_AT(pteval_t, 1) << _PAGE_BIT_PROTNONE)
>  
> +/*
> + * Cannot be set on pte. The fact it's in between _PAGE_FILE and
> + * _PAGE_PROTNONE avoids having to alter the swp entries.
> + */
> +#define _PAGE_NUMA_PTE	_PAGE_PSE
> +/*
> + * Cannot be set on pmd, if transparent hugepages will be swapped out
> + * the swap entry offset must start above it.
> + */
> +#define _PAGE_NUMA_PMD	_PAGE_UNUSED2
> +
>  #define _PAGE_TABLE	(_PAGE_PRESENT | _PAGE_RW | _PAGE_USER |	\
>  			 _PAGE_ACCESSED | _PAGE_DIRTY)
>  #define _KERNPG_TABLE	(_PAGE_PRESENT | _PAGE_RW | _PAGE_ACCESSED |	\
> 
> --
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