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Date: Tue, 13 Nov 2012 10:54:17 +0100
From: Ingo Molnar <mingo@...nel.org>
To: Mel Gorman <mgorman@...e.de>
Cc: Peter Zijlstra <a.p.zijlstra@...llo.nl>,
Andrea Arcangeli <aarcange@...hat.com>,
Rik van Riel <riel@...hat.com>,
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 05/19] mm: numa: pte_numa() and pmd_numa()
* Mel Gorman <mgorman@...e.de> wrote:
> From: Andrea Arcangeli <aarcange@...hat.com>
>
> Implement pte_numa and pmd_numa.
>
> We must atomically set the numa bit and clear the present bit to
> define a pte_numa or pmd_numa.
>
> Once a pte or pmd has been set as pte_numa or pmd_numa, the next time
> a thread touches a virtual address in the corresponding virtual range,
> a NUMA hinting page fault will trigger. The NUMA hinting page fault
> will clear the NUMA bit and set the present bit again to resolve the
> page fault.
>
> The expectation is that a NUMA hinting page fault is used as part
> of a placement policy that decides if a page should remain on the
> current node or migrated to a different node.
>
> Acked-by: Rik van Riel <riel@...hat.com>
> Signed-off-by: Andrea Arcangeli <aarcange@...hat.com>
> Signed-off-by: Mel Gorman <mgorman@...e.de>
> ---
> arch/x86/include/asm/pgtable.h | 65 ++++++++++++++++++++++++++++++++++++++--
> include/asm-generic/pgtable.h | 12 ++++++++
> 2 files changed, 75 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/include/asm/pgtable.h b/arch/x86/include/asm/pgtable.h
> index a1f780d..e075d57 100644
> --- a/arch/x86/include/asm/pgtable.h
> +++ b/arch/x86/include/asm/pgtable.h
> @@ -404,7 +404,8 @@ static inline int pte_same(pte_t a, pte_t b)
>
> static inline int pte_present(pte_t a)
> {
> - return pte_flags(a) & (_PAGE_PRESENT | _PAGE_PROTNONE);
> + return pte_flags(a) & (_PAGE_PRESENT | _PAGE_PROTNONE |
> + _PAGE_NUMA);
> }
>
> static inline int pte_hidden(pte_t pte)
> @@ -420,7 +421,63 @@ static inline int pmd_present(pmd_t pmd)
> * the _PAGE_PSE flag will remain set at all times while the
> * _PAGE_PRESENT bit is clear).
> */
> - return pmd_flags(pmd) & (_PAGE_PRESENT | _PAGE_PROTNONE | _PAGE_PSE);
> + return pmd_flags(pmd) & (_PAGE_PRESENT | _PAGE_PROTNONE | _PAGE_PSE |
> + _PAGE_NUMA);
> +}
> +
> +#ifdef CONFIG_BALANCE_NUMA
> +/*
> + * _PAGE_NUMA works identical to _PAGE_PROTNONE (it's actually the
> + * same bit too). It's set only when _PAGE_PRESET is not set and it's
> + * never set if _PAGE_PRESENT is set.
> + *
> + * pte/pmd_present() returns true if pte/pmd_numa returns true. Page
> + * fault triggers on those regions if pte/pmd_numa returns true
> + * (because _PAGE_PRESENT is not set).
> + */
> +static inline int pte_numa(pte_t pte)
> +{
> + return (pte_flags(pte) &
> + (_PAGE_NUMA|_PAGE_PRESENT)) == _PAGE_NUMA;
> +}
> +
> +static inline int pmd_numa(pmd_t pmd)
> +{
> + return (pmd_flags(pmd) &
> + (_PAGE_NUMA|_PAGE_PRESENT)) == _PAGE_NUMA;
> +}
> +#endif
> +
> +/*
> + * pte/pmd_mknuma sets the _PAGE_ACCESSED bitflag automatically
> + * because they're called by the NUMA hinting minor page fault. If we
> + * wouldn't set the _PAGE_ACCESSED bitflag here, the TLB miss handler
> + * would be forced to set it later while filling the TLB after we
> + * return to userland. That would trigger a second write to memory
> + * that we optimize away by setting _PAGE_ACCESSED here.
> + */
> +static inline pte_t pte_mknonnuma(pte_t pte)
> +{
> + pte = pte_clear_flags(pte, _PAGE_NUMA);
> + return pte_set_flags(pte, _PAGE_PRESENT|_PAGE_ACCESSED);
> +}
> +
> +static inline pmd_t pmd_mknonnuma(pmd_t pmd)
> +{
> + pmd = pmd_clear_flags(pmd, _PAGE_NUMA);
> + return pmd_set_flags(pmd, _PAGE_PRESENT|_PAGE_ACCESSED);
> +}
> +
> +static inline pte_t pte_mknuma(pte_t pte)
> +{
> + pte = pte_set_flags(pte, _PAGE_NUMA);
> + return pte_clear_flags(pte, _PAGE_PRESENT);
> +}
> +
> +static inline pmd_t pmd_mknuma(pmd_t pmd)
> +{
> + pmd = pmd_set_flags(pmd, _PAGE_NUMA);
> + return pmd_clear_flags(pmd, _PAGE_PRESENT);
> }
>
> static inline int pmd_none(pmd_t pmd)
> @@ -479,6 +536,10 @@ static inline pte_t *pte_offset_kernel(pmd_t *pmd, unsigned long address)
>
> static inline int pmd_bad(pmd_t pmd)
> {
> +#ifdef CONFIG_BALANCE_NUMA
> + if (pmd_numa(pmd))
> + return 0;
> +#endif
> return (pmd_flags(pmd) & ~_PAGE_USER) != _KERNPG_TABLE;
> }
>
> diff --git a/include/asm-generic/pgtable.h b/include/asm-generic/pgtable.h
> index b36ce40..896667e 100644
> --- a/include/asm-generic/pgtable.h
> +++ b/include/asm-generic/pgtable.h
> @@ -554,6 +554,18 @@ static inline int pmd_trans_unstable(pmd_t *pmd)
> #endif
> }
>
> +#ifndef CONFIG_BALANCE_NUMA
> +static inline int pte_numa(pte_t pte)
> +{
> + return 0;
> +}
> +
> +static inline int pmd_numa(pmd_t pmd)
> +{
> + return 0;
> +}
> +#endif /* CONFIG_BALANCE_NUMA */
> +
Hm, this overcomplicates things quite a bit and adds arch
specific code, and there's no explanation given for that
approach that I can see?
Basically, what's wrong with the generic approach that numa/core
has:
__weak bool pte_numa(struct vm_area_struct *vma, pte_t pte)
[see the full function below.]
Then we can reuse existing protection-changing functionality and
keep it all tidy.
an architecture that wants to do something special could
possibly override it in the future - but we want to keep the
generic logic in generic code.
Thanks,
Ingo
------------>
__weak bool pte_numa(struct vm_area_struct *vma, pte_t pte)
{
/*
* For NUMA page faults, we use PROT_NONE ptes in VMAs with
* "normal" vma->vm_page_prot protections. Genuine PROT_NONE
* VMAs should never get here, because the fault handling code
* will notice that the VMA has no read or write permissions.
*
* This means we cannot get 'special' PROT_NONE faults from genuine
* PROT_NONE maps, nor from PROT_WRITE file maps that do dirty
* tracking.
*
* Neither case is really interesting for our current use though so we
* don't care.
*/
if (pte_same(pte, pte_modify(pte, vma->vm_page_prot)))
return false;
return pte_same(pte, pte_modify(pte, vma_prot_none(vma)));
}
--
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