linux-kernel - Re: [patch -mm 1/2] i386: add ptep_test_and_clear

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Date:	Mon, 26 Mar 2007 06:53:50 +0100 (BST)
From:	Hugh Dickins <hugh@...itas.com>
To:	David Rientjes <rientjes@...gle.com>
cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...hat.com>,
	Zachary Amsden <zach@...are.com>, linux-kernel@...r.kernel.org
Subject: Re: [patch -mm 1/2] i386: add ptep_test_and_clear_{dirty,young}

On Sun, 25 Mar 2007, David Rientjes wrote:
> Add ptep_test_and_clear_{dirty,young} to i386.  They advertise that they
> have it and there is at least one place where it needs to be called
> without the page table lock: to clear the accessed bit on write to

Without the page table lock??

> /proc/pid/clear_refs.
> 
> ptep_clear_flush_{dirty,young} are updated to use the new functions.  The
> overall net effect to current users of ptep_clear_flush_{dirty,young} is
> that we introduce an additional branch.

We need to Cc Zach on this: git blame indicates it was he who replaced
i386's ptep_test_and_clear_{dirty,young} by that "We don't actually
have these" comment - it looks a bit as if what you want to do might
violate the assumptions he wants to make, but I don't grasp it.

Hugh

> 
> Cc: Hugh Dickins <hugh@...itas.com>
> Cc: Ingo Molnar <mingo@...hat.com>
> Signed-off-by: David Rientjes <rientjes@...gle.com>
> ---
>  include/asm-i386/pgtable.h |   25 +++++++++++++++++--------
>  1 files changed, 17 insertions(+), 8 deletions(-)
> 
> diff --git a/include/asm-i386/pgtable.h b/include/asm-i386/pgtable.h
> --- a/include/asm-i386/pgtable.h
> +++ b/include/asm-i386/pgtable.h
> @@ -283,12 +283,23 @@ do {									\
>  	}								\
>  } while (0)
>  
> -/*
> - * We don't actually have these, but we want to advertise them so that
> - * we can encompass the flush here.
> - */
>  #define __HAVE_ARCH_PTEP_TEST_AND_CLEAR_DIRTY
> +static inline int ptep_test_and_clear_dirty(struct vm_area_struct *vma,
> +					    unsigned long addr, pte_t *ptep)
> +{
> +	if (!pte_dirty(*ptep))
> +		return 0;
> +	return test_and_clear_bit(_PAGE_BIT_DIRTY, &ptep->pte_low);
> +}
> +
>  #define __HAVE_ARCH_PTEP_TEST_AND_CLEAR_YOUNG
> +static inline int ptep_test_and_clear_young(struct vm_area_struct *vma,
> +					    unsigned long addr, pte_t *ptep)
> +{
> +	if (!pte_young(*ptep))
> +		return 0;
> +	return test_and_clear_bit(_PAGE_BIT_ACCESSED, &ptep->pte_low);
> +}
>  
>  /*
>   * Rules for using ptep_establish: the pte MUST be a user pte, and
> @@ -305,9 +316,8 @@ do {									\
>  #define ptep_clear_flush_dirty(vma, address, ptep)			\
>  ({									\
>  	int __dirty;							\
> -	__dirty = pte_dirty(*(ptep));					\
> +	__dirty = ptep_test_and_clear_dirty((vma), (address), (ptep));	\
>  	if (__dirty) {							\
> -		clear_bit(_PAGE_BIT_DIRTY, &(ptep)->pte_low);		\
>  		pte_update_defer((vma)->vm_mm, (address), (ptep));	\
>  		flush_tlb_page(vma, address);				\
>  	}								\
> @@ -318,9 +328,8 @@ do {									\
>  #define ptep_clear_flush_young(vma, address, ptep)			\
>  ({									\
>  	int __young;							\
> -	__young = pte_young(*(ptep));					\
> +	__young = ptep_test_and_clear_young((vma), (address), (ptep));	\
>  	if (__young) {							\
> -		clear_bit(_PAGE_BIT_ACCESSED, &(ptep)->pte_low);	\
>  		pte_update_defer((vma)->vm_mm, (address), (ptep));	\
>  		flush_tlb_page(vma, address);				\
>  	}								\
-
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