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

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Date:	Sun, 25 Mar 2007 23:07:43 -0800
From:	Zachary Amsden <zach@...are.com>
To:	Hugh Dickins <hugh@...itas.com>
CC:	David Rientjes <rientjes@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: [patch -mm 1/2] i386: add ptep_test_and_clear_{dirty,young}

Hugh Dickins wrote:
> 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);
>> +}
>> +
>>     

If you actually clear the bit, you need to:

+         pte_update_defer(vma->vm_mm, addr, ptep);

The reason is, when updating PTEs, the hypervisor must be notified.  
Using atomic operations to do this is fine for all hypervisors I am 
aware of.  However, for hypervisors which shadow page tables, if these 
PTE modifications are not trapped, you need a post-modification call to 
fulfill the update of the shadow page table.

>>  #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);
>> +}
>>     

Same here.

Hugh, thanks for the cc.

Zach
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