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SubjectRe: [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@veritas.com>
>> Cc: Ingo Molnar <mingo@redhat.com>
>> Signed-off-by: David Rientjes <rientjes@google.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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