Re: [patch -mm 1/2] i386: add ptep_test_and_clear_{dirty,young}

[Zachary Amsden]

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@xxxxxxxxxxx>
> > Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> > Signed-off-by: David Rientjes <rientjes@xxxxxxxxxx>
> > ---
> >  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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