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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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