Messages in this thread | | | Date | Sun, 25 Mar 2007 23:07:43 -0800 | From | Zachary Amsden <> | Subject | Re: [patch -mm 1/2] i386: add ptep_test_and_clear_{dirty,young} |
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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.
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