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    SubjectRe: Page aging broken in 2.6
    Benjamin Herrenschmidt <benh@kernel.crashing.org> wrote:
    >
    > HI !
    >
    > I don't know if x86 is affected (I suspect not) but ppc and ppc64
    > definitely are.
    >
    > in mm/rmap.c, in page_referenced(), we do that twice:
    >
    > if (ptep_test_and_clear_young(pte))
    > referenced++;
    >
    > And we never flush the TLB entry.
    >
    > I don't know if x86 (or other archs really using page tables) will
    > actually set the referenced bit again in the PTE if it's already set
    > in the TLB, if not, then x86 needs a flush too.

    x86 needs a flush_tlb_page(), yes.

    > ppc and ppc64 need a flush to evict the entry from the hash table or
    > we'll never set the _PAGE_ACCESSED bit anymore.
    >
    > On the other hand, I'd like to propose a semantic change here, by
    > changing ptep_test_and_clear_dirty() as well so that the flush is done
    > by the arch function and not explicitely by the generic code in both
    > cases. (I'm not sure if it's worth adding an mm parameter to the call
    > or if the arch will figure it out, we don't have it at hand in
    > page_referenced()).
    >
    > That way, arch that don't need the flush (if any) can avoid it, and
    > in the case of ptep_test_and_clear_dirty, I may have a better way of
    > implementing it without a flush in mind.

    I don't feel particularly strongly either way, but the core mm code is
    sprinkled with flushes anyway; it would probably be more consistent to
    open-code it in rmap.c now.

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