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    SubjectRe: [patch 5/5] Optimize page_mkclean_one
    On Sun, 1 Jul 2007, Martin Schwidefsky wrote:
    > >
    > > Expect you're right, but I _really_ don't want to comment, when I don't
    > > understand that "|| pte_write" in the first place, and don't know the
    > > consequence of pte_dirty && !pte_write or !pte_dirty && pte_write there.
    > The pte_write() part is for the shared dirty page tracking. If you want
    > to make sure that a max of x% of your pages are dirty then you cannot
    > allow to have more than x% to be writable. Thats why page_mkclean_one
    > clears the dirty bit and makes the page read-only.

    The whole of page_mkclean_one is for the dirty page tracking: so it's
    obvious why it tests pte_dirty, but not obvious why it tests pte_write.

    > > My suspicion is that the "|| pte_write" is precisely to cover your
    > > s390 case where pte is never dirty (it may even have been me who got
    > > Peter to put it in for that reason). In which case your patch would
    > > be fine - though I think it'd be improved a lot by a comment or
    > > rearrangement or new macro in place of the pte_dirty || pte_write
    > > line (perhaps adjust my pte_maybe_dirty in asm-generic/pgtable.h,
    > > and use that - its former use in msync has gone away now).
    > No, s390 is covered by the page_test_dirty / page_clear_dirty pair in
    > page_mkclean.

    That's where its dirty page count comes from, yes: but since the s390
    pte_dirty just says no, if page_mkclean_one tested only pte_dirty,
    then it wouldn't do anything on s390, and in particular wouldn't
    write protect the ptes to re-enforce dirty counting from then on.

    So in answering your denials, I grow more confident that the pte_write
    test is precisely for the s390 case. Though it might also be to cover
    some defect in the write-protection scheme on other arches.

    Come to think of it, would your patch really make any difference?
    Although page_mkclean's "count" of dirty ptes on s390 will be nonsense,
    that count would anyway be unknown, and it's only used as a boolean;
    and now I don't think your patch changes the boolean value - if any
    pte is found writable (and if the scheme is working) that implies
    that the page was written to, and so should give the same answer
    as the page_test_dirty.

    But I could easily be overlooking something: Peter will recall.

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