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    SubjectRe: [patch 2.6.13-rc4] fix get_user_pages bug
    On Tue, 2 Aug 2005, Linus Torvalds wrote:
    >
    > On the other hand, this being s390, maybe nobody cares?

    You have a cruel streak.

    But have I just realized a non-s390 problem with your pte_dirty
    technique? The ptep_set_wrprotect in fork's copy_one_pte.

    That's specifically write-protecting the pte to force COW, but
    leaving the dirty bit: so now get_user_pages will skip COW-ing it
    (in all write cases, not just the peculiar ptrace force one).

    We really do need to COW those in get_user_pages, don't we?
    And although we could handle it by clearing dirty and doing a
    set_page_dirty, it's a path we don't want to clutter further.

    (Yes, there's another issue of a fork occurring while the pages
    are already under get_user_pages, which is a significant issue for
    InfiniBand; but I see that as a separate kind of race, which we can
    reasonably disregard in this discussion - though it will need proper
    attention, perhaps through Michael Tsirkin's PROT_DONTCOPY patch.)

    The simple answer to Robin's pagetable update race is to say that
    anyone using ptrace should be prepared for a pt race ;)

    Hugh
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