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    SubjectRe: [PATCH v4 3.0-rc2-tip 2/22] 2: uprobes: Breakground page replacement.
    On Tue, 2011-06-14 at 16:27 +0200, Oleg Nesterov wrote:
    > On 06/14, Peter Zijlstra wrote:
    > >
    > > On Mon, 2011-06-13 at 19:00 +0200, Oleg Nesterov wrote:
    > > >
    > > > Also. This is called under down_read(mmap_sem), can't we race with
    > > > access_process_vm() modifying the same memory?
    > >
    > > Shouldn't matter COW and similar things are serialized using the pte
    > > lock.
    > Yes, but afaics this doesn't matter. Suppose that write_opcode() is
    > called when access_process_vm() does copy_to_user_page(). We can cow
    > the page before memcpy() completes.

    access_process_vm() will end up doing a FOLL_WRITE itself when
    copy_to_user_page() is called since write=1 in that case.

    At that point we have a COW-race, someone wins, but the other will then
    return the same page.

    At this point further PTRACE pokes can indeed race with the memcpy in
    write_opcode(). A possible fix would be to lock_page() around
    copy_to_user_page() (its already done in set_page_dirty_lock(), so
    pulling it out shouldn't matter much).

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