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    SubjectRe: [PATCH 1/3] compound page: use page[1].lru
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    From: Andrew Morton <akpm@osdl.org>
    Date: Sun, 12 Feb 2006 18:13:12 -0800

    > We have a page which has no ->mapping, but lo, it's mmapped by userspace
    > and can be MAP_SHARED between different CPUs and processes.
    >
    > Yes, I suspect it'll do the wrong thing in unpleasantly subtle ways.
    >
    > (cc's davem and runs away).

    The ->mapping check is there essentially to hit user mapped pages that
    would be modified by the kernel using kernel space memory accesses
    other than those done by copy_user_page() and clear_user_page() (and
    their brothers copy_user_highpage() and clear_user_highpage() which
    just call the former directly on a non-HIGHPAGE platform like
    sparc64).

    Hugepages actually have no D-cache aliasing issues by definition on
    sparc64 because the smallest possible hugepage size is 64K which is
    larger than the D-cache aliasing factor (which is 16K).
    -
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