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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 23:12:23 -0800

    > "David S. Miller" <davem@davemloft.net> wrote:
    > >
    > > 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).
    >
    > The direct-io.c code just does memset. (That's very common - maybe
    > clear_user_highpage_partial() is needed?)

    Yes, something like clear_user_higpage_partial() is definitely needed
    for cases like direct-io.c.

    We have similar handling for when ptrace() uses get_user_pages() and
    tries to write to those pages, via copy_to_user_page() and
    copy_from_user_page(). But those interfaces don't clear memory and
    they only work in the ptrace path, and they are optimized for that
    usage.
    -
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