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    SubjectRe: [RFC PATCH 2/2] macvtap: TX zero copy between guest and host kernel
    From
    From: Shirley Ma <mashirle@us.ibm.com>
    Date: Mon, 13 Sep 2010 13:48:03 -0700

    > + base = (unsigned long)from->iov_base + offset1;
    > + size = ((base & ~PAGE_MASK) + len + ~PAGE_MASK) >> PAGE_SHIFT;
    > + num_pages = get_user_pages_fast(base, size, 0, &page[i]);
    > + if ((num_pages != size) ||
    > + (num_pages > MAX_SKB_FRAGS - skb_shinfo(skb)->nr_frags))
    > + /* put_page is in skb free */
    > + return -EFAULT;

    What keeps the user from writing to these pages in it's address space
    after the write call returns?

    A write() return of success means:

    "I wrote what you gave to me"

    not

    "I wrote what you gave to me, oh and BTW don't touch these
    pages for a while."

    In fact "a while" isn't even defined in any way, as there is no way
    for the write() invoker to know when the networking card is done with
    those pages.


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