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    SubjectRe: [PATCH] Describe race of direct read and fork for unaligned buffers
    On 2 May 2012 01:38, Jeff Moyer <> wrote:
    > KOSAKI Motohiro <> writes:
    >> On Tue, May 1, 2012 at 11:11 AM, Jeff Moyer <> wrote:
    >>> KOSAKI Motohiro <> writes:
    >>>>> Hello,
    >>>>> Thank you revisit this. But as far as my remember is correct, this issue is NOT
    >>>>> unaligned access issue. It's just get_user_pages(_fast) vs fork race issue. i.e.
    >>>>> DIRECT_IO w/ multi thread process should not use fork().
    >>>> The problem is, fork (and its COW logic) assume new access makes cow break,
    >>>> But page table protection can't detect a DMA write. Therefore DIO may override
    >>>> shared page data.
    >>> Hm, I've only seen this with misaligned or multiple sub-page-sized reads
    >>> in the same page.  AFAIR, aligned, page-sized I/O does not get split.
    >>> But, I could be wrong...
    >> If my remember is correct, the reproducer of past thread is misleading.
    >> dma_thread.c in
    >> has
    >> align parameter. But it doesn't only change align. Because of, every
    >> worker thread read 4K (pagesize), then
    >>  - when offset is page aligned
    >>     -> every page is accessed from only one worker
    >>  - when offset is not page aligned
    >>     -> every page is accessed from two workers
    >> But I don't remember why two threads are important things. hmm.. I'm
    >> looking into the code a while.
    >> Please don't 100% trust me.
    > I bet Andrea or Larry would remember the details.

    KOSAKI-san is correct, I think.

    The race is something like this:

    page = get_user_pages()

    So whether parent or child touches the page, determines who gets the
    actual DMA target, and who gets the copy.

    2 threads are not required, but it makes the race easier to code and a
    larger window, I suspect.

    It can also be hit with a single thread, using AIO.
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