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    SubjectRe: [PATCH] Describe race of direct read and fork for unaligned buffers
    Date
    KOSAKI Motohiro <kosaki.motohiro@gmail.com> writes:

    > On Tue, May 1, 2012 at 11:11 AM, Jeff Moyer <jmoyer@redhat.com> wrote:
    >> KOSAKI Motohiro <kosaki.motohiro@gmail.com> 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
    > http://lkml.indiana.edu/hypermail/linux/kernel/0903.1/01498.html 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.

    Cheers,
    Jeff
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