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    SubjectRe: [PATCH] Describe race of direct read and fork for unaligned buffers
    On Wed, 2 May 2012, Nick Piggin wrote:
    > On 2 May 2012 03:56, Michael Kerrisk (man-pages) <> wrote:
    > >
    > > In the light of all of the comments, can someone revise the man-pages
    > > patch that Jan sent?
    > This does not quite describe the entire situation, but something understandable
    > to developers:
    > O_DIRECT IOs should never be run concurrently with fork(2) system call,
    > when the memory buffer is anonymous memory, or comes from mmap(2)
    > with MAP_PRIVATE.
    > Any such IOs, whether submitted with asynchronous IO interface or from
    > another thread in the process, should be quiesced before fork(2) is called.
    > Failure to do so can result in data corruption and undefined behavior in
    > parent and child processes.
    > This restriction does not apply when the memory buffer for the O_DIRECT
    > IOs comes from mmap(2) with MAP_SHARED or from shmat(2).

    Nor does this restriction apply when the memory buffer has been advised
    as MADV_DONTFORK with madvise(2), ensuring that it will not be available
    to the child after fork(2).

    > Is that on the right track? I feel it might be necessary to describe this
    > allowance for MAP_SHARED, because some databases may be doing
    > such things, and anyway it gives apps a potential way to make this work
    > if concurrent fork + DIO is very important.

    Looks good, but we do need a reference to MADV_DONTFORK, perhaps as above.


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