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    SubjectRe: [PATCH] Describe race of direct read and fork for unaligned buffers
    On Wed 09-05-12 19:18:16, Michael Kerrisk (man-pages) wrote:
    > On Wed, May 9, 2012 at 7:01 PM, Nick Piggin <npiggin@gmail.com> wrote:
    > > On 9 May 2012 15:35, Michael Kerrisk (man-pages) <mtk.manpages@gmail.com> wrote:
    > >> On Wed, May 9, 2012 at 11:10 AM, Nick Piggin <npiggin@gmail.com> wrote:
    > >>> On 6 May 2012 01:29, KOSAKI Motohiro <kosaki.motohiro@gmail.com> wrote:
    > >>>>> So, am I correct to assume that right text to add to the page is as below?
    > >>>>>
    > >>>>> Nick, can you clarify what you mean by "quiesced"?
    > >>>>
    > >>>> finished?
    > >>>
    > >>> Yes exactly. That might be a simpler word. Thanks!
    > >>
    > >> Thanks.
    > >>
    > >> But see below. I realize the text is still ambiguous.
    > >>
    > >>>>> [[
    > >>>>> 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).
    > >>>>> ]]
    > >>
    > >> In the above, the status of a MAP_SHARED MAP_ANONYMOUS buffer is
    > >> unclear. The first paragraph implies that such a buffer is unsafe,
    > >> while the third paragraph implies that it *is* safe, thus
    > >> contradicting the first paragraph. Which is correct?
    > >
    > > Yes I see. It's because MAP_SHARED | MAP_ANONYMOUS isn't *really*
    > > anonymous from the virtual memory subsystem's point of view. But that
    > > just serves to confuse userspace I guess.
    > >
    > > Anything with MAP_SHARED, shmat, or MADV_DONTFORK is OK.
    > >
    > > Anything else (MAP_PRIVATE, brk, without MADV_DONTFORK) is
    > > dangerous. These type are used by standard heap allocators malloc,
    > > new, etc.
    >
    > So, would the following text be okay:
    >
    > O_DIRECT I/Os should never be run concurrently with the fork(2)
    > system call, if the memory buffer is a private mapping (i.e.,
    > any mapping created with the mmap(2) MAP_PRIVATE flag; this
    > includes memory allocated on the heap and statically allocated
    > buffers). Any such I/Os, whether submitted via an asynchronous
    > I/O interface or from another thread in the process, should be
    > completed 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 mem‐
    > ory buffer for the O_DIRECT I/Os was created using shmat(2) or
    > mmap(2) with the MAP_SHARED flag. 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).
    This text looks OK, to me. Thanks for putting it together.

    Honza
    --
    Jan Kara <jack@suse.cz>
    SUSE Labs, CR
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