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    SubjectRe: [PATCH] Describe race of direct read and fork for unaligned buffers
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    On 2 May 2012 18:17, Jan Kara <jack@suse.cz> wrote:
    > On Wed 02-05-12 01:50:46, Nick Piggin wrote:

    >> KOSAKI-san is correct, I think.
    >>
    >> The race is something like this:
    >>
    >> DIO-read
    >>     page = get_user_pages()
    >>                                                         fork()
    >>                                                             COW(page)
    >>                                                          touch(page)
    >>     DMA(page)
    >>     page_cache_release(page);
    >>
    >> So whether parent or child touches the page, determines who gets the
    >> actual DMA target, and who gets the copy.
    >  OK, this is roughly what I understood from original threads as well. So
    > if our buffer is page aligned and its size is page aligned, you would hit
    > the corruption only if you do modify the buffer while IO to / from that buffer
    > is in progress. And that would seem like a really bad programming practice
    > anyway. So I still believe that having everything page size aligned will
    > effectively remove the problem although I agree it does not aim at the core
    > of it.

    I see what you mean.

    I'm not sure, though. For most apps it's bad practice I think. If you get into
    realm of sophisticated, performance critical IO/storage managers, it would
    not surprise me if such concurrent buffer modifications could be allowed.
    We allow exactly such a thing in our pagecache layer. Although probably
    those would be using shared mmaps for their buffer cache.

    I think it is safest to make a default policy of asking for IOs against private
    cow-able mappings to be quiesced before fork, so there are no surprises
    or reliance on COW details in the mm. Do you think?
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