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    Subject[RFC][PATCH 0/6] IO pinning(get_user_pages()) vs fork race fix
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    Linux Device Drivers, Third Edition, Chapter 15: Memory Mapping and DMA says

    get_user_pages is a low-level memory management function, with a suitably complex
    interface. It also requires that the mmap reader/writer semaphore for the address
    space be obtained in read mode before the call. As a result, calls to get_user_pages
    usually look something like:

    down_read(&current->mm->mmap_sem);
    result = get_user_pages(current, current->mm, ...);
    up_read(&current->mm->mmap_sem);

    The return value is the number of pages actually mapped, which could be fewer than
    the number requested (but greater than zero).

    but, it isn't true. mmap_sem isn't only used for vma traversal, but also prevent vs-fork race.
    up_read(mmap_sem) mean end of critical section, IOW after up_read() code is fork unsafe.
    (access_process_vm() explain proper get_user_pages() usage)

    Oh well, We have many wrong caller now. What is the best fix method?

    Nick Piggin and Andrea Arcangeli proposed to change get_user_pages() semantics as caller expected.
    see "[PATCH] fork vs gup(-fast) fix" thead in linux-mm
    but Linus NACKed it.

    Thus I made caller change approach patch series. it is made for discuss to compare Nick's approach.
    I don't hope submit it yet.

    Nick, This version fixed vmsplice and aio issue (you pointed). I hope to hear your opiniton ;)



    ChangeLog:
    V2 -> V3
    o remove early decow logic
    o introduce prevent unmap logic
    o fix nfs-directio
    o fix aio
    o fix bio (only bandaid fix)

    V1 -> V2
    o fix aio+dio case

    TODO
    o implement down_write_killable()
    o fix kvm (need?)
    o fix get_arg_page() (Why this function don't use mmap_sem?)




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