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    SubjectRe: [PATCH 1/4] mm: Trial do_wp_page() simplification
    On Mon, Sep 14, 2020 at 03:59:31PM -0700, Linus Torvalds wrote:
    > On Mon, Sep 14, 2020 at 3:55 PM Jason Gunthorpe <jgg@nvidia.com> wrote:
    > >
    > > Just as an aside, the RDMA stuff is also supposed to set MADV_DONTFORK
    > > on these regions, so I'm a bit puzzled what is happening here.
    >
    > Did the fork perhaps happen _before_ , so the pages are shared when
    > you do the pin?

    Looking at the progam, it seems there are a number of forks for exec
    before and after pin_user_pages_fast(), but the parent process always
    does waitpid() after the fork.

    > MADV_DONTFORK doesn't mean COW doesn't happen. It just means that the
    > next fork() won't be copying that memory area.

    Yes, this stuff does pin_user_pages_fast() and MADV_DONTFORK
    together. It sets FOLL_FORCE and FOLL_WRITE to get an exclusive copy
    of the page and MADV_DONTFORK was needed to ensure that a future fork
    doesn't establish a COW that would break the DMA by moving the
    physical page over to the fork. DMA should stay with the process that
    called pin_user_pages_fast() (Is MADV_DONTFORK still needed with
    recent years work to GUP/etc? It is a pretty terrible ancient thing)

    > That said, it's possible that the test cases do something invalid - or
    > maybe we've broken MADV_DONTFORK - and it all just happened to work
    > before.

    Hmm. If symptoms stop with this patch should we investigate
    MADV_DONTFORK?

    Thanks,
    Jason

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