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    SubjectRe: [PATCH v3 08/23] vfio, mm: fix get_user_pages_remote() and FOLL_LONGTERM
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    On 11/12/19 2:45 PM, Dan Williams wrote:
    > On Tue, Nov 12, 2019 at 2:43 PM John Hubbard <jhubbard@nvidia.com> wrote:
    >>
    >> On 11/12/19 12:43 PM, Jason Gunthorpe wrote:
    >> ...
    >>>> - }
    >>>> + ret = get_user_pages_remote(NULL, mm, vaddr, 1, flags | FOLL_LONGTERM,
    >>>> + page, vmas, NULL);
    >>>> + /*
    >>>> + * The lifetime of a vaddr_get_pfn() page pin is
    >>>> + * userspace-controlled. In the fs-dax case this could
    >>>> + * lead to indefinite stalls in filesystem operations.
    >>>> + * Disallow attempts to pin fs-dax pages via this
    >>>> + * interface.
    >>>> + */
    >>>> + if (ret > 0 && vma_is_fsdax(vmas[0])) {
    >>>> + ret = -EOPNOTSUPP;
    >>>> + put_page(page[0]);
    >>>> }
    >>>
    >>> AFAIK this chunk is redundant now as it is some hack to emulate
    >>> FOLL_LONGTERM? So vmas can be deleted too.
    >>
    >> Let me first make sure I understand what Dan has in mind for the vma
    >> checking, in the other thread...
    >
    > It's not redundant relative to upstream which does not do anything the
    > FOLL_LONGTERM in the gup-slow path... but I have not looked at patches
    > 1-7 to see if something there made it redundant.
    >

    There is nothing in patches 1-7 that would make it redundant.

    About the only thing that you might find interesting in that subset is
    patch 4 ("mm: devmap: refactor 1-based refcounting for ZONE_DEVICE pages"),
    for devmap and ZONE_DEVICE interest. But it doesn't affect this
    discussion directly.


    thanks,
    --
    John Hubbard
    NVIDIA

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