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    SubjectRe: [PATCH v8 5/8] mm: Device exclusive memory access
    On Tue, May 18, 2021 at 01:27:42PM -0400, Peter Xu wrote:

    > I also have a pure and high level question regarding a process fork() when
    > there're device exclusive ptes: would the two processes then own the device
    > together? Is this a real usecase?

    If the pages are MAP_SHARED then yes. All VMAs should point at the
    same device_exclusive page and all VMA should migrate back to CPU
    pages together.

    > Indeed it'll be odd for a COW page since for COW page then it means after
    > parent/child writting to the page it'll clone into two, then it's a mistery on
    > which one will be the one that "exclusived owned" by the device..

    For COW pages it is like every other fork case.. We can't reliably
    write-protect the device_exclusive page during fork so we must copy it
    at fork time.

    Thus three reasonable choices:
    - Copy to a new CPU page
    - Migrate back to a CPU page and write protect it
    - Copy to a new device exclusive page

    Jason

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