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    SubjectRe: [RFC 0/3] mm: Discard lazily freed pages when migrating
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    Michal Hocko <mhocko@kernel.org> writes:

    > On Tue 03-03-20 09:30:28, Huang, Ying wrote:
    > [...]
    >> Yes. mmap() can control whether to populate the underlying physical
    >> pages.
    >
    > right because many usecases benefit from it. They simply know that the
    > mapping will be used completely and it is worth saving overhead for #PF.
    > See. there is a clear justification for that policy.
    >
    >> But for migrating MADV_FREE pages, there's no control, all pages
    >> will be populated again always by default. Maybe we should avoid to do
    >> that in some situations too.
    >
    > Now let's have a look here. It is the userspace that decided to mark
    > MADV_FREE pages. It is under its full control which pages are to be
    > freed lazily. If the userspace wants to move those pages then it is
    > likely aware they have been MADV_FREE, right? If the userspace wanted to
    > save migration overhead then it could either chose to not migrate those
    > pages or simply unmap them right away. So in the end we are talking
    > about saving munmap/MAMDV_DONTNEED or potentially more move_pages calls
    > to skip over MADV_FREE holes. Which is all nice but is there any
    > userspace that really does care? Because this is a fundamental question
    > here and it doesn't make much sense to discuss this left to right unless
    > this is clear.

    Although I don't agree with you, I don't want to continue. Because I
    feel that the discussion may be too general to go anywhere. I admit
    that I go to the general side firstly, sorry about that.

    Best Regards,
    Huang, Ying

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