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    SubjectRe: [PATCH v5 00/21] Free some vmemmap pages of hugetlb page
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    On 20.11.20 09:42, Michal Hocko wrote:
    > On Fri 20-11-20 14:43:04, Muchun Song wrote:
    > [...]
    >
    > Thanks for improving the cover letter and providing some numbers. I have
    > only glanced through the patchset because I didn't really have more time
    > to dive depply into them.
    >
    > Overall it looks promissing. To summarize. I would prefer to not have
    > the feature enablement controlled by compile time option and the kernel
    > command line option should be opt-in. I also do not like that freeing
    > the pool can trigger the oom killer or even shut the system down if no
    > oom victim is eligible.
    >
    > One thing that I didn't really get to think hard about is what is the
    > effect of vmemmap manipulation wrt pfn walkers. pfn_to_page can be
    > invalid when racing with the split. How do we enforce that this won't
    > blow up?

    I have the same concerns - the sections are online the whole time and
    anybody with pfn_to_online_page() can grab them

    I think we have similar issues with memory offlining when removing the
    vmemmap, it's just very hard to trigger and we can easily protect by
    grabbing the memhotplug lock. I once discussed with Dan using rcu to
    protect the SECTION_IS_ONLINE bit, to make sure anybody who did a
    pfn_to_online_page() stopped using the page. Of course, such an approach
    is not easy to use in this context where the sections stay online the
    whole time ... we would have to protect vmemmap table entries using rcu
    or similar, which can get quite ugly.

    To keep things easy, maybe simply never allow to free these hugetlb
    pages again for now? If they were reserved during boot and the vmemmap
    condensed, then just let them stick around for all eternity.

    Once we have a safe approach on how to modify an online vmemmap, we can
    enable this freeing, and eventually also dynamically manage vmemmaps for
    runtime-allocated huge pages.

    --
    Thanks,

    David / dhildenb

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