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    SubjectRe: [PATCH RFC] hotplug-memory: refactor online_pages to separate zone growth from page onlining
    Dave Hansen wrote:
    >>> and a flat sparsemem map, you're only looking at
    >>> ~500k of overhead for the sparsemem storage. Less if you use vmemmap.
    >>>
    >>>
    >> At the moment my concern is 32-bit x86, which doesn't support vmemmap or
    >> sections smaller than 512MB because of the shortage of page flags bits.
    >>
    >
    > Yeah, I forgot that we didn't have vmemmap on x86-32. Ugh.
    >
    > OK, here's another idea: Xen (and the balloon driver) already handle a
    > case where a guest boots up with 2GB of memory but only needs 1GB,
    > right? It will balloon the guest down to 1GB from 2GB.
    >

    Right.

    > Why don't we just have hotplug work that way? When we want to take a
    > guest from 1GB to 1GB+1 page (or whatever), we just hotplug the entire
    > section (512MB or 1GB or whatever), actually online the whole thing,
    > then make the balloon driver take it back to where it *should* be. That
    > way we're completely reusing existing components that have do be able to
    > handle this case anyway.
    >
    > Yeah, this is suboptimal, an it has a possibility of fragmenting the
    > memory, but it will only be used for the x86-32 case.
    >

    It also requires you actually have the memory on hand to populate the
    whole area. 512MB is still a significant chunk on a 2GB server; you may
    end up generating significant overall system memory pressure to scrape
    together the memory, only to immediately discard it again.

    J


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