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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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