Re: [PATCH RFC] hotplug-memory: refactor online_pages to separatezone growth from page onlining
From: Jeremy Fitzhardinge
Date: Wed Apr 02 2008 - 17:04:50 EST
Dave Hansen wrote:
On Wed, 2008-04-02 at 11:52 -0700, Jeremy Fitzhardinge wrote:
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.
That's a very good point. Can we make it so that the hypervisors don't
actually allocate the memory to the guest until its first touch? If the
pages are on the freelist, their *contents* shouldn't be touched at all
during the onlining process.
No, not in a Xen direct-pagetable guest. The guest actually sees real
hardware page numbers (mfns) when the hypervisor gives it a page. By
the time the hypervisor gives it a page reference, it already
guaranteeing that the page is available for guest use. The only thing
that we could do is prevent the guest from mapping the page, but that
doesn't really achieve much.
I think we're getting off track here; this is a lot of extra complexity
to justify allowing usermode to use /sys to online a chunk of hotplugged
memory.
J
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