Re: [PATCH RFC] hotplug-memory: refactor online_pages to separate zone growth from page onlining

[Jeremy Fitzhardinge <jeremy@goop.org>]

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