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

[Jeremy Fitzhardinge <jeremy@goop.org>]

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