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

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Message-ID: <47F3FC64.8050507@codemonkey.ws>
Date:	Wed, 02 Apr 2008 16:36:36 -0500
From:	Anthony Liguori <anthony@...emonkey.ws>
To:	Dave Hansen <dave@...ux.vnet.ibm.com>
CC:	Jeremy Fitzhardinge <jeremy@...p.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Yasunori Goto <y-goto@...fujitsu.com>,
	Christoph Lameter <clameter@....com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Mel Gorman <mel@....ul.ie>
Subject: Re: [PATCH RFC] hotplug-memory: refactor online_pages to separate
 zone growth from page onlining

Dave Hansen wrote:
> On Wed, 2008-04-02 at 14:03 -0700, Jeremy Fitzhardinge wrote:
>   
>> Dave Hansen wrote:
>> 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.
>>     
>
> Oh, once we've let Linux establish ptes to it, we've required that the
> hypervisor have it around?  How does that work with the balloon driver?
> Do we destroy the ptes when giving balloon memory back to the
> hypervisor?
>
> If we're talking about i386, then we're set.  We don't map the hot-added
> memory at all because we only add highmem on i386.  The only time we map
> these pages is *after* we actually allocate them when they get mapped
> into userspace or used as vmalloc() or they're kmap()'d.
>
>   
>> 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.
>>     
>
> Either that, or we're going to develop the entire Xen/kvm memory hotplug
> architecture around the soon-to-be-legacy i386 limitations. :)
>   

s:Xen/kvm:Xen:g

We don't need anything special for KVM.  Bare metal memory hotplug 
should be sufficient provided userspace udev scripts are properly 
configured to offline memory automatically.

Regards,

Anthony Liguori

> -- Dave
>
>   

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