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Re: [Xen-devel] [PATCH] linux/balloon: don't allow ballooningdown a doma

To: "Keir Fraser" <keir.fraser@xxxxxxxxxxxxx>
Subject: Re: [Xen-devel] [PATCH] linux/balloon: don't allow ballooningdown a domain below a reasonable limit
From: "Jan Beulich" <jbeulich@xxxxxxxxxx>
Date: Mon, 07 Apr 2008 08:10:21 +0100
Cc: Ky Srinivasan <KSrinivasan@xxxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxx, Kurt Garloff <garloff@xxxxxxx>
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>>> Keir Fraser <keir.fraser@xxxxxxxxxxxxx> 05.04.08 23:39 >>>
>On 4/4/08 16:07, "Jan Beulich" <jbeulich@xxxxxxxxxx> wrote:
>> +#ifndef CONFIG_XEN
>> +#define max_pfn totalram_pages
>> +#endif
>This is silly. We modify totalram_pages as we balloon up and down, so this
>really isn't very max_pfn-like after ballooning gets under way.

Indeed. It's been a very long time since I had to last touch this patch, so
I can only assume that originally was meant to address a build problem,
and then got forgotten about.

>So I've applied the patch but I made it a no-op if !defined(CONFIG_XEN),
>until/unless someone comes up with a better alternative to totalram_pages.
>Possibly just latching totalram_pages when we install the balloon driver
>would be sufficient?

That would be one option, though not exactly representing what is
intended here - the minimum memory requirement depends (at least for
FLATMEM) much more on the size of the 'struct page' array than on the
part of the array that's actually valid memory.
Since max_mapnr doesn't get initialized for x86-64 and end_pfn is no
longer being exported in 2.6.25, num_physpages would seem to be
the only other alternative.


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