Re: [PATCH] VMware Balloon driver

From: Andrew Morton
Date: Mon Apr 05 2010 - 18:18:11 EST

On Mon, 05 Apr 2010 15:03:08 -0700
Jeremy Fitzhardinge <jeremy@xxxxxxxx> wrote:

> On 04/05/2010 02:24 PM, Andrew Morton wrote:
> > I think I've forgotten what balloon drivers do. Are they as nasty a
> > hack as I remember believing them to be?
> >
> (I haven't looked at Dmitry's patch yet, so this is from the Xen
> perspective.)
> In the simplest form, they just look like a driver which allocates a
> pile of pages, and the underlying memory gets returned to the
> hypervisor. When you want the memory back, it reattaches memory to the
> pageframes and releases the memory back to the kernel. This allows a
> virtual machine to shrink with respect to its original size.
> Going the other way - expanding beyond the memory allocation - is a bit
> trickier because you need to get some new page structures from
> somewhere. We don't do this in Xen yet, but I've done some experiments
> with hotplug memory to implement this. Or a simpler approach is to fake
> up some reserved E820 ranges to grow into.

Lots of stuff for Dmitry to add to his changelog ;)

> > A summary of what this code sets out to do, and how it does it would be
> > useful.
> >
> > Also please explain the applicability of this driver. Will xen use it?
> > kvm? Out-of-tree code?
> >
> The basic idea of the driver is to allow a guest system to give up
> memory it isn't using so it can be reused by other virtual machines (or
> the host itself).

So... does this differ in any fundamental way from what hibernation
does, via shrink_all_memory()?

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