Re: [PATCH 2/2] Drivers: hv: Add Hyper-V balloon driver

[Jeremy Fitzhardinge <jeremy@goop.org>]

On 10/09/2012 06:14 PM, Andrew Morton wrote:
> On Wed, 10 Oct 2012 00:09:12 +0000 KY Srinivasan <kys@microsoft.com> wrote:
>
>>>> +		if (!pg) {
>>>> +			*alloc_error = true;
>>>> +			return i * alloc_unit;
>>>> +		}
>>>> +
>>>> +		totalram_pages -= alloc_unit;
>>> Well, I'd consider totalram_pages to be an mm-private thing which drivers
>>> shouldn't muck with.  Why is this done?
>> By modifying the totalram_pages, the information presented in /proc/meminfo
>> correctly reflects what is currently assigned to the guest (MemTotal).
> eh?  /proc/meminfo:MemTotal tells you the total memory in the machine. 
> The only thing which should change it after boot is memory hotplug. 
[...]
> Why on earth do balloon drivers do this?  If the amount of memory which
> is consumed by balloons is interesting then it should be exported via a
> standalone metric, not by mucking with totalram_pages.

Balloon drivers are trying to fake a form of page-by-page memory
hotplug.  When they allocate memory from the kernel, they're actually
giving the pages back to the hypervisor to redistribute to other
guests.  They reduce totalram_pages to try and reflect that the memory
is no longer the kernel (in Xen, at least, the pfns will no longer have
any physical page underlying them).

I agree this is pretty ugly; it would be nice to have some better
interface to indicate what's going on.  At one point I tried to use the
memory hotplug interfaces for larger-scale dynamic transfers of memory
between a domain and the host, but when I last looked at it, it was too
coarse grained and heavyweight to replace the balloon mechanism.

    J