RE: [PATCH 2/2] Drivers: hv: balloon: Support 2M page allocations for ballooning

[KY Srinivasan <kys@microsoft.com>]

> -----Original Message-----
> From: Michal Hocko [mailto:mhocko@suse.cz]
> Sent: Monday, March 18, 2013 6:53 AM
> To: KY Srinivasan
> Cc: gregkh@linuxfoundation.org; linux-kernel@vger.kernel.org;
> devel@linuxdriverproject.org; olaf@aepfle.de; apw@canonical.com;
> andi@firstfloor.org; akpm@linux-foundation.org; linux-mm@kvack.org;
> kamezawa.hiroyuki@gmail.com; hannes@cmpxchg.org; yinghan@google.com
> Subject: Re: [PATCH 2/2] Drivers: hv: balloon: Support 2M page allocations for
> ballooning
> 
> On Sat 16-03-13 14:42:05, K. Y. Srinivasan wrote:
> > While ballooning memory out of the guest, attempt 2M allocations first.
> > If 2M allocations fail, then go for 4K allocations. In cases where we
> > have performed 2M allocations, split this 2M page so that we can free this
> > page at 4K granularity (when the host returns the memory).
> 
> Maybe I am missing something but what is the advantage of 2M allocation
> when you split it up immediately so you are not using it as a huge page?

The Hyper-V ballooning protocol specifies the pages being ballooned as page ranges -
start_pfn: number_of_pfns. So, when the guest balloon is inflating and I am able to
allocate 2M pages, I will be able to represent 512 contiguous pages in one 64 bit entry and this makes
the ballooning operation that much more efficient. The reason I split the page is that the host does not
guarantee that when it returns the memory to the guest, it will return in any particular granularity and so
I have to be able to free this memory in 4K granularity. This is the corner case that I will have to handle.

Regards,

K. Y
> 
> [...]
> --
> Michal Hocko
> SUSE Labs
> 


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