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

[KY Srinivasan]

> -----Original Message-----
> From: Michal Hocko [mailto:mhocko at suse.cz]
> Sent: Monday, March 18, 2013 10:13 AM
> To: KY Srinivasan
> Cc: gregkh at linuxfoundation.org; linux-kernel at vger.kernel.org;
> devel at linuxdriverproject.org; olaf at aepfle.de; apw at canonical.com;
> andi at firstfloor.org; akpm at linux-foundation.org; linux-mm at kvack.org;
> kamezawa.hiroyuki at gmail.com; hannes at cmpxchg.org; yinghan at google.com
> Subject: Re: [PATCH 2/2] Drivers: hv: balloon: Support 2M page allocations for
> ballooning
> 
> On Mon 18-03-13 13:44:05, KY Srinivasan wrote:
> >
> >
> > > -----Original Message-----
> > > From: Michal Hocko [mailto:mhocko at suse.cz]
> > > Sent: Monday, March 18, 2013 6:53 AM
> > > To: KY Srinivasan
> > > Cc: gregkh at linuxfoundation.org; linux-kernel at vger.kernel.org;
> > > devel at linuxdriverproject.org; olaf at aepfle.de; apw at canonical.com;
> > > andi at firstfloor.org; akpm at linux-foundation.org; linux-mm at kvack.org;
> > > kamezawa.hiroyuki at gmail.com; hannes at cmpxchg.org;
> yinghan at 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.
> 
> Thanks for the clarification. I think this information would be valuable
> in the changelog.

Thanks Michal. I will resend the patches with the changes you have suggested. What is your
recommendation with regards which tree the mm patch needs to go through; the Hyper-V balloon
driver patch will go through Greg's tree.

Regards,

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