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

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Message-ID: <1701384b10204014b53acecb006521b0@SN2PR03MB061.namprd03.prod.outlook.com>
Date:	Mon, 18 Mar 2013 13:44:05 +0000
From:	KY Srinivasan <kys@...rosoft.com>
To:	Michal Hocko <mhocko@...e.cz>
CC:	"gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"devel@...uxdriverproject.org" <devel@...uxdriverproject.org>,
	"olaf@...fle.de" <olaf@...fle.de>,
	"apw@...onical.com" <apw@...onical.com>,
	"andi@...stfloor.org" <andi@...stfloor.org>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"kamezawa.hiroyuki@...il.com" <kamezawa.hiroyuki@...il.com>,
	"hannes@...xchg.org" <hannes@...xchg.org>,
	"yinghan@...gle.com" <yinghan@...gle.com>
Subject: RE: [PATCH 2/2] Drivers: hv: balloon: Support 2M page allocations
 for ballooning



> -----Original Message-----
> From: Michal Hocko [mailto:mhocko@...e.cz]
> Sent: Monday, March 18, 2013 6:53 AM
> To: KY Srinivasan
> Cc: gregkh@...uxfoundation.org; linux-kernel@...r.kernel.org;
> devel@...uxdriverproject.org; olaf@...fle.de; apw@...onical.com;
> andi@...stfloor.org; akpm@...ux-foundation.org; linux-mm@...ck.org;
> kamezawa.hiroyuki@...il.com; hannes@...xchg.org; yinghan@...gle.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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