linux-kernel - RE: [PATCH 1/2] mm: Export vm_committed

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Message-ID: <426367E2313C2449837CD2DE46E7EAF930DFA7B8@SN2PRD0310MB382.namprd03.prod.outlook.com>
Date:	Mon, 5 Nov 2012 22:12:25 +0000
From:	KY Srinivasan <kys@...rosoft.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	Greg KH <gregkh@...uxfoundation.org>,
	"olaf@...fle.de" <olaf@...fle.de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"andi@...stfloor.org" <andi@...stfloor.org>,
	"apw@...onical.com" <apw@...onical.com>,
	"devel@...uxdriverproject.org" <devel@...uxdriverproject.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	Hiroyuki Kamezawa <kamezawa.hiroyuki@...il.com>,
	Michal Hocko <mhocko@...e.cz>,
	Johannes Weiner <hannes@...xchg.org>,
	Ying Han <yinghan@...gle.com>
Subject: RE: [PATCH 1/2] mm: Export vm_committed_as



> -----Original Message-----
> From: Andrew Morton [mailto:akpm@...ux-foundation.org]
> Sent: Monday, November 05, 2012 4:45 PM
> To: KY Srinivasan
> Cc: Greg KH; olaf@...fle.de; linux-kernel@...r.kernel.org; andi@...stfloor.org;
> apw@...onical.com; devel@...uxdriverproject.org; linux-mm@...ck.org;
> Hiroyuki Kamezawa; Michal Hocko; Johannes Weiner; Ying Han
> Subject: Re: [PATCH 1/2] mm: Export vm_committed_as
> 
> On Sat, 3 Nov 2012 14:09:38 +0000
> KY Srinivasan <kys@...rosoft.com> wrote:
> 
> >
> >
> > > >
> > > > Ok, but you're going to have to get the -mm developers to agree that
> > > > this is ok before I can accept it.
> > >
> > > Well I guess it won't kill us.
> >
> > Andrew,
> >
> > I presumed this was an Ack from you with regards to exporting the
> > symbol. Looks like Greg is waiting to hear from you before he can check
> > these patches in. Could you provide an explicit Ack.
> >
> 
> Well, I do have some qualms about exporting vm_committed_as to modules.
> 
> vm_committed_as is a global thing and only really makes sense in a
> non-containerised system.  If the application is running within a
> memory cgroup then vm_enough_memory() and the global overcommit policy
> are at best irrelevant and misleading.
> 
> If use of vm_committed_as is indeed a bad thing, then exporting it to
> modules might increase the amount of badness in the kernel.
> 
> 
> I don't think these qualms are serious enough to stand in the way of
> this patch, but I'd be interested in hearing the memcg developers'
> thoughts on the matter?
> 
> 
> Perhaps you could provide a detailed description of why your module
> actually needs this?  Precisely what information is it looking for
> and why?  If we know that then perhaps a more comfortable alternative
> can be found.

The Hyper-V host has a policy engine for managing available physical memory across
competing virtual machines. This policy decision is based on a number of parameters
including the memory pressure reported by the guest. Currently, the pressure calculation is
based on the memory commitment made by the guest. From what I can tell, the ratio of
currently allocated physical memory to the current memory commitment made by the guest
(vm_committed_as) is used as one of the parameters in making the memory balancing decision on
the host. This is what Windows guests report to the host. So, I need some measure of memory
commitments made by the Linux guest. This is the reason I want export vm_committed_as. 

Regards,

K. Y  



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