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

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Date:	Tue, 6 Nov 2012 15:46:44 +0100
From:	Michal Hocko <mhocko@...e.cz>
To:	David Rientjes <rientjes@...gle.com>
Cc:	KY Srinivasan <kys@...rosoft.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	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>,
	Johannes Weiner <hannes@...xchg.org>,
	Ying Han <yinghan@...gle.com>
Subject: Re: [PATCH 1/2] mm: Export vm_committed_as

On Mon 05-11-12 14:33:12, David Rientjes wrote:
> On Mon, 5 Nov 2012, KY Srinivasan wrote:
> 
> > 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. 
> > 
> 
> I don't think you should export the symbol itself to modules but rather a 
> helper function that returns s64 that just wraps 
> percpu_counter_read_positive() which your driver could use instead.

Agreed, we should rather make sure that nobody can manipulate the value
from modules.

> (And why percpu_counter_read_positive() returns a signed type is a 
> mystery.)

Strange indeed. The last commit changed it from long to s64 to suport
values bigger than 2^31 but even the original long doesn't make much
sense to me.

-- 
Michal Hocko
SUSE Labs
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