Re: [PATCH 1/2] mm: Export vm_committed_as

[Andrew Morton]

On Thu, 8 Nov 2012 22:01:33 +0000
KY Srinivasan <kys@xxxxxxxxxxxxx> wrote:

> 
> 
> > -----Original Message-----
> > From: David Rientjes [mailto:rientjes@xxxxxxxxxx]
> > Sent: Monday, November 05, 2012 5:33 PM
> > To: KY Srinivasan
> > Cc: Andrew Morton; Greg KH; olaf@xxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx;
> > andi@xxxxxxxxxxxxxx; apw@xxxxxxxxxxxxx; devel@xxxxxxxxxxxxxxxxxxxxxx; linux-
> > mm@xxxxxxxxx; Hiroyuki Kamezawa; Michal Hocko; Johannes Weiner; Ying Han
> > Subject: RE: [PATCH 1/2] mm: Export vm_committed_as
> > 
> > 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.
> > 
> > (And why percpu_counter_read_positive() returns a signed type is a
> > mystery.)
> 
> Yes, this makes sense. I just want to access (read) this metric. Andrew, if you are willing to
> take this patch, I could send one.

Sure.  I suppose that's better, although any module which modifies
committed_as would never pass review (rofl).

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