[PATCH 1/2] mm: Export vm_committed_as

[KY Srinivasan]

> -----Original Message-----
> From: Michal Hocko [mailto:mstsxfx at gmail.com] On Behalf Of Michal Hocko
> Sent: Tuesday, November 06, 2012 4:06 AM
> To: KY Srinivasan
> Cc: Andrew Morton; Greg KH; olaf at aepfle.de; linux-kernel at vger.kernel.org;
> andi at firstfloor.org; apw at canonical.com; devel at linuxdriverproject.org; linux-
> mm at kvack.org; Hiroyuki Kamezawa; Johannes Weiner; Ying Han
> Subject: Re: [PATCH 1/2] mm: Export vm_committed_as
> 
> On Mon 05-11-12 22:12:25, KY Srinivasan wrote:
> >
> >
> > > -----Original Message-----
> > > From: Andrew Morton [mailto:akpm at linux-foundation.org]
> > > Sent: Monday, November 05, 2012 4:45 PM
> > > To: KY Srinivasan
> > > Cc: Greg KH; olaf at aepfle.de; linux-kernel at vger.kernel.org;
> andi at firstfloor.org;
> > > apw at canonical.com; devel at linuxdriverproject.org; linux-mm at kvack.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 at microsoft.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.
> 
> So IIUC it will be guest who reports the value and the guest runs in the
> ring-0 so it is not in any user process context, right?
> If this is correct then memcg doesn't play any role here.

Thanks Michal. Yes, the kernel driver reports this metric to the host.
Andrew, let me know how I should proceed here.

Thanks,

K. Y