linux-kernel - RE: [PATCH 1/1] mm: Export a function to read vm_committed

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Date:	Sun, 11 Nov 2012 09:24:22 +0000
From:	KY Srinivasan <kys@...rosoft.com>
To:	David Rientjes <rientjes@...gle.com>
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>,
	"mhocko@...e.cz" <mhocko@...e.cz>,
	"hannes@...xchg.org" <hannes@...xchg.org>,
	"yinghan@...gle.com" <yinghan@...gle.com>
Subject: RE: [PATCH 1/1] mm: Export a function to read vm_committed_as



> -----Original Message-----
> From: David Rientjes [mailto:rientjes@...gle.com]
> Sent: Saturday, November 10, 2012 9:35 PM
> 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; mhocko@...e.cz; hannes@...xchg.org;
> yinghan@...gle.com
> Subject: Re: [PATCH 1/1] mm: Export a function to read vm_committed_as
> 
> On Sat, 10 Nov 2012, K. Y. Srinivasan wrote:
> 
> > diff --git a/mm/mmap.c b/mm/mmap.c
> > index 2d94235..e527239 100644
> > --- a/mm/mmap.c
> > +++ b/mm/mmap.c
> > @@ -89,6 +89,17 @@ int sysctl_max_map_count __read_mostly =
> DEFAULT_MAX_MAP_COUNT;
> >  struct percpu_counter vm_committed_as ____cacheline_aligned_in_smp;
> >
> >  /*
> > + * A wrapper to read vm_committed_as that can be used by external
> modules.
> > + */
> > +
> > +unsigned long read_vm_committed_as(void)
> > +{
> > +	return percpu_counter_read_positive(&vm_committed_as);
> > +}
> > +
> > +EXPORT_SYMBOL_GPL(read_vm_committed_as);
> > +
> > +/*
> >   * Check that a process has enough memory to allocate a new virtual
> >   * mapping. 0 means there is enough memory for the allocation to
> >   * succeed and -ENOMEM implies there is not.
> 
> This is precisely what I didn't want to see; I was expecting that this
> function was going to have some name that would describe what a hypervisor
> would use it for, regardless of its implementation and current use of
> vm_committed_as.  read_vm_committed_as() misses the entire point of the
> suggestion and a few people have mentioned that they think this
> implementation will evolve over time.
> 
> Please think of what you're trying to determine in the code that will
> depend on this and then convert the existing user in
> drivers/xen/xen-selfballoon.c.

David,

Thanks for the prompt response. For the Linux balloon driver for Hyper-V, I need access
to the metric that reflects the system wide memory commitment made by the guest kernel. 
In the Hyper-V case, this information is one of the many metrics used to drive the policy engine
on the host. Granted, the interface name I have chosen here could be more generic; how about
read_mem_commit_info(void). I am open to suggestions here.

With regards to making changes to the Xen self ballooning code, I would like to separate that patch
from the patch that implements the exported mechanism to access the memory commitment information.
Once we settle on this patch, I can submit the patch to fix the Xen self ballooning driver to use this new
interface along with the Hyper-V balloon driver that is currently blocked on resolving this issue.

Regards,

K. Y



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