Re: [Qemu-devel] [PATCH 2/2] balloon: add a feature bit to let Guest OS deflate balloon on oom

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Apr 01, 2015 at 12:51:42PM +0300, James Bottomley wrote:
> On Wed, 2015-04-01 at 11:50 +0200, Michael S. Tsirkin wrote:
> > On Wed, Apr 01, 2015 at 12:44:28PM +0300, James Bottomley wrote:
> > > On Fri, 2015-02-27 at 09:57 +0300, Denis V. Lunev wrote:
> > > > Excessive virtio_balloon inflation can cause invocation of OOM-killer,
> > > > when Linux is under severe memory pressure. Various mechanisms are
> > > > responsible for correct virtio_balloon memory management. Nevertheless it
> > > > is often the case that these control tools does not have enough time to
> > > > react on fast changing memory load. As a result OS runs out of memory and
> > > > invokes OOM-killer. The balancing of memory by use of the virtio balloon
> > > > should not cause the termination of processes while there are pages in the
> > > > balloon. Now there is no way for virtio balloon driver to free memory at
> > > > the last moment before some process get killed by OOM-killer.
> > > > 
> > > > This does not provide a security breach as balloon itself is running
> > > > inside Guest OS and is working in the cooperation with the host. Thus
> > > > some improvements from Guest side should be considered as normal.
> > > > 
> > > > To solve the problem, introduce a virtio_balloon callback which is
> > > > expected to be called from the oom notifier call chain in out_of_memory()
> > > > function. If virtio balloon could release some memory, it will make the
> > > > system return and retry the allocation that forced the out of memory
> > > > killer to run.
> > > > 
> > > > This behavior should be enabled if and only if appropriate feature bit
> > > > is set on the device. It is off by default.
> > > > 
> > > > This functionality was recently merged into vanilla Linux.
> > > > 
> > > >   commit 5a10b7dbf904bfe01bb9fcc6298f7df09eed77d5
> > > >   Author: Raushaniya Maksudova <rmaksudova@parallels.com>
> > > >   Date:   Mon Nov 10 09:36:29 2014 +1030
> > > > 
> > > > This patch adds respective control bits into QEMU. It introduces
> > > > deflate-on-oom option for balloon device which does the trick.
> > > 
> > > What's the status on this, please?  It's been over a month since this
> > > was posted with no further review feedback, so I think it's ready.
> > > Getting this into qemu is blocking our next step which would be adding
> > > the feature bit to the virtio spec.
> > > 
> > > James
> > 
> > This was posted after soft feature freeze for 2.3, so it'll have to go
> > into 2.4.  I don't see why would this block your work on the spec: you
> > should make progress on this meanwhile.
> 
> I can do that ... I just thought the spec was trailing edge, so I was
> waiting to have the patch accepted, which confirms the implementation.
> I didn't want to write it into the spec and have the actual
> implementation changed by review later.
> 
> James
> 

It's up to you really, I would just like to point out two things:
- spec process is a long one, assuming we accept a spec change,
  we go though a public review period, multiple votes etc.
  About half a year to release a spec revision with
  new features.
  So time enough to make minor changes.
- oasis process works like this (roughly):
	spec is written
	spec goes through a public review process
	community standard is published
	3 implementations are reported
	spec becomes an oasis standard
  so implementations aren't required at early stages
-- 
MST