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 Mon, Jun 08, 2015 at 07:54:42AM -0700, James Bottomley wrote:
> On Mon, 2015-05-04 at 12:47 +0300, Denis V. Lunev wrote:
> > On 01/04/15 13:18, Michael S. Tsirkin wrote:
> > > 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
> > 2.3 is done, 2.4 window is opened....
> > 
> > The patch is applicable for both
> > git://git.kernel.org/pub/scm/virt/kvm/mst/qemu.git
> > and vanilla qemu.
> > 
> > How can we proceed?
> 
> The spec update supporting this feature is published for review:
> 
> https://www.oasis-open.org/committees/download.php/55709/virtio-v1.0-csprd04.zip
> 
> It's probably a good idea to have the implementation there as well.  Do
> we need to resend these patches?
> 
> James
> 

Yes, please do.