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

[James Bottomley]

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 <address@hidden>
> > >   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