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

[Michael S. Tsirkin]

On Thu, Nov 27, 2014 at 03:50:11PM +0400, Andrey Korolyov wrote:
> On Thu, Nov 27, 2014 at 2:45 PM, Denis V. Lunev <address@hidden> 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 to 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 (actually in
> > linux-next at the moment)
> >
> >   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 baloon device which do the trick.
> >
> > Signed-off-by: Denis V. Lunev <address@hidden>
> > CC: Raushaniya Maksudova <address@hidden>
> > CC: Anthony Liguori <address@hidden>
> > CC: Michael S. Tsirkin <address@hidden>

...

> Had you tried this with a system-wide OOM on a real workload? This
> behavior can work perfectly with dedicated memory cgroups, but I`m
> afraid it would be unusable when entire system stalls and waits for a
> balloon deflation.

That's really a question about guest drivers though, isn't it?
So you aren't responding to correct patches, and aren't copying
the correct people.

-- 
MST