Re: [Qemu-devel] [PATCH v7 0/1] balloon: add a feature bit to let Guest OS deflate

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

On Mon, Jun 22, 2015 at 02:09:48PM -0700, James Bottomley wrote:
> On Mon, 2015-06-22 at 15:56 +0300, Denis V. Lunev wrote:
> > On 15/06/15 13:52, 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 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 <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 baloon device which do the trick.
> > >
> > > Changes from v6:
> > > - ported to virtio_add_feature
> > >
> > > Changes from v5:
> > > - ported to QEMU current
> > >
> > > Changes from v4:
> > > - spelling corrected according to suggestions from Eric Blake
> > >
> > > Changes from v3:
> > > - ported to git://git.kernel.org/pub/scm/virt/kvm/mst/qemu.git tags/for_upstream_rebased
> > >
> > > Changes from v2:
> > > - fixed mistake with bit number in virtio_balloon_get_features
> > >
> > > Changes from v1:
> > > - From: in patch 1 according to the original ownership
> > > - feature processing in patch 2 as suggested by Michael. It could be done
> > >    without additional field, but this will require to move the property
> > >    level up, i.e. to PCI & CCW level.
> > >
> > > Signed-off-by: Raushaniya Maksudova <rmaksudova@virtuozzo.com>
> > > Signed-off-by: Denis V. Lunev <den@openvz.org>
> > > CC: Anthony Liguori <aliguori@amazon.com>
> > > CC: Michael S. Tsirkin <mst@redhat.com>
> > >
> > > P.S. Sorry for resend (if you have caught additional patch), I have
> > >       expirienced some troubles in the process
> > >
> > ping
> 
> We seem to have become bogged down in a dispute over whether this flag
> should be automatically enabled or disabled.  To be honest, we don't
> care, since we're going to be shipping qemu configured according to our
> requirements (as are most other distros anyway).
> 
> However, for the sake of getting the patch in, what about putting it in
> as is (default disable) because that has no impact to the status quo.
> If it later turns out everyone ships in a default enabled configuration,
> why then someone can patch upstream qemu to match.
> 
> James
> 

Great, so I get your ack on it?