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

["Denis V. Lunev" <den@openvz.org>]

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?