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

From: Denis V. Lunev
Subject: Re: [Qemu-devel] [PATCH 2/2] balloon: add a feature bit to let Guest OS deflate balloon on oom
Date: Mon, 4 May 2015 12:47:38 +0300
User-agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.10; rv:31.0) Gecko/20100101 Thunderbird/31.6.0

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 <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.

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.


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
and vanilla qemu.

How can we proceed?

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