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

[Eric Blake <eblake@redhat.com>]

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On 02/26/2015 10:39 AM, 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

s/to return/return/

> 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 baloon device which do the trick.

s/baloon/balloon/
s/do/does/

I'll leave the content review to someone more familiar with the code.

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org


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