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

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

On Thu, Nov 27, 2014 at 02:45:42PM +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 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.
> 
> Signed-off-by: Denis V. Lunev <den@openvz.org>
> CC: Raushaniya Maksudova <rmaksudova@parallels.com>
> CC: Anthony Liguori <aliguori@amazon.com>
> CC: Michael S. Tsirkin <mst@redhat.com>
> ---
>  hw/virtio/virtio-balloon.c         | 6 ++++--
>  include/hw/virtio/virtio-balloon.h | 2 ++
>  2 files changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/hw/virtio/virtio-balloon.c b/hw/virtio/virtio-balloon.c
> index 7bfbb75..4d043ce 100644
> --- a/hw/virtio/virtio-balloon.c
> +++ b/hw/virtio/virtio-balloon.c
> @@ -305,8 +305,8 @@ static void virtio_balloon_set_config(VirtIODevice *vdev,
>  
>  static uint32_t virtio_balloon_get_features(VirtIODevice *vdev, uint32_t f)
>  {
> -    f |= (1 << VIRTIO_BALLOON_F_STATS_VQ);
> -    return f;
> +    VirtIOBalloon *dev = VIRTIO_BALLOON(vdev);
> +    return (f | VIRTIO_BALLOON_F_STATS_VQ) | dev->host_features;

This looks very strange.
You use | on the bit number?

Also, no need for () above.

>  }
>  
>  static void virtio_balloon_stat(void *opaque, BalloonInfo *info)
> @@ -409,6 +409,8 @@ static void virtio_balloon_device_unrealize(DeviceState *dev, Error **errp)
>  }
>  
>  static Property virtio_balloon_properties[] = {
> +    DEFINE_PROP_BIT("deflate-on-oom", VirtIOBalloon, host_features,
> +                    VIRTIO_BALLOON_F_DEFLATE_ON_OOM, false),
>      DEFINE_PROP_END_OF_LIST(),
>  };
>  
> diff --git a/include/hw/virtio/virtio-balloon.h b/include/hw/virtio/virtio-balloon.h
> index f863bfe..2e1ccd9 100644
> --- a/include/hw/virtio/virtio-balloon.h
> +++ b/include/hw/virtio/virtio-balloon.h
> @@ -30,6 +30,7 @@
>  /* The feature bitmap for virtio balloon */
>  #define VIRTIO_BALLOON_F_MUST_TELL_HOST 0 /* Tell before reclaiming pages */
>  #define VIRTIO_BALLOON_F_STATS_VQ 1       /* Memory stats virtqueue */
> +#define VIRTIO_BALLOON_F_DEFLATE_ON_OOM 2 /* Deflate balloon on OOM */
>  
>  /* Size of a PFN in the balloon interface. */
>  #define VIRTIO_BALLOON_PFN_SHIFT 12
> @@ -67,6 +68,7 @@ typedef struct VirtIOBalloon {
>      QEMUTimer *stats_timer;
>      int64_t stats_last_update;
>      int64_t stats_poll_interval;
> +    uint32_t host_features;
>  } VirtIOBalloon;
>  
>  #endif
> -- 
> 1.9.1