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Date:	Mon, 20 Oct 2014 10:01:37 +1030
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	"Denis V. Lunev" <den@...nvz.org>
Cc:	virtualization@...ts.linux-foundation.org,
	linux-kernel@...r.kernel.org,
	Raushaniya Maksudova <rmaksudova@...allels.com>,
	"Denis V. Lunev" <den@...nvz.org>,
	"Michael S. Tsirkin" <mst@...hat.com>
Subject: Re: [PATCH 2/2] virtio_balloon: free some memory from balloon on OOM

"Denis V. Lunev" <den@...nvz.org> writes:

> From: Raushaniya Maksudova <rmaksudova@...allels.com>
>
> 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 some memory at the last moment before some process will be 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.
>
> Allocate virtio  feature bit for this: it is not set by default,
> the the guest will not deflate virtio balloon on OOM without explicit
> permission from host.
>
> Signed-off-by: Raushaniya Maksudova <rmaksudova@...allels.com>
> Signed-off-by: Denis V. Lunev <den@...nvz.org>
> CC: Rusty Russell <rusty@...tcorp.com.au>
> CC: Michael S. Tsirkin <mst@...hat.com>
> ---
>  drivers/virtio/virtio_balloon.c     | 52 +++++++++++++++++++++++++++++++++++++
>  include/uapi/linux/virtio_balloon.h |  1 +
>  2 files changed, 53 insertions(+)
>
> diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
> index 66cac10..88d73a0 100644
> --- a/drivers/virtio/virtio_balloon.c
> +++ b/drivers/virtio/virtio_balloon.c
> @@ -28,6 +28,7 @@
>  #include <linux/slab.h>
>  #include <linux/module.h>
>  #include <linux/balloon_compaction.h>
> +#include <linux/oom.h>
>  
>  /*
>   * Balloon device works in 4K page units.  So each page is pointed to by
> @@ -36,6 +37,12 @@
>   */
>  #define VIRTIO_BALLOON_PAGES_PER_PAGE (unsigned)(PAGE_SIZE >> VIRTIO_BALLOON_PFN_SHIFT)
>  #define VIRTIO_BALLOON_ARRAY_PFNS_MAX 256
> +#define OOM_VBALLOON_DEFAULT_PAGES 256
> +#define VIRTBALLOON_OOM_NOTIFY_PRIORITY 80
> +
> +static int oom_pages = OOM_VBALLOON_DEFAULT_PAGES;
> +module_param(oom_pages, int, S_IRUSR | S_IWUSR);
> +MODULE_PARM_DESC(oom_pages, "pages to free on OOM");
>  
>  struct virtio_balloon
>  {
> @@ -71,6 +78,9 @@ struct virtio_balloon
>  	/* Memory statistics */
>  	int need_stats_update;
>  	struct virtio_balloon_stat stats[VIRTIO_BALLOON_S_NR];
> +
> +	/* To register callback in oom notifier call chain */
> +	struct notifier_block nb;
>  };
>  
>  static struct virtio_device_id id_table[] = {
> @@ -290,6 +300,38 @@ static void update_balloon_size(struct virtio_balloon *vb)
>  		      &actual);
>  }
>  
> +/*
> + * virtballoon_oom_notify - release pages when system is under severe
> + *			    memory pressure (called from out_of_memory())
> + * @self : notifier block struct
> + * @dummy: not used
> + * @parm : returned - number of freed pages
> + *
> + * The balancing of memory by use of the virtio balloon should not cause
> + * the termination of processes while there are pages in the balloon.
> + * If virtio balloon manages to release some memory, it will make the
> + * system return and retry the allocation that forced the OOM killer
> + * to run.
> + */
> +static int virtballoon_oom_notify(struct notifier_block *self,
> +				  unsigned long dummy, void *parm)
> +{
> +	struct virtio_balloon *vb;
> +	unsigned long *freed;
> +	unsigned num_freed_pages;
> +
> +	vb = container_of(self, struct virtio_balloon, nb);
> +	if (!virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_DEFLATE_ON_OOM))
> +		return NOTIFY_OK;
> +
> +	freed = parm;
> +	num_freed_pages = leak_balloon(vb, oom_pages);
> +	update_balloon_size(vb);
> +	*freed += num_freed_pages;
> +
> +	return NOTIFY_OK;
> +}
> +
>  static int balloon(void *_vballoon)
>  {
>  	struct virtio_balloon *vb = _vballoon;

OK, I applied these.  I thought it a bit weird that you're checking the
feature in the callback, rather than only registering the callback
if the feature exists.  But it's clear enough.

Thanks,
Rusty.
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