Linux Virtualization - Oct 2014 - [PATCH 2/2] virtio_balloon: free some memory from baloon on OOM

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 baloon 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.

Patch 1 of this series adds support for implementation of virtio_balloon
callback, so now leak_balloon() function returns number of freed pages.
Patch 2 implements virtio_balloon callback itself.

Signed-off-by: Raushaniya Maksudova <rmaksudova at parallels.com>
Signed-off-by: Denis V. Lunev <den at openvz.org>
CC: Rusty Russell <rusty at rustcorp.com.au>
CC: Michael S. Tsirkin <mst at redhat.com>
CC: virtualization at lists.linux-foundation.org

From: Raushaniya Maksudova <rmaksudova at parallels.com>

This value would be useful in the next patch to provide the amount of
the freed memory for OOM killer.

Accessing to vb->num_pfns outside of vb->balloon_lock is wrong and unsafe.

Signed-off-by: Raushaniya Maksudova <rmaksudova at parallels.com>
Signed-off-by: Denis V. Lunev <den at openvz.org>
CC: Rusty Russell <rusty at rustcorp.com.au>
CC: Michael S. Tsirkin <mst at redhat.com>
CC: virtualization at lists.linux-foundation.org
---
 drivers/virtio/virtio_balloon.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
index 25ebe8e..213da41 100644
--- a/drivers/virtio/virtio_balloon.c
+++ b/drivers/virtio/virtio_balloon.c
@@ -168,8 +168,9 @@ static void release_pages_by_pfn(const u32 pfns[], unsigned
int num)
 	}
 }
 
-static void leak_balloon(struct virtio_balloon *vb, size_t num)
+static unsigned leak_balloon(struct virtio_balloon *vb, size_t num)
 {
+	unsigned num_freed_pages;
 	struct page *page;
 	struct balloon_dev_info *vb_dev_info = vb->vb_dev_info;
 
@@ -186,6 +187,7 @@ static void leak_balloon(struct virtio_balloon *vb, size_t
num)
 		vb->num_pages -= VIRTIO_BALLOON_PAGES_PER_PAGE;
 	}
 
+	num_freed_pages = vb->num_pfns;
 	/*
 	 * Note that if
 	 * virtio_has_feature(vdev, VIRTIO_BALLOON_F_MUST_TELL_HOST);
@@ -195,6 +197,7 @@ static void leak_balloon(struct virtio_balloon *vb, size_t
num)
 		tell_host(vb, vb->deflate_vq);
 	mutex_unlock(&vb->balloon_lock);
 	release_pages_by_pfn(vb->pfns, vb->num_pfns);
+	return num_freed_pages;
 }
 
 static inline void update_stat(struct virtio_balloon *vb, int idx,
-- 
1.9.1

From: Raushaniya Maksudova <rmaksudova at parallels.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 baloon 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.

Signed-off-by: Raushaniya Maksudova <rmaksudova at parallels.com>
Signed-off-by: Denis V. Lunev <den at openvz.org>
CC: Rusty Russell <rusty at rustcorp.com.au>
CC: Michael S. Tsirkin <mst at redhat.com>
CC: virtualization at lists.linux-foundation.org
---
 drivers/virtio/virtio_balloon.c | 46 +++++++++++++++++++++++++++++++++++++++++
 1 file changed, 46 insertions(+)

diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
index 213da41..ca77831 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_vballoon_pages = OOM_VBALLOON_DEFAULT_PAGES;
+module_param(oom_vballoon_pages, int, S_IRUSR | S_IWUSR);
+MODULE_PARM_DESC(oom_vballoon_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 +300,33 @@ 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)
+{
+	unsigned int num_freed_pages;
+	unsigned long *freed = (unsigned long *)parm;
+	struct virtio_balloon *vb = container_of((struct notifier_block *)self,
+						 struct virtio_balloon, nb);
+
+	num_freed_pages = leak_balloon(vb, oom_vballoon_pages);
+	update_balloon_size(vb);
+	*freed += num_freed_pages;
+
+	return NOTIFY_OK;
+}
+
 static int balloon(void *_vballoon)
 {
 	struct virtio_balloon *vb = _vballoon;
@@ -474,6 +511,12 @@ static int virtballoon_probe(struct virtio_device *vdev)
 	if (err)
 		goto out_free_vb_mapping;
 
+	vb->nb.notifier_call = virtballoon_oom_notify;
+	vb->nb.priority = VIRTBALLOON_OOM_NOTIFY_PRIORITY;
+	err = register_oom_notifier(&vb->nb);
+	if (err < 0)
+		goto out_oom_notify;
+
 	vb->thread = kthread_run(balloon, vb, "vballoon");
 	if (IS_ERR(vb->thread)) {
 		err = PTR_ERR(vb->thread);
@@ -483,6 +526,8 @@ static int virtballoon_probe(struct virtio_device *vdev)
 	return 0;
 
 out_del_vqs:
+	unregister_oom_notifier(&vb->nb);
+out_oom_notify:
 	vdev->config->del_vqs(vdev);
 out_free_vb_mapping:
 	balloon_mapping_free(vb_mapping);
@@ -511,6 +556,7 @@ static void virtballoon_remove(struct virtio_device *vdev)
 {
 	struct virtio_balloon *vb = vdev->priv;
 
+	unregister_oom_notifier(&vb->nb);
 	kthread_stop(vb->thread);
 	remove_common(vb);
 	balloon_mapping_free(vb->vb_dev_info->mapping);
-- 
1.9.1

"Denis V. Lunev" <den at parallels.com>
writes:
> From: Raushaniya Maksudova <rmaksudova at parallels.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 makes some amount of sense.

But I suggest a few minor changes:
> +static int oom_vballoon_pages = OOM_VBALLOON_DEFAULT_PAGES;
> +module_param(oom_vballoon_pages, int, S_IRUSR | S_IWUSR);
> +MODULE_PARM_DESC(oom_vballoon_pages, "pages to free on OOM");
Since this is already prefixed with "virtio_balloon." I suggest just
calling it "oom_pages".
> +static int virtballoon_oom_notify(struct notifier_block *self,
> +				  unsigned long dummy, void *parm)
> +{
> +	unsigned int num_freed_pages;
> +	unsigned long *freed = (unsigned long *)parm;
> +	struct virtio_balloon *vb = container_of((struct notifier_block *)self,
> +						 struct virtio_balloon, nb);
Why cast self here?
> +	num_freed_pages = leak_balloon(vb, oom_vballoon_pages);
> +	update_balloon_size(vb);
> +	*freed += num_freed_pages;
> +
> +	return NOTIFY_OK;
> +}
Cheers,
Rusty.