Re: [PATCH 1/1] balloon: stop inflate balloon after oom notification

[Michael S. Tsirkin]

On Fri, Sep 09, 2016 at 04:54:44PM +0300, Denis V. Lunev wrote:
> From: Konstantin Neumoin 
> 
> At this moment oom notification in balloon does not work as expected.
> After virtballoon_oom_notify there is an infinitive loop:
>  - virtballoon_oom_notify was called and balloon was deflated
>  - balloon get notification that config was changed, compare target and
>actual and try to reach target again.
> 
> This patch adds global variable fail_counter which indicates that oom has
> been happened. We check that fail_counter was changed between calls
> update_balloon_size_func. In this case we should not try to inflate balloon
> even if actual != target.

Doesn't look right to me. What if it triggered an hour ago?
We really need mm core to expose an oom_in_progress flag that we can test.

More implementation comments below.

> 
> Signed-off-by: Konstantin Neumoin 
> Signed-off-by: Denis V. Lunev 
> CC: Michael S. Tsirkin 
> ---
>  drivers/virtio/virtio_balloon.c | 15 +++
>  1 file changed, 15 insertions(+)
> 
> diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
> index 4e7003d..253bf05 100644
> --- a/drivers/virtio/virtio_balloon.c
> +++ b/drivers/virtio/virtio_balloon.c
> @@ -50,6 +50,8 @@ MODULE_PARM_DESC(oom_pages, "pages to free on OOM");
>  static struct vfsmount *balloon_mnt;
>  #endif
>  
> +static unsigned long fail_count;
> +
>  struct virtio_balloon {
>   struct virtio_device *vdev;
>   struct virtqueue *inflate_vq, *deflate_vq, *stats_vq;
> @@ -361,6 +363,8 @@ static int virtballoon_oom_notify(struct notifier_block 
> *self,
>   unsigned long *freed;
>   unsigned num_freed_pages;
>  
> + fail_count++;
> +
>   vb = container_of(self, struct virtio_balloon, nb);
>   if (!virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_DEFLATE_ON_OOM))
>   return NOTIFY_OK;

OOM should not interfere with ballooning unless
VIRTIO_BALLOON_F_DEFLATE_ON_OOM has been negotiated.


> @@ -386,11 +390,22 @@ static void update_balloon_size_func(struct work_struct 
> *work)
>  {
>   struct virtio_balloon *vb;
>   s64 diff;
> + static unsigned long fc;
> +
> + if (fc == 0)
> + fc = fail_count;

Why?

>  
>   vb = container_of(work, struct virtio_balloon,
> update_balloon_size_work);
>   diff = towards_target(vb);
>  
> + if (fc != fail_count) {
> + fc = fail_count;

Unlikely to work correctly if there are multiple balloon devices.


> + /* Don't inflate balloon after oom notification */
> + if (diff > 0)
> + return;
> + }
> +
>   if (diff > 0)
>   diff -= fill_balloon(vb, diff);
>   else if (diff < 0)

I'd rather make it per-device.
Absence of memory ordering primitives of any kind also
looks suspicious.

> -- 
> 2.7.4
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[PATCH 1/1] balloon: stop inflate balloon after oom notification

[Denis V. Lunev]

From: Konstantin Neumoin 

At this moment oom notification in balloon does not work as expected.
After virtballoon_oom_notify there is an infinitive loop:
 - virtballoon_oom_notify was called and balloon was deflated
 - balloon get notification that config was changed, compare target and
   actual and try to reach target again.

This patch adds global variable fail_counter which indicates that oom has
been happened. We check that fail_counter was changed between calls
update_balloon_size_func. In this case we should not try to inflate balloon
even if actual != target.

Signed-off-by: Konstantin Neumoin 
Signed-off-by: Denis V. Lunev 
CC: Michael S. Tsirkin 
---
 drivers/virtio/virtio_balloon.c | 15 +++
 1 file changed, 15 insertions(+)

diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
index 4e7003d..253bf05 100644
--- a/drivers/virtio/virtio_balloon.c
+++ b/drivers/virtio/virtio_balloon.c
@@ -50,6 +50,8 @@ MODULE_PARM_DESC(oom_pages, "pages to free on OOM");
 static struct vfsmount *balloon_mnt;
 #endif
 
+static unsigned long fail_count;
+
 struct virtio_balloon {
struct virtio_device *vdev;
struct virtqueue *inflate_vq, *deflate_vq, *stats_vq;
@@ -361,6 +363,8 @@ static int virtballoon_oom_notify(struct notifier_block 
*self,
unsigned long *freed;
unsigned num_freed_pages;
 
+   fail_count++;
+
vb = container_of(self, struct virtio_balloon, nb);
if (!virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_DEFLATE_ON_OOM))
return NOTIFY_OK;
@@ -386,11 +390,22 @@ static void update_balloon_size_func(struct work_struct 
*work)
 {
struct virtio_balloon *vb;
s64 diff;
+   static unsigned long fc;
+
+   if (fc == 0)
+   fc = fail_count;
 
vb = container_of(work, struct virtio_balloon,
  update_balloon_size_work);
diff = towards_target(vb);
 
+   if (fc != fail_count) {
+   fc = fail_count;
+   /* Don't inflate balloon after oom notification */
+   if (diff > 0)
+   return;
+   }
+
if (diff > 0)
diff -= fill_balloon(vb, diff);
else if (diff < 0)
-- 
2.7.4

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