[PATCH 0/2] shrink virtio baloon on OOM in guest

[PATCH 0/2] shrink virtio baloon on OOM in guest

[Denis V. Lunev]

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

[PATCH 1/2] virtio_balloon: return the amount of freed memory from leak_balloon()

[Denis V. Lunev]

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

[PATCH 2/2] virtio_balloon: free some memory from baloon on OOM

[Denis V. Lunev]

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

Re: [PATCH 2/2] virtio_balloon: free some memory from baloon on OOM

[Rusty Russell]

"Denis V. Lunev" <den@parallels.com> writes:
> From: Raushaniya Maksudova <rmaksudova@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.

Re: [PATCH 2/2] virtio_balloon: free some memory from baloon on OOM

[Denis V. Lunev]

On 13/10/14 09:32, Rusty Russell wrote:
> "Denis V. Lunev" <den@parallels.com> writes:
>> From: Raushaniya Maksudova <rmaksudova@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".
ok, will do

>> +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?
this is a piece from a previous version of the patch, I'll fix this and 
resend.

>> +	num_freed_pages = leak_balloon(vb, oom_vballoon_pages);
>> +	update_balloon_size(vb);
>> +	*freed += num_freed_pages;
>> +
>> +	return NOTIFY_OK;
>> +}
> Cheers,
> Rusty.

Re: [PATCH 2/2] virtio_balloon: free some memory from baloon on OOM

[Michael S. Tsirkin]

On Mon, Oct 13, 2014 at 04:02:52PM +1030, Rusty Russell wrote:
> "Denis V. Lunev" <den@parallels.com> writes:
> > From: Raushaniya Maksudova <rmaksudova@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.

This reminds me of the balloon fs that Google once proposed.
This really needs to be controlled from host though.
At the moment host does not expect guest to deflate before
requests.
So as a minimum, add a feature bit for this.  what if you want a mix of
mandatory and optional balooning? I guess we can use multiple balloons,
is that the idea?


> 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.

Re: [PATCH 2/2] virtio_balloon: free some memory from baloon on OOM

[Rusty Russell]

"Michael S. Tsirkin" <mst@redhat.com> writes:

> On Mon, Oct 13, 2014 at 04:02:52PM +1030, Rusty Russell wrote:
>> "Denis V. Lunev" <den@parallels.com> writes:
>> > From: Raushaniya Maksudova <rmaksudova@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.
>
> This reminds me of the balloon fs that Google once proposed.
> This really needs to be controlled from host though.
> At the moment host does not expect guest to deflate before
> requests.
> So as a minimum, add a feature bit for this.  what if you want a mix of
> mandatory and optional balooning? I guess we can use multiple balloons,
> is that the idea?

Trying to claw back some pages on OOM is almost certainly correct,
even if the host doesn't expect it.  It's roughly equivalent to not
giving up pages in the first place.

Cheers,
Rusty.
PS.  Yes, a real guest-driven balloon is preferable, but that's a much
     larger task.

Re: [PATCH 2/2] virtio_balloon: free some memory from baloon on OOM

[Michael S. Tsirkin]

On Tue, Oct 14, 2014 at 10:14:05AM +1030, Rusty Russell wrote:
> "Michael S. Tsirkin" <mst@redhat.com> writes:
> 
> > On Mon, Oct 13, 2014 at 04:02:52PM +1030, Rusty Russell wrote:
> >> "Denis V. Lunev" <den@parallels.com> writes:
> >> > From: Raushaniya Maksudova <rmaksudova@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.
> >
> > This reminds me of the balloon fs that Google once proposed.
> > This really needs to be controlled from host though.
> > At the moment host does not expect guest to deflate before
> > requests.
> > So as a minimum, add a feature bit for this.  what if you want a mix of
> > mandatory and optional balooning? I guess we can use multiple balloons,
> > is that the idea?
> 
> Trying to claw back some pages on OOM is almost certainly correct,
> even if the host doesn't expect it.  It's roughly equivalent to not
> giving up pages in the first place.

Well the difference is that there are management tools that
poll balloon in host until they see balloon size reaches
the expected value.

They don't expect balloon to shrink below num_pages and will respond in various
unexpected ways like e.g. killing the VM if it does.
Killing a userspace process within the guest might be better
for VM health.

Besides the fact that we always did it like this, these tools seem to have
basis in the spec.
Specifically, this is based on this text from the spec:
	the device asks for a certain amount of memory, and the driver
	supplies it (or withdraws it, if the device has more than it asks for).
	This allows the guest to adapt to changes in allowance of underlying
	physical memory.

and

	The device is driven by the receipt of a configuration change interrupt.



> Cheers,
> Rusty.
> PS.  Yes, a real guest-driven balloon is preferable, but that's a much
>      larger task.


Any objection to making the feature depend on a feature flag?

-- 
MST

Re: [PATCH 2/2] virtio_balloon: free some memory from baloon on OOM

[Rusty Russell]

"Michael S. Tsirkin" <mst@redhat.com> writes:
> On Tue, Oct 14, 2014 at 10:14:05AM +1030, Rusty Russell wrote:
>> "Michael S. Tsirkin" <mst@redhat.com> writes:
>> 
>> > On Mon, Oct 13, 2014 at 04:02:52PM +1030, Rusty Russell wrote:
>> >> "Denis V. Lunev" <den@parallels.com> writes:
>> >> > From: Raushaniya Maksudova <rmaksudova@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.
>> >
>> > This reminds me of the balloon fs that Google once proposed.
>> > This really needs to be controlled from host though.
>> > At the moment host does not expect guest to deflate before
>> > requests.
>> > So as a minimum, add a feature bit for this.  what if you want a mix of
>> > mandatory and optional balooning? I guess we can use multiple balloons,
>> > is that the idea?
>> 
>> Trying to claw back some pages on OOM is almost certainly correct,
>> even if the host doesn't expect it.  It's roughly equivalent to not
>> giving up pages in the first place.
>
> Well the difference is that there are management tools that
> poll balloon in host until they see balloon size reaches
> the expected value.
>
> They don't expect balloon to shrink below num_pages and will respond in various
> unexpected ways like e.g. killing the VM if it does.
> Killing a userspace process within the guest might be better
> for VM health.
>
> Besides the fact that we always did it like this, these tools seem to have
> basis in the spec.
> Specifically, this is based on this text from the spec:
> 	the device asks for a certain amount of memory, and the driver
> 	supplies it (or withdraws it, if the device has more than it asks for).
> 	This allows the guest to adapt to changes in allowance of underlying
> 	physical memory.
>
> and
>
> 	The device is driven by the receipt of a configuration change interrupt.
>
>
>
>> Cheers,
>> Rusty.
>> PS.  Yes, a real guest-driven balloon is preferable, but that's a much
>>      larger task.
>
>
> Any objection to making the feature depend on a feature flag?

If you believe a guest which does this will cause drastic failure on the
host side (ie. killing the VM), then yes, we can do this.

However, I'm not aware of anything that sophisticated...

Cheers,
Rusty.

Re: [PATCH 2/2] virtio_balloon: free some memory from baloon on OOM

[Denis V. Lunev]

On 14/10/14 13:10, Michael S. Tsirkin wrote:
> On Tue, Oct 14, 2014 at 10:14:05AM +1030, Rusty Russell wrote:
>> "Michael S. Tsirkin" <mst@redhat.com> writes:
>>
>>> On Mon, Oct 13, 2014 at 04:02:52PM +1030, Rusty Russell wrote:
>>>> "Denis V. Lunev" <den@parallels.com> writes:
>>>>> From: Raushaniya Maksudova <rmaksudova@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.
>>>
>>> This reminds me of the balloon fs that Google once proposed.
>>> This really needs to be controlled from host though.
>>> At the moment host does not expect guest to deflate before
>>> requests.
>>> So as a minimum, add a feature bit for this.  what if you want a mix of
>>> mandatory and optional balooning? I guess we can use multiple balloons,
>>> is that the idea?
>>
>> Trying to claw back some pages on OOM is almost certainly correct,
>> even if the host doesn't expect it.  It's roughly equivalent to not
>> giving up pages in the first place.
>
> Well the difference is that there are management tools that
> poll balloon in host until they see balloon size reaches
> the expected value.
>
> They don't expect balloon to shrink below num_pages and will respond in various
> unexpected ways like e.g. killing the VM if it does.
> Killing a userspace process within the guest might be better
> for VM health.
>
> Besides the fact that we always did it like this, these tools seem to have
> basis in the spec.
> Specifically, this is based on this text from the spec:
> 	the device asks for a certain amount of memory, and the driver
> 	supplies it (or withdraws it, if the device has more than it asks for).
> 	This allows the guest to adapt to changes in allowance of underlying
> 	physical memory.
>
> and
>
> 	The device is driven by the receipt of a configuration change interrupt.
>
>
>
>> Cheers,
>> Rusty.
>> PS.  Yes, a real guest-driven balloon is preferable, but that's a much
>>       larger task.
>
>
> Any objection to making the feature depend on a feature flag?
>


OK. I got the point. This sounds good for me. We will prepare patch for
kernel and proper bits (command line option) for QEMU.