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

[Denis V. Lunev]

From: Raushaniya Maksudova <address@hidden>

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 <address@hidden>
  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: Raushaniya Maksudova <address@hidden>
Signed-off-by: Denis V. Lunev <address@hidden>
CC: Anthony Liguori <address@hidden>
CC: Michael S. Tsirkin <address@hidden>
---
 hw/virtio/virtio-balloon.c         | 7 +++++++
 include/hw/virtio/virtio-balloon.h | 2 ++
 qemu-options.hx                    | 6 +++++-
 3 files changed, 14 insertions(+), 1 deletion(-)

diff --git a/hw/virtio/virtio-balloon.c b/hw/virtio/virtio-balloon.c
index 7bfbb75..9d145fa 100644
--- a/hw/virtio/virtio-balloon.c
+++ b/hw/virtio/virtio-balloon.c
@@ -305,7 +305,12 @@ static void virtio_balloon_set_config(VirtIODevice *vdev,
 
 static uint32_t virtio_balloon_get_features(VirtIODevice *vdev, uint32_t f)
 {
+    VirtIOBalloon *dev = VIRTIO_BALLOON(vdev);
     f |= (1 << VIRTIO_BALLOON_F_STATS_VQ);
+    if (dev->deflate_on_oom) {
+        f |= (1 << VIRTIO_BALLOON_F_DEFLATE_ON_OOM);
+    }
+
     return f;
 }
 
@@ -409,6 +414,7 @@ static void virtio_balloon_device_unrealize(DeviceState 
*dev, Error **errp)
 }
 
 static Property virtio_balloon_properties[] = {
+    DEFINE_PROP_BOOL("deflate-on-oom", VirtIOBalloon, 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..45cc55a 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;
+    bool deflate_on_oom;
 } VirtIOBalloon;
 
 #endif
diff --git a/qemu-options.hx b/qemu-options.hx
index da9851d..14ede0b 100644
--- a/qemu-options.hx
+++ b/qemu-options.hx
@@ -324,7 +324,8 @@ ETEXI
 DEF("balloon", HAS_ARG, QEMU_OPTION_balloon,
     "-balloon none   disable balloon device\n"
     "-balloon virtio[,addr=str]\n"
-    "                enable virtio balloon device (default)\n", QEMU_ARCH_ALL)
+    "                enable virtio balloon device (default)\n"
+    "               [,deflate-on-oom=on|off]\n", QEMU_ARCH_ALL)
 STEXI
 @item -balloon none
 @findex -balloon
@@ -332,6 +333,9 @@ Disable balloon device.
 @item -balloon virtio[,address@hidden
 Enable virtio balloon device (default), optionally with PCI address
 @var{addr}.
address@hidden -balloon virtio[,address@hidden
address@hidden is "on" or "off" and enables whether to let Guest OS
+to deflate virtio balloon on OOM. Default is off.
 ETEXI
 
 DEF("device", HAS_ARG, QEMU_OPTION_device,
-- 
1.9.1