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

[Michael S. Tsirkin]

On Thu, Nov 27, 2014 at 02:04:55PM +0300, Denis V. Lunev wrote:
> On 26/11/14 14:16, Michael S. Tsirkin wrote:
> >On Wed, Nov 26, 2014 at 01:11:25PM +0300, Denis V. Lunev wrote:
> >>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
> >You don't need an extra bool, and open-coding.
> >Do it same as we do for other features please,
> >set bit in feature mask directly.
> there is no host_features placeholder at this level.
> 
> We could add it and propagate proper bit in get_features
> callback like
>    return f | dev->host_features
> or we have to move this stuff a little bit up into
> CCW & PCI code like done in virtscsi with
>   #define DEFINE_VIRTIO_SCSI_FEATURES(_state, _feature_field)
> 
> The first approach keeps bits in the same place,
> the second follows current approach. If you prefer unification way,
> I think that other devices could be re-written this way.
> 
> Any opinion?

Balloon is the weird one out here.
Look at how feature bits are defined for e.g.
DEFINE_PROP_BIT("any_layout", _state, _field, VIRTIO_F_ANY_LAYOUT, true),

you should do something similar.


> >>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,
> >Please don't add stuff to legacy -balloon.
> >New -device is enough, you don't need to touch qemu-options.hx
> >for it.
> >
> sure