[PATCH] virtio_balloon: Fix endless deflation and inflation on arm64

[Gavin Shan]

The deflation request to the target, which isn't unaligned to the
guest page size causes endless deflation and inflation actions. For
example, we receive the flooding QMP events for the changes on memory
balloon's size after a deflation request to the unaligned target is
sent for the ARM64 guest, where we have 64KB base page size.

  /home/gavin/sandbox/qemu.main/build/qemu-system-aarch64      \
  -accel kvm -machine virt,gic-version=host -cpu host          \
  -smp maxcpus=8,cpus=8,sockets=2,clusters=2,cores=2,threads=1 \
  -m 1024M,slots=16,maxmem=64G                                 \
  -object memory-backend-ram,id=mem0,size=512M                 \
  -object memory-backend-ram,id=mem1,size=512M                 \
  -numa node,nodeid=0,memdev=mem0,cpus=0-3                     \
  -numa node,nodeid=1,memdev=mem1,cpus=4-7                     \
    :                                                          \
  -device virtio-balloon-pci,id=balloon0,bus=pcie.10

  { "execute" : "balloon", "arguments": { "value" : 1073672192 } }
  {"return": {}}
  {"timestamp": {"seconds": 1693272173, "microseconds": 88667},   \
   "event": "BALLOON_CHANGE", "data": {"actual": 1073610752}}
  {"timestamp": {"seconds": 1693272174, "microseconds": 89704},   \
   "event": "BALLOON_CHANGE", "data": {"actual": 1073610752}}
  {"timestamp": {"seconds": 1693272175, "microseconds": 90819},   \
   "event": "BALLOON_CHANGE", "data": {"actual": 1073610752}}
  {"timestamp": {"seconds": 1693272176, "microseconds": 91961},   \
   "event": "BALLOON_CHANGE", "data": {"actual": 1073610752}}
  {"timestamp": {"seconds": 1693272177, "microseconds": 93040},   \
   "event": "BALLOON_CHANGE", "data": {"actual": 1073676288}}
  {"timestamp": {"seconds": 1693272178, "microseconds": 94117},   \
   "event": "BALLOON_CHANGE", "data": {"actual": 1073676288}}
  {"timestamp": {"seconds": 1693272179, "microseconds": 95337},   \
   "event": "BALLOON_CHANGE", "data": {"actual": 1073610752}}
  {"timestamp": {"seconds": 1693272180, "microseconds": 96615},   \
   "event": "BALLOON_CHANGE", "data": {"actual": 1073676288}}
  {"timestamp": {"seconds": 1693272181, "microseconds": 97626},   \
   "event": "BALLOON_CHANGE", "data": {"actual": 1073610752}}
  {"timestamp": {"seconds": 1693272182, "microseconds": 98693},   \
   "event": "BALLOON_CHANGE", "data": {"actual": 1073676288}}
  {"timestamp": {"seconds": 1693272183, "microseconds": 99698},   \
   "event": "BALLOON_CHANGE", "data": {"actual": 1073610752}}
  {"timestamp": {"seconds": 1693272184, "microseconds": 100727},  \
   "event": "BALLOON_CHANGE", "data": {"actual": 1073610752}}
  {"timestamp": {"seconds": 1693272185, "microseconds": 90430},   \
   "event": "BALLOON_CHANGE", "data": {"actual": 1073610752}}
  {"timestamp": {"seconds": 1693272186, "microseconds": 102999},  \
   "event": "BALLOON_CHANGE", "data": {"actual": 1073676288}}
     :
  <The similar QMP events repeat>

Fix it by having the target aligned to the guest page size, 64KB
in this specific case. With this applied, no flooding QMP event
is observed and the memory balloon's size can be stablizied to
0x3ffe0000 soon after the deflation request is sent.

  { "execute" : "balloon", "arguments": { "value" : 1073672192 } }
  {"return": {}}
  {"timestamp": {"seconds": 1693273328, "microseconds": 793075},  \
   "event": "BALLOON_CHANGE", "data": {"actual": 1073610752}}
  { "execute" : "query-balloon" }
  {"return": {"actual": 1073610752}}

Signed-off-by: Gavin Shan <gshan@xxxxxxxxxx>
---
 drivers/virtio/virtio_balloon.c | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
index 5b15936a5214..625caac35264 100644
--- a/drivers/virtio/virtio_balloon.c
+++ b/drivers/virtio/virtio_balloon.c
@@ -386,6 +386,17 @@ static void stats_handle_request(struct virtio_balloon *vb)
 	virtqueue_kick(vq);
 }
 
+static inline s64 align_pages_up(s64 diff)
+{
+	if (diff == 0)
+		return diff;
+
+	if (diff > 0)
+		return ALIGN(diff, VIRTIO_BALLOON_PAGES_PER_PAGE);
+
+	return -ALIGN(-diff, VIRTIO_BALLOON_PAGES_PER_PAGE);
+}
+
 static inline s64 towards_target(struct virtio_balloon *vb)
 {
 	s64 target;
@@ -396,7 +407,7 @@ static inline s64 towards_target(struct virtio_balloon *vb)
 			&num_pages);
 
 	target = num_pages;
-	return target - vb->num_pages;
+	return align_pages_up(target - vb->num_pages);
 }
 
 /* Gives back @num_to_return blocks of free pages to mm. */
-- 
2.41.0