[PATCH v20 7/7] virtio-balloon: don't report free pages when page poisoning is enabled

[Wei Wang]

The guest free pages should not be discarded by the live migration thread
when page poisoning is enabled with PAGE_POISONING_NO_SANITY=n, because
skipping the transfer of such poisoned free pages will trigger false
positive when new pages are allocated and checked on the destination.
This patch adds a config field, poison_val. Guest writes to the config
field to tell the host about the poisoning value. The value will be 0 in
the following cases:
1) PAGE_POISONING_NO_SANITY is enabled;
2) page poisoning is disabled; or
3) PAGE_POISONING_ZERO is enabled.

Signed-off-by: Wei Wang <wei.w.wang@xxxxxxxxx>
Suggested-by: Michael S. Tsirkin <mst@xxxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxx>
---
 drivers/virtio/virtio_balloon.c     | 8 ++++++++
 include/uapi/linux/virtio_balloon.h | 2 ++
 2 files changed, 10 insertions(+)

diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
index eae65c1..1fa8598 100644
--- a/drivers/virtio/virtio_balloon.c
+++ b/drivers/virtio/virtio_balloon.c
@@ -860,6 +860,7 @@ static struct file_system_type balloon_fs = {
 static int virtballoon_probe(struct virtio_device *vdev)
 {
 	struct virtio_balloon *vb;
+	__u32 poison_val;
 	int err;
 
 	if (!vdev->config->get) {
@@ -897,6 +898,13 @@ static int virtballoon_probe(struct virtio_device *vdev)
 					WQ_FREEZABLE | WQ_CPU_INTENSIVE, 0);
 		INIT_WORK(&vb->report_free_page_work, report_free_page);
 		vb->stop_cmd_id = VIRTIO_BALLOON_FREE_PAGE_REPORT_STOP_ID;
+		if (IS_ENABLED(CONFIG_PAGE_POISONING_NO_SANITY) ||
+		    !page_poisoning_enabled())
+			poison_val = 0;
+		else
+			poison_val = PAGE_POISON;
+		virtio_cwrite(vb->vdev, struct virtio_balloon_config,
+			      poison_val, &poison_val);
 	}
 
 	vb->nb.notifier_call = virtballoon_oom_notify;
diff --git a/include/uapi/linux/virtio_balloon.h b/include/uapi/linux/virtio_balloon.h
index 58f1274..f270e9e 100644
--- a/include/uapi/linux/virtio_balloon.h
+++ b/include/uapi/linux/virtio_balloon.h
@@ -48,6 +48,8 @@ struct virtio_balloon_config {
 	__u32 actual;
 	/* Free page report command id, readonly by guest */
 	__u32 free_page_report_cmd_id;
+	/* Stores PAGE_POISON if page poisoning with sanity check is in use */
+	__u32 poison_val;
 };
 
 #define VIRTIO_BALLOON_S_SWAP_IN  0   /* Amount of memory swapped in */
-- 
2.7.4