virtio-balloon: add deflate_on_oom support

Sometimes we need balloon deflate automatically to give memory
back to guest, especially for some low priority guest processes
under memory pressure. Enable deflate_on_oom to support this.

Usage: --balloon "size=0,deflate_on_oom=on" \

Signed-off-by: Fei Li <lifei.shirley@bytedance.com>

vmm/src/api/openapi/cloud-hypervisor.yaml

@@ -745,6 +745,10 @@ components:
         size:
           type: integer
           format: int64
+        deflate_on_oom:
+          type: boolean
+          default: false
+          description: Whether the balloon should deflate when the guest is under memory pressure.
 
     FsConfig:
       required:

vmm/src/config.rs

@@ -1201,14 +1201,19 @@ impl Default for RngConfig {
 #[derive(Clone, Debug, PartialEq, Deserialize, Serialize)]
 pub struct BalloonConfig {
     pub size: u64,
+    /// Option to deflate the balloon in case the guest is out of memory.
+    #[serde(default)]
+    pub deflate_on_oom: bool,
 }
 
 impl BalloonConfig {
-    pub const SYNTAX: &'static str = "Balloon parameters \"size=<balloon_size>\"";
+    pub const SYNTAX: &'static str =
+        "Balloon parameters \"size=<balloon_size>,deflate_on_oom=on|off\"";
 
     pub fn parse(balloon: &str) -> Result<Self> {
         let mut parser = OptionParser::new();
         parser.add("size");
+        parser.add("deflate_on_oom");
         parser.parse(balloon).map_err(Error::ParseBalloon)?;
 
         let size = parser
@@ -1217,7 +1222,16 @@ impl BalloonConfig {
             .map(|v| v.0)
             .unwrap_or(0);
 
-        Ok(BalloonConfig { size })
+        let deflate_on_oom = parser
+            .convert::<Toggle>("deflate_on_oom")
+            .map_err(Error::ParseBalloon)?
+            .unwrap_or(Toggle(false))
+            .0;
+
+        Ok(BalloonConfig {
+            size,
+            deflate_on_oom,
+        })
     }
 }
 

vmm/src/device_manager.rs

@@ -2610,6 +2610,7 @@ impl DeviceManager {
                 virtio_devices::Balloon::new(
                     id.clone(),
                     balloon_config.size,
+                    balloon_config.deflate_on_oom,
                     self.seccomp_action.clone(),
                 )
                 .map_err(DeviceManagerError::CreateVirtioBalloon)?,