virtio-devices: balloon: add Linux extended statistics tags

Add support for balloon stats tags 10-15, which are Linux kernel
extensions reporting VM memory pressure metrics: OOM kills, allocation
stalls, async/direct page scans, and async/direct page reclaims.

Also downgrade the balloon statistics API request log and the unknown
stats tag warning to debug level to reduce log noise.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

virtio-devices/src/balloon.rs

@@ -86,6 +86,13 @@ const VIRTIO_BALLOON_S_AVAIL: u16 = 6;
 const VIRTIO_BALLOON_S_CACHES: u16 = 7;
 const VIRTIO_BALLOON_S_HTLB_PGALLOC: u16 = 8;
 const VIRTIO_BALLOON_S_HTLB_PGFAIL: u16 = 9;
+// Linux kernel extensions (not yet in the virtio spec).
+const VIRTIO_BALLOON_S_OOM_KILL: u16 = 10;
+const VIRTIO_BALLOON_S_ALLOC_STALL: u16 = 11;
+const VIRTIO_BALLOON_S_ASYNC_SCAN: u16 = 12;
+const VIRTIO_BALLOON_S_DIRECT_SCAN: u16 = 13;
+const VIRTIO_BALLOON_S_ASYNC_RECLAIM: u16 = 14;
+const VIRTIO_BALLOON_S_DIRECT_RECLAIM: u16 = 15;
 
 #[derive(Error, Debug)]
 pub enum Error {
@@ -201,6 +208,18 @@ pub struct BalloonStatistics {
     pub hugetlb_allocations: Option<u64>,
     #[serde(skip_serializing_if = "Option::is_none")]
     pub hugetlb_failures: Option<u64>,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub oom_kills: Option<u64>,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub alloc_stalls: Option<u64>,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub async_scans: Option<u64>,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub direct_scans: Option<u64>,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub async_reclaims: Option<u64>,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub direct_reclaims: Option<u64>,
 }
 
 pub struct BalloonStatsState {
@@ -445,8 +464,14 @@ impl BalloonEpollHandler {
                 VIRTIO_BALLOON_S_CACHES => stats.disk_caches = Some(val),
                 VIRTIO_BALLOON_S_HTLB_PGALLOC => stats.hugetlb_allocations = Some(val),
                 VIRTIO_BALLOON_S_HTLB_PGFAIL => stats.hugetlb_failures = Some(val),
+                VIRTIO_BALLOON_S_OOM_KILL => stats.oom_kills = Some(val),
+                VIRTIO_BALLOON_S_ALLOC_STALL => stats.alloc_stalls = Some(val),
+                VIRTIO_BALLOON_S_ASYNC_SCAN => stats.async_scans = Some(val),
+                VIRTIO_BALLOON_S_DIRECT_SCAN => stats.direct_scans = Some(val),
+                VIRTIO_BALLOON_S_ASYNC_RECLAIM => stats.async_reclaims = Some(val),
+                VIRTIO_BALLOON_S_DIRECT_RECLAIM => stats.direct_reclaims = Some(val),
                 _ => {
-                    log::warn!("Unknown balloon stats tag: {tag}");
+                    log::debug!("Unknown balloon stats tag: {tag}");
                 }
             }
             offset += entry_size as u64;

vmm/src/api/mod.rs

@@ -36,7 +36,7 @@ pub mod http;
 use std::io;
 use std::sync::mpsc::{RecvError, SendError, Sender, channel};
 
-use log::info;
+use log::{debug, info};
 use micro_http::Body;
 use serde::{Deserialize, Serialize};
 use thiserror::Error;
@@ -895,7 +895,7 @@ impl ApiAction for VmBalloonStatistics {
 
     fn request(&self, _: Self::RequestBody, response_sender: Sender<ApiResponse>) -> ApiRequest {
         Box::new(move |vmm| {
-            info!("API request event: VmBalloonStatistics");
+            debug!("API request event: VmBalloonStatistics");
             let response = vmm
                 .vm_balloon_statistics()
                 .map_err(ApiError::VmBalloonStatistics)

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

@@ -1102,6 +1102,36 @@ components:
           type: integer
           format: int64
           nullable: true
+        oom_kills:
+          description: Number of OOM killer invocations
+          type: integer
+          format: int64
+          nullable: true
+        alloc_stalls:
+          description: Number of memory allocation stalls
+          type: integer
+          format: int64
+          nullable: true
+        async_scans:
+          description: Amount of memory scanned asynchronously by kswapd (pages)
+          type: integer
+          format: int64
+          nullable: true
+        direct_scans:
+          description: Amount of memory scanned by direct reclaim (pages)
+          type: integer
+          format: int64
+          nullable: true
+        async_reclaims:
+          description: Amount of memory reclaimed asynchronously by kswapd (pages)
+          type: integer
+          format: int64
+          nullable: true
+        direct_reclaims:
+          description: Amount of memory reclaimed by direct reclaim (pages)
+          type: integer
+          format: int64
+          nullable: true
         balloon_actual_bytes:
           description: Current balloon size in bytes (guest-confirmed)
           type: integer