Disallow simultaneous usage of balloon device and huge pages

The balloon device does not work with huge pages, so for now disallow
using them together.

Signed-off-by: Patrick Roy <roypat@amazon.co.uk>

src/vmm/src/resources.rs

@@ -261,6 +261,10 @@ impl VmResources {
             return Err(VmConfigError::IncompatibleBalloonSize);
         }
 
+        if self.balloon.get().is_some() && updated.huge_pages != HugePageConfig::None {
+            return Err(VmConfigError::BalloonAndHugePages);
+        }
+
         self.vm_config = updated;
 
         Ok(())
@@ -325,6 +329,10 @@ impl VmResources {
             return Err(BalloonConfigError::TooManyPagesRequested);
         }
 
+        if self.vm_config.huge_pages != HugePageConfig::None {
+            return Err(BalloonConfigError::HugePages);
+        }
+
         self.balloon.set(config)
     }
 
@@ -1389,6 +1397,9 @@ mod tests {
 
         if KernelVersion::get().unwrap() >= KernelVersion::new(5, 10, 0) {
             aux_vm_config.mem_size_mib = Some(2048);
+            // Remove the balloon device config that's added by `default_vm_resources` as it would
+            // trigger the "ballooning incompatible with huge pages" check.
+            vm_resources.balloon = BalloonBuilder::new();
             vm_resources.update_vm_config(&aux_vm_config).unwrap();
         }
     }

src/vmm/src/vmm_config/balloon.rs

@@ -28,6 +28,8 @@ pub enum BalloonConfigError {
     CreateFailure(crate::devices::virtio::balloon::BalloonError),
     /// Error updating the balloon device configuration: {0:?}
     UpdateFailure(std::io::Error),
+    /// Firecracker's huge pages support is incompatible with memory ballooning.
+    HugePages,
 }
 
 /// This struct represents the strongly typed equivalent of the json body

src/vmm/src/vmm_config/machine_config.rs

@@ -33,6 +33,8 @@ pub enum VmConfigError {
     KernelVersion,
     /// Firecracker's hugetlbfs support requires at least host kernel 5.10.
     HugetlbfsNotSupported,
+    /// Firecracker's huge pages support is incompatible with memory ballooning.
+    BalloonAndHugePages,
 }
 
 // We cannot do a `KernelVersion(kernel_version::Error)` variant because `kernel_version::Error`

tests/integration_tests/performance/test_huge_pages.py

@@ -112,3 +112,30 @@ def test_hugetlbfs_snapshot(
     assert not rc
 
     check_hugetlbfs_in_use(vm.firecracker_pid, "/anon_hugepage")
+
+
+@pytest.mark.skipif(
+    global_props.host_linux_version == "4.14",
+    reason="MFD_HUGETLB | MFD_ALLOW_SEALING only supported on kernels >= 4.16",
+)
+def test_negative_huge_pages_plus_balloon(uvm_plain):
+    """Tests that huge pages and memory ballooning cannot be used together"""
+    uvm_plain.memory_monitor = None
+    uvm_plain.spawn()
+
+    # Ensure setting huge pages and then adding a balloon device doesn't work
+    uvm_plain.basic_config(huge_pages=HugePagesConfig.HUGETLBFS_2MB)
+    with pytest.raises(
+        RuntimeError,
+        match="Firecracker's huge pages support is incompatible with memory ballooning.",
+    ):
+        uvm_plain.api.balloon.put(amount_mib=0, deflate_on_oom=False)
+
+    # Ensure adding a balloon device and then setting huge pages doesn't work
+    uvm_plain.basic_config(huge_pages=HugePagesConfig.NONE)
+    uvm_plain.api.balloon.put(amount_mib=0, deflate_on_oom=False)
+    with pytest.raises(
+        RuntimeError,
+        match="Machine config error: Firecracker's huge pages support is incompatible with memory ballooning.",
+    ):
+        uvm_plain.basic_config(huge_pages=HugePagesConfig.HUGETLBFS_2MB)