uffd: unconditionally enable UFFD_FEATURE_EVENT_REMOVE

Only enabling it when a balloon device is present doesn't really gain us
anything, because the only time this feature is checked in the kernel is
as part of madvise handling - but without balloon devices we wont ever
call madvise, so it makes no difference whether we enabled the feature
or not (it is unconditionally available since linux 4.10).

Signed-off-by: Patrick Roy <[email protected]>

Author: roypat

src/vmm/src/persist.rs

@@ -393,9 +393,6 @@ pub fn restore_from_snapshot(
             mem_backend_path,
             mem_state,
             track_dirty_pages,
-            // We enable the UFFD_FEATURE_EVENT_REMOVE feature only if a balloon device
-            // is present in the microVM state.
-            microvm_state.device_states.balloon_device.is_some(),
             vm_resources.machine_config.huge_pages,
         )
         .map_err(RestoreFromSnapshotGuestMemoryError::Uffd)?,
@@ -479,19 +476,18 @@ fn guest_memory_from_uffd(
     mem_uds_path: &Path,
     mem_state: &GuestMemoryState,
     track_dirty_pages: bool,
-    enable_balloon: bool,
     huge_pages: HugePageConfig,
 ) -> Result<(Vec<GuestRegionMmap>, Option<Uffd>), GuestMemoryFromUffdError> {
     let (guest_memory, backend_mappings) =
         create_guest_memory(mem_state, track_dirty_pages, huge_pages)?;
 
     let mut uffd_builder = UffdBuilder::new();
 
-    if enable_balloon {
-        // We enable this so that the page fault handler can add logic
-        // for treating madvise(MADV_DONTNEED) events triggerd by balloon inflation.
-        uffd_builder.require_features(FeatureFlags::EVENT_REMOVE);
-    }
+    // We only make use of this if balloon devices are present, but we can enable it unconditionally
+    // because the only place the kernel checks this is in a hook from madvise, e.g. it doesn't
+    // actively change the behavior of UFFD, only passively. Without balloon devices
+    // we never call madvise anyway, so no need to put this into a conditional.
+    uffd_builder.require_features(FeatureFlags::EVENT_REMOVE);
 
     let uffd = uffd_builder
         .close_on_exec(true)