vm-migration: Optimize downtime by moving stop_dirty_log()

The larger the VM memory, the greater the memory pressure, and the
greater the stop_dirty_log() overhead. Moving stop_dirty_log() outside
the downtime period can reduce downtime.

Signed-off-by: Jinrong Liang <cloudliang@tencent.com>

vmm/src/lib.rs

@@ -1209,9 +1209,6 @@ impl Vmm {
 
             // Send last batch of dirty pages
             Self::vm_maybe_send_dirty_pages(vm, &mut socket)?;
-
-            // Stop logging dirty pages
-            vm.stop_dirty_log()?;
         }
         // Capture snapshot and send it
         let vm_snapshot = vm.snapshot()?;
@@ -1231,6 +1228,11 @@ impl Vmm {
             MigratableError::MigrateSend(anyhow!("Error completing migration")),
         )?;
 
+        // Stop logging dirty pages
+        if !send_data_migration.local {
+            vm.stop_dirty_log()?;
+        }
+
         info!("Migration complete");
 
         // Let every Migratable object know about the migration being complete