diff --git a/pkg/sentry/fsimpl/overlay/copy_up.go b/pkg/sentry/fsimpl/overlay/copy_up.go
index f073b2369..e6c12e0c3 100644
--- a/pkg/sentry/fsimpl/overlay/copy_up.go
+++ b/pkg/sentry/fsimpl/overlay/copy_up.go
@@ -144,8 +144,6 @@ func (d *dentry) copyUpMaybeSyntheticMountpointLocked(ctx context.Context, forSy
 			cleanupUndoCopyUp()
 			return err
 		}
-		d.mapsMu.Lock()
-		defer d.mapsMu.Unlock()
 		if d.wrappedMappable != nil {
 			// We may have memory mappings of the file on the lower layer.
 			// Switch to mapping the file on the upper layer instead.
@@ -305,8 +303,8 @@ func (d *dentry) copyUpMaybeSyntheticMountpointLocked(ctx context.Context, forSy
 	}
 
 	if mmapOpts != nil && mmapOpts.Mappable != nil {
-		// Note that if mmapOpts != nil, then d.mapsMu is locked for writing
-		// (from the S_IFREG path above).
+		d.mapsMu.Lock()
+		defer d.mapsMu.Unlock()
 
 		// Propagate mappings of d to the new Mappable. Remember which mappings
 		// we added so we can remove them on failure.
diff --git a/pkg/sentry/fsimpl/overlay/overlay.go b/pkg/sentry/fsimpl/overlay/overlay.go
index c2e3e783c..84027a557 100644
--- a/pkg/sentry/fsimpl/overlay/overlay.go
+++ b/pkg/sentry/fsimpl/overlay/overlay.go
@@ -533,6 +533,10 @@ type dentry struct {
 	//
 	//	- isMappable is non-zero iff wrappedMappable is non-nil. isMappable is
 	//		accessed using atomic memory operations.
+	//
+	//	- wrappedMappable is protected by mapsMu and dataMu. In addition,
+	//	  it has to be immutable if copyMu is taken for write.
+	//        copyUpMaybeSyntheticMountpointLocked relies on this behavior.
 	mapsMu          mapsMutex `state:"nosave"`
 	lowerMappings   memmap.MappingSet
 	dataMu          dataRWMutex `state:"nosave"`