diff --git a/br/pkg/lightning/restore/restore.go b/br/pkg/lightning/restore/restore.go
index 1fcc5e844703c..329ef29c98667 100644
--- a/br/pkg/lightning/restore/restore.go
+++ b/br/pkg/lightning/restore/restore.go
@@ -2300,7 +2300,7 @@ type deliveredKVs struct {
 	offset  int64
 	rowID   int64
 
-	realOffset int64
+	realOffset int64 // indicates file reader's current position, only used for compressed files
 }
 
 type deliverResult struct {