diff --git a/mm/mmap.c b/mm/mmap.c
index 56eb871..b6d74b3 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -570,9 +570,9 @@
 
 	/*
 	 * When changing only vma->vm_end, we don't really need
-	 * anon_vma lock: but is that case worth optimizing out?
+	 * anon_vma lock.
 	 */
-	if (vma->anon_vma)
+	if (vma->anon_vma && (insert || importer || start != vma->vm_start))
 		anon_vma = vma->anon_vma;
 	if (anon_vma) {
 		spin_lock(&anon_vma->lock);