diff --git a/lib/Support/MemoryBuffer.cpp b/lib/Support/MemoryBuffer.cpp
index cab45c736b5..1c45c69ff3f 100644
--- a/lib/Support/MemoryBuffer.cpp
+++ b/lib/Support/MemoryBuffer.cpp
@@ -276,7 +276,7 @@ static bool shouldUseMmap(int FD,
                           int PageSize) {
   // We don't use mmap for small files because this can severely fragment our
   // address space.
-  if (MapSize < 4096*4)
+  if (MapSize < 4 * 4096 || MapSize < (unsigned)PageSize)
     return false;
 
   if (!RequiresNullTerminator)