diff --git a/libc/bionic/mmap.cpp b/libc/bionic/mmap.cpp
index 8301445..57a8cdf 100644
--- a/libc/bionic/mmap.cpp
+++ b/libc/bionic/mmap.cpp
@@ -54,10 +54,14 @@
     return MAP_FAILED;
   }
 
-  bool is_private_anonymous = (flags & (MAP_PRIVATE | MAP_ANONYMOUS)) != 0;
+  bool is_private_anonymous =
+      (flags & (MAP_PRIVATE | MAP_ANONYMOUS)) == (MAP_PRIVATE | MAP_ANONYMOUS);
+  bool is_stack_or_grows_down = (flags & (MAP_STACK | MAP_GROWSDOWN)) != 0;
+
   void* result = __mmap2(addr, size, prot, flags, fd, offset >> MMAP2_SHIFT);
 
-  if (result != MAP_FAILED && kernel_has_MADV_MERGEABLE && is_private_anonymous) {
+  if (result != MAP_FAILED && kernel_has_MADV_MERGEABLE &&
+      is_private_anonymous && !is_stack_or_grows_down) {
     ErrnoRestorer errno_restorer;
     int rc = madvise(result, size, MADV_MERGEABLE);
     if (rc == -1 && errno == EINVAL) {