diff --git a/fs/ext4/super.c b/fs/ext4/super.c
index cf2f612..a8c599a 100644
--- a/fs/ext4/super.c
+++ b/fs/ext4/super.c
@@ -1991,6 +1991,17 @@ static int ext4_fill_super (struct super_block *sb, void *data, int silent)
 		printk(KERN_WARNING
 		       "EXT4-fs warning: feature flags set on rev 0 fs, "
 		       "running e2fsck is recommended\n");
+
+	/*
+	 * Since ext4 is still considered development code, we require
+	 * that the TEST_FILESYS flag in s->flags be set.
+	 */
+	if (!(le32_to_cpu(es->s_flags) & EXT2_FLAGS_TEST_FILESYS)) {
+		printk(KERN_WARNING, "EXT4-fs: %s: test filesystem flag "
+		       "not set.\n");
+		goto failed_mount;
+	}
+
 	/*
 	 * Check feature flags regardless of the revision level, since we
 	 * previously didn't change the revision level when setting the flags,
diff --git a/include/linux/ext4_fs.h b/include/linux/ext4_fs.h
index 98d1c3c..d203543 100644
--- a/include/linux/ext4_fs.h
+++ b/include/linux/ext4_fs.h
@@ -547,6 +547,13 @@ do {									       \
 #define	EXT4_ORPHAN_FS			0x0004	/* Orphans being recovered */
 
 /*
+ * Misc. filesystem flags
+ */
+#define EXT2_FLAGS_SIGNED_HASH		0x0001  /* Signed dirhash in use */
+#define EXT2_FLAGS_UNSIGNED_HASH	0x0002  /* Unsigned dirhash in use */
+#define EXT2_FLAGS_TEST_FILESYS		0x0004	/* OK for use on development code */
+
+/*
  * Mount flags
  */
 #define EXT4_MOUNT_CHECK		0x00001	/* Do mount-time checks */