diff --git a/Documentation/kbuild/makefiles.rst b/Documentation/kbuild/makefiles.rst
index b9b50553bfc5..d7e6534a8505 100644
--- a/Documentation/kbuild/makefiles.rst
+++ b/Documentation/kbuild/makefiles.rst
@@ -297,9 +297,19 @@ more details, with real examples.
 	If CONFIG_EXT2_FS is set to either 'y' (built-in) or 'm' (modular)
 	the corresponding obj- variable will be set, and kbuild will descend
 	down in the ext2 directory.
-	Kbuild only uses this information to decide that it needs to visit
-	the directory, it is the Makefile in the subdirectory that
-	specifies what is modular and what is built-in.
+
+	Kbuild uses this information not only to decide that it needs to visit
+	the directory, but also to decide whether or not to link objects from
+	the directory into vmlinux.
+
+	When Kbuild descends into the directory with 'y', all built-in objects
+	from that directory are combined into the built-in.a, which will be
+	eventually linked into vmlinux.
+
+	When Kbuild descends into the directory with 'm', in contrast, nothing
+	from that directory will be linked into vmlinux. If the Makefile in
+	that directory specifies obj-y, those objects will be left orphan.
+	It is very likely a bug of the Makefile or of dependencies in Kconfig.
 
 	It is good practice to use a `CONFIG_` variable when assigning directory
 	names. This allows kbuild to totally skip the directory if the