Index: kernel/generic/include/lib/ra.h
===================================================================
--- kernel/generic/include/lib/ra.h	(revision 300f4c4dd43b87fb910ea1cfccdc3280c2bdd2ca)
+++ kernel/generic/include/lib/ra.h	(revision b08941d1678cbbd629779b42bcfc4b47c1a39134)
@@ -71,5 +71,13 @@
 typedef struct {
 	link_t segment_link;	/**< Span's segment list link. */
-	link_t fu_link;		/**< Span's free list or used hash link. */
+
+	/*
+	 * A segment cannot be both on the free list and in the used hash.
+	 * Their respective links can therefore occupy the same space.
+	 */
+	union {
+		link_t fl_link;		/**< Span's free list link. */
+		ht_link_t uh_link;	/**< Span's used hash link. */
+	};
 
 	uintptr_t base;		/**< Segment base. */