struct arena_chunk_map_misc_s {
	/*
	 * Linkage for run trees.  There are two disjoint uses:
	 *
	 * 1) arena_t's runs_avail tree.
	 * 2) arena_run_t conceptually uses this linkage for in-use non-full
	 *    runs, rather than directly embedding linkage.
	 */
	rb_node(arena_chunk_map_misc_t)		rb_link;

	union {
		/* Linkage for list of dirty runs. */
		arena_runs_dirty_link_t		rd;

		/* Profile counters, used for large object runs. */
		union {
			void				*prof_tctx_pun;
			prof_tctx_t			*prof_tctx;
		};

		/* Small region run metadata. */
		arena_run_t			run;
	};
};