struct arena_run_s {
	/* Index of bin this run is associated with. */
	szind_t		binind;

	/* Number of free regions in run. */
	unsigned	nfree;

	/* Per region allocated/deallocated bitmap. */
        // 记录 run 中 region 的分配情况，每 bit 对应1个 region
	bitmap_t	bitmap[BITMAP_GROUPS_MAX];
};