uma_slab
LIST_SWAP(&freeslabs, &dom->ud_free_slab, uma_slab, us_link);
s = sizeof(struct uma_slab) + BITSET_SIZE(nitems) * SLAB_BITSETS;
LIST_HEAD(slabhead, uma_slab);
LIST_ENTRY(uma_slab) us_link; /* slabs in zone */
_Static_assert(sizeof(struct uma_slab) == __offsetof(struct uma_slab, us_free),
typedef struct uma_slab * uma_slab_t;
struct uma_slab uhs_slab; /* Must be last. */