IDR_BITS           27 include/linux/idr.h #define IDR_SIZE (1 << IDR_BITS)
IDR_BITS           28 include/linux/idr.h #define IDR_MASK ((1 << IDR_BITS)-1)
IDR_BITS           33 include/linux/idr.h 	struct idr_layer __rcu	*ary[1<<IDR_BITS];