aligned_byte_mask
# define aligned_byte_mask(n) ((1UL << 8*(n))-1)
# define aligned_byte_mask(n) (~0xffUL << (BITS_PER_LONG - 8 - 8*(n)))
# define aligned_byte_mask(n) ((1UL << 8*(n))-1)
# define aligned_byte_mask(n) (~0xffUL << (BITS_PER_LONG - 8 - 8*(n)))