PAGE_PTRS_PER_BVEC
static_assert(PAGE_PTRS_PER_BVEC > 1);
pages += nr_segs * (PAGE_PTRS_PER_BVEC - 1);
BUILD_BUG_ON(PAGE_PTRS_PER_BVEC < 2);
entries_left * (PAGE_PTRS_PER_BVEC - 1);