VMAP_BLOCK_SIZE
int index = (addr / VMAP_BLOCK_SIZE) % nr_cpu_ids;
addr -= VMALLOC_START & ~(VMAP_BLOCK_SIZE-1);
addr /= VMAP_BLOCK_SIZE;
va = alloc_vmap_area(VMAP_BLOCK_SIZE, VMAP_BLOCK_SIZE,
offset = (addr & (VMAP_BLOCK_SIZE - 1)) >> PAGE_SHIFT;