VMALLOC_TOTAL
if (mem > mult_frac(VMALLOC_TOTAL, DM_BUFIO_VMALLOC_PERCENT, 100))
mem = mult_frac(VMALLOC_TOTAL, DM_BUFIO_VMALLOC_PERCENT, 100);
(unsigned long)VMALLOC_TOTAL >> 10);
if (max_distance > VMALLOC_TOTAL * 3 / 4) {
max_distance, VMALLOC_TOTAL);