ZERO_REGION_SIZE
if (len > ZERO_REGION_SIZE)
len = ZERO_REGION_SIZE;
if (copylen > ZERO_REGION_SIZE)
copylen = ZERO_REGION_SIZE;
CTASSERT(ZERO_REGION_SIZE >= TARFS_BLOCKSIZE);
iosize = min(iosize * 8, ZERO_REGION_SIZE);
tlen -= MIN(ZERO_REGION_SIZE, tlen);
liovec[i].iov_len = MIN(ZERO_REGION_SIZE, tlen);
tlen = min(xfersize, ZERO_REGION_SIZE);
CTASSERT((ZERO_REGION_SIZE & PAGE_MASK) == 0);
addr = kva_alloc(ZERO_REGION_SIZE);
for (i = 0; i < ZERO_REGION_SIZE; i += PAGE_SIZE)
pmap_protect(kernel_pmap, addr, addr + ZERO_REGION_SIZE, VM_PROT_READ);