LOWER_SZ
res = vm_munmap_memseg(ctx, LOWER_OFF, LOWER_SZ);
#define UPPER_SZ LOWER_SZ
#define TOTAL_SZ (LOWER_SZ + UPPER_SZ)
#define UPPER_OFF LOWER_SZ
res = alloc_memseg(ctx, MSEG_LOW, LOWER_SZ, "mseg_low");
res = vm_mmap_memseg(ctx, LOWER_OFF, MSEG_LOW, 0, LOWER_SZ, PROT_ALL);