Behavior of process_vm_* syscalls on various architectures

Results (aarch64)

Size 2 copy at page_size - 1: 0
	1 bytes less than could have been written
Size 3 copy at page_size - 1: 0
	1 bytes less than could have been written
Size 4 copy at page_size - 1: 0
	1 bytes less than could have been written
Size 5 copy at page_size - 1: 0
	1 bytes less than could have been written
Size 6 copy at page_size - 1: 0
	1 bytes less than could have been written
Size 7 copy at page_size - 1: 0
	1 bytes less than could have been written
Size 8 copy at page_size - 1: 0
	1 bytes less than could have been written
Size 3 copy at page_size - 2: 2
Size 4 copy at page_size - 2: 0
	2 bytes less than could have been written
Size 5 copy at page_size - 2: 0
	2 bytes less than could have been written
Size 6 copy at page_size - 2: 0
	2 bytes less than could have been written
Size 7 copy at page_size - 2: 0
	2 bytes less than could have been written
Size 8 copy at page_size - 2: 0
	2 bytes less than could have been written
Size 9 copy at page_size - 2: 0
	2 bytes less than could have been written
Size 4 copy at page_size - 3: 0
	3 bytes less than could have been written
Size 5 copy at page_size - 3: 0
	3 bytes less than could have been written
Size 6 copy at page_size - 3: 0
	3 bytes less than could have been written
Size 7 copy at page_size - 3: 0
	3 bytes less than could have been written
Size 8 copy at page_size - 3: 0
	3 bytes less than could have been written
Size 9 copy at page_size - 3: 0
	3 bytes less than could have been written
Size 10 copy at page_size - 3: 0
	3 bytes less than could have been written
Size 5 copy at page_size - 4: 4
Size 6 copy at page_size - 4: 4
Size 7 copy at page_size - 4: 4
Size 8 copy at page_size - 4: 0
	4 bytes less than could have been written
Size 9 copy at page_size - 4: 0
	4 bytes less than could have been written
Size 10 copy at page_size - 4: 0
	4 bytes less than could have been written
Size 11 copy at page_size - 4: 0
	4 bytes less than could have been written
Size 6 copy at page_size - 5: 4
	1 bytes less than could have been written
Size 7 copy at page_size - 5: 4
	1 bytes less than could have been written
Size 8 copy at page_size - 5: 0
	5 bytes less than could have been written
Size 9 copy at page_size - 5: 0
	5 bytes less than could have been written
Size 10 copy at page_size - 5: 0
	5 bytes less than could have been written
Size 11 copy at page_size - 5: 0
	5 bytes less than could have been written
Size 12 copy at page_size - 5: 0
	5 bytes less than could have been written
Size 7 copy at page_size - 6: 6
Size 8 copy at page_size - 6: 0
	6 bytes less than could have been written
Size 9 copy at page_size - 6: 0
	6 bytes less than could have been written
Size 10 copy at page_size - 6: 0
	6 bytes less than could have been written
Size 11 copy at page_size - 6: 0
	6 bytes less than could have been written
Size 12 copy at page_size - 6: 0
	6 bytes less than could have been written
Size 13 copy at page_size - 6: 0
	6 bytes less than could have been written
Size 8 copy at page_size - 7: 0
	7 bytes less than could have been written
Size 9 copy at page_size - 7: 0
	7 bytes less than could have been written
Size 10 copy at page_size - 7: 0
	7 bytes less than could have been written
Size 11 copy at page_size - 7: 0
	7 bytes less than could have been written
Size 12 copy at page_size - 7: 0
	7 bytes less than could have been written
Size 13 copy at page_size - 7: 0
	7 bytes less than could have been written
Size 14 copy at page_size - 7: 0
	7 bytes less than could have been written
Size 9 copy at page_size - 8: 8
Size 10 copy at page_size - 8: 8
Size 11 copy at page_size - 8: 8
Size 12 copy at page_size - 8: 8
Size 13 copy at page_size - 8: 8
Size 14 copy at page_size - 8: 8
Size 15 copy at page_size - 8: 8
Size 10 copy at page_size - 9: 8
	1 bytes less than could have been written
Size 11 copy at page_size - 9: 8
	1 bytes less than could have been written
Size 12 copy at page_size - 9: 8
	1 bytes less than could have been written
Size 13 copy at page_size - 9: 8
	1 bytes less than could have been written
Size 14 copy at page_size - 9: 8
	1 bytes less than could have been written
Size 15 copy at page_size - 9: 8
	1 bytes less than could have been written
Size 16 copy at page_size - 9: 0
	9 bytes less than could have been written
	8 bytes more were written than the syscall indicated
Size 11 copy at page_size - 10: 10
Size 12 copy at page_size - 10: 8
	2 bytes less than could have been written
Size 13 copy at page_size - 10: 8
	2 bytes less than could have been written
Size 14 copy at page_size - 10: 8
	2 bytes less than could have been written
Size 15 copy at page_size - 10: 8
	2 bytes less than could have been written
Size 16 copy at page_size - 10: 0
	10 bytes less than could have been written
	8 bytes more were written than the syscall indicated
Size 17 copy at page_size - 10: 0
	10 bytes less than could have been written
	8 bytes more were written than the syscall indicated
Size 12 copy at page_size - 11: 8
	3 bytes less than could have been written
Size 13 copy at page_size - 11: 8
	3 bytes less than could have been written
Size 14 copy at page_size - 11: 8
	3 bytes less than could have been written
Size 15 copy at page_size - 11: 8
	3 bytes less than could have been written
Size 16 copy at page_size - 11: 0
	11 bytes less than could have been written
	8 bytes more were written than the syscall indicated
Size 17 copy at page_size - 11: 0
	11 bytes less than could have been written
	8 bytes more were written than the syscall indicated
Size 18 copy at page_size - 11: 0
	11 bytes less than could have been written
	8 bytes more were written than the syscall indicated
Size 13 copy at page_size - 12: 12
Size 14 copy at page_size - 12: 12
Size 15 copy at page_size - 12: 12
Size 16 copy at page_size - 12: 0
	12 bytes less than could have been written
	8 bytes more were written than the syscall indicated
Size 17 copy at page_size - 12: 0
	12 bytes less than could have been written
	8 bytes more were written than the syscall indicated
Size 18 copy at page_size - 12: 0
	12 bytes less than could have been written
	8 bytes more were written than the syscall indicated
Size 19 copy at page_size - 12: 0
	12 bytes less than could have been written
	8 bytes more were written than the syscall indicated
Size 14 copy at page_size - 13: 12
	1 bytes less than could have been written
Size 15 copy at page_size - 13: 12
	1 bytes less than could have been written
Size 16 copy at page_size - 13: 0
	13 bytes less than could have been written
	8 bytes more were written than the syscall indicated
Size 17 copy at page_size - 13: 0
	13 bytes less than could have been written
	8 bytes more were written than the syscall indicated
Size 18 copy at page_size - 13: 0
	13 bytes less than could have been written
	8 bytes more were written than the syscall indicated
Size 19 copy at page_size - 13: 0
	13 bytes less than could have been written
	8 bytes more were written than the syscall indicated
Size 20 copy at page_size - 13: 0
	13 bytes less than could have been written
	8 bytes more were written than the syscall indicated
Size 15 copy at page_size - 14: 14
Size 16 copy at page_size - 14: 0
	14 bytes less than could have been written
	8 bytes more were written than the syscall indicated
Size 17 copy at page_size - 14: 0
	14 bytes less than could have been written
	8 bytes more were written than the syscall indicated
Size 18 copy at page_size - 14: 0
	14 bytes less than could have been written
	8 bytes more were written than the syscall indicated
Size 19 copy at page_size - 14: 0
	14 bytes less than could have been written
	8 bytes more were written than the syscall indicated
Size 20 copy at page_size - 14: 0
	14 bytes less than could have been written
	8 bytes more were written than the syscall indicated
Size 21 copy at page_size - 14: 0
	14 bytes less than could have been written
	8 bytes more were written than the syscall indicated
Size 16 copy at page_size - 15: 0
	15 bytes less than could have been written
	8 bytes more were written than the syscall indicated
Size 17 copy at page_size - 15: 0
	15 bytes less than could have been written
	8 bytes more were written than the syscall indicated
Size 18 copy at page_size - 15: 0
	15 bytes less than could have been written
	8 bytes more were written than the syscall indicated
Size 19 copy at page_size - 15: 0
	15 bytes less than could have been written
	8 bytes more were written than the syscall indicated
Size 20 copy at page_size - 15: 0
	15 bytes less than could have been written
	8 bytes more were written than the syscall indicated
Size 21 copy at page_size - 15: 0
	15 bytes less than could have been written
	8 bytes more were written than the syscall indicated
Size 22 copy at page_size - 15: 0
	15 bytes less than could have been written
	8 bytes more were written than the syscall indicated
Size 17 copy at page_size - 16: 16
Size 18 copy at page_size - 16: 16
Size 19 copy at page_size - 16: 16
Size 20 copy at page_size - 16: 16
Size 21 copy at page_size - 16: 16
Size 22 copy at page_size - 16: 16
Size 23 copy at page_size - 16: 16
Size 18 copy at page_size - 17: 16
	1 bytes less than could have been written
Size 19 copy at page_size - 17: 16
	1 bytes less than could have been written
Size 20 copy at page_size - 17: 16
	1 bytes less than could have been written
Size 21 copy at page_size - 17: 16
	1 bytes less than could have been written
Size 22 copy at page_size - 17: 16
	1 bytes less than could have been written
Size 23 copy at page_size - 17: 16
	1 bytes less than could have been written
Size 24 copy at page_size - 17: 16
	1 bytes less than could have been written
Size 19 copy at page_size - 18: 18
Size 20 copy at page_size - 18: 16
	2 bytes less than could have been written
Size 21 copy at page_size - 18: 16
	2 bytes less than could have been written
Size 22 copy at page_size - 18: 16
	2 bytes less than could have been written
Size 23 copy at page_size - 18: 16
	2 bytes less than could have been written
Size 24 copy at page_size - 18: 16
	2 bytes less than could have been written
Size 25 copy at page_size - 18: 16
	2 bytes less than could have been written
Size 20 copy at page_size - 19: 16
	3 bytes less than could have been written
Size 21 copy at page_size - 19: 16
	3 bytes less than could have been written
Size 22 copy at page_size - 19: 16
	3 bytes less than could have been written
Size 23 copy at page_size - 19: 16
	3 bytes less than could have been written
Size 24 copy at page_size - 19: 16
	3 bytes less than could have been written
Size 25 copy at page_size - 19: 16
	3 bytes less than could have been written
Size 26 copy at page_size - 19: 16
	3 bytes less than could have been written

Results (powerpc)

Size 2 copy at page_size - 1: 0
	1 bytes less than could have been written
Size 3 copy at page_size - 1: 0
	1 bytes less than could have been written
Size 4 copy at page_size - 1: 0
	1 bytes less than could have been written
Size 5 copy at page_size - 1: 0
	1 bytes less than could have been written
Size 6 copy at page_size - 1: 0
	1 bytes less than could have been written
Size 7 copy at page_size - 1: 0
	1 bytes less than could have been written
Size 8 copy at page_size - 1: 0
	1 bytes less than could have been written
Size 3 copy at page_size - 2: 2
Size 4 copy at page_size - 2: 0
	2 bytes less than could have been written
Size 5 copy at page_size - 2: 0
	2 bytes less than could have been written
Size 6 copy at page_size - 2: 0
	2 bytes less than could have been written
Size 7 copy at page_size - 2: 0
	2 bytes less than could have been written
Size 8 copy at page_size - 2: 0
	2 bytes less than could have been written
Size 9 copy at page_size - 2: 0
	2 bytes less than could have been written
Size 4 copy at page_size - 3: 0
	3 bytes less than could have been written
Size 5 copy at page_size - 3: 0
	3 bytes less than could have been written
Size 6 copy at page_size - 3: 0
	3 bytes less than could have been written
Size 7 copy at page_size - 3: 0
	3 bytes less than could have been written
Size 8 copy at page_size - 3: 0
	3 bytes less than could have been written
Size 9 copy at page_size - 3: 0
	3 bytes less than could have been written
Size 10 copy at page_size - 3: 0
	3 bytes less than could have been written
Size 5 copy at page_size - 4: 4
Size 6 copy at page_size - 4: 4
Size 7 copy at page_size - 4: 4
Size 8 copy at page_size - 4: 4
Size 9 copy at page_size - 4: 4
Size 10 copy at page_size - 4: 4
Size 11 copy at page_size - 4: 4
Size 6 copy at page_size - 5: 4
	1 bytes less than could have been written
Size 7 copy at page_size - 5: 4
	1 bytes less than could have been written
Size 8 copy at page_size - 5: 4
	1 bytes less than could have been written
Size 9 copy at page_size - 5: 4
	1 bytes less than could have been written
Size 10 copy at page_size - 5: 4
	1 bytes less than could have been written
Size 11 copy at page_size - 5: 4
	1 bytes less than could have been written
Size 12 copy at page_size - 5: 4
	1 bytes less than could have been written
Size 7 copy at page_size - 6: 6
Size 8 copy at page_size - 6: 4
	2 bytes less than could have been written
Size 9 copy at page_size - 6: 4
	2 bytes less than could have been written
Size 10 copy at page_size - 6: 4
	2 bytes less than could have been written
Size 11 copy at page_size - 6: 4
	2 bytes less than could have been written
Size 12 copy at page_size - 6: 4
	2 bytes less than could have been written
Size 13 copy at page_size - 6: 4
	2 bytes less than could have been written
Size 8 copy at page_size - 7: 4
	3 bytes less than could have been written
Size 9 copy at page_size - 7: 4
	3 bytes less than could have been written
Size 10 copy at page_size - 7: 4
	3 bytes less than could have been written
Size 11 copy at page_size - 7: 4
	3 bytes less than could have been written
Size 12 copy at page_size - 7: 4
	3 bytes less than could have been written
Size 13 copy at page_size - 7: 4
	3 bytes less than could have been written
Size 14 copy at page_size - 7: 4
	3 bytes less than could have been written
Size 9 copy at page_size - 8: 8
Size 10 copy at page_size - 8: 8
Size 11 copy at page_size - 8: 8
Size 12 copy at page_size - 8: 8
Size 13 copy at page_size - 8: 8
Size 14 copy at page_size - 8: 8
Size 15 copy at page_size - 8: 8
Size 10 copy at page_size - 9: 8
	1 bytes less than could have been written
Size 11 copy at page_size - 9: 8
	1 bytes less than could have been written
Size 12 copy at page_size - 9: 8
	1 bytes less than could have been written
Size 13 copy at page_size - 9: 8
	1 bytes less than could have been written
Size 14 copy at page_size - 9: 8
	1 bytes less than could have been written
Size 15 copy at page_size - 9: 8
	1 bytes less than could have been written
Size 16 copy at page_size - 9: 9
Size 11 copy at page_size - 10: 10
Size 12 copy at page_size - 10: 8
	2 bytes less than could have been written
Size 13 copy at page_size - 10: 8
	2 bytes less than could have been written
Size 14 copy at page_size - 10: 8
	2 bytes less than could have been written
Size 15 copy at page_size - 10: 8
	2 bytes less than could have been written
Size 16 copy at page_size - 10: 10
Size 17 copy at page_size - 10: 10
Size 12 copy at page_size - 11: 8
	3 bytes less than could have been written
Size 13 copy at page_size - 11: 8
	3 bytes less than could have been written
Size 14 copy at page_size - 11: 8
	3 bytes less than could have been written
Size 15 copy at page_size - 11: 8
	3 bytes less than could have been written
Size 16 copy at page_size - 11: 11
Size 17 copy at page_size - 11: 11
Size 18 copy at page_size - 11: 11
Size 13 copy at page_size - 12: 12
Size 14 copy at page_size - 12: 12
Size 15 copy at page_size - 12: 12
Size 16 copy at page_size - 12: 12
Size 17 copy at page_size - 12: 12
Size 18 copy at page_size - 12: 12
Size 19 copy at page_size - 12: 12
Size 14 copy at page_size - 13: 12
	1 bytes less than could have been written
Size 15 copy at page_size - 13: 12
	1 bytes less than could have been written
Size 16 copy at page_size - 13: 13
Size 17 copy at page_size - 13: 13
Size 18 copy at page_size - 13: 13
Size 19 copy at page_size - 13: 13
Size 20 copy at page_size - 13: 13
Size 15 copy at page_size - 14: 14
Size 16 copy at page_size - 14: 14
Size 17 copy at page_size - 14: 14
Size 18 copy at page_size - 14: 14
Size 19 copy at page_size - 14: 14
Size 20 copy at page_size - 14: 14
Size 21 copy at page_size - 14: 14
Size 16 copy at page_size - 15: 15
Size 17 copy at page_size - 15: 15
Size 18 copy at page_size - 15: 15
Size 19 copy at page_size - 15: 15
Size 20 copy at page_size - 15: 15
Size 21 copy at page_size - 15: 15
Size 22 copy at page_size - 15: 15
Size 17 copy at page_size - 16: 16
Size 18 copy at page_size - 16: 16
Size 19 copy at page_size - 16: 16
Size 20 copy at page_size - 16: 16
Size 21 copy at page_size - 16: 16
Size 22 copy at page_size - 16: 16
Size 23 copy at page_size - 16: 16
Size 18 copy at page_size - 17: 17
Size 19 copy at page_size - 17: 17
Size 20 copy at page_size - 17: 17
Size 21 copy at page_size - 17: 17
Size 22 copy at page_size - 17: 17
Size 23 copy at page_size - 17: 17
Size 24 copy at page_size - 17: 17
Size 19 copy at page_size - 18: 18
Size 20 copy at page_size - 18: 18
Size 21 copy at page_size - 18: 18
Size 22 copy at page_size - 18: 18
Size 23 copy at page_size - 18: 18
Size 24 copy at page_size - 18: 18
Size 25 copy at page_size - 18: 18
Size 20 copy at page_size - 19: 19
Size 21 copy at page_size - 19: 19
Size 22 copy at page_size - 19: 19
Size 23 copy at page_size - 19: 19
Size 24 copy at page_size - 19: 19
Size 25 copy at page_size - 19: 19
Size 26 copy at page_size - 19: 19

Results (x86)

Size 2 copy at page_size - 1: 1
Size 3 copy at page_size - 1: 1
Size 4 copy at page_size - 1: 1
Size 5 copy at page_size - 1: 1
Size 6 copy at page_size - 1: 1
Size 7 copy at page_size - 1: 1
Size 8 copy at page_size - 1: 1
Size 3 copy at page_size - 2: 2
Size 4 copy at page_size - 2: 2
Size 5 copy at page_size - 2: 2
Size 6 copy at page_size - 2: 2
Size 7 copy at page_size - 2: 2
Size 8 copy at page_size - 2: 2
Size 9 copy at page_size - 2: 2
Size 4 copy at page_size - 3: 3
Size 5 copy at page_size - 3: 3
Size 6 copy at page_size - 3: 3
Size 7 copy at page_size - 3: 3
Size 8 copy at page_size - 3: 3
Size 9 copy at page_size - 3: 3
Size 10 copy at page_size - 3: 3
Size 5 copy at page_size - 4: 4
Size 6 copy at page_size - 4: 4
Size 7 copy at page_size - 4: 4
Size 8 copy at page_size - 4: 4
Size 9 copy at page_size - 4: 4
Size 10 copy at page_size - 4: 4
Size 11 copy at page_size - 4: 4
Size 6 copy at page_size - 5: 5
Size 7 copy at page_size - 5: 5
Size 8 copy at page_size - 5: 5
Size 9 copy at page_size - 5: 5
Size 10 copy at page_size - 5: 5
Size 11 copy at page_size - 5: 5
Size 12 copy at page_size - 5: 5
Size 7 copy at page_size - 6: 6
Size 8 copy at page_size - 6: 6
Size 9 copy at page_size - 6: 6
Size 10 copy at page_size - 6: 6
Size 11 copy at page_size - 6: 6
Size 12 copy at page_size - 6: 6
Size 13 copy at page_size - 6: 6
Size 8 copy at page_size - 7: 7
Size 9 copy at page_size - 7: 7
Size 10 copy at page_size - 7: 7
Size 11 copy at page_size - 7: 7
Size 12 copy at page_size - 7: 7
Size 13 copy at page_size - 7: 7
Size 14 copy at page_size - 7: 7
Size 9 copy at page_size - 8: 8
Size 10 copy at page_size - 8: 8
Size 11 copy at page_size - 8: 8
Size 12 copy at page_size - 8: 8
Size 13 copy at page_size - 8: 8
Size 14 copy at page_size - 8: 8
Size 15 copy at page_size - 8: 8
Size 10 copy at page_size - 9: 9
Size 11 copy at page_size - 9: 9
Size 12 copy at page_size - 9: 9
Size 13 copy at page_size - 9: 9
Size 14 copy at page_size - 9: 9
Size 15 copy at page_size - 9: 9
Size 16 copy at page_size - 9: 9
Size 11 copy at page_size - 10: 10
Size 12 copy at page_size - 10: 10
Size 13 copy at page_size - 10: 10
Size 14 copy at page_size - 10: 10
Size 15 copy at page_size - 10: 10
Size 16 copy at page_size - 10: 10
Size 17 copy at page_size - 10: 10
Size 12 copy at page_size - 11: 11
Size 13 copy at page_size - 11: 11
Size 14 copy at page_size - 11: 11
Size 15 copy at page_size - 11: 11
Size 16 copy at page_size - 11: 11
Size 17 copy at page_size - 11: 11
Size 18 copy at page_size - 11: 11
Size 13 copy at page_size - 12: 12
Size 14 copy at page_size - 12: 12
Size 15 copy at page_size - 12: 12
Size 16 copy at page_size - 12: 12
Size 17 copy at page_size - 12: 12
Size 18 copy at page_size - 12: 12
Size 19 copy at page_size - 12: 12
Size 14 copy at page_size - 13: 13
Size 15 copy at page_size - 13: 13
Size 16 copy at page_size - 13: 13
Size 17 copy at page_size - 13: 13
Size 18 copy at page_size - 13: 13
Size 19 copy at page_size - 13: 13
Size 20 copy at page_size - 13: 13
Size 15 copy at page_size - 14: 14
Size 16 copy at page_size - 14: 14
Size 17 copy at page_size - 14: 14
Size 18 copy at page_size - 14: 14
Size 19 copy at page_size - 14: 14
Size 20 copy at page_size - 14: 14
Size 21 copy at page_size - 14: 14
Size 16 copy at page_size - 15: 15
Size 17 copy at page_size - 15: 15
Size 18 copy at page_size - 15: 15
Size 19 copy at page_size - 15: 15
Size 20 copy at page_size - 15: 15
Size 21 copy at page_size - 15: 15
Size 22 copy at page_size - 15: 15
Size 17 copy at page_size - 16: 16
Size 18 copy at page_size - 16: 16
Size 19 copy at page_size - 16: 16
Size 20 copy at page_size - 16: 16
Size 21 copy at page_size - 16: 16
Size 22 copy at page_size - 16: 16
Size 23 copy at page_size - 16: 16
Size 18 copy at page_size - 17: 17
Size 19 copy at page_size - 17: 17
Size 20 copy at page_size - 17: 17
Size 21 copy at page_size - 17: 17
Size 22 copy at page_size - 17: 17
Size 23 copy at page_size - 17: 17
Size 24 copy at page_size - 17: 17
Size 19 copy at page_size - 18: 18
Size 20 copy at page_size - 18: 18
Size 21 copy at page_size - 18: 18
Size 22 copy at page_size - 18: 18
Size 23 copy at page_size - 18: 18
Size 24 copy at page_size - 18: 18
Size 25 copy at page_size - 18: 18
Size 20 copy at page_size - 19: 19
Size 21 copy at page_size - 19: 19
Size 22 copy at page_size - 19: 19
Size 23 copy at page_size - 19: 19
Size 24 copy at page_size - 19: 19
Size 25 copy at page_size - 19: 19
Size 26 copy at page_size - 19: 19

vmtest.c

#define _GNU_SOURCE
#include <assert.h>
#include <stdio.h>
#include <sys/mman.h>
#include <sys/uio.h>
#include <unistd.h>
#include <string.h>
#include <stdint.h>
int main(void) {
  size_t page_size = sysconf(_SC_PAGESIZE);
  void* p = mmap(NULL, page_size, PROT_READ | PROT_WRITE,
                           MAP_ANONYMOUS | MAP_PRIVATE, -1, 0);
  size_t first_half_size = page_size / 2;
  assert(p != MAP_FAILED);
  struct iovec in_iov[2];
  struct iovec out_iov[2];
  out_iov[0].iov_base = in_iov[0].iov_base = in_iov[1].iov_base = p;
  out_iov[0].iov_len = in_iov[0].iov_len = 1;

  for (int i = 1; i < 20; ++i) {
    for (int j = 1; j < 8; ++j) {
      // initialize the page
      memset(p, 0x11, first_half_size);
      memset(p+first_half_size, 0x22, page_size-first_half_size);
      int len = i+j;
      in_iov[1].iov_len = len;
      out_iov[1].iov_len = len;
      out_iov[1].iov_base = p + page_size - i;
      int nbytes = process_vm_readv(getpid(), out_iov, 2, in_iov, 2, 0)-1;
      // Check what actually happened
      int correct = 1;
      int more = 0;
      int torn = 0;
      for (int k = 0; k < i; ++k) {
        uint8_t *ptr = p + page_size - i + k;
        if (k < nbytes) {
          if (*ptr != 0x11)
            correct = 0;
        } else {
          if (*ptr == 0x22)
            ;
          else if (*ptr == 0x11)
            ++more;
          else
            torn = 1;
        }
      }
      printf("Size %d copy at page_size - %d: %d\n", len, i, nbytes);
      if (nbytes < i) {
        printf("\t%d bytes less than could have been written\n", i-nbytes);
      }
      if (more) {
        printf("\t%d bytes more were written than the syscall indicated\n", more);
      }
      if (torn) {
        printf("\tThe kernel did something strange\n");
      }
    }
  }

  return 0;
}