Why could my program execute non-executable data?

Why does this not fail? I mean, if I compile and execute the following code, I get

mmap 4KB as readable/writeable, but not executable. write some code there. and call there. this should fail! but does not? thats weird! 

I expected I had to use syscall sys_mprotect to mark some memory to be executable, but this works, even if it should not?

format elf64 executable use64 entry start macro echo message { mov rdx, message#.size lea rsi, [ message ] mov rdi, 1 mov rax, 1 syscall } struc db [ data ] { common . db data .size = $ - . } segment executable start: echo msg0 mov r10, 0x22 ;MMAP_Private | MMAP_Anonymous mov rdx, 0x03 ;readable | writeable mov rsi, 4096 xor rdi, rdi mov rax, 9 syscall mov qword [ buffer ], rax echo msg1 mov rcx, stub.size mov rdi, qword [ buffer ] lea rsi, [ stub ] rep movsb echo msg2 mov rdx, msg3.size lea rsi, [ msg3 ] mov rdi, 1 mov rax, 1 call qword [ buffer ] echo msg4 exit: xor rdi, rdi mov rax, 60 syscall segment readable writeable stub: syscall ret stub.size = $ - stub msg0 db 'mmap 4KB as readable/writeable, but not executable.', 10, 0 msg1 db 'write some code there.', 10, 0 msg2 db 'and call there. this should fail!', 10, 0 msg3 db 'but does not?', 10, 0 msg4 db 'thats weird!', 10, 0 buffer rq 1 

So the actual question is: How can I get this to fail? I expected, that Linux uses the NX-bit for such memory and my PC, I checked it with

grep ^flags /proc/cpuinfo | head -n1 | egrep --color=auto ' (pae|nx) ' 

and also checked it in the BIOS, allows such kind of memory protection.