proof that mlock doesn't prevent dirty writeback

test.c

$cat test.cc
#include <sys/mman.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <string.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

#define CHECK(x) do { if (!(x)) { perror(#x); abort(); }} while (0)

int main() {
  const size_t SZ = 100ul * 1024 * 1024;
  int fd = open("test.bin", O_CREAT | O_RDWR, 0666);
  CHECK(fd > 0);
  CHECK(ftruncate(fd, SZ) == 0);
  void* m = mmap(0, SZ, PROT_WRITE|PROT_READ, MAP_SHARED, fd, 0);
  CHECK(m);
  CHECK(mlock(m, SZ) == 0);
  memset(m, 0x42, SZ);

  for(;;) {
    char cmd[64];
    sprintf(cmd,"cat /proc/%d/smaps | grep test.bin -A8 | grep Private_; echo", getpid());
    system(cmd);
    sleep(5);
  }
}


$ make test && ./test
g++     test.cc   -o test
Private_Clean:         0 kB
Private_Dirty:    102400 kB

Private_Clean:         0 kB
Private_Dirty:    102400 kB

Private_Clean:    102400 kB
Private_Dirty:         0 kB  # Writeback happened at this point