$ cat mmap.c
#include <stdio.h>
#include <sys/mman.h>

int
main (void)
{
  void *A;
  size_t L = 0, U, Max = 0;
  for (U = 1; ; U *= 2)
    {
      A = mmap (0, U, PROT_NONE, MAP_PRIVATE | MAP_ANONYMOUS | MAP_NORESERVE,
-1, 0);
      if (A == (void *) -1)
    break;
      else
    munmap (A, U);
    }
  while (L + 1 < U)
    {
      size_t M = L + (U - L) / 2;
      A = mmap (0, M, PROT_NONE, MAP_PRIVATE | MAP_ANONYMOUS | MAP_NORESERVE,
-1, 0);
      if (A == (void *) -1)
    U = M;
      else
    {
      Max = M;
      munmap(A, M);
      L = M;
    }
    }
  printf ("%zx\n", Max);
}
$ cc -O2 -g -Wall -std=gnu99 -g     mmap.c   -o mmap
$ ./mmap 
7f672e5ff000
$ qemu-x86_64 ./mmap
Killed