testprogram:

#include <sys/mman.h>
#include <stdio.h>
#include <fcntl.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/wait.h>

#define SIZE (512*1024*1024)

int main(int argc, char ** argv)
{
  int fd = -1, level, max_level;
  char * start, * end, * tmp;

  max_level = atoi(argv[1]);

  if ((start = mmap(0, SIZE, PROT_READ|PROT_WRITE, MAP_ANONYMOUS|MAP_PRIVATE, fd, 0)) == MAP_FAILED)
    perror("mmap"), exit(1);
  end = start + SIZE;
  
  for (tmp = start; tmp < end; tmp += 4096) {
    *tmp = 0;
  }

  for (level = 0; level < max_level; level++) {
    int pid = fork();
    if (pid < 0)
      perror("fork"), exit(1);
    for (tmp = start; tmp < end; tmp += 4096) {
      *(volatile char *)tmp;
    }
  }
  for (;;)
	if (wait(NULL) < 0)
		break;
  return 0;
}

2.4.21 based kernel:

6:

real    0m6.548s
user    0m11.340s
sys     0m10.730s

real    0m6.684s
user    0m11.120s
sys     0m11.090s

7:

real    0m12.145s
user    0m22.180s
sys     0m21.330s

real    0m11.862s
user    0m21.680s
sys     0m21.100s


2.6.5-rc2-aa1 (== 2.6 + objrmap + anon_vma):

6:

real    0m6.520s
user    0m9.926s
sys     0m11.791s

real    0m6.527s
user    0m9.823s
sys     0m11.975s

7:

real    0m12.072s
user    0m21.913s
sys     0m22.034s

real    0m12.062s
user    0m21.331s
sys     0m22.161s

2.6.2 (the last non-objrmap kernel I had installed on the test box)

6:

real    0m8.359s
user    0m10.490s
sys     0m17.840s

real    0m8.185s
user    0m9.684s
sys     0m18.078s

7:

real    0m16.150s
user    0m21.184s
sys     0m38.473s

real    0m16.082s
user    0m21.441s
sys     0m38.025s


from number 8 2.6.2 runs out of memory in fork, 2.6.5-rc1-aa1
reaches 9 without problems filling around 400m with pagetables.