Timestamp of file modified through mmap are not changed in 2.6

[Xavier Roche <roche+kml2@exalead.com>]

Timestamp of file modified through mmap are not changed in 2.6 (even 
after msync()). Observations on 2.4 and 2.6 kernels:
- on 2.4, timestamps are altered a few seconds after the program exits.
- on 2.6, timestamps are never altered.

Is this behaviour a normal behaviour ?

Program example to reproduce the bug (you need to create a "test" file 
in the current directory first):

#include <sys/mman.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <unistd.h>
#include <fcntl.h>

int main(void) {
   int fd = open("test", O_RDWR);
   struct stat st;
   char* file;
   if (fd == -1) {
     printf("error opening file\n");
     return 1;
   }
   if (fstat(fd, &st) != 0) {
     printf("error fstating file\n");
     return 1;
   }

   if (st.st_size == 0) {
     printf("error empty file\n");
     return 1;
   }

   printf("Modified date before change: %u\n", st.st_mtime);

   file = mmap(NULL, st.st_size, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);

   if (file == NULL) {
     printf("error mmaping file");
     return 1;
   }

   file[0] = file[0] + 1;

   if (msync((void*) file, st.st_size, MS_SYNC) != 0) {
     printf("error syncing file");
     return 1;
   }

   if (munmap(file, st.st_size) != 0) {
     printf("error closing file");
     return 1;
   }

   if (fstat(fd, &st) != 0) {
     printf("error fstating file\n");
     return 1;
   }

   printf("Modified date after change: %u\n", st.st_mtime);

   return 0;
}