2.4.18 no timestamp update on modified mmapped files

[Keith Owens <kaos@ocs.com.au>]

fd = open("foo", O_RDWR);
map = mmap(NULL, size, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
... modify the mapped pages ...
munmap(map, size);
close(fd);

The timestamp on foo is not updated, even though the contents have
changed.  Adding msync(map, size, MS_[A]SYNC) before munmap makes no
difference.  2.4.19-pre10 has no obvious fixes for this problem.

I was tearing my hair out wondering why some files were not being
rsynced.  No change on size or timestamp tells rsync that the file is
"unchanged".  I had to add a dummy write(map, fd, 1) to force a
timestamp update.