The virtual and real memory usage of mmap()'ed memory

# stat doesn't give us the real usage, only the virtual
famzah@vbox64:~$ sudo stat -L /proc/9256/map_files/7f052ea9b000-7f05aba9b000
  File: /proc/9256/map_files/7f052ea9b000-7f05aba9b000
  Size: 2097152000      Blocks: 2048000    IO Block: 4096   regular file
Device: 5h/5d   Inode: 733825      Links: 0
Access: (0777/-rwxrwxrwx)  Uid: ( 1000/  famzah)   Gid: ( 1000/  famzah)

# VSS (virtual memory size)
famzah@vbox64:~$ sudo du --apparent-size -BM -D /proc/9256/map_files/7f052ea9b000-7f05aba9b000
2000M   /proc/9256/map_files/7f052ea9b000-7f05aba9b000

# RSS (resident set size, or simply "the real usage")
famzah@vbox64:~$ sudo du -BM -D /proc/9256/map_files/7f052ea9b000-7f05aba9b000
1000M   /proc/9256/map_files/7f052ea9b000-7f05aba9b000