df not showing all mounts

On a server, df not showing all mounts

root@server20 [~]# df -h
Filesystem            Size  Used Avail Use% Mounted on
tmpfs                 3.9G     0  3.9G   0% /dev/shm
root@server20 [~]#

This is caused by corrupt /etc/mtab

To fix

mv /etc/mtab /etc/mtab.old
cat /proc/mounts > /etc/mtab

See df