why it didn't say that a tmpfs was mounted on /dev
 

I found that the whole /dev is of tmpfs filesystem type, while when we type mount, we didn't find a tmpfs mounted on /dev, it is mounted on /dev/shm
can anyone tell me what's going on?
Thanks.

Hi,

Please post your /etc/fstab

/dev/shm is a different tmpfs. /dev may be of type "devtmpfs", and is frequently not shown by the mount command. If you look at the output from "cat /proc/mounts" you should see it, perhaps more than once.

That's how I see the thing after I have looked up some information, mount command print the content of /etc/mtab, mtab is maintained by mount. Because the devtmpfs and rootfs devices are mounted at boot phase before kernel calls mount to mount fstab, so, they are not shown in mtab as well as mount.
when I use the stat -f command on /dev, it says the filesystem is tmpfs, while it is not. As well as /.