$ df Filesystem 1K-blocks Used Available Use% Mounted on /dev/sda1 19615736 18618140 1168 100% /
I googled a while and noticed many people mention it is because of some stupid backup program. But, I didn't use it.
So I focused on what I have downloaded since yesterday. Oh, I figured out, it must because of the phoronix-test-suite. I checked their doc and deleted its files under my home directory. And things became good again.
So, if you get the same trouble, just think what you have downloaded *unintentionally*.