umount无法拆载文件系统

自从使用了NFS后, 问题就一箩箩. 还出现了这种情况:
# umount -f /mnt/nfs
device is busy

使用-f这种暴力方法都无法拆载, 到牛B的,google了一把, 看到了一个-l的选项.

如下是man的内容:
-l Lazy unmount. Detach the filesystem from the filesystem hier‐archy now, and cleanup all references to the filesystem as soon as it is not busy anymore. (Requires kernel 2.4.11 or later.)