delete file entry and free space

["Anthony Chee" <anthony.chee@polyu.edu.hk>]

I am doing kernel module work on file system. The module holds two variable
dir and dentry, and pass to myunlink function, which is below.

int myunlink(struct inode *dir, struct dentry *dentry) {

    int error = 0;

 down(&dir->i_sem);
 down(&dir->i_zombie);

 if (dir->i_op && dir->i_op->unlink) {
  if (d_mountpoint(dentry))
   error = -EBUSY;
  else {
   lock_kernel();
   error = dir->i_op->unlink(dir, dentry);
   unlock_kernel();

   if (!error) {
    d_delete(dentry);
   }
  }
 }

 up(&dir->i_zombie);
 if (!error)
  inode_dir_notify(dir, DN_DELETE);

 dput(dentry);
 up(&dir->i_sem);
 return error;
 }

The myunlink function is a merge from sys_unlink() and vfs_unlink(), and
remove some permission and quota syntax, as I need to force to delete the
entry. After passing the variables to the function, the entry is disappeared
while using "ls", but I found that it still occupy disk space by using
command "df". How can I also free the disk space. My target is on releasing
the disk space, and I don't care any permission or others. Or any other
method on releasing the disk space by using dir and dentry? Thanks.