Potential empty page leaks in mapped areas

[greatbridf]

in mm_list.rs:784:

fn set_mapped(&mut self, execute: bool) {
    // Writable flag is set during page fault handling while executable flag is
    // preserved across page faults, so we set executable flag now.
    let mut attr = PageAttribute::READ | PageAttribute::USER | PageAttribute::MAPPED;
    attr.set(PageAttribute::EXECUTE, execute);

    self.set(EMPTY_PAGE.clone().into_raw(), T::Attr::from(attr));
}

in mm_area.rs:153:

let map_page = |cache_page: &CachePage| {
    if !self.permission.write {
        assert!(!write, "Write fault on read-only mapping");

        *pfn = cache_page.add_mapping();
        return;
    }

The empty pages mapped here are not released. Since the empty page will reside in memory forever, this won't be a huge issue. But it may cause more issues if not handled properly.