forktree worked with depth=3 on k210 platform.

os/src/mm/memory_set.rs

@@ -212,7 +212,7 @@ impl MemorySet {
     pub fn translate(&self, vpn: VirtPageNum) -> Option<PageTableEntry> {
         self.page_table.translate(vpn)
     }
-    pub fn clear(&mut self) {
+    pub fn recycle_data_pages(&mut self) {
         //*self = Self::new_bare();
         self.areas.clear();
     }