Special treat VM Space clearing

ostd/src/mm/page_table/mod.rs

@@ -92,6 +92,26 @@ impl PageTable<UserMode> {
             self.root.activate();
         }
     }
+
+    /// Clear the page table.
+    ///
+    /// # Safety
+    ///
+    /// The caller must ensure that:
+    ///  1. No other cursors are accessing the page table.
+    ///  2. No other CPUs activates the page table.
+    pub(in crate::mm) unsafe fn clear(&self) {
+        let mut root_node = self.root.clone_shallow().lock();
+        const NR_PTES_PER_NODE: usize = nr_subpage_per_huge::<PagingConsts>();
+        for i in 0..NR_PTES_PER_NODE / 2 {
+            let root_entry = root_node.entry(i);
+            if !root_entry.is_none() {
+                let old = root_entry.replace(Child::None);
+                // Since no others are accessing the old child, dropping it is fine.
+                drop(old);
+            }
+        }
+    }
 }
 
 impl PageTable<KernelMode> {