Special treat VM Space clearing

kernel/src/vm/vmar/mod.rs

@@ -339,8 +339,7 @@ impl Vmar_ {
     }
 
     fn clear_vm_space(&self) {
-        let mut cursor = self.vm_space.cursor_mut(&(0..ROOT_VMAR_CAP_ADDR)).unwrap();
-        cursor.unmap(ROOT_VMAR_CAP_ADDR);
+        self.vm_space.clear().unwrap();
     }
 
     pub fn destroy(&self, range: Range<usize>) -> Result<()> {

ostd/src/mm/page_table/cursor.rs

@@ -76,10 +76,7 @@ use super::{
 use crate::{
     mm::{
         kspace::should_map_as_tracked,
-        page::{
-            meta::{MapTrackingStatus, PageTablePageMeta},
-            DynPage, Page,
-        },
+        page::{meta::MapTrackingStatus, DynPage},
         Paddr, PageProperty, Vaddr,
     },
     task::{disable_preempt, DisabledPreemptGuard},
@@ -593,8 +590,11 @@ where
                 continue;
             }
 
-            // Go down if not applicable.
-            if cur_va % page_size::<C>(cur_level) != 0 || cur_va + page_size::<C>(cur_level) > end {
+            // Go down if not applicable or if the entry points to a child page table.
+            if cur_entry.is_node()
+                || cur_va % page_size::<C>(cur_level) != 0
+                || cur_va + page_size::<C>(cur_level) > end
+            {
                 let child = cur_entry.to_owned();
                 match child {
                     Child::PageTable(pt) => {
@@ -645,10 +645,7 @@ where
                         prop,
                     }
                 }
-                Child::PageTable(node) => PageTableItem::PageTableNode {
-                    page: Page::<PageTablePageMeta<E, C>>::from(node).into(),
-                },
-                Child::None => unreachable!(),
+                Child::PageTable(_) | Child::None => unreachable!(),
             };
         }
 

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> {

ostd/src/mm/vm_space.rs

@@ -12,7 +12,9 @@
 use core::{ops::Range, sync::atomic::Ordering};
 
 use crate::{
-    arch::mm::{current_page_table_paddr, PageTableEntry, PagingConsts},
+    arch::mm::{
+        current_page_table_paddr, tlb_flush_all_excluding_global, PageTableEntry, PagingConsts,
+    },
     cpu::{AtomicCpuSet, CpuExceptionInfo, CpuSet, PinCurrentCpu},
     cpu_local_cell,
     mm::{
@@ -64,6 +66,35 @@ impl VmSpace {
         }
     }
 
+    /// Clears the user space mappings in the page table.
+    ///
+    /// This method returns error if the page table is activated on any other
+    /// CPUs or there are any cursors alive.
+    pub fn clear(&self) -> core::result::Result<(), VmSpaceClearError> {
+        let preempt_guard = disable_preempt();
+        let _guard = self
+            .activation_lock
+            .try_write()
+            .ok_or(VmSpaceClearError::CursorsAlive)?;
+
+        let cpus = self.cpus.load();
+        let cpu = preempt_guard.current_cpu();
+        let cpus_set_is_empty = cpus.is_empty();
+        let cpus_set_is_single_self = cpus.count() == 1 && cpus.contains(cpu);
+
+        if cpus_set_is_empty || cpus_set_is_single_self {
+            // SAFETY: We have ensured that the page table is not activated on
+            // other CPUs and no cursors are alive.
+            unsafe { self.pt.clear() };
+            if cpus_set_is_single_self {
+                tlb_flush_all_excluding_global();
+            }
+            Ok(())
+        } else {
+            Err(VmSpaceClearError::PageTableActivated(cpus))
+        }
+    }
+
     /// Gets an immutable cursor in the virtual address range.
     ///
     /// The cursor behaves like a lock guard, exclusively owning a sub-tree of
@@ -195,6 +226,17 @@ impl Default for VmSpace {
     }
 }
 
+/// An error that may occur when doing [`VmSpace::clear`].
+#[derive(Debug)]
+pub enum VmSpaceClearError {
+    /// The page table is activated on other CPUs.
+    ///
+    /// The activated CPUs detected are contained in the error.
+    PageTableActivated(CpuSet),
+    /// There are still cursors alive.
+    CursorsAlive,
+}
+
 /// The cursor for querying over the VM space without modifying it.
 ///
 /// It exclusively owns a sub-tree of the page table, preventing others from