diff --git a/crate/memory/src/cow.rs b/crate/memory/src/cow.rs
index 1325d331..3f132be0 100644
--- a/crate/memory/src/cow.rs
+++ b/crate/memory/src/cow.rs
@@ -103,31 +103,39 @@ impl<T: PageTable> DerefMut for CowExt<T> {
 }
 
 /// A map contains reference count for shared frame
+///
+/// It will lazily construct the `BTreeMap`, to avoid heap alloc when heap is unavailable.
 #[derive(Default)]
-struct FrameRcMap(BTreeMap<Frame, (u16, u16)>);
+struct FrameRcMap(Option<BTreeMap<Frame, (u16, u16)>>);
 
 type Frame = usize;
 
 impl FrameRcMap {
     fn read_count(&mut self, frame: &Frame) -> u16 {
-        self.0.get(frame).unwrap_or(&(0, 0)).0
+        self.map().get(frame).unwrap_or(&(0, 0)).0
     }
     fn write_count(&mut self, frame: &Frame) -> u16 {
-        self.0.get(frame).unwrap_or(&(0, 0)).1
+        self.map().get(frame).unwrap_or(&(0, 0)).1
     }
     fn read_increase(&mut self, frame: &Frame) {
-        let (r, w) = self.0.get(&frame).unwrap_or(&(0, 0)).clone();
-        self.0.insert(frame.clone(), (r + 1, w));
+        let (r, w) = self.map().get(&frame).unwrap_or(&(0, 0)).clone();
+        self.map().insert(frame.clone(), (r + 1, w));
     }
     fn read_decrease(&mut self, frame: &Frame) {
-        self.0.get_mut(frame).unwrap().0 -= 1;
+        self.map().get_mut(frame).unwrap().0 -= 1;
     }
     fn write_increase(&mut self, frame: &Frame) {
-        let (r, w) = self.0.get(&frame).unwrap_or(&(0, 0)).clone();
-        self.0.insert(frame.clone(), (r, w + 1));
+        let (r, w) = self.map().get(&frame).unwrap_or(&(0, 0)).clone();
+        self.map().insert(frame.clone(), (r, w + 1));
     }
     fn write_decrease(&mut self, frame: &Frame) {
-        self.0.get_mut(frame).unwrap().1 -= 1;
+        self.map().get_mut(frame).unwrap().1 -= 1;
+    }
+    fn map(&mut self) -> &mut BTreeMap<Frame, (u16, u16)> {
+        if self.0.is_none() {
+            self.0 = Some(BTreeMap::new());
+        }
+        self.0.as_mut().unwrap()
     }
 }
 
diff --git a/src/memory.rs b/src/memory.rs
index 62eea2db..54735d55 100644
--- a/src/memory.rs
+++ b/src/memory.rs
@@ -4,7 +4,6 @@ use consts::MEMORY_OFFSET;
 use spin::{Mutex, MutexGuard};
 use super::HEAP_ALLOCATOR;
 use ucore_memory::{*, paging::PageTable};
-#[cfg(target_arch = "x86_64")]
 use ucore_memory::cow::CowExt;
 pub use ucore_memory::memory_set::{MemoryArea, MemoryAttr, MemorySet as MemorySet_, Stack};
 
@@ -34,44 +33,24 @@ pub fn alloc_stack() -> Stack {
     Stack { top, bottom }
 }
 
-#[cfg(target_arch = "x86_64")]
 lazy_static! {
     static ref ACTIVE_TABLE: Mutex<CowExt<ActivePageTable>> = Mutex::new(unsafe {
         CowExt::new(ActivePageTable::new())
     });
 }
 
-#[cfg(target_arch = "riscv")]
-lazy_static! {
-    static ref ACTIVE_TABLE: Mutex<ActivePageTable> = Mutex::new(unsafe {
-        ActivePageTable::new()
-    });
-}
-
 /// The only way to get active page table
-#[cfg(target_arch = "x86_64")]
 pub fn active_table() -> MutexGuard<'static, CowExt<ActivePageTable>> {
     ACTIVE_TABLE.lock()
 }
 
-#[cfg(target_arch = "riscv")]
-pub fn active_table() -> MutexGuard<'static, ActivePageTable> {
-    ACTIVE_TABLE.lock()
-}
-
 // Return true to continue, false to halt
-#[cfg(target_arch = "x86_64")]
 pub fn page_fault_handler(addr: usize) -> bool {
     // Handle copy on write
     unsafe { ACTIVE_TABLE.force_unlock(); }
     active_table().page_fault_handler(addr, || alloc_frame().unwrap())
 }
 
-#[cfg(target_arch = "riscv")]
-pub fn page_fault_handler(addr: usize) -> bool {
-    false
-}
-
 pub fn init_heap() {
     use consts::{KERNEL_HEAP_OFFSET, KERNEL_HEAP_SIZE};
     unsafe { HEAP_ALLOCATOR.lock().init(KERNEL_HEAP_OFFSET, KERNEL_HEAP_SIZE); }