diff --git a/crate/bit-allocator/src/lib.rs b/crate/bit-allocator/src/lib.rs
index bc49dcb..02c38c2 100644
--- a/crate/bit-allocator/src/lib.rs
+++ b/crate/bit-allocator/src/lib.rs
@@ -6,6 +6,18 @@ extern crate bit_field;
 use bit_field::BitField;
 use core::ops::Range;
 
+/// Allocator of a bitmap, able to allocate / free bits.
+///
+/// CAP: the bitmap has a total of CAP bits, numbered from 0 to CAP-1 inclusively.
+///
+/// alloc: allocate a free bit.
+/// dealloc: free an allocated bit.
+///
+/// insert: mark bits in the range as allocated
+/// remove: reverse of insert
+///
+/// any: whether there are free bits remaining
+/// test: whether a specific bit is free
 pub trait BitAlloc: Default {
     const CAP: usize;
     fn alloc(&mut self) -> Option<usize>;
@@ -23,6 +35,7 @@ pub type BitAlloc1M = BitAllocCascade16<BitAlloc64K>;
 pub type BitAlloc16M = BitAllocCascade16<BitAlloc1M>;
 pub type BitAlloc256M = BitAllocCascade16<BitAlloc16M>;
 
+/// Implement the bit allocator by segment tree algorithm.
 #[derive(Default)]
 pub struct BitAllocCascade16<T: BitAlloc> {
     bitset: u16,
@@ -77,6 +90,8 @@ impl<T: BitAlloc> BitAllocCascade16<T> {
 #[derive(Default)]
 pub struct BitAlloc16(u16);
 
+/// BitAlloc16 acts as the leaf (except the leaf bits of course) nodes
+/// in the segment trees.
 impl BitAlloc for BitAlloc16 {
     const CAP: usize = 16;
 
diff --git a/crate/process/src/scheduler.rs b/crate/process/src/scheduler.rs
index f29df1f..4d8e696 100644
--- a/crate/process/src/scheduler.rs
+++ b/crate/process/src/scheduler.rs
@@ -211,4 +211,5 @@ mod stride {
 fn expand<T: Default + Clone>(vec: &mut Vec<T>, id: usize) {
     let len = vec.len();
     vec.resize(len.max(id + 1), T::default());
-}
\ No newline at end of file
+}
+
diff --git a/crate/riscv b/crate/riscv
index ed6e4c5..48dffe3 160000
--- a/crate/riscv
+++ b/crate/riscv
@@ -1 +1 @@
-Subproject commit ed6e4c5b935d9d027303da829a7508c105df3139
+Subproject commit 48dffe3f9aa6404a1bfe53de6645b53401d7499e
diff --git a/kernel/Makefile b/kernel/Makefile
index 1213feb..5e58276 100644
--- a/kernel/Makefile
+++ b/kernel/Makefile
@@ -143,6 +143,7 @@ $(kernel): kernel $(assembly_object_files) $(linker_script)
 		$(assembly_object_files) target/x86_64-blog_os/$(mode)/libucore.a
 
 kernel:
+	@mkdir build/$(arch) -p
 	@CC=$(cc) cargo xbuild $(build_args)
 
 # compile assembly files
diff --git a/kernel/src/arch/riscv32/paging.rs b/kernel/src/arch/riscv32/paging.rs
index 95ecefe..680448b 100644
--- a/kernel/src/arch/riscv32/paging.rs
+++ b/kernel/src/arch/riscv32/paging.rs
@@ -17,9 +17,11 @@ pub fn setup_page_table(frame: Frame) {
     p2.set_recursive(RECURSIVE_PAGE_PML4, frame.clone());
 
     // Set kernel identity map
-    p2[0x40].set(Frame::of_addr(PhysAddr::new(0x10000000)), EF::VALID | EF::READABLE | EF::WRITABLE);
-    p2[KERNEL_PML4].set(Frame::of_addr(PhysAddr::new((KERNEL_PML4 as u32) << 22)), EF::VALID | EF::READABLE | EF::WRITABLE | EF::EXECUTABLE);
-    p2[KERNEL_PML4 + 1].set(Frame::of_addr(PhysAddr::new((KERNEL_PML4 as u32 + 1) << 22)), EF::VALID | EF::READABLE | EF::WRITABLE | EF::EXECUTABLE);
+    // 0x10000000 ~ 1K area
+    p2.map_identity(0x40, EF::VALID | EF::READABLE | EF::WRITABLE);
+    // 0x80000000 ~ 8K area
+    p2.map_identity(KERNEL_PML4, EF::VALID | EF::READABLE | EF::WRITABLE | EF::EXECUTABLE);
+    p2.map_identity(KERNEL_PML4 + 1, EF::VALID | EF::READABLE | EF::WRITABLE | EF::EXECUTABLE);
 
     use super::riscv::register::satp;
     unsafe { satp::set(satp::Mode::Sv32, 0, frame); }
@@ -249,4 +251,4 @@ impl FrameDeallocator for FrameAllocatorForRiscv {
     fn dealloc(&mut self, frame: Frame) {
         dealloc_frame(frame.start_address().as_u32() as usize);
     }
-}
\ No newline at end of file
+}