diff --git a/Makefile b/Makefile
index 9dad6ad..01b26d2 100644
--- a/Makefile
+++ b/Makefile
@@ -70,7 +70,7 @@ USER_OBJS  		:= $(addprefix $(OBJ_DIR)/, $(patsubst %.c,%.o,$(USER_CPPS)))
 
 
 
-USER_TARGET 	:= $(OBJ_DIR)/app_naive_malloc
+USER_TARGET 	:= $(OBJ_DIR)/app_sum_sequence
 #------------------------targets------------------------
 $(OBJ_DIR):
 	@-mkdir -p $(OBJ_DIR)	
diff --git a/kernel/strap.c b/kernel/strap.c
index 8f3086b..3e2ff08 100644
--- a/kernel/strap.c
+++ b/kernel/strap.c
@@ -6,6 +6,9 @@
 #include "process.h"
 #include "strap.h"
 #include "syscall.h"
+#include "pmm.h"
+#include "vmm.h"
+#include "util/functions.h"
 
 #include "spike_interface/spike_utils.h"
 
@@ -41,6 +44,28 @@ void handle_mtimer_trap() {
 
 }
 
+//
+// the page fault handler. added @lab2_3. parameters:
+// sepc: the pc when fault happens;
+// stval: the virtual address that causes pagefault when being accessed.
+//
+void handle_user_page_fault(uint64 mcause, uint64 sepc, uint64 stval) {
+  sprint("handle_page_fault: %lx\n", stval);
+  switch (mcause) {
+    case CAUSE_STORE_PAGE_FAULT:
+      // TODO (lab2_3): implement the operations that solve the page fault to
+      // dynamically increase application stack.
+      // hint: first allocate a new physical page, and then, maps the new page to the
+      // virtual address that causes the page fault.
+      panic( "You need to implement the operations that actually handle the page fault in lab2_3.\n" );
+
+      break;
+    default:
+      sprint("unknown page fault.\n");
+      break;
+  }
+}
+
 //
 // kernel/smode_trap.S will pass control to smode_trap_handler, when a trap happens
 // in S-mode.
@@ -58,15 +83,25 @@ void smode_trap_handler(void) {
   // read_csr() and CAUSE_USER_ECALL are macros defined in kernel/riscv.h
   uint64 cause = read_csr(scause);
 
-  // we need to handle the timer trap @lab1_3.
-  if (cause == CAUSE_USER_ECALL) {
-    handle_syscall(current->trapframe);
-  } else if (cause == CAUSE_MTIMER_S_TRAP) {  //soft trap generated by timer interrupt in M mode
-    handle_mtimer_trap();
-  } else {
-    sprint("smode_trap_handler(): unexpected scause %p\n", read_csr(scause));
-    sprint("            sepc=%p stval=%p\n", read_csr(sepc), read_csr(stval));
-    panic( "unexpected exception happened.\n" );
+  // use switch-case instead of if-else, as there are many cases since lab2_3.
+  switch (cause) {
+    case CAUSE_USER_ECALL:
+      handle_syscall(current->trapframe);
+      break;
+    case CAUSE_MTIMER_S_TRAP:
+      handle_mtimer_trap();
+      break;
+    case CAUSE_STORE_PAGE_FAULT:
+    case CAUSE_LOAD_PAGE_FAULT:
+      // the address of missing page is stored in stval
+      // call handle_user_page_fault to process page faults
+      handle_user_page_fault(cause, read_csr(sepc), read_csr(stval));
+      break;
+    default:
+      sprint("smode_trap_handler(): unexpected scause %p\n", read_csr(scause));
+      sprint("            sepc=%p stval=%p\n", read_csr(sepc), read_csr(stval));
+      panic( "unexpected exception happened.\n" );
+      break;
   }
 
   // continue (come back to) the execution of current process.
diff --git a/user/app_naive_malloc.c b/user/app_naive_malloc.c
deleted file mode 100644
index adf40bf..0000000
--- a/user/app_naive_malloc.c
+++ /dev/null
@@ -1,22 +0,0 @@
-/*
- * Below is the given application for lab2_2.
- */
-
-#include "user_lib.h"
-#include "util/types.h"
-
-struct my_structure {
-  char c;
-  int n;
-};
-
-int main(void) {
-  struct my_structure* s = (struct my_structure*)naive_malloc();
-  s->c = 'a';
-  s->n = 1;
-
-  printu("s: %lx, {%c %d}\n", s, s->c, s->n);
-
-  naive_free(s);
-  exit(0);
-}
diff --git a/user/app_sum_sequence.c b/user/app_sum_sequence.c
new file mode 100644
index 0000000..809d341
--- /dev/null
+++ b/user/app_sum_sequence.c
@@ -0,0 +1,28 @@
+/*
+ * The application of lab2_3.
+ */
+
+#include "user_lib.h"
+#include "util/types.h"
+
+//
+// compute the summation of an arithmetic sequence. for a given "n", compute
+// result = n + (n-1) + (n-2) + ... + 0
+// sum_sequence() calls itself recursively till 0. The recursive call, however,
+// may consume more memory (from stack) than a physical 4KB page, leading to a page fault.
+// PKE kernel needs to improved to handle such page fault by expanding the stack.
+//
+uint64 sum_sequence(uint64 n) {
+  if (n == 0)
+    return 0;
+  else
+    return sum_sequence( n-1 ) + n;
+}
+
+int main(void) {
+  // we need a large enough "n" to trigger pagefaults in the user stack
+  uint64 n = 1000;
+
+  printu("Summation of an arithmetic sequence from 0 to %ld is: %ld \n", n, sum_sequence(1000) );
+  exit(0);
+}