diff --git a/ChangeLog b/ChangeLog
index 3f0f4f3bc8..2370e92d7b 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,11 @@
+Wed Jul  6 21:24:53 2011  KOSAKI Motohiro  <kosaki.motohiro@gmail.com>
+
+	* cont.c (fiber_machine_stack_alloc): fix mprotect misuse. A stack
+	  guard page should have PROT_NONE.
+	* cont.c (fiber_initialize_machine_stack_context):
+	  th->machine_stack_maxsize shouldn't be included guard pages size.
+	  [Bug #4983][ruby-dev:44043]
+
 Wed Jul  6 21:23:38 2011  KOSAKI Motohiro  <kosaki.motohiro@gmail.com>
 
 	* cont.c (fiber_machine_stack_alloc): use MAP_STACK if it's provided.
diff --git a/cont.c b/cont.c
index e46f52ccf0..beaa5c8e5f 100644
--- a/cont.c
+++ b/cont.c
@@ -543,12 +543,15 @@ fiber_machine_stack_alloc(size_t size)
     else {
 	void *page;
 	STACK_GROW_DIR_DETECTION;
+
 	ptr = mmap(NULL, size, PROT_READ | PROT_WRITE, FIBER_STACK_FLAGS, -1, 0);
 	if (ptr == MAP_FAILED) {
 	    rb_raise(rb_eFiberError, "can't alloc machine stack to fiber");
 	}
+
+	/* guard page setup */
 	page = ptr + STACK_DIR_UPPER((size - RB_PAGE_SIZE) / sizeof(VALUE), 0);
-	if (mprotect(page, RB_PAGE_SIZE, PROT_READ | PROT_WRITE) < 0) {
+	if (mprotect(page, RB_PAGE_SIZE, PROT_NONE) < 0) {
 	    rb_raise(rb_eFiberError, "mprotect failed");
 	}
     }
@@ -572,6 +575,7 @@ fiber_initialize_machine_stack_context(rb_fiber_t *fib, size_t size)
 	    rb_raise(rb_eFiberError, "can't create fiber");
 	}
     }
+    sth->machine_stack_maxsize = size;
 #else /* not WIN32 */
     ucontext_t *context = &fib->context;
     VALUE *ptr;
@@ -584,9 +588,8 @@ fiber_initialize_machine_stack_context(rb_fiber_t *fib, size_t size)
     context->uc_stack.ss_size = size;
     makecontext(context, rb_fiber_start, 0);
     sth->machine_stack_start = ptr + STACK_DIR_UPPER(0, size / sizeof(VALUE));
+    sth->machine_stack_maxsize = size - RB_PAGE_SIZE;
 #endif
-
-    sth->machine_stack_maxsize = size;
 #ifdef __ia64
     sth->machine_register_stack_maxsize = sth->machine_stack_maxsize;
 #endif