src/userprog/exception.c

@@ -155,11 +155,15 @@ page_fault (struct intr_frame *f)
   user = (f->error_code & PF_U) != 0;
 
 #ifdef VM
+  struct thread *t = thread_current ();
   if (user)
     {
       if (not_present)
         {
-          struct thread *t = thread_current ();
+          /* Check if the non-present user page is in the swap partition.
+             If so, swap it back into main memory, updating the PTE for
+             the faulted virtual address to point to the newly allocated
+             frame. */
           if (page_in_swap (t, fault_addr))
             {
               size_t swap_slot = page_get_swap (t, fault_addr);
@@ -171,15 +175,18 @@ page_fault (struct intr_frame *f)
               if (pagedir_set_page (t->pagedir, upage, kpage, writeable)) return;
             }
 
-          /* Handle page faults that need to be resolved by dynamic stack growth
-             by checking if this is such a fault and resolving it appropriately. */
+          /* Handle user page faults that need to be resolved by dynamic
+             stack growth by checking if this is such a fault and responding
+             accordingly. */
           if (handle_stack_fault (fault_addr, f->esp)) return;
         }
     }
   else
     { 
-      /* Allows for stack growth in kernel context, due to syscall failure */
-      if (handle_stack_fault (fault_addr, thread_current ()->curr_esp)) return;
+      /* Handle kernel page faults that need to be resolved by dynamic stack
+         growth by checking if this is such a fault and responding
+         accordingly. */
+      if (not_present && handle_stack_fault (fault_addr, t->curr_esp)) return;
 
       f->eip = (void *)f->eax;
       f->eax = 0xffffffff;