Implement page fault for lazy loading executables, w/ G

src/userprog/exception.c

@@ -2,14 +2,19 @@
 #include <inttypes.h>
 #include <stdio.h>
 #include "userprog/gdt.h"
+#include "userprog/pagedir.h"
 #include "threads/interrupt.h"
 #include "threads/thread.h"
+#include "threads/vaddr.h"
+#include "vm/page.h"
 
 /* Number of page faults processed. */
 static long long page_fault_cnt;
 
 static void kill (struct intr_frame *);
 static void page_fault (struct intr_frame *);
+static bool try_fetch_page (void *upage, bool not_present, bool write,
+                            bool user);
 
 /* Registers handlers for interrupts that can be caused by user
    programs.
@@ -145,6 +150,10 @@ page_fault (struct intr_frame *f)
   write = (f->error_code & PF_W) != 0;
   user = (f->error_code & PF_U) != 0;
 
+  void *upage = pg_round_down (fault_addr);
+  if (try_fetch_page (upage, not_present, write, user))
+    return;
+
   /* To implement virtual memory, delete the rest of the function
      body, and replace it with code that brings in the page to
      which fault_addr refers. */
@@ -156,3 +165,27 @@ page_fault (struct intr_frame *f)
   kill (f);
 }
 
+static bool
+try_fetch_page (void *upage, bool not_present, bool write, bool user)
+{
+  if (!not_present || !is_user_vaddr (upage) || upage == NULL)
+    return false;
+
+  struct page_entry *page = page_get (upage);
+  if (page == NULL)
+    return false;
+
+  if (write && !page->writable)
+    {
+      pagedir_set_writable(thread_current()->pagedir, upage, write);
+      page->writable = true;
+      return true;
+    }
+
+  switch (page->type) {
+    case PAGE_EXECUTABLE:
+      return page_load (page, write);
+    default:
+      return false;
+  }
+}