Fix: Avoid closing the file after loading segments to be able to lazily load from file without opening, w/ G

src/userprog/exception.c

@@ -13,8 +13,7 @@ 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);
+static bool try_fetch_page (void *upage, bool write);
 
 /* Registers handlers for interrupts that can be caused by user
    programs.
@@ -150,9 +149,15 @@ page_fault (struct intr_frame *f)
   write = (f->error_code & PF_W) != 0;
   user = (f->error_code & PF_U) != 0;
 
+  /* If the fault address is in a user page that is not present, then it might
+     just need to be lazily loaded. So, we check our SPT to see if the page
+     is expected to have data loaded in memory. */
   void *upage = pg_round_down (fault_addr);
-  if (try_fetch_page (upage, not_present, write, user))
-    return;
+  if (not_present && is_user_vaddr (upage) && upage != NULL)
+    {
+      if (try_fetch_page (upage, write))
+        return;
+    }
 
   /* To implement virtual memory, delete the rest of the function
      body, and replace it with code that brings in the page to
@@ -166,11 +171,10 @@ page_fault (struct intr_frame *f)
 }
 
 static bool
-try_fetch_page (void *upage, bool not_present, bool write, bool user)
+try_fetch_page (void *upage, bool write)
 {
-  if (!not_present || !is_user_vaddr (upage) || upage == NULL)
-    return false;
-
+  /* Check if the page is in the supplemental page table. That is, it is a page
+     that is expected to be in memory. */
   struct page_entry *page = page_get (upage);
   if (page == NULL)
     return false;
@@ -182,6 +186,7 @@ try_fetch_page (void *upage, bool not_present, bool write, bool user)
       return true;
     }
 
+  /* Load the page into memory based on the type of data it is expecting. */
   switch (page->type) {
     case PAGE_EXECUTABLE:
       return page_load (page, write);