Add data structures for shared executable files along with helper methods for them, w/ G

src/vm/page.h

@@ -23,6 +23,21 @@ struct page_entry {
   struct hash_elem elem; /* An elem for the hash table. */
 };
 
+struct shared_file_entry {
+  struct file *file;  /* Pointer to the file. */
+  struct hash pages;
+  int ref_count;
+
+  struct hash_elem elem;
+};
+
+struct shared_page_entry {
+  void *upage;
+  void *frame;
+
+  struct hash_elem elem;
+};
+
 unsigned page_hash (const struct hash_elem *e, void *aux);
 bool page_less (const struct hash_elem *a_, const struct hash_elem *b_,
                 void *aux);