refactor: document shared_file_page

src/vm/page.h

@@ -9,7 +9,6 @@ enum page_type
 {
   PAGE_FILE,
   PAGE_MMAP,
-  PAGE_EMPTY,
   PAGE_SHARED
 };
 
@@ -35,13 +34,18 @@ struct page_entry
 
 struct shared_file_page
 {
-  struct file *file;
-  void *upage;
-  void *frame;
-  size_t swap_slot;
-  int ref_count;
+  struct file *file; /* The shared file page's source file, used for indexing
+                        the table. */
+  void *upage; /* The shared page's upage which is the same across all process
+                  using it. Used for indexing the table. */
+  void *frame; /* Set to the frame address of the page when it is in memory.
+                  Set to NULL when the page is in swap. */
+  size_t swap_slot; /* Set to the swap_slot of the shared paged if it is
+                      currently in swap. Should not be used when frame is not
+                      NULL.*/
+  int ref_count; /* Number of processes that are using this shared page. */
 
-  struct hash_elem elem;
+  struct hash_elem elem; /* AN elem for the hash table. */
 };
 
 bool init_pages (struct hash *pages);