feat: implement proper destruction of pages, including for shared pages

src/vm/frame.h

@@ -4,6 +4,12 @@
 #include "threads/thread.h"
 #include "threads/palloc.h"
 
+struct frame_owner
+{
+  struct thread *owner;  /* The thread that owns the frame. */
+  struct list_elem elem; /* List element for the list of owners. */
+};
+
 void frame_init (void);
 void *frame_alloc (enum palloc_flags, void *, struct thread *);
 void frame_free (void *frame);