Update frame table to add a pinned flag and protect those from being evicted

src/vm/frame.c

@@ -37,6 +37,8 @@ struct frame_metadata
      void *frame;  /* The kernel virtual address holding the frame. */
      void *upage; /* The user virtual address pointing to the frame. */
      struct thread *owner; /* Pointer to the thread that owns the frame. */
+     bool pinned;
+
      struct hash_elem hash_elem;  /* Tracks the position of the frame metadata
                                      within 'frame_table', whose key is the 
                                      kernel virtual address of the frame. */
@@ -50,6 +52,7 @@ hash_less_func frame_metadata_less;
 
 static struct list_elem *lru_next (struct list_elem *e);
 static struct list_elem *lru_prev (struct list_elem *e);
+static struct frame_metadata *frame_metadata_get (void *frame);
 static struct frame_metadata *get_victim (void);
 
 /* Initialize the frame system by initializing the frame (hash) table with
@@ -143,11 +146,34 @@ frame_alloc (enum palloc_flags flags, void *upage, struct thread *owner)
 
   frame_metadata->upage = upage;
   frame_metadata->owner = owner;
+  frame_metadata->pinned = false;
   lock_release (&lru_lock);
 
   return frame_metadata->frame;
 }
 
+void
+frame_pin (void *frame)
+{
+  struct frame_metadata *frame_metadata = frame_metadata_get (frame);
+  if (frame_metadata == NULL)
+    PANIC ("Attempted to pin a frame at an unallocated kernel address '%p'\n",
+           frame);
+
+  frame_metadata->pinned = true;
+}
+
+void
+frame_unpin (void *frame)
+{
+  struct frame_metadata *frame_metadata = frame_metadata_get (frame);
+  if (frame_metadata == NULL)
+    PANIC ("Attempted to unpin a frame at an unallocated kernel address '%p'\n",
+           frame);
+
+  frame_metadata->pinned = false;
+}
+
 /* Attempt to deallocate a frame for a user process by removing it from the
    frame table as well as lru_list, and freeing the underlying page
    memory & metadata struct. Panics if the frame isn't active in memory. */
@@ -201,6 +227,10 @@ get_victim (void)
         upage = frame_metadata->upage;
         e = lru_next (e);
 
+        /* Skip pinned frames */
+        if (frame_metadata->pinned)
+          continue;
+
         if (!pagedir_is_accessed (pd, upage))
           break;
 
@@ -228,14 +258,26 @@ frame_metadata_hash (const struct hash_elem *e, void *aux UNUSED)
 bool
 frame_metadata_less (const struct hash_elem *a_, const struct hash_elem *b_,
                      void *aux UNUSED)
-  {
-    struct frame_metadata *a =
-      hash_entry (a_, struct frame_metadata, hash_elem);
-    struct frame_metadata *b =
-      hash_entry (b_, struct frame_metadata, hash_elem);
+{
+  struct frame_metadata *a =
+    hash_entry (a_, struct frame_metadata, hash_elem);
+  struct frame_metadata *b =
+    hash_entry (b_, struct frame_metadata, hash_elem);
 
-    return a->frame < b->frame;
-  }
+  return a->frame < b->frame;
+}
+
+static struct frame_metadata *
+frame_metadata_get (void *frame)
+{
+  struct frame_metadata key_metadata;
+  key_metadata.frame = frame;
+
+  struct hash_elem *e = hash_find (&frame_table, &key_metadata.hash_elem);
+  if (e == NULL) return NULL;
+
+  return hash_entry (e, struct frame_metadata, hash_elem);
+}
 
 /* Returns the next recently used element after the one provided, which
    is achieved by iterating through lru_list like a circular queue

src/vm/frame.h

@@ -6,6 +6,8 @@
 
 void frame_init (void);
 void *frame_alloc (enum palloc_flags, void *, struct thread *);
+void frame_pin (void *frame);
+void frame_unpin (void *frame);
 void frame_free (void *frame);
 
 #endif /* vm/frame.h */