diff --git a/drivers/md/bcache/bcache.h b/drivers/md/bcache/bcache.h
index b61b83bbcfffc0104827619f2eab689ff7ececb9..96d2213f279ed39b0ea6f61836043caa63906eec 100644
--- a/drivers/md/bcache/bcache.h
+++ b/drivers/md/bcache/bcache.h
@@ -658,7 +658,11 @@ struct cache_set {
 
 	/*
 	 * A btree node on disk could have too many bsets for an iterator to fit
-	 * on the stack - have to dynamically allocate them
+	 * on the stack - have to dynamically allocate them.
+	 * bch_cache_set_alloc() will make sure the pool can allocate iterators
+	 * equipped with enough room that can host
+	 *     (sb.bucket_size / sb.block_size)
+	 * btree_iter_sets, which is more than static MAX_BSETS.
 	 */
 	mempool_t		fill_iter;
 
diff --git a/drivers/md/bcache/btree.c b/drivers/md/bcache/btree.c
index 3f4211b5cd3347329c16bac78b3fe2c04804d3ae..23cb1dc7296bf2701fa93659bfeef3b8b66621ca 100644
--- a/drivers/md/bcache/btree.c
+++ b/drivers/md/bcache/btree.c
@@ -207,6 +207,11 @@ void bch_btree_node_read_done(struct btree *b)
 	struct bset *i = btree_bset_first(b);
 	struct btree_iter *iter;
 
+	/*
+	 * c->fill_iter can allocate an iterator with more memory space
+	 * than static MAX_BSETS.
+	 * See the comment arount cache_set->fill_iter.
+	 */
 	iter = mempool_alloc(&b->c->fill_iter, GFP_NOIO);
 	iter->size = b->c->sb.bucket_size / b->c->sb.block_size;
 	iter->used = 0;