diff --git a/test/test_pallas.py b/test/test_pallas.py
index ab88bf6070c..7b8755fc71e 100644
--- a/test/test_pallas.py
+++ b/test/test_pallas.py
@@ -22,6 +22,9 @@
 
 class PallasTest(unittest.TestCase):
 
+  # This is to create a diagonal mask where only elements within the same segment
+  # can attend to each other. Since the mask is to mask out the unrelevant parts,
+  # therefore we use != instead of ==.
   def _make_attention_mask_from_segment_ids(self, q_segment_ids,
                                             kv_segment_ids):
     return q_segment_ids.view(q_segment_ids.shape[0], 1,
@@ -32,6 +35,7 @@ def _make_attention_mask_from_segment_ids(self, q_segment_ids,
   def _attention(self, q, k, v, *, attn_mask=None):
     attn_weight = q @ k.transpose(-2, -1)
     if attn_mask is not None:
+      # Masked out the unrelevant parts.
       attn_weight = attn_weight.masked_fill(attn_mask,
                                             torch.finfo(attn_weight.dtype).min)
     attn_weight = nn.functional.softmax(attn_weight, dim=-1)