Fix: FA2 with packed training

src/transformers/modeling_flash_attention_utils.py

@@ -264,11 +264,10 @@ def _flash_attention_forward(
         )
         attn_output = pad_input(attn_output_unpad, indices_q, batch_size, query_length)
 
-    # if position_ids is provided and check not all examples (row) contain only 1 sequence, and is in pre-fill/training stage
-    # then use `flash_attn_varlen_func` to prevent cross-example attention and also allow padding free approach
-    elif (
-        position_ids is not None and not (position_ids[:, -1] == position_ids.size(1) - 1).all() and query_length != 1
-    ):
+    # If position_ids is provided and check all examples do not contain only 1 sequence, If tensor in increasing
+    # then we probably have one sequence, otherwise it is packed. Additionally check we are in pre-fill/training stage.
+    # Use `flash_attn_varlen_func` to prevent cross-example attention and also allow padding free approach
+    elif position_ids is not None and not (torch.diff(position_ids, dim=-1) >= 0).all() and query_length != 1:
         batch_size = query_states.size(0)
         query_states, key_states, value_states, indices_q, cu_seq_lens, max_seq_lens = prepare_fa2_from_position_ids(
             query_states, key_states, value_states, position_ids

src/transformers/models/phi3/modeling_phi3.py

@@ -540,7 +540,7 @@ def forward(
             max(kv_seq_len, position_ids[:, -1].max().item() + 1) if position_ids is not None else kv_seq_len
         )
 
-        cos, sin = self.rotary_emb(value_states, seq_len=rotary_seq_len)
+        cos, sin = self.rotary_emb(value_states, seq_len=rotary_seq_len, position_ids=position_ids)
 
         query_states, key_states = apply_rotary_pos_emb(query_states, key_states, cos, sin, position_ids)
 

tests/test_modeling_common.py

@@ -4284,6 +4284,63 @@ def test_flash_attn_2_generate_use_cache(self):
                     use_cache=True,
                 )
 
+    @require_flash_attn
+    @require_torch_gpu
+    @mark.flash_attn_test
+    @slow
+    def test_flash_attn_2_generate_reuse_cache(self):
+        if not self.has_attentions:
+            self.skipTest(reason="Model architecture does not support attentions")
+
+        max_new_tokens = 2
+
+        for model_class in self.all_generative_model_classes:
+            if not model_class._supports_flash_attn_2:
+                self.skipTest(f"{model_class.__name__} does not support Flash Attention 2")
+
+            config, inputs_dict = self.model_tester.prepare_config_and_inputs_for_common()
+
+            dummy_input = inputs_dict[model_class.main_input_name]
+            if dummy_input.dtype in [torch.float32, torch.bfloat16]:
+                dummy_input = dummy_input.to(torch.float16)
+
+            # make sure that all models have enough positions for generation
+            if hasattr(config, "max_position_embeddings"):
+                config.max_position_embeddings = dummy_input.shape[1] * 2 + max_new_tokens * 2 + 1
+
+            model = model_class(config)
+
+            with tempfile.TemporaryDirectory() as tmpdirname:
+                model.save_pretrained(tmpdirname)
+
+                model = model_class.from_pretrained(
+                    tmpdirname,
+                    torch_dtype=torch.float16,
+                    attn_implementation="flash_attention_2",
+                    low_cpu_mem_usage=True,
+                ).to(torch_device)
+
+                # run generate once to get filled cache
+                output = model.generate(
+                    dummy_input,
+                    max_new_tokens=max_new_tokens,
+                    do_sample=False,
+                    use_cache=True,
+                    return_dict_in_generate=True,
+                )
+                past_key_values = output.past_key_values
+
+                # Try to continue generation from where we left, given that we have more than 1 new token to process
+                # e.g. this can happen in speculative decoding when feeding candidate tokens back to target model
+                dummy_input_updated = torch.cat([dummy_input, output.sequences], dim=-1)
+                _ = model.generate(
+                    dummy_input_updated,
+                    max_new_tokens=max_new_tokens,
+                    do_sample=False,
+                    use_cache=True,
+                    past_key_values=past_key_values,
+                )
+
     @require_flash_attn
     @require_torch_gpu
     @require_bitsandbytes