[megatron] fix: add megatron checkpoint patch

tests/special_sanity/check_device_api_usage.py

@@ -48,6 +48,7 @@
     "verl/workers/rollout/trtllm_rollout/trtllm_async_server.py",  # appear in config.cudagraph_capture_sizes
     "verl/workers/rollout/replica.py",  # appear in default device_name
     "verl/checkpoint_engine",  # checkpoint engine backend are device specific
+    "verl/models/mcore/patch.py",  # checkpoint patch only on cuda
 ]
 
 # directory or file path must contain keyword "nccl"

verl/models/mcore/patch.py

@@ -362,3 +362,67 @@ def get_tensor_model_parallel_group_if_none(tp_group, is_expert=False, check_ini
             return tp_group
 
         megatron.core.utils.get_tensor_model_parallel_group_if_none = get_tensor_model_parallel_group_if_none
+
+
+# When using checkpoint + MoE models (like Qwen3-30B-A3B and Qwen3-VL-30B-A3B),
+# input tensors and their grads will stay in gpu memory after forward_backward completes.
+def apply_patch_checkpoint():
+    import megatron.core.tensor_parallel.random as rd
+    import torch
+
+    _fork_rng = rd._fork_rng
+    _set_all_rng_states = rd._set_all_rng_states
+    detach_variable = rd.detach_variable
+    gather_split_1d_tensor = rd.gather_split_1d_tensor
+    safely_set_viewless_tensor_data = rd.safely_set_viewless_tensor_data
+
+    @staticmethod
+    def patch_backward(ctx, *args):
+        """Backward pass."""
+        if not torch.autograd._is_checkpoint_valid():
+            raise RuntimeError("Checkpointing is not compatible with .grad(), please use .backward() if possible")
+        inputs = ctx.saved_tensors
+        if ctx.distribute_saved_activations:
+            safely_set_viewless_tensor_data(inputs[0], gather_split_1d_tensor(inputs[0].data).view(ctx.input_0_shape))
+
+        with _fork_rng():
+            # Set the states to what it used to be before the forward pass.
+            _set_all_rng_states(*ctx.rng_states)
+
+            # Compute the forward pass.
+            detached_inputs = detach_variable(inputs)
+
+            with torch.enable_grad():
+                outputs = ctx.run_function(*detached_inputs)
+
+        if isinstance(outputs, torch.Tensor):
+            outputs = (outputs,)
+
+        # filter out non tensor outputs for backward pass
+        outputs, args = zip(
+            *filter(lambda x: torch.is_tensor(x[0]) and x[0].requires_grad, zip(outputs, args, strict=False)),
+            strict=False,
+        )
+        torch.autograd.backward(outputs, args)
+        # Clone grads to return
+        grads = tuple(
+            inp.grad.clone()
+            if isinstance(inp, torch.Tensor) and inp.grad is not None
+            else inp.grad
+            if isinstance(inp, torch.Tensor)
+            else inp
+            for inp in detached_inputs
+        )
+        cur_stream = torch.cuda.current_stream()
+        # Release original input and grad tensors
+        for t in detached_inputs:
+            if isinstance(t, torch.Tensor) and t.requires_grad:
+                t.record_stream(cur_stream)
+                t.untyped_storage().resize_(0)
+                if t.grad is not None:
+                    t.grad.record_stream(cur_stream)
+                    t.grad.untyped_storage().resize_(0)
+        # ctx.saved_tensors = None
+        return (None, None) + grads
+
+    rd.CheckpointFunction.backward = patch_backward

verl/workers/engine/megatron/transformer_impl.py

@@ -98,6 +98,13 @@ def __init__(
         logger.info(f"enable_routing_replay in MegatronEngine: {self.enable_routing_replay}")
         if self.enable_routing_replay:
             apply_router_replay_patch()
+        # Apply checkpoint patch for MoE models
+        from verl.utils.device import is_cuda_available
+
+        if is_cuda_available:
+            from verl.models.mcore.patch import apply_patch_checkpoint
+
+            apply_patch_checkpoint()
 
     def _init_device_mesh(self):
         # TODO: set different parallelism for actor, critic, ref