[Fix] Support fp32 param preservation during FSDP save and load

- Add `fp32_keys_pattern` to `HFSaveCfg` to specify params that should
  be saved in fp32 regardless of the global save dtype
- Add `_fully_shard()` to `BaseModel` which wraps `fully_shard()` and
  distributes matched params as Replicate DTensors so FSDP ignores them
- Replace all direct `fully_shard()` call sites with `self._fully_shard()`
- Add `_get_save_dtype()` to select fp32 per-param at save time
- Fix load path in `_load_same_hf_param` to skip shard-offset logic for
  Replicate DTensors (only apply for Shard-placed params)
- Fix `world_mesh` property setter bug: use `_world_mesh` directly
  instead of assigning through the read-only property
- Fix gradient reduce in `MoE.scale_and_reduce_grad` to correctly
  identify the Replicate mesh dimension via DTensor placements
- Preserve `requires_grad` when distributing params in EP and fp32 paths
- Configure `Qwen3_5_VLTextMoEConfig` with fp32 patterns for
  `linear_attn.norm.weight` and `linear_attn.A_log`
- Add `test_save_hf_with_mtp` to verify round-trip weight preservation

Author: HAOCHENYE

tests/model/test_qwen3_5.py

@@ -15,6 +15,11 @@
 from xtuner.v1.config import FSDPConfig
 from xtuner.v1.model.compose.qwen3_vl.modeling_vision import init_world_mesh
 
+import tempfile
+from pathlib import Path
+import json
+from safetensors import safe_open
+
 
 VIDEO_ROOT = os.environ["VIDEO_ROOT"]
 
@@ -216,6 +221,82 @@ def test_qwen3_5_vl_run(self, device, sp_size, tol):
         self.assertTrue(torch.allclose(loss_xtuner_image_fsdp, loss_xtuner_image, atol=tol, rtol=tol))
         self.assertTrue(torch.allclose(loss_xtuner_video_fsdp, loss_xtuner_video, atol=tol, rtol=tol))
 
+    @parametrize.parametrize(
+        "device,sp_size",
+        [
+            ("cuda", 1),
+        ],
+    )
+    def test_save_hf_with_mtp(self, device, sp_size):
+        self.create_pg(device)
+        QWEN3_VL_MOE_PATH = os.environ["QWEN3_5_MOE_PATH"]
+
+        with torch.device("meta"):
+            model_cfg = Qwen3_5_VLMoE35BA3Config(compile_cfg=False)
+            qwen3vl_model = model_cfg.build().to(torch.bfloat16)
+
+        fsdp_config = FSDPConfig(cpu_offload=False)
+        fsdp_mesh = init_world_mesh()
+        qwen3vl_model.vision_tower.fsdp_mesh = fsdp_mesh
+        qwen3vl_model.vision_tower.fsdp_config = fsdp_config
+        qwen3vl_model.fully_shard(fsdp_config=fsdp_config)
+
+        with tempfile.TemporaryDirectory() as tmpdir:
+            syncdir = [tmpdir]
+            dist.broadcast_object_list(syncdir, src=0)
+            tmpdir = Path(syncdir[0])
+            qwen3vl_model.from_hf(QWEN3_VL_MOE_PATH)
+            qwen3vl_model.save_hf(tmpdir)
+
+            origin_hf_path = Path(QWEN3_VL_MOE_PATH)
+            origin_index_path = origin_hf_path / "model.safetensors.index.json"
+            saved_index_path = tmpdir / "model.safetensors.index.json"
+
+            if dist.get_rank() == 0:
+                with open(origin_index_path, "r") as f:
+                    origin_index = json.load(f)
+                with open(saved_index_path, "r") as f:
+                    saved_index = json.load(f)
+
+                cache_save_fh: dict = {}
+
+                # Verify all original HF weights are preserved correctly
+                for key in origin_index["weight_map"].keys():
+                    if "mtp" in key:
+                        continue  # TODO: remove this after MTP is implemented
+                    origin_safetensor_name = origin_index["weight_map"][key]
+                    saved_safetensor_name = saved_index["weight_map"][key]
+
+                    origin_sf_fh_name = str(origin_hf_path / origin_safetensor_name)
+                    saved_sf_fh_name = str(tmpdir / saved_safetensor_name)
+
+                    if origin_sf_fh_name not in cache_save_fh:
+                        cache_save_fh[origin_sf_fh_name] = safe_open(origin_sf_fh_name, framework="pt")
+                    if saved_sf_fh_name not in cache_save_fh:
+                        cache_save_fh[saved_sf_fh_name] = safe_open(saved_sf_fh_name, framework="pt")
+
+                    origin_tensor = cache_save_fh[origin_sf_fh_name].get_tensor(key)
+                    saved_tensor = cache_save_fh[saved_sf_fh_name].get_tensor(key)
+
+                    self.assertTrue(torch.equal(origin_tensor, saved_tensor), f"Tensor mismatch for key: {key}")
+
+                # Verify MTP weights are present in the saved output
+                mtp_keys = [key for key in saved_index["weight_map"].keys() if key.startswith("mtp.")]
+                # TODO: remove skip after MTP is implemented
+                _ = mtp_keys
+
+                # Verify the tensor count in safetensors matches the saved index
+                safetensor_keys: list[str] = []
+                for safetensor_path in tmpdir.glob("*.safetensors"):
+                    fh = safe_open(str(safetensor_path), framework="pt")
+                    safetensor_keys.extend(fh.keys())
+                safetensor_keys.sort()
+                model_index_keys = list(saved_index["weight_map"].keys())
+                model_index_keys.sort()
+                self.assertListEqual(safetensor_keys, model_index_keys)
+
+        dist.barrier()
+
     @property
     def world_size(self) -> int:
         return int(os.getenv("XTUNER_TEST_WORLD_SIZE", "4"))

xtuner/v1/model/base.py

@@ -26,7 +26,7 @@
     MixedPrecisionPolicy,
     fully_shard,
 )
-from torch.distributed.tensor import DTensor, Placement, Shard
+from torch.distributed.tensor import DTensor, Placement, Replicate, Shard, distribute_tensor
 from torch.distributed.tensor._utils import compute_local_shape_and_global_offset
 from typing_extensions import NotRequired, Self, TypedDict, overload
 
@@ -82,6 +82,12 @@ class HFSaveCfg(PydanticBaseModel):
     worker_per_rank: Annotated[int, Parameter(group="model")] = 16
     max_save_rank: Annotated[int, Parameter(group="model")] = 16
     bucket_size: Annotated[int, Parameter(group="model")] = 1024**3 * 4
+    # TODO: `XTunerBaseModel` should also be able to specify which parameters to be trained in fp32,
+    # currently it could only be specified in HFSaveCfg
+    # Each entry is a **regex** pattern (passed to `re.search`) matched against the HF parameter name.
+    # Remember to escape literal dots, e.g. use r"model\.layers\.\d+\.weight" instead of
+    # r"model.layers.\d+.weight" to avoid unintended wildcard matches.
+    fp32_keys_pattern: Annotated[list[str] | None, Parameter(group="model")] = None
 
 
 class XTunerBaseModelConfig(PydanticBaseModel):
@@ -313,6 +319,7 @@ def fully_shard(
         """Fully shard the model parameters."""
         self.fsdp_config = fsdp_config
         self.fsdp_mesh = self._init_world_mesh()
+        self._world_mesh = self.fsdp_mesh
 
         if self.fsdp_config.requires_grad:
             for name, module in self.named_modules():
@@ -337,15 +344,79 @@ def fully_shard(
         mp_policy = MixedPrecisionPolicy(
             param_dtype=self.fsdp_config.param_dtype, reduce_dtype=fsdp_config.reduce_dtype
         )
-        fully_shard(
-            self,
+        self._fully_shard(
             mesh=self.fsdp_mesh,
             mp_policy=mp_policy,
             reshard_after_forward=fsdp_config.reshard_after_forward,
             offload_policy=CPUOffloadPolicy() if self.fsdp_config.cpu_offload else None,
         )
         return self
 
+    def _fully_shard(
+        self,
+        mesh: DeviceMesh,
+        mp_policy: MixedPrecisionPolicy,
+        reshard_after_forward: bool,
+        offload_policy: CPUOffloadPolicy | None,
+        module: nn.Module | None = None,
+    ) -> None:
+        def traverse(module):
+            for name, param in module.named_parameters(recurse=False):
+                full_name = full_param_name_mapping[id(param)]
+                full_name = self._clean_param_name(full_name)
+                hf_name_list = self.to_hf_key_list(full_name)
+
+                for hf_name in hf_name_list:
+                    if any(re.search(p, hf_name) for p in patterns):  # type: ignore
+                        if not isinstance(param, DTensor):
+                            dist_param = nn.Parameter(
+                                distribute_tensor(
+                                    param, self.world_mesh, [Replicate() for _ in range(self.world_mesh.ndim)]
+                                ),
+                                requires_grad=param.requires_grad,
+                            )
+                            module.register_parameter(name, dist_param)
+                            ignored_params.add(dist_param)
+                        else:
+                            # param is already a DTensor (e.g. distributed by
+                            # MoE._replicate_other_params on ep_mesh before _fully_shard
+                            # is called). We skip re-distributing on world_mesh and just
+                            # add it to ignored_params so FSDP leaves it alone.
+                            # ASSUMPTION: fp32 distribution always happens AFTER any
+                            # prior EP distribution, so the existing placement is correct.
+                            ignored_params.add(param)
+                        break
+
+            for child in module.children():
+                traverse(child)
+
+        # Collect the parameters of `target` that match any fp32 pattern so they can be
+        # excluded from FSDP sharding (passed as `ignored_params`).
+        #
+        # We intentionally iterate over `self.named_parameters()` rather than
+        # `target.named_parameters()` so that `name` is always relative to the root model
+        # (`self`). This matters when `target` is a sub-module (e.g. `self.embed_tokens`):
+        # `target.named_parameters()` would yield bare names like `"weight"`, which
+        # `to_hf_key_list` cannot resolve correctly. By iterating from `self` we get the
+        # full path (e.g. `"embed_tokens.weight"`) and filter to `target`'s parameters
+        # using identity comparison.
+        full_param_name_mapping = {id(param): name for name, param in self.named_parameters()}
+        ignored_params: set[nn.Parameter] = set()
+        patterns = self.config.hf_save_cfg.fp32_keys_pattern
+
+        target = module or self
+        if patterns:
+            traverse(target)
+
+        fully_shard(
+            target,
+            mesh=mesh,
+            mp_policy=mp_policy,
+            reshard_after_forward=reshard_after_forward,
+            offload_policy=offload_policy,
+            ignored_params=ignored_params if ignored_params else None,
+        )
+
     def save_hf(self, hf_dir: Path | str, save_dtype: torch.dtype = torch.bfloat16, safetensors_prefix: str = "model"):
         with profile_time_and_memory(f"[Saving HF to [{safetensors_prefix}]{hf_dir} cost]"):
             self._save_hf(hf_dir=hf_dir, save_dtype=save_dtype, safetensors_prefix=safetensors_prefix)
@@ -396,6 +467,12 @@ def device(self) -> torch.device:
             return torch.device("cpu")
         return torch.device(DEVICE)
 
+    @property
+    def world_mesh(self) -> DeviceMesh | None:
+        if not hasattr(self, "_world_mesh"):
+            self._world_mesh = self._init_world_mesh()
+        return self._world_mesh
+
     @property
     def default_compile_cfg(self) -> dict[str, TorchCompileOption]:
         return {}
@@ -670,6 +747,12 @@ def post_micro_batch_forward(self, batch_outputs: Sequence[ModelOutputs]) -> Bat
         )
         return ret
 
+    def _get_save_dtype(self, name: str, dtype: torch.dtype) -> torch.dtype:
+        patterns = self.config.hf_save_cfg.fp32_keys_pattern
+        if patterns and any(re.search(p, name) for p in patterns):
+            return torch.float32
+        return dtype
+
     def _get_shard_hf_param(
         self,
         params: list[tuple[torch.Tensor, LoadSpec]],
@@ -679,6 +762,16 @@ def _get_shard_hf_param(
     ) -> Generator[tuple[list[str], list[torch.Tensor]], None, None]:
         if not params:
             return
+
+        ignored_params, params = self._split_ignored_params(params)
+        if ignored_params:
+            name_list: list[str] = [load_spec.hf_keys[0] for _, load_spec in ignored_params]
+            hf_params = [param._local_tensor if isinstance(param, DTensor) else param for param, _ in ignored_params]
+            yield name_list, hf_params
+
+        if not params:
+            return
+
         if dtype != torch.bfloat16:
             raise NotImplementedError
 
@@ -696,7 +789,7 @@ def _get_hf_params(fsdp_tensor_list: list[tuple[torch.Tensor, LoadSpec]]) -> lis
             # Get unsharded params
             _unsharded_tensor_list = foreach_all_gather(fsdp_unsharded_tensor_list, load_spec0.group)
             unsharded_tensor_list = [
-                torch.cat([i.to(dtype) for i in tensors], dim=load_spec0.dim) for tensors in _unsharded_tensor_list
+                torch.cat(list(tensors), dim=load_spec0.dim) for tensors in _unsharded_tensor_list
             ]
             name_list = [spec.hf_keys[0] for _, spec in fsdp_tensor_list]
             unsharded_tensor_list = [
@@ -711,11 +804,11 @@ def _get_hf_params(fsdp_tensor_list: list[tuple[torch.Tensor, LoadSpec]]) -> lis
 
         safetensor_size = 0
         tensor_list: list[tuple[torch.Tensor, LoadSpec]] = []
-        name_list: list[str] = []
+        name_list = []
 
         for param, load_spec in params:
             local_tensor = param._local_tensor if isinstance(param, DTensor) else param
-            local_tensor = local_tensor.to(dtype=dtype)
+            local_tensor = local_tensor.to(dtype=self._get_save_dtype(load_spec.hf_keys[0], torch.bfloat16))
             tensor_size = self._get_tensor_size(param, dtype)
             if safetensor_size + tensor_size > bucket_size and tensor_list:
                 hf_params = _get_hf_params(tensor_list)
@@ -744,6 +837,12 @@ def _get_fused_hf_param(
         if not params:
             return
 
+        ignored_params, params = self._split_ignored_params(params)
+        if ignored_params:
+            fp32_name_list: list[str] = [load_spec.hf_keys[0] for _, load_spec in ignored_params]
+            fp32_params = [param._local_tensor if isinstance(param, DTensor) else param for param, _ in ignored_params]
+            yield fp32_name_list, fp32_params
+
         def _get_hf_params(
             fsdp_tensor_list: list[tuple[torch.Tensor, LoadSpec]],
             name_list: list[str],
@@ -867,7 +966,7 @@ def _get_hf_params(
 
         for param, load_spec in params:
             local_tensor = param._local_tensor if isinstance(param, DTensor) else param
-            local_tensor = local_tensor.bfloat16()
+            local_tensor = local_tensor.to(dtype=self._get_save_dtype(load_spec.hf_keys[0], torch.bfloat16))
             tensor_size = self._get_tensor_size(param, dtype)
             if safetensor_size + tensor_size > bucket_size and tensor_list:
                 hf_params, name_list = _get_hf_params(tensor_list, name_list)
@@ -893,6 +992,15 @@ def _get_same_hf_param(
     ) -> Generator[tuple[list[str], list[torch.Tensor]], None, None]:
         if not params:
             return
+
+        ignored_params, params = self._split_ignored_params(params)
+        if ignored_params:
+            fp32_name_list: list[str] = [load_spec.hf_keys[0] for _, load_spec in ignored_params]
+            fp32_tensor_list: list[torch.Tensor] = [
+                param._local_tensor if isinstance(param, DTensor) else param for param, _ in ignored_params
+            ]
+            yield fp32_name_list, fp32_tensor_list
+
         if bucket_size is None:
             bucket_size = self.config.hf_save_cfg.bucket_size
         safetensor_size = 0
@@ -909,7 +1017,7 @@ def _get_same_hf_param(
                 buffer_name_list.append(load_spec.hf_keys[0])
                 continue
             local_tensor = param._local_tensor if isinstance(param, DTensor) else param
-            local_tensor = local_tensor.bfloat16()
+            local_tensor = local_tensor.to(dtype=self._get_save_dtype(load_spec.hf_keys[0], torch.bfloat16))
             tensor_size = self._get_tensor_size(param, dtype)
             if safetensor_size + tensor_size > bucket_size and tensor_list:
                 if self.fsdp_mesh is not None:
@@ -953,6 +1061,21 @@ def _get_same_hf_param(
         if buffer_tensor_list:
             yield buffer_name_list, buffer_tensor_list
 
+    def _is_ignored_params(self, key: str):
+        patterns = self.config.hf_save_cfg.fp32_keys_pattern
+        if patterns is None:
+            return False
+        return any(re.search(p, key) for p in patterns)
+
+    def _split_ignored_params(
+        self, params: list[tuple[torch.Tensor, LoadSpec]]
+    ) -> tuple[list[tuple[torch.Tensor, LoadSpec]], list[tuple[torch.Tensor, LoadSpec]]]:
+        if not self.config.hf_save_cfg.fp32_keys_pattern:
+            return [], params
+        ignored_params = [(p, l) for p, l in params if self._is_ignored_params(l.hf_keys[0])]
+        remaining = [(p, l) for p, l in params if not self._is_ignored_params(l.hf_keys[0])]
+        return ignored_params, remaining
+
     # TODO: Using `xtuenr.v1.utils.misc.clean_param_name`
     def _clean_param_name(self, name: str) -> str:
         if "_checkpoint_wrapped_module." in name:
@@ -1230,7 +1353,12 @@ def _load_same_hf_param(
 
         loaded_tensor = loaded_tensor.to(local_tensor.device)
 
-        if self.fsdp_mesh is not None and isinstance(param, nn.Parameter):
+        if (
+            self.fsdp_mesh is not None
+            and isinstance(param, nn.Parameter)
+            and isinstance(param, DTensor)
+            and any(isinstance(p, Shard) for p in param.placements)
+        ):
             shape_before_fsdp = load_spec.shape
             _, _offset = compute_local_shape_and_global_offset(
                 shape_before_fsdp, self.fsdp_mesh, [Shard(self.FSDP_SHARD_DIM)]

xtuner/v1/model/compose/base.py

@@ -10,7 +10,6 @@
     CPUOffloadPolicy,
     FSDPModule,
     MixedPrecisionPolicy,
-    fully_shard,
 )
 from typing_extensions import override
 
@@ -108,8 +107,7 @@ def fully_shard(
         # Note: 非常关键，不能删除这个 assert
         assert self.fsdp_mesh is not None
 
-        fully_shard(
-            self,
+        self._fully_shard(
             mesh=self.fsdp_mesh,
             mp_policy=mp_policy,
             reshard_after_forward=fsdp_config.reshard_after_forward,

xtuner/v1/model/compose/intern_s1/modeling_intern_s1.py

@@ -83,8 +83,7 @@ def fully_shard(
         # Note: 非常关键，不能删除这个 assert
         assert self.fsdp_mesh is not None
 
-        fully_shard(
-            self,
+        self._fully_shard(
             mesh=self.fsdp_mesh,
             mp_policy=mp_policy,
             reshard_after_forward=fsdp_config.reshard_after_forward,

xtuner/v1/model/compose/intern_s1/modeling_projector.py

@@ -70,8 +70,7 @@ def fully_shard(
             for param in self.parameters():
                 param.requires_grad = False
 
-        fully_shard(
-            self,
+        self._fully_shard(
             mesh=self.fsdp_mesh,
             mp_policy=mp_policy,
             reshard_after_forward=True,