fix(nvfp4): make process_weights_after_loading hot-reload-safe via alias-when-same-shape (#25190)

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

python/sglang/srt/layers/quantization/modelopt_quant.py

@@ -57,7 +57,7 @@ from sglang.srt.layers.quantization.utils import (
     swizzle_blockscale,
 )
 from sglang.srt.layers.radix_attention import RadixAttention
-from sglang.srt.layers.utils import copy_or_rebind_param
+from sglang.srt.layers.utils import alias_or_bind_derived_param, copy_or_rebind_param
 from sglang.srt.utils.common import (
     is_cuda,
     is_sm120_supported,
@@ -1363,13 +1363,15 @@ class ModelOptFp4LinearMethod(LinearMethodBase):
         input_scale_2 = layer.input_scale.max().to(torch.float32)
         weight_scale_2 = layer.weight_scale_2.max().to(torch.float32)
 
+        # alpha / input_scale_inv stay as scalar Parameters. Aliasing them into
+        # the [N_partitions] source slot breaks fused-QKV linears whose
+        # downstream kernels assume scalar input scale.
         copy_or_rebind_param(
             layer, "alpha", (input_scale_2 * weight_scale_2).to(torch.float32)
         )
         copy_or_rebind_param(
             layer, "input_scale_inv", (1 / input_scale_2).to(torch.float32)
         )
-        del layer.input_scale, layer.weight_scale_2
 
         # Store original output size before any padding
         layer.output_size_per_partition = layer.weight.shape[0]
@@ -1420,10 +1422,11 @@ class ModelOptFp4LinearMethod(LinearMethodBase):
                 .view(torch.float8_e4m3fn)
             )
 
-            copy_or_rebind_param(layer, "weight_scale_interleaved", scale)
+            alias_or_bind_derived_param(
+                layer, "weight_scale", "weight_scale_interleaved", scale
+            )
             copy_or_rebind_param(layer, "weight", weight)
             layer.weights_padding_cols = weights_padding_cols
-            del layer.weight_scale
             return
 
         # Pad weights for CUTLASS/FlashInfer kernel alignment (K and N divisible by 32)
@@ -1453,8 +1456,9 @@ class ModelOptFp4LinearMethod(LinearMethodBase):
             if scale_ndim == 2
             else padded_scales.reshape(B, M_padded, K_padded)
         )
-        copy_or_rebind_param(layer, "weight_scale_interleaved", padded_scales)
-        del layer.weight_scale
+        alias_or_bind_derived_param(
+            layer, "weight_scale", "weight_scale_interleaved", padded_scales
+        )
 
     def apply(
         self,
@@ -1704,7 +1708,6 @@ class ModelOptNvFp4FusedMoEMethod(FusedMoEMethodBase):
 
         Only supports pre-quantized checkpoints with FP8 weights and scales.
         """
-
         # GEMM 1 scale processing
         if layer.moe_runner_config.is_gated:
             if layer.w13_weight_scale_2.dim() == 1:
@@ -1777,12 +1780,6 @@ class ModelOptNvFp4FusedMoEMethod(FusedMoEMethodBase):
             "w2_input_scale_quant",
             (1 / w2_input_scale).to(torch.float32),
         )
-        del layer.w13_input_scale, layer.w2_input_scale
-        # TODO: w13_weight_scale_2 / w2_weight_scale_2 are also unused by apply()
-        # after this point. flashinfer_cutedsl reads them via hasattr() but has a
-        # mathematically-equivalent fallback through w13_input_scale_quant and
-        # g1_alphas. Kept for now to avoid a silent code-path switch and possible
-        # sub-ULP precision drift; revisit once the fallback is validated.
 
         # TODO: for flashinfer always do MOE_NVFP4_DISPATCH
         layer.dispatcher.set_quant_config(
@@ -1835,7 +1832,10 @@ class ModelOptNvFp4FusedMoEMethod(FusedMoEMethodBase):
 
             # FlashInfer TRTLLM processing - handles both w13 and w2
             align_fp4_moe_weights_for_flashinfer_trtllm(layer)
-            del layer.w13_blockscale_swizzled, layer.w2_blockscale_swizzled
+            # TRTLLM doesn't read *_blockscale_swizzled; alias to free the
+            # placeholders from create_weights.
+            layer.w13_blockscale_swizzled = layer.w13_weight_scale
+            layer.w2_blockscale_swizzled = layer.w2_weight_scale
 
         else:
             # CUTLASS processing - handle w13 and w2 separately
@@ -1864,8 +1864,11 @@ class ModelOptNvFp4FusedMoEMethod(FusedMoEMethodBase):
 
             # Process w13 weights
             w13_blockscale_swizzled = swizzle_blockscale(layer.w13_weight_scale)
-            copy_or_rebind_param(
-                layer, "w13_blockscale_swizzled", w13_blockscale_swizzled
+            alias_or_bind_derived_param(
+                layer,
+                "w13_weight_scale",
+                "w13_blockscale_swizzled",
+                w13_blockscale_swizzled,
             )
 
             w13_weight = layer.w13_weight
@@ -1902,8 +1905,11 @@ class ModelOptNvFp4FusedMoEMethod(FusedMoEMethodBase):
 
             # Process w2 weights
             w2_blockscale_swizzled = swizzle_blockscale(layer.w2_weight_scale)
-            copy_or_rebind_param(
-                layer, "w2_blockscale_swizzled", w2_blockscale_swizzled
+            alias_or_bind_derived_param(
+                layer,
+                "w2_weight_scale",
+                "w2_blockscale_swizzled",
+                w2_blockscale_swizzled,
             )
 
             if self._is_cutedsl_v2_standard:
@@ -1969,7 +1975,6 @@ class ModelOptNvFp4FusedMoEMethod(FusedMoEMethodBase):
                     intermediate_size_per_partition=inter_size,  # n
                     hidden_size=hidden_size,
                 )  # k
-            del layer.w13_weight_scale, layer.w2_weight_scale
 
     @property
     def load_up_proj_weight_first(self) -> bool:

python/sglang/srt/layers/utils/common.py

@@ -71,6 +71,41 @@ def copy_or_rebind_param(
         setattr(module, name, Parameter(new_value, requires_grad=False))
 
 
+def alias_or_bind_derived_param(
+    module: torch.nn.Module,
+    source_name: str,
+    derived_name: str,
+    derived_value: torch.Tensor,
+) -> None:
+    """Bind a post-processed (derived) tensor to a derived attribute name.
+
+    When `derived_value` is broadcastable to the source Parameter's shape (and
+    dtype matches), write it broadcast-filled into the source's storage in
+    place and register `derived_name` as an alias of the source Parameter. The
+    two attribute names then share one underlying buffer, so:
+      - apply() can read via `derived_name`
+      - update_weights_from_disk can keep refilling `source_name` (the loader
+        re-runs process_weights_after_loading which re-derives in place)
+      - peak GPU memory is the source size, not source + derived.
+
+    When the shapes are not broadcast-compatible, fall back to allocating a
+    separate Parameter under `derived_name` via copy_or_rebind_param.
+    """
+    derived_value = derived_value.detach()
+    source = getattr(module, source_name, None)
+    if isinstance(source, Parameter) and source.data.dtype == derived_value.dtype:
+        try:
+            broadcast = torch.broadcast_to(derived_value, source.data.shape)
+        except RuntimeError:
+            broadcast = None
+        if broadcast is not None:
+            source.data.copy_(broadcast)
+            source.requires_grad_(False)
+            setattr(module, derived_name, source)
+            return
+    copy_or_rebind_param(module, derived_name, derived_value)
+
+
 class PPMissingLayer(torch.nn.Identity):
     # Adapted from
     # https://github.com/vllm-project/vllm/blob/18ed3132d2bfe1df9a74729457b69243955221e8/vllm/model_executor/models/utils.py#L468C1-L486C1