[Bugfix] Fix Qwen3.5 LoRA IndexError in packed_modules_mapping

vllm/lora/layers/column_parallel_linear.py

@@ -283,10 +283,9 @@ def can_replace_layer(
         packed_modules_list: list,
         model_config: PretrainedConfig | None = None,
     ) -> bool:
-        return (
-            type(source_layer) is MergedColumnParallelLinear
-            and len(packed_modules_list) == 2
-        )
+        return type(source_layer) is MergedColumnParallelLinear and len(
+            packed_modules_list
+        ) == len(source_layer.output_sizes)
 
 
 class QKVParallelLinearWithLoRA(ColumnParallelLinearWithLoRA):
@@ -613,8 +612,17 @@ def can_replace_layer(
         if len(packed_modules_list) >= 3:
             return True
 
-        # If packed_modules_list has exactly 2 items, let
-        # MergedColumnParallelLinearWithLoRA handle it
+        # Handle mismatch between packed modules count and output_sizes
+        # count. E.g. Qwen3.5 GDN layers have 2 packed modules
+        # (in_proj_qkv, in_proj_z) but 4 output_sizes — one HF weight
+        # covers multiple output slices.
+        if hasattr(source_layer, "output_sizes") and len(packed_modules_list) != len(
+            source_layer.output_sizes
+        ):
+            return True
+
+        # If packed_modules_list has exactly 2 items matching output_sizes,
+        # let MergedColumnParallelLinearWithLoRA handle it
         if len(packed_modules_list) == 2:
             return False
 
@@ -633,10 +641,19 @@ def set_lora(
         lora_a: torch.Tensor | list[torch.Tensor],
         lora_b: torch.Tensor | list[torch.Tensor],
     ):
-        """Override to handle single tensor weights
-        that need to be split into slices."""
+        """Override to handle weights that need to be split into slices.
+
+        Handles three cases:
+        1. Single tensor: split lora_b by output_sizes, duplicate lora_a
+        2. List matching n_slices: pass through directly
+        3. List shorter than n_slices: expand by splitting lora_b entries
+           that cover multiple output slices (e.g., 2 packed modules
+           mapping to 4 output_sizes in Qwen3.5 GDN layers)
+        """
         self.reset_lora(index)
 
+        output_sizes = self.base_layer.output_sizes
+
         # Handle case where checkpoint has single tensor weights
         # lora_a shape: (rank, input_size) - same for all slices, duplicate it
         if isinstance(lora_a, torch.Tensor):
@@ -645,7 +662,6 @@ def set_lora(
         # lora_b shape: (total_output_size, rank) -
         # split along dim 0 based on output_sizes
         if isinstance(lora_b, torch.Tensor):
-            output_sizes = self.base_layer.output_sizes
             lora_b_list = []
             start_idx = 0
             for output_size in output_sizes:
@@ -654,5 +670,56 @@ def set_lora(
                 start_idx = end_idx
             lora_b = lora_b_list
 
+        # Handle list shorter than n_slices: one packed module may cover
+        # multiple output slices. Greedily match each lora_b entry to
+        # consecutive output_sizes by dimension, then split accordingly.
+        if isinstance(lora_a, list) and len(lora_a) < self.n_slices:
+            expanded_a: list[torch.Tensor | None] = []
+            expanded_b: list[torch.Tensor | None] = []
+            slice_idx = 0
+            for i, (a_i, b_i) in enumerate(zip(lora_a, lora_b)):
+                if b_i is None:
+                    # Figure out how many slices this None covers
+                    remaining = len(lora_a) - i - 1
+                    remaining_slices = self.n_slices - slice_idx
+                    count = remaining_slices - remaining
+                    expanded_a.extend([None] * count)
+                    expanded_b.extend([None] * count)
+                    slice_idx += count
+                else:
+                    b_dim = b_i.shape[0]
+                    # Greedily consume output_sizes until we match b_dim
+                    consumed = 0
+                    start_slice = slice_idx
+                    while slice_idx < self.n_slices and consumed < b_dim:
+                        consumed += output_sizes[slice_idx]
+                        slice_idx += 1
+                    if consumed != b_dim:
+                        raise ValueError(
+                            f"Packed LoRA B dimension {b_dim} does not match "
+                            f"the sum of output sizes {consumed} for LoRA {i}."
+                        )
+                    num_covered = slice_idx - start_slice
+                    if num_covered == 1:
+                        expanded_a.append(a_i)
+                        expanded_b.append(b_i)
+                    else:
+                        # Split lora_b and duplicate lora_a
+                        split_start = 0
+                        for j in range(start_slice, slice_idx):
+                            sz = output_sizes[j]
+                            expanded_a.append(a_i)
+                            expanded_b.append(b_i[split_start : split_start + sz, :])
+                            split_start += sz
+
+            # Pad remaining slices with None (e.g. dummy LoRA warmup
+            # where per-slice dimensions don't span multiple output_sizes)
+            while len(expanded_a) < self.n_slices:
+                expanded_a.append(None)
+                expanded_b.append(None)
+
+            lora_a = expanded_a
+            lora_b = expanded_b
+
         # Now call parent's set_lora which expects lists
         super().set_lora(index, lora_a, lora_b)