[Bugfix][LoRA][FusedMoE] Select MxFP4 Backend based on LoRA Enablement (#27487)

Signed-off-by: Varun Sundar Rabindranath <vsundarr@redhat.com>
Co-authored-by: Varun Sundar Rabindranath <vsundarr@redhat.com>

vllm/model_executor/layers/fused_moe/config.py

@@ -825,6 +825,8 @@ class FusedMoEConfig:
 
     is_act_and_mul: bool = True
 
+    is_lora_enabled: bool = False
+
     def __post_init__(self):
         if self.dp_size > 1:
             logger.debug_once(

vllm/model_executor/layers/fused_moe/layer.py

@@ -982,6 +982,7 @@ def maybe_roundup_hidden_size(
     act_dtype: torch.dtype,
     quant_config: QuantizationConfig | None,
     moe_parallel_config: FusedMoEParallelConfig,
+    is_lora_enabled: bool,
 ) -> int:
     """
     Given layer hidden size and MoE configurations, round up hidden_size
@@ -992,6 +993,9 @@ def maybe_roundup_hidden_size(
         act_dtype: Data type of the layer activations.
         quant_config: Fused MoE quantization configuration.
         moe_parallel_config: Fused MoE parallelization strategy configuration.
+        is_lora_enabled: True if the engine is enabled with LoRA. This
+            is used in the case of mxfp4 quantization in selecting the
+            MxFP4Backend.
 
     Return:
         Rounded up hidden_size if rounding up is required based on the configs.
@@ -1015,7 +1019,7 @@ def maybe_roundup_hidden_size(
             get_mxfp4_backend,
         )
 
-        current_mxfp4_backend = get_mxfp4_backend()
+        current_mxfp4_backend = get_mxfp4_backend(is_lora_enabled)
         if (
             current_mxfp4_backend == Mxfp4Backend.SM90_FI_MXFP4_BF16
             or current_mxfp4_backend == Mxfp4Backend.SM100_FI_MXFP4_MXFP8_CUTLASS
@@ -1139,7 +1143,11 @@ class FusedMoE(CustomOp):
 
         # Round up hidden size if needed.
         hidden_size = maybe_roundup_hidden_size(
-            hidden_size, moe_in_dtype, quant_config, self.moe_parallel_config
+            hidden_size,
+            moe_in_dtype,
+            quant_config,
+            self.moe_parallel_config,
+            is_lora_enabled=self.vllm_config.lora_config is not None,
         )
 
         # For smuggling this layer into the fused moe custom op
@@ -1270,8 +1278,9 @@ class FusedMoE(CustomOp):
             max_num_tokens=envs.VLLM_MOE_DP_CHUNK_SIZE,
             has_bias=has_bias,
             is_act_and_mul=is_act_and_mul,
+            is_lora_enabled=vllm_config.lora_config is not None,
         )
-        self.moe_config = moe
+        self.moe_config: FusedMoEConfig = moe
         self.moe_quant_config: FusedMoEQuantConfig | None = None
         self.quant_config = quant_config
 

vllm/model_executor/layers/quantization/mxfp4.py

@@ -73,8 +73,24 @@ class Mxfp4Backend(Enum):
     TRITON = 6
 
 
-def get_mxfp4_backend():
+def get_mxfp4_backend_with_lora() -> Mxfp4Backend:
+    """
+    Not all MXFP4 backends support LoRA. Select backends that are known to
+    have LoRA support.
+    """
+    if not current_platform.is_cuda():
+        return Mxfp4Backend.NONE
+
+    logger.info_once("[get_mxfp4_backend_with_lora] Using Marlin backend")
+    return Mxfp4Backend.MARLIN
+
+
+def get_mxfp4_backend(with_lora_support: bool) -> Mxfp4Backend:
     # Backend Selection
+
+    if with_lora_support:
+        return get_mxfp4_backend_with_lora()
+
     if current_platform.is_cuda():
         if (
             current_platform.is_device_capability(90)
@@ -183,13 +199,14 @@ class Mxfp4MoEMethod(FusedMoEMethodBase):
         super().__init__(moe)
         self.topk_indices_dtype = None
         self.moe = moe
-        self.mxfp4_backend = get_mxfp4_backend()
+        self.mxfp4_backend = get_mxfp4_backend(moe.is_lora_enabled)
         self.max_capture_size = (
             get_current_vllm_config().compilation_config.max_cudagraph_capture_size
         )
 
         assert self.mxfp4_backend != Mxfp4Backend.NONE, (
-            "No MXFP4 MoE backend (FlashInfer/Marlin/Triton) available."
+            f"get_mxfp4_backend(with_lora_support={moe.is_lora_enabled}) found"
+            "no compatible MXFP4 MoE backend (FlashInfer/Marlin/Triton)."
             "Please check your environment and try again."
         )
         self._cache_permute_indices: dict[torch.Size, torch.Tensor] = {}