[Bugfix][Attention] Fix FlashInfer MLA block size logic (vllm-project#24692)

Signed-off-by: Matthew Bonanni <mbonanni001@gmail.com>

Author: MatthewBonanni

vllm/platforms/cuda.py

@@ -146,6 +146,7 @@ def check_and_update_config(cls, vllm_config: "VllmConfig") -> None:
             # required block_size.
             use_flashmla = False
             use_cutlass_mla = False
+            use_flashinfer_mla = False
 
             if envs.VLLM_ATTENTION_BACKEND is None:
                 # Default case
@@ -164,6 +165,8 @@ def check_and_update_config(cls, vllm_config: "VllmConfig") -> None:
                 use_flashmla = (envs.VLLM_ATTENTION_BACKEND == "FLASHMLA")
                 use_cutlass_mla = (
                     envs.VLLM_ATTENTION_BACKEND == "CUTLASS_MLA")
+                use_flashinfer_mla = (
+                    envs.VLLM_ATTENTION_BACKEND == "FLASHINFER_MLA")
 
             from vllm.attention.ops.flashmla import is_flashmla_supported
             if use_flashmla and is_flashmla_supported()[0] \
@@ -176,6 +179,11 @@ def check_and_update_config(cls, vllm_config: "VllmConfig") -> None:
                 cache_config.block_size = 128
                 logger.info("Forcing kv cache block size to 128 for "
                             "CUTLASS_MLA backend.")
+            if use_flashinfer_mla and cache_config.block_size not in [32, 64]:
+                cache_config.block_size = 64
+                logger.info(
+                    "Forcing kv cache block size to 64 for FlashInferMLA "
+                    "backend.")
 
         # lazy import to avoid circular import
         from vllm.config import CUDAGraphMode
@@ -228,8 +236,9 @@ def get_attn_backend_cls(cls, selected_backend, head_size, dtype,
             use_cutlassmla = selected_backend == _Backend.CUTLASS_MLA or (
                 selected_backend is None and cls.is_device_capability(100)
                 and block_size == 128)
-            use_flashinfermla = (selected_backend == _Backend.FLASHINFER_MLA
-                                 and cls.has_device_capability(100))
+            use_flashinfermla = selected_backend == _Backend.FLASHINFER_MLA or (
+                selected_backend is None and cls.is_device_capability(100)
+                and block_size in [32, 64])
             use_flashmla = selected_backend in [
                 _Backend.FLASHMLA, _Backend.FLASHMLA_VLLM_V1
             ] or (selected_backend is None and is_flashmla_supported()[0])