[Bugfix] detect alibi and revert to FA2 (#15231)

Signed-off-by: Travis Johnson <tsjohnso@us.ibm.com>

vllm/attention/backends/flash_attn.py

@@ -630,7 +630,8 @@ class FlashAttentionImpl(AttentionImpl):
         self.sliding_window = ((sliding_window - 1,
                                 0) if sliding_window is not None else (-1, -1))
         self.kv_cache_dtype = kv_cache_dtype
-        self.vllm_flash_attn_version = get_flash_attn_version()
+        self.vllm_flash_attn_version = get_flash_attn_version(
+            requires_alibi=self.alibi_slopes is not None)
         if (is_quantized_kv_cache(self.kv_cache_dtype)
                 and self.vllm_flash_attn_version != 3):
             raise NotImplementedError(

vllm/fa_utils.py

@@ -7,7 +7,7 @@ from vllm.logger import init_logger
 logger = init_logger(__name__)
 
 
-def get_flash_attn_version() -> Optional[int]:
+def get_flash_attn_version(requires_alibi: bool = False) -> Optional[int]:
     # import here to avoid circular dependencies
     from vllm.platforms import current_platform
     try:
@@ -28,7 +28,13 @@ def get_flash_attn_version() -> Optional[int]:
 
         # 3. fallback for unsupported combinations
         if device_capability.major == 10 and fa_version == 3:
-            logger.warning("Cannot use FA version 3 on Blackwell platform",
+            logger.warning_once(
+                "Cannot use FA version 3 on Blackwell platform "
+                "defaulting to FA version 2.")
+            fa_version = 2
+
+        if requires_alibi and fa_version == 3:
+            logger.warning_once("Cannot use FA version 3 with ALiBi, "
                                 "defaulting to FA version 2.")
             fa_version = 2