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(bugfix): block_size check for flex attn #41363

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vllm-bot merged 4 commits into from
May 1, 2026
+6 −1
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(bugfix): block_size check for flex attn #41363

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dbfe6be
(bugfix): flex attn not support block_size < 16
JisoLya Apr 30, 2026
ce97067
Merge branch 'main' into invalid_block_size
JisoLya Apr 30, 2026
561581a
reformat&update docs
JisoLya Apr 30, 2026
f9ec521
Merge branch 'main' into invalid_block_size
MatthewBonanni Apr 30, 2026
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2 changes: 1 addition & 1 deletion docs/design/attention_backends.md
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Original file line number Diff line number Diff line change
Expand Up @@ -174,7 +174,7 @@ Priority is **1 = highest** (tried first).
| `FLASH_ATTN` | FA3* | fp16, bf16 | `auto`, `float16`, `bfloat16`, `fp8`, `fp8_e4m3`, `fp8_e5m2` | %16 | Any | ✅ | ❌ | ✅ | All | 9.x |
| `FLASH_ATTN` | FA4* | fp16, bf16 | `auto`, `float16`, `bfloat16` | %16 | Any | ✅ | ❌ | ✅ | All | ≥10.0 |
| `FLASH_ATTN_DIFFKV` | | fp16, bf16 | `auto` | Any | Any | ❌ | ❌ | ✅ | Decoder | Any |
| `FLEX_ATTENTION` | | fp16, bf16, fp32 | `auto`, `float16`, `bfloat16` | Any | Any | ❌ | ✅ | ❌ | Decoder, Encoder Only | Any |
| `FLEX_ATTENTION` | | fp16, bf16, fp32 | `auto`, `float16`, `bfloat16` | %16 | Any | ❌ | ✅ | ❌ | Decoder, Encoder Only | Any |
| `ROCM_AITER_FA` | | fp16, bf16 | `auto`, `float16`, `bfloat16`, `fp8`, `fp8_e4m3`, `fp8_e5m2` | 16, 32 | 64, 128, 256 | ❌ | ❌ | ❌ | Decoder | N/A |
| `ROCM_AITER_UNIFIED_ATTN` | | fp16, bf16 | `auto` | %16 | Any | ✅ | ✅ | ❌ | All | N/A |
| `ROCM_ATTN` | | fp16, bf16, fp32 | `auto`, `float16`, `bfloat16`, `fp8`, `fp8_e4m3`, `fp8_e5m2` | %16 | 32, 64, 80, 96, 128, 160, 192, 224, 256 | ❌ | ✅ | ❌ | Decoder, Encoder, Encoder Only | N/A |
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5 changes: 5 additions & 0 deletions vllm/v1/attention/backends/flex_attention.py
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Expand Up @@ -35,6 +35,7 @@
AttentionMetadataBuilder,
AttentionType,
CommonAttentionMetadata,
MultipleOf,
)
from vllm.v1.kv_cache_interface import AttentionSpec, EncoderOnlyAttentionSpec

Expand Down Expand Up @@ -134,6 +135,10 @@ def use_cascade_attention(*args, **kwargs) -> bool:
def get_supported_head_sizes(cls) -> list[int]:
return []

@staticmethod
def get_supported_kernel_block_sizes() -> list[int | MultipleOf]:
return [MultipleOf(16)]


# @torch.compile(fullgraph=True, mode="reduce-overhead")
def physical_to_logical_mapping(
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