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[BugFix] Cosmos-Reason1-7B Model Flash Attention requires head dim to be a multiple of 32 #29615

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ECMGit wants to merge 1 commit into from
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[BugFix] Cosmos-Reason1-7B Model Flash Attention requires head dim to be a multiple of 32 #29615

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13 changes: 13 additions & 0 deletions vllm/model_executor/models/qwen2_5_vl.py
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Original file line number Diff line number Diff line change
Expand Up @@ -362,6 +362,19 @@
self.use_upstream_fa = True
if current_platform.is_xpu():
self.use_upstream_fa = False
# Flash attention requires head_dim to be a multiple of 32
# Fall back to TORCH_SDPA if the head dimension is incompatible
if self.attn_backend in {
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@tjtanaa tjtanaa Nov 28, 2025

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@ECMGit can you skip this check on rocm for now? We have different conditions.

AttentionBackendEnum.FLASH_ATTN,
AttentionBackendEnum.ROCM_AITER_FA,
} and self.hidden_size_per_attention_head % 32 != 0:
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@Isotr0py Isotr0py Nov 27, 2025

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I think #28763 has added head_size=80 (used by cosmos-7b's ViT) support to FA. Can you try to install the latest nightly wheel?

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@ECMGit ECMGit Dec 1, 2025

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Thanks! I have verify this bug does not occur in latest nightly build, verify to close this PR and issue.

logger.warning(
f"Flash attention backend requires head_dim to be a multiple of 32, "
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@DarkLight1337 DarkLight1337 Nov 27, 2025

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Please fix pre-commit

f"but got {self.hidden_size_per_attention_head}. "
f"Falling back to TORCH_SDPA backend."

Check failure on line 374 in vllm/model_executor/models/qwen2_5_vl.py

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Ruff (G004) 

vllm/model_executor/models/qwen2_5_vl.py:372:17: G004 Logging statement uses f-string
)
self.attn_backend = AttentionBackendEnum.TORCH_SDPA
self.flash_attn_varlen_func = None
self.is_flash_attn_backend = self.attn_backend in {
AttentionBackendEnum.FLASH_ATTN,
AttentionBackendEnum.ROCM_AITER_FA,
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