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[Bugfix] [Kernel] Triton attention kernels: mask out V blocks that fall outside sliding window #30887

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Merged
Isotr0py merged 4 commits into from
Dec 19, 2025
Merged

[Bugfix] [Kernel] Triton attention kernels: mask out V blocks that fall outside sliding window #30887

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0703d40
Mask out V blocks that are out of sliding window
tdoublep Dec 17, 2025
7ce3a87
Apply same fix to 2D kernel
tdoublep Dec 18, 2025
16f6d5c
Merge branch 'main' into fix-3d-kernel-swa
tdoublep Dec 18, 2025
3de9911
Merge branch 'main' into fix-3d-kernel-swa
tdoublep Dec 18, 2025
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12 changes: 12 additions & 0 deletions vllm/attention/ops/triton_unified_attention.py
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Original file line number Diff line number Diff line change
Expand Up @@ -357,6 +357,12 @@ def kernel_unified_attention_2d(
L = L * alpha + l_j
M = m_j

if SLIDING_WINDOW:
qpos_lo = q_block_local_idx * BLOCK_Q
V = tl.where(
(context_len + qpos_lo - seq_offset[:, None]) < SLIDING_WINDOW, V, 0.0
)

# acc : (BLOCK_M, HEAD_SIZE_PADDED)
acc += tl.dot(P.to(V.dtype), V)

Expand Down Expand Up @@ -672,6 +678,12 @@ def kernel_unified_attention_3d(
L = L * alpha + l_j
M = m_j

if SLIDING_WINDOW:
qpos_lo = q_block_local_idx * BLOCK_Q
V = tl.where(
(context_len + qpos_lo - seq_offset[:, None]) < SLIDING_WINDOW, V, 0.0
)

# acc : (BLOCK_M, HEAD_SIZE_PADDED)
acc += tl.dot(P.to(V.dtype), V)

Expand Down