[Bugfix] Fix TurboQuant KV cache index-out-of-bounds in Triton decode kernel

[devarakondasrikanth]

Purpose

Clamp masked-out SIMD lanes to page_idx=0 before block table pointer arithmetic. Triton's bounds checker fires on the address even when the output is masked, causing an index error on long (e.g. 32k) sequences.

Fixes issue #39998 #39998

Test Plan

Test Result

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[gemini-code-assist]

Code Review

This pull request modifies the _tq_decode_stage1 function in vllm/v1/attention/ops/triton_turboquant_decode.py to prevent out-of-bounds pointer arithmetic in Triton. It introduces a safe_page_idx that clamps masked-out lanes to index 0 before loading from the block table, ensuring the bounds checker does not trigger on masked lanes. I have no feedback to provide.

[devarakondasrikanth]

@LucasWilkinson and @MatthewBonanni i started working with vlllm and came across this bug, i implemented minimal fix to solve the issue (harmless). As this is my first contribution "pre-commit / pre-run-check (pull_request)Failing after 5s" can you help here to run ci.

[vibhavagarwal5]

A better solution is to clamp kv_offs itself at the source by just adding this

kv_offs = tl.where(kv_mask, kv_offs, split_start) 

[jhsmith409]

Tried to reproduce the OOB crash on RTX 5090 (sm_120) + vllm/vllm-openai:cu130-nightly (0.19.2rc1.dev21+g893611813), running RedHatAI/Qwen3.6-35B-A3B-NVFP4 with --kv-cache-dtype=turboquant_k8v4 on top of JartX#10 overlay.

Experiment — 8 concurrent ×31 632 prompt tokens, 256 decode tokens each, ignore_eos=true:

Image	#40074 applied	Result
treatment	yes	8/8 OK, 57% peak KV usage, 42 tok/s decode
control	no	8/8 OK, 57% peak KV usage, 42 tok/s decode

Could not reproduce #39998's index out of bounds: 0 <= tmp16 < 40960 on this config — likely needs the exact combination from that issue (Qwen3-0.6B + turboquant_4bit_nc + 4090, where Triton's JIT presumably picks a tile/block layout that exposes the masked-lane address). sm_120 with turboquant_k8v4 on a 35 B MoE apparently picks a variant that avoids the OOB path.

That said, the fix is provably correct: mask= in tl.load only silences the value, not the address arithmetic, so a masked-out kv_offs lane can still compute Block_table_ptr + bt_base + page_idx outside the tensor's bounds-checker range. Clamping via tl.where(kv_mask, page_idx, 0) before the pointer math is the standard Triton hygiene and the PR's 5-line implementation is exactly that.

+1 to merge; applies cleanly on top of #39931 as well.

(AI-assisted verification run; human submitter reviewed and ran both A/B configurations.)