[Bugfix] Keep the GLM-5.3 kpool tail and Qwen4 QSA ring out of the null block

[ivanium]

Purpose

Block id 0 is the null block. It is shared by every KV cache group and is assumed to stay zero. Since #35431, the runners mark rows that own no state with 0 in the block table, not PAD_SLOT_ID: dummy runs, CUDA graph capture and padding rows all carry 0. Two ring caches that address their pages through column 0 of the block table did not treat 0 as "no ring", so dummy runs wrote their rows into block 0:

• GLM-5.3-Flash kpool tail: _kpool_tail_slot_mapping_kernel maps every token to block_table[req][0] * kpool + pos % kpool. Its torch fallback does the same.
• Qwen4 QSA circular ring: circular_qsa_slot_mapping and _build_qsa_metadata_kernel map tokens to block_table[req, 0] and kept every block >= 0. The compressed QSA path already keeps dummy slots inert (test_qsa_compressed_metadata_keeps_dummy_slots_inert); the ring path did not.

Every scheduled request owns its ring block, so a request on the null block has none. Its tokens now map to PAD, as the generic slot mapping does. The ring writers already skip PAD slots.

This is the same pattern as #58560, which fixes the DeepSeek-V4.1 compressor ring.

Changes

• vllm/v1/attention/backends/mla/indexer.py: the kpool tail slot mapping (Triton and torch) emits PAD for requests on the null block.
• vllm/models/qwen4_exp/common/qsa_cache.py: the QSA circular ring slot mapping (torch and Triton) requires block > 0.
• Tests: the existing QSA circular metadata test, which runs the Triton builder, also checks a dummy batch (all-zero block table) and expects PAD. QSA tests that used block 0 as a real ring block now start real blocks at 1. This includes _make_block_table in the AMD pre-indexer test, which could randomly hand out block 0. For the kpool tail, the existing CPU padding test now ends with a padding request on the null block, and the existing Triton-vs-CPU test adds a mixed batch and an all-null dummy batch. Both check that the null-block request maps to PAD.

Not a duplicate

• [Bugfix][DSv4.1] Keep the compressor ring out of the null block #58560 covers only the DeepSeek-V4.1 compressor ring.
• [Bugfix][Attention] Zero the null KV block after CUDA graph capture #57158 zeroes block 0 after capture. That does not cover DP idle-rank dummy batches during serving.
• [KV Cache] GLM-5.3-Flash: use the generic packed KV layout; CircularBufferSpec tail with speculative ring slots #57169 and [KV Cache] Map kernel blocks per attention group; GLM-5.3-Flash on the generic packed layout #59297 move the kpool tail onto CircularBufferSpec / the packed layout but keep its slot kernel unchanged.
• [Bugfix] Guard slot mapping block table loads #54296 masks out-of-range block indices in the generic slot mapping kernel, which neither ring uses.

Test Plan

pytest tests/models/qwen4_exp/test_qsa_reference.py tests/models/qwen4_exp/test_qsa_pre_indexer_amd.py tests/v1/attention/test_kpool_tail_slot_mapping.py tests/v1/worker/test_gpu_kpool_tail_slot_mapping.py
pre-commit run --files <changed files>

Test Result

• Rebased on main d6fe5dca68. 102 passed, 16 skipped on GB200. The 16 skips are test_qsa_pre_indexer_amd.py, which requires ROCm.
• Control: with the QSA change reverted, the dummy-batch check fails: dummy tokens map to real slots in block 0.
• Control: with the kpool tail change reverted, the CPU padding test and the two null-block Triton cases fail.
• pre-commit passes.
• No model eval. Only tokens of requests on the null block change, and those are dummy or padding rows whose outputs are discarded. Real requests never own block 0 (BlockPool pops it as the null block at init), so their mappings and outputs are unchanged.

AI assistance (Claude Code) was used for this PR. The submitter reviewed the changes and ran the tests above.

🤖 Generated with Claude Code

[claude]

Claude Code Review

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[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @ivanium.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork

[JaredforReal]

LGTM @ivanium
nit: tests for diffs in vllm/v1/attention/backends/mla/indexer.py also needed

[ivanium]

/ci run

[github-actions]

❌ This PR is 1 commit behind upstream main. Your branch must contain every commit currently on upstream main. No new CI build was started. Merge or rebase onto the latest main, then rerun /ci run. To test this branch at your own risk, use /ci run --allow-stale.

[njhill]

/ci run

[github-actions]

✅ Triggered Buildkite CI #93683 for commit c25289d5cf19.