Fix NIXL Mamba prefix-cache block trimming

[nealvaidya]

Trim remote Mamba block groups to the decode worker's local receive slots before descriptor expansion. This keeps local and remote NIXL descriptor counts aligned for partial prefix-cache hits on hybrid SSM/Mamba models.

Purpose

This fixes a decode-side NIXL assertion when prefix caching is enabled for hybrid SSM/Mamba models under disaggregated prefill/decode:

vllm/distributed/kv_transfer/kv_connector/v1/nixl/worker.py
assert len(local_block_descs_ids) == len(remote_block_descs_ids)

In the failing case, the decode worker can already have some prefix state in its local prefix cache, while the prefill worker's remote metadata still includes block IDs from the beginning of the prefetched prompt. That means the remote side can have leading prefix/null/alignment entries that do not have corresponding local receive slots:

local_block_ids  = [[16], [17], [18], [19], [20, 21]]
remote_block_ids = [[0, 17], [0, 19], [0, 21], [0, 23], [24, 25]]

The existing NIXL path already trimmed remote attention groups to the local suffix length for partial prefix-cache hits. A previous Mamba/HMA change skipped that trimming for Mamba groups because Mamba blocks represent full conv+SSM state rather than per-token KV. That concern is valid for slicing inside a single Mamba state, but this PR only drops unmatched leading whole Mamba block IDs before descriptor expansion. Each retained Mamba block still expands into the complete transfer unit, including x/B/C conv regions and SSM state.

The fix applies the same cardinality invariant to Mamba groups:

• each local receive slot must have one corresponding remote block;
• remote may be longer because of prefix-cache hits, so we keep the remote tail;
• if there are zero local receive slots, the remote group must become empty instead of accidentally preserving the full group via Python's [-0:] slice behavior.

Duplicate-work check: searched open PRs for nixl mamba prefix cache and NixlConnector Mamba prefix caching. The only nearby open PR was #40826, which handles heterogeneous split K/V policy support and does not address Mamba prefix-cache block trimming or this descriptor-count assertion.

This PR was prepared with AI assistance; I reviewed the changed lines and validated the repro/test results below.

Test Plan

Unit coverage:

python -m pytest tests/v1/kv_connector/unit/test_nixl_connector_hma.py -m cpu_test -q

Manual repro validation:

• model: nvidia/NVIDIA-Nemotron-3-Nano-30B-A3B-FP8
• setup: Dynamo disaggregated prefill/decode, one H100 for prefill and one H100 for decode
• connector: {"kv_connector":"NixlConnector","kv_role":"kv_both"}
• important env: VLLM_SSM_CONV_STATE_LAYOUT=DS
• prefix caching enabled
• loadgen:
  • 8 shared-prefix chat-completion requests with a long BrowseComp-style prompt
  • AIPerf chat profile with one shared 768-token prefix, concurrency 2, 1 warmup + 8 profiled requests, forced 2048 output tokens with min_tokens:2048 and ignore_eos:true

Test Result

Unit/lint:

21 passed, 1 deselected
ruff-check passed
ruff-format passed

Manual repro:

• simple shared-prefix driver completed 8/8 requests with HTTP 200
• AIPerf completed 8/8 profiled requests with 0 errors and 0 cancellations
• AIPerf run details:
  • average input length: 848.9 tokens
  • output length: 2048 tokens for every profiled request
  • average TTFT: ~209 ms
  • average request latency: ~18.0 s
• searched decode/prefill/frontend logs for AssertionError, local_block_ids, remote.block_ids, connector_prefix_cache_stats, Traceback, ERROR, and read_blocks; no NIXL assertion or block-group mismatch appeared
• /v1/models continued returning 200 after the run, confirming the disaggregated endpoints survived the workload

---

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• The test plan, such as providing test command.
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[gemini-code-assist]

Code Review

This pull request updates the _read_blocks logic in the NIXL connector to correctly handle partial prefix cache hits for Mamba groups. It ensures that remote block IDs are trimmed to match local transfer slots and includes a fix to prevent incorrect slicing when zero local blocks are present. New unit tests have been added to verify these trimming behaviors and prevent regressions. I have no feedback to provide as no review comments were submitted.

[claude]

Claude Code Review

This pull request is from a fork — automated review is disabled. A repository maintainer can comment @claude review to run a one-time review.

[nealvaidya]

Fixed in #40731