[zhou-yuhan]

Overview

Address premature D-side KV block reclamation when one Mooncake P peer fails while another peer can still write to the request's destination blocks (write-after-free).

Related

• Improving MooncakeConnector #60523.
• Mooncake bootstrap and KV transfer failures can leave PD requests waiting without terminal error propagation #55870
• Report failed remote KV loads to the scheduler #50984: adds error handling at D-side but leaves this bug.
• #61006: guards stale scheduler notifications.
• #50565: suppresses cleanup-only completions.
• #57529: adds an unwired failure ledger.

These related PRs do not track outstanding peer results for a failed receive.

Motivation

Failure mechanism

A D request can receive KV slices from multiple P peers (P0, P1), if a peer (say P0) fails to transfer a request's KV:

1. P0 reports a transfer error while P1 still holds the destination addresses.
2. _handle_failed_recv() marks the request failed, reports its invalid blocks, and adds it to finished_recving_reqs without waiting for the other peers.
3. With kv_load_failure_policy=fail, the scheduler marks the request FINISHED_ERROR and consumes that completion to release its blocks.
4. P1 can subsequently write to those addresses, after the blocks have returned to the pool or been allocated to another request.

Failure notification therefore permits reclamation before all writers have finished. This is a KV-block write-after-free; the underlying registered GPU allocation remains valid, so the stale WRITE can succeed. #50984 fixed failure propagation and stranded requests; this reproducer exposes the remaining multi-peer destination-lifetime gap.

Reproduction

Configs:

• Qwen3-32B BF16, P TP2 on node 0, D TP1 on node 1, Nvidia H20. Nodes are interconnected with 200 Gbps RoCE RDMA (per rail)
• Workload: replay Alibaba bailian open-source in-production trace qwen_coder_blksz_16.jsonl with trace-replayer. Replayed for 1800s

Launch commands

# P instance
vllm serve /home/models/Qwen3-32B \
    --dtype bfloat16 --max-model-len 40960 \
    --gpu-memory-utilization 0.9 --enable-prefix-caching --block-size 16 \
    --host 0.0.0.0 \
    --tensor-parallel-size 2 --port 8100 --kv-transfer-config \
    '{"kv_connector":"MooncakeConnector","kv_role":"kv_producer","kv_load_failure_policy":"fail","kv_connector_extra_config":{"mooncake_protocol":"rdma"}}'

# D instance
vllm serve /home/models/Qwen3-32B \
    --dtype bfloat16 --max-model-len 40960 \
    --gpu-memory-utilization 0.9 --enable-prefix-caching --block-size 16 \
    --host 0.0.0.0 \
    --tensor-parallel-size 1 --port 8200 --kv-transfer-config \
    '{"kv_connector":"MooncakeConnector","kv_role":"kv_consumer","kv_load_failure_policy":"fail","kv_connector_extra_config":{"mooncake_protocol":"rdma"}}'

# vllm-router
vllm-router --vllm-pd-disaggregation --kv-connector mooncake \
    --prefill "http://$P_IP:8100" 8998 --decode "http://$D_HOST:8200" \
    --intra-node-data-parallel-size 1 --policy round_robin --disable-retries \
    --request-timeout-secs "$REQUEST_TIMEOUT_SECS" --host 0.0.0.0 --port 8000 \
    --prometheus-host 127.0.0.1 --prometheus-port 29000

# trace-replayer
trace-replayer --tokenizer /home/models/Qwen3-32B/tokenizer.json \
    --tokenizer-config /home/models/Qwen3-32B/tokenizer_config.json \
    --api openai --dataset bailian --hash-block-size 16 --dataset-path qwen_coder_blksz_16.jsonl \
    --stream --scale-factor 0.1 --ttft-slo "$REQUEST_TIMEOUT_SECS" --tpot-slo 0 \
    --threads 16 --num-producer 16 --channel-capacity 1024 \
    --endpoint http://127.0.0.1:8000/v1/chat/completions \
    --time-in-secs 1800

To trigger and observe the subtle write-after-free bug, we manually simulate P0 failed write and P1 delayed write: P0 reports err_reqs through its existing MooncakeXferResponse after a successful WRITE with 5% failure rate per request; P1 waits for a randomly sampled 0–500 ms before submitting WRITE (batch delay = maximum selected-request delay). D's failure/reclamation logic and RDMA network configuration are unchanged.

We add tracing hooks for each kv block and analyze the dumped traces offline after experimental runs, matching successful WRITE starts and target addresses to block generations, pool returns, and subsequent owners:

Requests	Count
Total; all with actual writes from both P peers	791
P0 experimental failures	33
P1 WRITE started after destination block free	21
Of those, P1 WRITE started after block reallocation	20

Solution

Each peer keeps a pending request map sharing the existing PullReqMeta (pull_metas.copy() shallow copy). Success and failure both consume that peer's entry and decrement pull_tasks_count, including success after another peer failed. Report the first failure once; publish receives completion only when the count reaches zero. Partial replies, peer-wide errors, and receive exceptions settle only pending entries.

On FINISH, requests still pending for that peer are considered to be failed and are settled, covering expired requests omitted by P (otherwise, P0 expires + P1 successes -> pull_tasks_count remains positive and D's KV blocks would never be released -- also a mem-leak bug left in the current implementation).

Bootstrap failures with no peer tasks still complete; empty pulls produce no D completion. The fix is confined to connector bookkeeping. It covers error replies followed by the other peers' results; remote quiescence after a local
socket timeout remains outside its scope.

Validation

Unit Tests

All relevant unit tests pass (with two added unit tests related to this bug)

Test commands

.venv/bin/python -m pytest -q --tb=short \
    tests/v1/kv_connector/unit/test_mooncake_connector.py \
    tests/v1/kv_connector/unit/test_mooncake_connector_hma.py \
    tests/v1/kv_connector/unit/test_mooncake_connector_hybrid_mamba.py \
    tests/v1/kv_connector/unit/test_mooncake_stats.py \
    tests/v1/kv_connector/unit/test_kv_load_failure_recovery.py \
    tests/v1/kv_connector/unit/test_output_aggregator.py \
    tests/v1/kv_connector/unit/test_error_propagation.py \
    tests/v1/kv_connector/unit/test_kv_connector_lifecycle.py

.venv/bin/python -m pytest -q --tb=short \
    tests/v1/kv_connector/unit/test_remote_prefill_lifecycle.py \
    tests/v1/kv_connector/unit/test_invalid_blocks_correctness.py

.venv/bin/python -m pre_commit run --files \
    vllm/distributed/kv_transfer/kv_connector/v1/mooncake/mooncake_connector.py \
    tests/v1/kv_connector/unit/test_mooncake_connector.py \
    tests/v1/kv_connector/unit/test_mooncake_connector_hma.py

E2E Result

The E2E run config is the same as in the Reproduction section. With fixed:

Requests	Before	After
Total; all with actual writes from both P peers	791	791
P0 experimental failures	33	39
P1 WRITE started after destination block free	21	0
Of those, after block reallocation	20	0

All 39 P0 errors reached D before P1's WRITE ended; 31 arrived before it started. Every failed receive followed all WRITE ends -> receive completion -> scheduler consumption -> block pool return. Thus the reproduced write-after-free did not recur.

---

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[claude]

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[liusy58]

@zhou-yuhan One edge case I’m wondering about: in send_kv_to_decode(), when a request has expired, P skips it without adding it to err_reqs. If P later sends FINISH, D exits the receive loop, but that request never gets its counter decremented for this peer. Could this leave pull_tasks_count above zero and prevent its KV blocks from being released? Should P include expired requests in err_reqs so D can account for them? I haven’t reproduced this end to end, so I’d like to confirm whether this path is possible.

[zhou-yuhan]

@zhou-yuhan One edge case I’m wondering about: in send_kv_to_decode(), when a request has expired, P skips it without adding it to err_reqs. If P later sends FINISH, D exits the receive loop, but that request never gets its counter decremented for this peer. Could this leave pull_tasks_count above zero and prevent its KV blocks from being released? Should P include expired requests in err_reqs so D can account for them? I haven’t reproduced this end to end, so I’d like to confirm whether this path is possible.

Thanks for catching this! After a careful code investigation I find it to be a new mem-leak bug that already exists before this PR: if P0 succeeds but P1 expires without reporting a result, pull_tasks_count stays above zero and D's blocks remain held. Previously, an explicit failure from P0 would immediately publish receive completion and allow reclamation.

This PR waits for all peer results after a failure, so an expired peer’s omitted result can now reserve D's blocks even when another peer explicitly fails. After processing a FINISH response, we should add a fallback:

if response.status == MooncakeXferResponseStatus.FINISH:
    self._handle_failed_recv(
        pull_metas,
        list(pull_metas),
        "peer finished without reporting a transfer result",
    )
    break

It settles the current peer’s pending entries (consider remaining pending requests as failures) and still waits for other peers. I will add another commit with this fallback to cover both the write-after-free and this mem-leak.

[mergify]

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

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

[zhou-yuhan]

@wangxiyuan Could you help take a review at this, maybe also ref it as robustness issue/phase in #60523?

[github-actions]

✅ @zhou-yuhan, CI is now available for this PR.

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[wangxiyuan]

The fix LGTM.

[wangxiyuan]

@ivanium Please take a look at this change. It fixes a real bug where a failed peer could release the request's D-side KV blocks while another peer was still writing into them. Thanks

[zhou-yuhan]

/ci run

[github-actions]

✅ Triggered Buildkite CI #96043 for commit f1e5576213be.

ℹ️ This PR is 41 commits behind upstream main. Merged upstream main into this branch as f1e5576213be before starting CI. Pull the branch before pushing more changes.

[mergify]

Hi @zhou-yuhan, the pre-commit checks have failed. Please run:

uv pip install pre-commit>=4.5.1
pre-commit install
pre-commit run --all-files

Then, commit the changes and push to your branch.

For future commits, pre-commit will run automatically on changed files before each commit.

[mergify]

Hi @zhou-yuhan, the pre-commit checks have failed. Please run:

uv pip install pre-commit>=4.5.1
pre-commit install
pre-commit run --all-files

Then, commit the changes and push to your branch.

For future commits, pre-commit will run automatically on changed files before each commit.

[Isotr0py]

/ci run

[github-actions]

✅ Triggered Buildkite CI #96061 for commit 5bb4da232d6e.