[Core] Allow verified copy-based KV connectors with expandable segments

[chriswritescode-dev]

Summary

Backports vLLM PR vllm-project#48243 / commit 81866d82 into the local inference lab fork.

• Adds an explicit, default-off kv_connector_supports_expandable_segments assertion.
• Allows verified copy-based connectors such as LMCacheConnectorV1 to run with PYTORCH_CUDA_ALLOC_CONF=expandable_segments:True.
• Preserves the existing rejection for connectors that pin or register KV memory.
• Documents the corruption/RDMA risk of opting in an incompatible connector.

Scope

This PR intentionally contains only the vLLM-owned configuration change. The DCP gather/scatter, heterogeneous KV packing, DCP-local token validation, and LMCache allocator logging changes belong to the external LMCache package and are not duplicated in vLLM.

Duplicate-work check

No open PR in local-inference-lab/vllm covers this change. The corresponding upstream work remains open as vllm-project#48243; this PR is a direct backport for the fork rather than a competing upstream implementation.

Validation

• .venv/bin/python -m pytest tests/v1/kv_connector/unit/test_config.py -v — 16 passed
• .venv/bin/pre-commit run --files tests/v1/kv_connector/unit/test_config.py vllm/config/kv_transfer.py vllm/config/vllm.py — all applicable hooks passed, including ruff and mypy
• .venv/bin/python -m compileall -q tests/v1/kv_connector/unit/test_config.py vllm/config/kv_transfer.py vllm/config/vllm.py — passed
• git diff main...HEAD --check — passed

Model evaluation is not applicable because this changes startup configuration validation only and does not alter model execution or outputs.

Assistance disclosure

AI assistance was used to review repository ownership, transplant the existing upstream commit, and run validation. The substantive patch is the unchanged upstream commit authored by Carlos Molina.

Summary by CodeRabbit

• New Features

  • Added an opt-in KV transfer configuration for connectors that support PyTorch’s expandable_segments memory allocator.
  • Copy-based LMCache connectors can now be used with expandable_segments:True when explicitly enabled.

• Bug Fixes

  • Valid compatible configurations are no longer rejected.
  • Improved configuration guidance explains available alternatives when compatibility is not enabled.

[github-actions]

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ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro Plus

Run ID: da3143ea-e0eb-416b-9783-7707896e59d7

📥 Commits

Reviewing files that changed from the base of the PR and between e12b91b and b8441cb.

📒 Files selected for processing (3)

• tests/v1/kv_connector/unit/test_config.py
• vllm/config/kv_transfer.py
• vllm/config/vllm.py

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📝 Walkthrough

Walkthrough

Adds an operator-declared KV connector capability for PyTorch expandable_segments, permits compatible connectors during validation, updates rejection guidance, and tests both opt-in acceptance and default rejection for LMCache connectors.

Changes

Expandable Segments KV Compatibility

Layer / File(s)	Summary
Capability declaration and compatibility gate vllm/config/kv_transfer.py, vllm/config/vllm.py	KVTransferConfig adds kv_connector_supports_expandable_segments; compatibility validation allows declared support and updates the incompatibility message.
Compatibility validation tests tests/v1/kv_connector/unit/test_config.py	Configuration helpers pass the capability flag, and LMCache tests cover opt-in acceptance and default rejection.

Estimated code review effort: 3 (Moderate) | ~20 minutes

Suggested reviewers: nicklucche

Sequence Diagram(s)

sequenceDiagram
  participant VllmConfig
  participant KVTransferConfig
  participant LMCacheConnector
  VllmConfig->>KVTransferConfig: Read connector capability flag
  KVTransferConfig->>LMCacheConnector: Identify configured connector
  VllmConfig->>LMCacheConnector: Allow or reject expandable_segments

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🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly matches the main change: allowing verified copy-based KV connectors to work with expandable segments.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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