[kv_offload] Use Hugepages for kv_offload tiering setup

[ofer]

Purpose

Add an opt-in hugepage-backed host-memory backend for the V1 KV offload TieringOffloadingSpec CPU primary tier.

Now you can specify to use hugepages instead of the old shared memory in the configuration:

{
  "kv_connector": "OffloadingConnector",
  "kv_role": "kv_both",
  "kv_connector_extra_config": {
    "spec_name": "TieringOffloadingSpec",
    "cpu_memory_backend": "hugetlbfs",
    "cpu_memory_path": "/dev/hugepages/vllm",
    "cpu_hugepage_block_size": "2MB",
    "cpu_bytes_to_use": 10737418240,
    "block_size": 256,
    "eviction_policy": "lru",
    "offload_prompt_only": true,
  }
}

New kv_connector_extra_config keys introduced:

• cpu_memory_backend: default, shm, or hugetlbfs
• cpu_memory_path: directory for shared mmap files, required for hugetlbfs
• cpu_hugepage_block_size: 2MB or 1GB

The implementation adds shared-memory backend parsing/validation, maps hugetlbfs files with hugepage-aligned size while exposing only logical KV capacity, threads the selected backend through scheduler and worker SharedOffloadRegion creation, keeps creator/joiner cleanup semantics, and documents setup/verification guidance.

AI assistance was used in preparing this change.

Test Plan

• Run focused KV offload unit tests, including config parsing, shared mmap allocation, spec wiring, GPU-worker handler wiring, and tiering manager coverage.
• Run the optional real hugetlbfs smoke test with VLLM_TEST_HUGETLBFS_PATH=/mnt/huge (requires hugepages setup)
  Set up a hugepages directory:

sudo sysctl -w vm.nr_hugepages=10000

sudo mkdir -p /mnt/huge

sudo mount -t hugetlbfs nodev /mnt/huge

Set up permissions so the test user can read write and execute on the /mnt/huge directory

• Run CPU-only hardware-gating coverage for GPU transfer tests.
• Run relevant ruff hooks and documentation/example validation.

Test Result

Passed:

VLLM_TEST_HUGETLBFS_PATH=/mnt/huge .venv/bin/python -m pytest \
  tests/v1/kv_offload/cpu/test_shared_offload_region.py \
  tests/v1/kv_connector/unit/test_config.py \
  tests/v1/kv_offload/cpu/test_spec_memory_config.py \
  tests/v1/kv_offload/cpu/test_gpu_worker.py \
  tests/v1/kv_offload/tiering/test_tiering_offloading.py -q
Result: 100 passed on a host with one visible CUDA device and /mnt/huge mounted as hugetlbfs.
Passed:
.venv/bin/python -m pytest tests/v1/kv_offload/cpu/test_gpu_worker.py -q
Result: 1 passed, 24 skipped in a CPU-only environment.
Passed:
pre-commit run ruff-check --files \
  tests/v1/kv_connector/unit/test_config.py \
  tests/v1/kv_offload/cpu/test_shared_offload_region.py \
  tests/v1/kv_offload/cpu/test_gpu_worker.py \
  tests/v1/kv_offload/tiering/test_tiering_offloading.py
Also passed:
.venv/bin/python -m json.tool examples/features/kv_offloading/doca_memos/kv_transfer_config.doca_memos_hugepages.json
git diff --check origin/main...HEAD

[github-actions]

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[mergify]

Documentation preview: https://vllm--45693.org.readthedocs.build/en/45693/

[mergify]

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

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

[orozery]

Thanks @ofer !
I'm trying to understand in what cases can this feature be valuable, and to what extent.
It would be useful if you can elaborate and include performance numbers.