fix(kv_cache): domain-tag LoRA and cache_salt prefix-cache keys to prevent collision (#44701)

[Anai-Guo]

Summary

generate_block_hash_extra_keys appends LoRA names and cache_salt into the same extra_keys tuple as bare strings, with no domain separation:

lora_extra_keys: list[str] = _gen_lora_extra_hash_keys(request)   # [lora_name]
cache_salt_keys: list[str] = [request.cache_salt] if ... else []  # [cache_salt]
extra_keys = lora_extra_keys + mm_extra_keys + cache_salt_keys + prompt_embeds_keys

Because both are untagged single strings, a LoRA request with lora_name == "X" and no cache_salt produces the same first-block hash as a base-model request with cache_salt == "X" and no LoRA, for identical prompt tokens. Reported and dynamically reproduced in #44701.

This is a correctness/isolation problem:

• cache_salt exists specifically to isolate prefix-cache entries between callers. A LoRA adapter whose name equals someone's salt silently defeats that isolation.
• A base-model request can receive a prefix-cache hit on a block whose KV was computed under LoRA weights, yielding wrong outputs.

Fix

Domain-tag the two sources so their keys live in distinct namespaces and can no longer collide:

return [("lora", request.lora_request.lora_name)]
...
[("cache_salt", request.cache_salt)]

mm_extra_keys already use (identifier, offset) tuples and prompt-embeds keys are raw bytes, so all four sources are now mutually distinguishable. The change is confined to key construction; hashing logic is untouched.

Test plan

• Updated existing assertions in test_kv_cache_utils.py to the new tagged shape (("cache_salt", "salt"), ("lora", "test_lora_adapter")).
• Added test_generate_block_hash_extra_keys_lora_cache_salt_no_collision — a LoRA adapter and a base request sharing the string "COLLIDE" must now produce different extra keys.

🤖 Generated with Claude Code

[claude]

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[Anai-Guo]

Friendly note for maintainers: pre-run-check is failing only because it requires the ready/verified label or 4+ merged PRs from the author (I currently have 0 merged here). The change itself is ready for review — could a maintainer add the ready label so CI can run? Thanks!

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[nickharris808]

Independent reproduction of #44701 on different hardware at a pinned release, plus one constraint I think the patch should state explicitly.

Reproduction. vllm==0.24.0, NVIDIA L4, TinyLlama-1.1B-Chat-v1.0 with zjudai/flowertune-general-nlp-lora-tinyllama-1.1b-chat-v1.0, seed 20260725, outcome pre-registered before the run.

arm	cached tokens
first touch, LoRA request	0
LoRA self-reuse (positive control)	640
base + a distinct cache_salt (negative control)	0
base + cache_salt equal to the LoRA name	16

All four controls pass. The last row is the defect: a base-model request whose cache_salt happens to equal a registered adapter's name is served 16 cached tokens computed under the adapter's weights.

The 16 is not incidental — it is structural, and I think it is worth stating in the PR. It is exactly one block (block_size=16). extra_keys participates only in the first block's hash, so the blast radius of this collision is bounded above by one block, and every subsequent block diverges. That bound is a property of where the key enters the hash and holds regardless of prompt length. I did not find it stated anywhere upstream, and it is the sort of thing a reviewer will want before deciding severity.

Eviction is ruled out by measurement, not assumption — the engine reported 48,265 GPU blocks = 772,240 KV tokens against a workload orders of magnitude smaller, so nothing was evicted between arms.

One thing the patch does not currently say: the concatenation order is load-bearing. The order of the four lists is part of the key. Re-ordering them changes every resulting hash and silently invalidates the entire prefix cache. The fix needs to preserve order as well as add tags, and it may be worth a comment at the construction site — otherwise a later well-meaning tidy-up (sorting for determinism, say) reintroduces a much larger problem than the one being fixed here.

On the two-source scope, since it is the obvious reviewer question. Tagging only lora and cache_salt is not a partial fix — it is the complete one. mm_extra_keys are int/tuple and prompt_embeds_keys are tensor hashes, so neither can compare equal to a str under Python's own typing. Of the six cross-source pairs, exactly one is reachable, and it is the one this PR tags. Tagging all four would cost a second cache invalidation and close nothing further. (I checked this mechanically — an earlier draft of my own notes proposed the four-source version and was wrong.)

A trivial process note: the title may need a [Bugfix] or [Core] prefix to match the contributing guide's convention.

And the actual blocker: this PR has had no human review in 54 days and CI has never run on it, because pre-run-check requires the ready label or 4+ merged PRs from the author. Could a maintainer add ready so the tests can run? The change is small, it targets a defect that is verbatim present in main today, and it now has an independent reproduction on second hardware.

Disclosure: this analysis was produced with AI assistance. I reviewed every line, ran the reproduction on the hardware and version stated, and the numbers above come from a committed certificate rather than from a model.

[Anai-Guo]

Thank you @nickharris808 — that's a remarkably thorough independent reproduction, and the one-block blast-radius bound (extra_keys only enters the first block's hash) is a valuable framing I hadn't stated explicitly.

I've taken your load-bearing-order point directly (702a3eb): added a NOTE at the concatenation site so a future determinism-motivated reorder can't silently invalidate the whole prefix cache. I also agree the two-source scope (lora + cache_salt) is the complete fix, not a partial one, for the type-disjointness reason you give.

The remaining blocker is unchanged: pre-run-check still needs a maintainer to add the ready label before CI can run. The defect is verbatim on main today and now has reproductions on two different GPUs, so a maintainer label would be much appreciated.

🤖 AI-assisted, human reviewed.

[Hotragn]

Heads up: this will fail tests/v1/core/test_prefix_caching.py::test_cache_key_salting, which isn't in the diff.

That test rebuilds the expected block hash itself and hard-codes the untagged salt shape in two places:

# tests/v1/core/test_prefix_caching.py:1813
assert block_hashes[0] == sha256(
    (kv_cache_utils.NONE_HASH, tuple(token_ids[:block_size]), ("salt1",))
)

# tests/v1/core/test_prefix_caching.py:1858
assert block_hashes[0] == sha256(
    (kv_cache_utils.NONE_HASH, tuple(token_ids[:block_size]), ("salt2",))
)

Both need to become ("cache_salt", "salt1") / ("cache_salt", "salt2"). Only the first one shows up as a failure, because the assert at 1813 stops the test before 1858 runs.

I reached the same defect from an independent audit of generate_block_hash_extra_keys, then found this PR and #44701 and dropped my branch rather than open a duplicate — so this is just the test run I already had. Applying only the cache_salt half on top of main (96242aa), on ubuntu-24.04 / Python 3.12.3 / torch 2.13.0+cpu:

$ python -m pytest -v -m cpu_test tests/v1/core

>       assert block_hashes[0] == sha256(
            (kv_cache_utils.NONE_HASH, tuple(token_ids[:block_size]), ("salt1",))
        )
E       AssertionError: assert b'(\xd7\xcc@o...0\x90\xa5\xcd' == b'5\xd1s\x10\...8a\xce\xeb[sI'
E         
E         At index 0 diff: b'(' != b'5'
E         
E         Full diff:
E         - (b'5\xd1s\x10\xfc\x01 \xc5\xde\xe3\xaf\x94oA\xce)\x86p\xbb\xe3\x8d\xce]\x03'
E         -  b'\xa6\xb8\x8a\xce\xeb[sI')
E         + (b'(\xd7\xcc@o\xefZ\x0c`1\x18\x12.Z\x84#\x96%\x8du4\x15\x7f\xdf.\x80\xaf-'
E         +  b'\x10\x90\xa5\xcd')

tests/v1/core/test_prefix_caching.py:1812: AssertionError
=========================== short test summary info ============================
FAILED tests/v1/core/test_prefix_caching.py::test_cache_key_salting - AssertionError: assert b'(\xd7\xcc@o...0\x90\xa5\xcd' == b'5\xd1s\x10\...8a\xce\xeb[sI'
  
  At index 0 diff: b'(' != b'5'
  
  Full diff:
  - (b'5\xd1s\x10\xfc\x01 \xc5\xde\xe3\xaf\x94oA\xce)\x86p\xbb\xe3\x8d\xce]\x03'
  -  b'\xa6\xb8\x8a\xce\xeb[sI')
  + (b'(\xd7\xcc@o\xefZ\x0c`1\x18\x12.Z\x84#\x96%\x8du4\x15\x7f\xdf.\x80\xaf-'
  +  b'\x10\x90\xa5\xcd')
==== 1 failed, 470 passed, 80 deselected, 14 warnings in 441.97s (0:07:21) =====

The LoRA half looks clear against that suite: the LoRA test in test_prefix_caching.py asserts lora_id and the block-hash count rather than a literal hash, so tagging the name doesn't move it.

(AI assistance was used for this analysis and to run the suite; I have reviewed it.)

[Anai-Guo]

Thanks @Hotragn — that's a real gap, and a genuinely useful test run. Fixed in 6c79a6f.

Both hand-rebuilt hashes in test_cache_key_salting now use the tagged shape:

(kv_cache_utils.NONE_HASH, tuple(token_ids[:block_size]), (("cache_salt", "salt1"),))

and the same for salt2 — so the second assertion is corrected too, not just the one that happened to fail first. ruff format (v0.14.0, the pinned pre-commit rev) reflowed both call sites; the diff is confined to those two assertions.

Also agree with your read on the LoRA half: the LoRA test asserts lora_id and the block-hash count rather than a literal hash, so tagging the adapter name doesn't move it.

Appreciate you dropping your branch instead of opening a duplicate.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

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📥 Commits

Reviewing files that changed from the base of the PR and between 07ee97e and 20993e5.

📒 Files selected for processing (3)

• tests/v1/core/test_kv_cache_utils.py
• tests/v1/core/test_prefix_caching.py
• vllm/v1/core/kv_cache_utils.py

Included review availability: Your plan provides up to 10 included reviews per hour; 9 remain after this review.

---

📝 Summary

Summary by CodeRabbit

• Bug Fixes
  • Improved cache key generation to distinguish cache salts from LoRA adapter metadata, preventing collisions when values are identical.
  • Ensured cache entries are correctly separated when different cache salts are used, preserving expected cache hits and misses.
  • Stabilized the ordering of metadata used in block hashes for more consistent cache behavior.

Walkthrough

KV-cache hash metadata now uses distinct tagged tuples for cache salts and LoRA adapters. Tests update expected hashes and verify that identical values from the two namespaces do not collide.

Changes

KV-cache hash key namespaces

Layer / File(s)	Summary
Tag extra hash keys vllm/v1/core/kv_cache_utils.py	LoRA and cache-salt extra keys now use tagged tuples. The block-hash key order is documented as stable.
Validate tagged hash keys tests/v1/core/test_kv_cache_utils.py, tests/v1/core/test_prefix_caching.py	Tests expect tagged cache-salt and LoRA keys. A new test verifies that equal LoRA and cache-salt values produce different keys.

Estimated code review effort: 2 (Simple) | ~10 minutes

Merge Risk: ⚪ Minimal · up to 20993

KV-cache keys now distinguish LoRA adapter names from cache-salt values with the same text, preventing incorrect first-block cache reuse. The updated cache-key expectations and collision coverage support merge readiness.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 66.67% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 9 functions across 3 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely describes the domain-tagging fix for LoRA and cache_salt prefix-cache keys and identifies the collision issue.
Description check	✅ Passed	The description directly explains the collision, the correctness risk, the domain-tagging fix, and the regression tests.
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.

• Fix all pre-merge checks with AI

✨ Finishing Touches 💡 1

🛠️ Fix failing CI checks 💡

• Create stacked PR
• Commit on current branch

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[njhill]

@Anai-Guo thanks for this. I'm going to close it in favor of related PR #51899 but will add you as coauthor.