[G-haoyu]

Fixes #57661

• Root cause: NIXL region registration deduped transfer regions by base_addr alone, and the collapsed branch only refreshed block geometry for MLA regions. In hybrid-attention models, a layer with a different block length aliasing another layer's base address (DeepSeek-V4.1-Flash compressor state_cache / indexer k_cache overlaid on swa_cache) was silently folded and inherited the first region's block length. PD transfers of its state were truncated and misaligned, producing garbled output with no errors.
• Fix: dedup key is now (base_addr, block_len). Identical-geometry aliases still fold (test_overlaid_transfer_groups_share_region_geometry continues to pass, confirming the lossless path is unchanged).
• Behavior change (maintainer sanity-check welcome): test_ring_scratch_without_mamba_registers_and_addresses_its_own_regions now expects _scratch_region_indices == [2] instead of [1]. That fixture (introduced in [Model] Support DeepSeek-V4.1-Flash #56214) overlays rings on a paged region whose block_len differs from the rings' page size — the old expectation had recorded the benign form of this very bug (the ring inheriting the paged region's geometry). If overlay folding was intentional design, this one assertion is trivially revertible.
• Tests: added test_shared_base_regions_with_distinct_block_lens_stay_separate (two geometries at one base address — 256B MLA swa page vs 128B compressor ring page — must register as two regions with their own block lens and descriptor geometry). Verified in a container: target test fails on HEAD / passes with the fix; test_nixl_connector_hma/push_connector/tp_mapping failure sets are identical before/after. ruff check + ruff format pass.

Why this is not a duplicate: searched open issues/PRs (nixl region dedup, deepseek pd disagg, base_addr, garbled output); #52627 is Mooncake-specific and #54926 is un-transferred draft KV — different root causes.

Model evals: 2×8 H200 cluster, NIXL P/D disagg — before: fully garbled output; after: clean English/Chinese generations, 0% garble rate, 6 concurrent streams clean (details in #57661).

AI assistance disclosure: root-cause analysis and this fix were produced with ZCode (local agent); cluster diagnosis, the initial mitigation patch, and remote test runs were done with Chaterm (remote agent). The submitter has reviewed every changed line and performed the end-to-end cluster verification above.

🤖 Generated with ZCode

[claude]

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[snadampal]

Should the key also include block_stride (and num_blocks)?

[G-haoyu]

Good question — I walked the registration branches before settling on (base_addr, block_len):

num_blocks isn't per-layer geometry: it's logical_num_blocks (mamba) or logical_num_blocks * physical_blocks_per_logical (attention), both worker/group-global, so specs sharing a base and block_len already share num_blocks. Host-resident groups could differ, but those have a different mem_type and the existing fold assert rejects that.

block_stride is pinned to block_len in every branch that can emit a shared base: the mamba branch (stride = page/ratio = len), the flat 1-D view (stride = page = len), virtual transfer pages (len = stride = page), the packed branch (len = stride = registration_len // num_blocks), and the multi-segment branch (len = stride). The remaining block-major branch takes stride from the view, but views at the same data_ptr with the same page size come from the same KVCacheTensor, which fixes one block_stride.

That said, this is an invariant argument, and an invariant argument is exactly what let the base_addr-only key survive until DeepSeek-V4.1-Flash's layout showed up. I'm inclined to keep the key minimal for now: we can't predict which assumption the next hybrid layout breaks, so if we want protection beyond the argument above, an assert in the fold branch (rejecting a fold whose stride/num_blocks disagree) seems more appropriate than widening the key.

[mergify]

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

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

[ovidiusm]

How will this work with Mamba?

See Mamba path of _compute_desc_ids:

vllm/vllm/distributed/kv_transfer/kv_connector/v1/nixl/base_worker.py

Line 285 in a4b85c2

elif _is_ssm_spec(spec_type):

I think it causes some overlaps.

Could you please check?

[G-haoyu]

Checked. No overlap is introduced on the Mamba path. Walking through the two cases:

1. Aliased / folded layers — behavior is identical

The MLA block-major branch emits (cache.data_ptr(), physical_page_size, block_stride) (L1675-1677); the mamba branch emits (cache.data_ptr(), physical_page_size // physical_ratio, block_stride // physical_ratio) (L1598-1610).

When the views share a storage base, cache.data_ptr() coincides with it, and for aliased specs the MLA physical page equals the mamba page divided by ppl — so base, block_len, and stride all agree, the key matches at L1711-1712, and the fold is exactly what it was before. No change.

2. Split (un-folded) layers — the SSM id layout at L285-303 cannot collide

This is the case you flagged. register_local_xfer_handler concatenates

[ FA descs (global region order) | mamba descs (_ssm_region_indices order) ]

(L2141-2159), and the SSM descriptor id is

ssm_id = ordinal * logical_blocks + block + num_fa_descs

where ordinal is the position within _ssm_region_indices. A split converts one region into two:

• num_regions grows by 1, num_fa_descs grows by num_blocks (one full FA region of descriptors).
• The mamba layer keeps its ordinal (list position) in _ssm_region_indices — only the region index it points at changes, from the folded MLA region to its own new region.
• So ssm_id = ordinal * logical_blocks + block + num_fa_descs: the ordinal is unchanged, num_fa_descs shifts up by exactly num_blocks.

The whole SSM block shifts by exactly the FA section's growth. Ordinals are unchanged and distinct, so no two SSM ids collide; and since + num_fa_descs tracks the FA length exactly, no SSM id lands inside the FA range. No overlap.

Where block_len differs, the pre-PR fold was already mis-addressing the mamba state through the folded MLA region's block_stride_per_layer[i] (L1941-1944) — the same silent misalignment this PR fixes for the compressor case. The split does not introduce a new failure mode; it replaces a silent one with a loud assert (see below).

3. One caveat: PP > 1 pull layouts

For PP > 1 pull layouts, the num_regions % num_local_layers assert at L1803 would now fire at startup for a model whose aliased layers split (previously they folded and were silently mis-transferred). This is a hard stop before any token is processed — no KV data is ever written to the wrong offset. This is a behavior change for that narrow shape; we believe fail-loud at registration is preferable to the previous silent mis-transfer.

4. Test coverage

_make_mla_hybrid_worker exercises exactly this shape (mla.i / kda_a.i / kda_b.i alias one tensor); the hybrid suites (hetero_ppl, mla_hybrid, csa_linear — 202 tests) pass identically before and after this change, including the byte-range overlap invariants in _assert_local_writes_within. Full file is 223/223 green.

[ovidiusm]

Thanks, makes sense: the Mamba page is padded to match the size of the attention page, so these alias pairs still fold.

[xzwgit]

Verification on a second platform: 2-node SM120 (8× RTX PRO 6000, PCIe-only) P/D disaggregation with DeepSeek-V4.1-Flash (TP8 + TP8, NixlConnector, DSpark speculative decoding enabled in both pools, vision tower enabled, KV fp8 + indexer fp8, vLLM dev382 + this patch).

Results with this patch applied (16+ benchmark tiers + correctness suites through the router → P → KV transfer → D chain):

• Correctness: text sanity (17×19 = 323), a 10/10 program-generated vision objective suite (OCR, in-image math, color counting, table cell, two-image comparison, bar chart, small text, solid color, text control — image requests carry 228–438 prompt tokens vs 43 for the text control, confirming images actually flow through P's encoder), and long-form Chinese generation — zero garbled/empty outputs anywhere, the failure mode described in [Bug]: NIXL P/D disagg silently corrupts output when hybrid-attention layers share a storage base with different block lengths (DeepSeek-V4.1-Flash) #57661.
• Transfers: zero NIXL transfer failure entries; every decode worker reports NIXL compatibility check passed with the same hash on both ends and Transfer plan: local_tp=8, remote_tp=8.
• Region layout: no interaction issues with the hybrid-attention groups (MLA + SWA + compressor circular buffer) under DSpark dual-pool KV layouts or with the vision tower enabled.

To be explicit about scope: we did not run a no-patch corruption baseline on this hardware (the author's H200 before/after already establishes that), so this is a correctness/no-regression confirmation on a different platform and a fuller-featured config (spec decode + vision), not an independent reproduction of the corruption itself.

Thanks for the fix — happy to share logs/configs if useful.

[liulanze]

This test does not show the bug. The ring page (128 B) is smaller than the swa page (256 B), so on main the merged region is still 256 B and covers the ring page. No data is lost. The test fails on main only because of the region count.

[G-haoyu]

Good catch — you're right. With the original page widths, folding only over-read the smaller ring page, so the test did not model the truncation failure. I swapped the geometry so the ring page (256 B) is wider than the aliased SWA page (128 B), matching the relative compressor/SWA shape in DeepSeek-V4.1-Flash. On main, folding these regions leaves the ring transfer descriptors at the SWA length, reproducing the actual truncation path. Verified: the targeted tests pass with the fix (2 passed), while the base version fails because the two same-base regions are incorrectly folded; the full file passes (224 passed).

[G-haoyu]

Thanks for #62317’s per-group region addressing (cc @NickLucche). This branch is now rebased onto it.

The rebase was mechanical:

• In register_kv_caches, the existing region_groups[...] bookkeeping composes cleanly with the (base_addr, block_len) key; no semantic change was needed.
• The ring test now asserts _group_region_indices[1] == [2], while test_shared_base_regions_with_distinct_block_lens_stay_separate asserts _group_region_indices[1] == [1]. The ring group still addresses only its own region, so its descriptor-id semantics remain unchanged.
• The wider PLE-page case is handled the same way: the PLE page gets its own region, with _ple_region_index == 1 and _ple_block_len matching that region’s block length.

Regarding the duplicate-byte accounting in #62317, the 283,136 B remainder appears to correspond to the alias pair this PR separates. The split should therefore eliminate that duplicate-byte remainder rather than move it, while making region_groups more precise.

Validation in the remote test container:

• test_nixl_desc_geometry.py: 257 passed, 0 failed; the base version had the six expected failures.
• test_nixl_push_connector.py and test_tp_mapping.py: passed.
• test_fewer_blocks_with_hma[google/gemma-3-1b-it-512] passed after adding the missing model.
• The remaining test_mamba_n1_p_side_truncation failure is identical on the base and fixed versions and is unrelated to this change.

[NickLucche]

Apologies for the delay @G-haoyu and thanks a lot for fixing this!
I think this works, unless we make multi-descs views a standard for kv groups, this is the cleanest way. Will have to keep an eye out for other models falling into this category though

[NickLucche]

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✅ Triggered Buildkite CI #98264 for commit 29242d067db4.

ℹ️ This PR is 101 commits behind upstream main. Merged upstream main into this branch as 29242d067db4 before starting CI. Pull the branch before pushing more changes.

[NickLucche]

/ci run

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✅ Triggered Buildkite CI #98269 for commit 449b700edb09.

ℹ️ This PR is 2 commits behind upstream main. Merged upstream main into this branch as 449b700edb09 before starting CI. Pull the branch before pushing more changes.

[hmellor]

@G-haoyu this PR appears to break tests/v1/kv_connector/unit/test_nixl_connector.py::test_register_packed_dsv4_mla_cache_as_single_region[v41-aliased-groups], which is now failing in the Intel BMG V1 job (e.g. https://buildkite.com/vllm/ci/builds/98344) and also fails locally on CUDA (B200):

>       assert connector.connector_worker.kv_caches_base_addr[
            connector.connector_worker.engine_id
        ][0] == [raw.data_ptr()] + [addr for addr, _ in ring_pages]
E       At index 1 diff: 131481278087168 != 131481278154752
E       Left contains one more item: 131481278154752

With vllm/distributed/kv_transfer/kv_connector/v1/nixl/base_worker.py restored to its state before this PR (on otherwise current main), both cases pass:

pytest tests/v1/kv_connector/unit/test_nixl_connector.py -k test_register_packed_dsv4_mla_cache_as_single_region

The nightly that ran before this merged (#98222) didn't include it, so there is no main alert yet. #61121 touches the same test and registration code, in case it is related.

[G-haoyu]

Thanks to @NickLucche for helping run CI yesterday and moving this PR through the merge process.

Also thanks to @hmellor for catching the regression in the packed DSv4.1 alias case. The issue involves several interacting region-geometry and aliasing conditions, so I’m carefully checking all boundary cases and expanding the test coverage. I’ll prepare and submit a follow-up fix PR as soon as the behavior is fully validated.

[G-haoyu]

@hmellor Thanks for catching this and for providing the reproducer. You were right: while #57662 fixed the truncation case, its region split also regressed the packed DeepSeek-V4.1 alias layout, causing v41-aliased-groups to register the same base twice.

I opened #64661 as a follow-up. It preserves exact-length aliases, allows shorter non-MLA views to reuse only compatible packed whole-row regions, and keeps wider or incompatible scratch/PLE views separate.

In the same source-covered container, the original mainline had 5 failed / 670 passed, while the fix branch had 4 failed / 671 passed. The only mainline-only failure was v41-aliased-groups; no fix-only failures appeared across the 15 runnable NIXL unit suites.

We currently do not have P/D resources for end-to-end DeepSeek-V4.1 validation, so community validation in a real P/D deployment would be appreciated. #61121 is related to the packed-MLA path but addresses a different layer of the problem.

[hmellor]

Thanks for the swift fix! I'll leave the review to @NickLucche et al