[Bugfix] Fix prefix caching for Mamba 'all' mode (Nemotron models) (#34874)

Signed-off-by: haosdent <haosdent@gmail.com>

tests/v1/attention/test_mamba_update_block_table.py

+# SPDX-License-Identifier: Apache-2.0
+# SPDX-FileCopyrightText: Copyright contributors to the vLLM project
+"""
+Regression test for https://github.com/vllm-project/vllm/issues/34865
+
+When multiple KV cache groups share the same MambaSpec (as in Nemotron
+hybrid models), the metadata caching optimization reuses metadata from
+an earlier group via update_block_table(). In 'all' mode with CUDA graphs,
+update_block_table() must copy block_idx_last_scheduled_token and
+block_idx_last_computed_token to the *current* builder's persistent
+buffers, otherwise CUDA graph replay reads stale values from uninitialized
+buffers.
+"""
+
+from types import SimpleNamespace
+
+import torch
+
+from vllm.config.compilation import CUDAGraphMode
+from vllm.v1.attention.backends.mamba_attn import (
+    BaseMambaAttentionMetadata,
+    BaseMambaAttentionMetadataBuilder,
+)
+from vllm.v1.kv_cache_interface import MambaSpec
+
+
+class _ConcreteMambaBuilder(
+    BaseMambaAttentionMetadataBuilder[BaseMambaAttentionMetadata]
+):
+    """Minimal concrete subclass for testing (base class is ABC)."""
+
+    metadata_cls = BaseMambaAttentionMetadata
+
+
+def _make_vllm_config(block_size, max_model_len, max_num_seqs):
+    """Create a minimal mock VllmConfig with only the fields the builder
+    accesses, avoiding any model download / HF config inspection."""
+    return SimpleNamespace(
+        cache_config=SimpleNamespace(mamba_cache_mode="all"),
+        compilation_config=SimpleNamespace(
+            cudagraph_mode=CUDAGraphMode.FULL,
+            max_cudagraph_capture_size=None,
+        ),
+        scheduler_config=SimpleNamespace(max_num_seqs=max_num_seqs),
+        model_config=SimpleNamespace(max_model_len=max_model_len),
+    )
+
+
+def test_update_block_table_copies_block_idx_to_persistent_buffers():
+    """update_block_table() must write block_idx tensors to the current
+    builder's persistent buffers, not leave them pointing to a different
+    builder's buffers."""
+
+    block_size = 16
+    max_model_len = 256
+    num_reqs = 4
+    device = torch.device("cpu")
+
+    vllm_config = _make_vllm_config(block_size, max_model_len, num_reqs)
+
+    spec = MambaSpec(
+        block_size=block_size,
+        shapes=((1,), (1,)),
+        dtypes=(torch.float32,),
+        mamba_cache_mode="all",
+    )
+
+    # Two builders simulating two KV cache groups with the same MambaSpec.
+    builder_a = _ConcreteMambaBuilder(spec, ["layer0"], vllm_config, device)
+    builder_b = _ConcreteMambaBuilder(spec, ["layer1"], vllm_config, device)
+
+    # Sanity: each builder has its own persistent buffer.
+    assert (
+        builder_a.block_idx_last_scheduled_token.data_ptr()
+        != builder_b.block_idx_last_scheduled_token.data_ptr()
+    )
+
+    # Construct decode-only metadata as if builder_a.build() produced it.
+    max_blocks = max_model_len // block_size
+    seq_lens = torch.full((num_reqs,), 64, dtype=torch.int32, device=device)
+    block_idx_vals = (seq_lens - 1) // block_size  # [3, 3, 3, 3]
+
+    builder_a.block_idx_last_scheduled_token[:num_reqs].copy_(block_idx_vals)
+    builder_a.block_idx_last_computed_token[:num_reqs].copy_(block_idx_vals)
+
+    metadata_a = BaseMambaAttentionMetadata(
+        num_prefills=0,
+        num_prefill_tokens=0,
+        num_decodes=num_reqs,
+        num_decode_tokens=num_reqs,
+        num_reqs=num_reqs,
+        has_initial_states_p=None,
+        query_start_loc_p=None,
+        num_computed_tokens_p=None,
+        state_indices_tensor=builder_a.state_indices_tensor[:num_reqs],
+        block_idx_last_scheduled_token=(
+            builder_a.block_idx_last_scheduled_token[:num_reqs]
+        ),
+        block_idx_first_scheduled_token_p=None,
+        block_idx_last_computed_token=(
+            builder_a.block_idx_last_computed_token[:num_reqs]
+        ),
+        seq_lens=seq_lens,
+    )
+
+    # Call update_block_table on builder_b (simulates the metadata caching
+    # optimization reusing metadata from builder_a's group).
+    blk_table = torch.randint(
+        0, 100, (num_reqs, max_blocks), dtype=torch.int32, device=device
+    )
+    slot_mapping = torch.zeros(num_reqs, dtype=torch.int64, device=device)
+
+    metadata_b = builder_b.update_block_table(metadata_a, blk_table, slot_mapping)
+
+    # block_idx tensors must live in builder_b's persistent buffers.
+    def shares_storage(tensor, buffer):
+        return (
+            tensor.untyped_storage().data_ptr() == buffer.untyped_storage().data_ptr()
+        )
+
+    assert shares_storage(
+        metadata_b.block_idx_last_scheduled_token,
+        builder_b.block_idx_last_scheduled_token,
+    ), "block_idx_last_scheduled_token not in builder_b's persistent buffer"
+
+    assert shares_storage(
+        metadata_b.block_idx_last_computed_token,
+        builder_b.block_idx_last_computed_token,
+    ), "block_idx_last_computed_token not in builder_b's persistent buffer"
+
+    # Must NOT point to builder_a's buffers.
+    assert not shares_storage(
+        metadata_b.block_idx_last_scheduled_token,
+        builder_a.block_idx_last_scheduled_token,
+    ), "block_idx_last_scheduled_token still points to builder_a's buffer"
+
+    # Values must be correct (copied from metadata_a).
+    torch.testing.assert_close(
+        metadata_b.block_idx_last_scheduled_token,
+        block_idx_vals,
+    )
+    torch.testing.assert_close(
+        metadata_b.block_idx_last_computed_token,
+        block_idx_vals,
+    )

vllm/v1/attention/backends/mamba_attn.py

@@ -331,5 +331,26 @@ class BaseMambaAttentionMetadataBuilder(AttentionMetadataBuilder[M], abc.ABC):
             persistent_state_indices_t.copy_(state_indices_t, non_blocking=True)
             state_indices_t = persistent_state_indices_t
 
+            # For 'all' mode, also update prefix caching block indices
+            # to use this builder's persistent buffers (required for CUDA
+            # graph replay to read from the correct memory addresses).
+            if self.vllm_config.cache_config.mamba_cache_mode == "all":
+                assert metadata.block_idx_last_scheduled_token is not None
+                assert metadata.block_idx_last_computed_token is not None
+                self.block_idx_last_scheduled_token[:num_reqs].copy_(
+                    metadata.block_idx_last_scheduled_token[:num_reqs],
+                    non_blocking=True,
+                )
+                new_metadata.block_idx_last_scheduled_token = (
+                    self.block_idx_last_scheduled_token[: metadata.num_decode_tokens]
+                )
+                self.block_idx_last_computed_token[:num_reqs].copy_(
+                    metadata.block_idx_last_computed_token[:num_reqs],
+                    non_blocking=True,
+                )
+                new_metadata.block_idx_last_computed_token = (
+                    self.block_idx_last_computed_token[: metadata.num_decode_tokens]
+                )
+
         new_metadata.state_indices_tensor = state_indices_t
         return new_metadata