[Fix][FlexAttention] return max logical block index to handle reused blocks (#30915)

Signed-off-by: Yifan Qiao <yifanqiao@berkeley.edu>

tests/kernels/test_flex_attention.py

@@ -15,7 +15,10 @@ from tests.v1.attention.utils import (
     create_standard_kv_cache_spec,
     create_vllm_config,
 )
-from vllm.v1.attention.backends.flex_attention import FlexAttentionMetadataBuilder
+from vllm.v1.attention.backends.flex_attention import (
+    FlexAttentionMetadataBuilder,
+    physical_to_logical_mapping,
+)
 
 from ..models.utils import check_embeddings_close, check_logprobs_close
 
@@ -205,5 +208,31 @@ def test_block_mask_direct_vs_slow_path():
     )
 
 
+def test_physical_to_logical_mapping_handles_reused_blocks():
+    """Regression test: reused physical blocks map to the latest logical block.
+
+    For sliding-window / hybrid attention layers, physical KV-cache blocks can be
+    reused over time. The inverse mapping must therefore select the latest
+    logical block index for a physical block id.
+    """
+    # Padding should not make physical block 0 look live.
+    block_table = torch.tensor([[6, 0, 0, 0]], dtype=torch.int32)
+    seq_lens = torch.tensor([1 * 16], dtype=torch.int32)  # only 1 block valid
+    out = physical_to_logical_mapping(
+        block_table=block_table, seq_lens=seq_lens, block_size=16, total_blocks=10
+    )
+    assert out[0, 0].item() == -1
+    assert out[0, 6].item() == 0
+
+    # If a physical block id appears multiple times (block reuse), mapping should
+    # point to the latest logical block index.
+    block_table2 = torch.tensor([[2, 2, 5]], dtype=torch.int32)
+    seq_lens2 = torch.tensor([3 * 16], dtype=torch.int32)
+    out2 = physical_to_logical_mapping(
+        block_table=block_table2, seq_lens=seq_lens2, block_size=16, total_blocks=8
+    )
+    assert out2[0, 2].item() == 1
+
+
 if __name__ == "__main__":
     pytest.main([__file__])

vllm/v1/attention/backends/flex_attention.py

@@ -160,7 +160,7 @@ def physical_to_logical_mapping(
     └───────────────────────────────────────────┘
 
     If multiple logical blocks map to the same physical block,
-    this function returns the first (minimum) logical block index.
+    this function returns the latest (maximum) logical block index.
 
     If a physical block is not mapped to by any logical block,
     its value in the result will be -1.
@@ -183,6 +183,15 @@ def physical_to_logical_mapping(
     To prevent this, we use seq_lens and block_size to mask out unused
     entries, ensuring only valid block references are processed.
 
+    IMPORTANT: Reused physical blocks (sliding-window / hybrid attention)
+    ────────────────────────────────────────────────────────────────────
+    For some attention types, physical cache blocks can be reused over time.
+    This can cause the same physical block id to appear multiple times in a row
+    of `block_table` at different logical block indices. In that case, only the
+    latest logical block index corresponds to the current contents of that
+    physical block. Therefore, the inverse mapping must pick the maximum logical
+    block index for each physical block id.
+
     Args:
         block_table: Tensor of shape [max_reqs, max_num_blocks]
             mapping logical blocks to physical locations. May contain
@@ -217,8 +226,8 @@ def physical_to_logical_mapping(
         mask, torch.arange(max_num_blocks, device=device)[None, :], 0
     )
 
-    physical_to_logical.scatter_(
-        -1, valid_block_table.to(torch.int64), valid_logical_indices
+    physical_to_logical.scatter_reduce_(
+        -1, valid_block_table.to(torch.int64), valid_logical_indices, reduce="amax"
     )
     # NB - Seems like block 0 is always empty so we reset it manually
     physical_to_logical[:, 0] = -1