f59d30dcb2
Random CCCL pick: cub/test/test_device_scan_warpspeed_shifted_output.cu (40 lines, full read — minimal reproducer for CCCL issue #8838) CCCL bug: InclusiveScan with out+1 (shifted output pointer) caused illegal memory access in lookahead scan warpspeed path. Root cause: uninitialized memory before the output offset was read by the kernel. Our V2 attention has analogous shifted outputs: tmp_output[seq_idx, :, :num_partitions, :] — only first num_partitions written, rest is max_num_partitions-sized buffer with garbage. Change: torch.empty → torch.zeros for tmp_output and exp_sums, torch.empty_like → torch.full(fill_value=-inf) for max_logits. This is defensive: paged_attention_v2_pytorch.py already initializes these in its body, but if any code path skips that (early return, exception), the caller's buffers are now safe by construction. Cost: one extra memset per decode step. For max_num_seqs=1: tmp_output: 1×24×200×256×2B = 2.4MB memset (negligible vs matmul) exp_sums+max_logits: 1×24×200×4B = 19KB each Base file modified: qwen3_6_scripts/paged_attn.py (deployed via patch_ops.sh)