Constant MASKED_ATTN_MAX_KEYS 

pub const MASKED_ATTN_MAX_KEYS: usize = 12_288;

🔴 dsa_mla_masked_attn is an oracle, not a serve path. Resolved from source 2026-08-27; do not re-derive.

It stages the whole [S] score row in shared memory, so 4·S ≤ 49,152 caps it at 12,288 keys — a limit that does not shrink with sparsity, because the dense mask and not the selected set sets the footprint. GLM-5.3 advertises 262,144.

The production path gathers the selected tokens through the page table instead:

• HF transformers 5.16.1 builds the dense [B, Q, kv_len] mask and sets _supports_flash_attn = False, saying so in its own docstring — “cannot be mapped to FA without a custom kernel that can select on a per indices bases per row”. The mask is pure set membership (scatter_add(...).ne(0), duplicates collapse, no additive weighting), so a per-row gather is exactly equivalent, not an approximation.
• vLLM ships that kernel (FlashMLA sparse / FlashInfer paged MLA), and the SM121 backend serving our own frozen oracle is the SM90 NoPE sparse-MLA path over a plain bf16 paged cache.

⇒ Atlas’s serve path is mla_paged_decode{,_fp8} — block-table paged, online softmax, no S term in shared memory — taught to walk a selected-index row instead of 0..seq_len. That kernel variant is NOT yet written; until it is, DSA decode has no production consumer and these two handles must stay test-only.