fix: stabilize paged decode attention kernels

- zero-fill split partials so combine skips unwritten splits deterministically
- skip loading masked KV in paged decode kernels to avoid 0*NaN output poisoning
- zero-fill shared memory tile buffers to prevent stale NaN leaking into softmax

csrc/kernels/attn_entry_utils.cuh

@@ -1,4 +1,5 @@
 #pragma once
+#include <float.h>
 #include <torch/extension.h>
 #include <c10/cuda/CUDAGuard.h>
 #include "attn_common.h"
@@ -23,8 +24,8 @@ using bf16 = __nv_bfloat16;
 template<typename P>
 inline void alloc_split_partials(P& p) {
     auto fopt = torch::TensorOptions().dtype(torch::kFloat32).device(torch::kCUDA);
-    auto o_part = torch::empty(at::IntArrayRef{p.batch, p.q_head, MAX_SPLITS, p.head_dim}, fopt);
+    auto o_part = torch::zeros(at::IntArrayRef{p.batch, p.q_head, MAX_SPLITS, p.head_dim}, fopt);
-    auto ml_part = torch::empty(at::IntArrayRef{p.batch, p.q_head, MAX_SPLITS, 2}, fopt);
+    auto ml_part = torch::full(at::IntArrayRef{p.batch, p.q_head, MAX_SPLITS, 2}, -FLT_MAX, fopt);
     p.o_part = (float*)o_part.data_ptr();
     p.ml_part = (float*)ml_part.data_ptr();
 }

csrc/kernels/attn_paged_decode_split_kv.cuh

@@ -67,14 +67,17 @@ __global__ void paged_attn_decode_split_kv_kernel(PagedAttentionParams<bf16> p)
             partial = warp_reduce_sum(partial) * p.scale;
 
             int kv_idx = chunk_start + s;
+            bool masked = false;
             if constexpr (HasMask) {
                 if (!p.mask[mask_base + kv_idx])
-                    partial = -FLT_MAX;
+                    masked = true;
             }
             if constexpr (IsCausal) {
                 if (kv_idx > p.causal_offset)
-                    partial = -FLT_MAX;
+                    masked = true;
             }
+            if (masked)
+                partial = -FLT_MAX;
 
             float new_m = fmaxf(m, partial);
             float alpha = __expf(m - new_m);
@@ -85,7 +88,11 @@ __global__ void paged_attn_decode_split_kv_kernel(PagedAttentionParams<bf16> p)
             int logical_page = pos / p.page_size;
             int page_offset = pos % p.page_size;
             int phys_page = p.page_table[batch * p.max_pages + logical_page];
-            if (phys_page >= 0) {
+            if (masked) {
+                #pragma unroll
+                for (int i = 0; i < hd_per_thread; i++)
+                    acc_reg[i] = fmaf(acc_reg[i], alpha, 0.0f);
+            } else if (phys_page >= 0) {
                 int64_t v_base = (int64_t)phys_page * p.page_size * p.kv_head * p.head_dim
                                + (int64_t)page_offset * p.kv_head * p.head_dim
                                + (int64_t)kv_head * p.head_dim;

csrc/kernels/attn_paged_decode_split_kv_mma.cuh

@@ -31,6 +31,13 @@ __global__ void paged_attn_decode_split_kv_mma_kernel(PagedAttentionParams<bf16>
     __shared__ __align__(16) bf16 sK[Traits::STAGES * Traits::BC * Traits::LD];
     __shared__ __align__(16) bf16 sV[Traits::STAGES * Traits::BC * Traits::LD];
 
+    #pragma unroll
+    for (int i = lane; i < Traits::STAGES * Traits::BC * Traits::LD; i += 32) {
+        sK[i] = __float2bfloat16(0.0f);
+        sV[i] = __float2bfloat16(0.0f);
+    }
+    __syncwarp();
+
     const int q_base = batch * p.q_stride_b + q_head0 * p.q_stride_h;
     const int qra = gid;
     const int qrb = gid + 8;
@@ -68,6 +75,9 @@ __global__ void paged_attn_decode_split_kv_mma_kernel(PagedAttentionParams<bf16>
             int r = i / Traits::HEAD_DIM, d = i % Traits::HEAD_DIM;
             int kc = kv0 + r;
             bool valid = (kc < p.kv_len);
+            if constexpr (HasMask) {
+                valid = valid && p.mask[batch * p.mask_b_stride + kc];
+            }
             int phys_page = valid ? p.page_table[batch * p.max_pages + kc] : 0;
             valid = valid && (phys_page >= 0);
             int page_off = kc % p.page_size;