Allow rel_pos_embedding with use_flash_attention in SABlock and CrossAttentionBlock

monai/networks/blocks/crossattention.py

@@ -61,8 +61,11 @@ def __init__(
             input_size (tuple(spatial_dim), optional): Input resolution for calculating the relative positional
                 parameter size.
             attention_dtype: cast attention operations to this dtype.
-            use_flash_attention: if True, use Pytorch's inbuilt flash attention for a memory efficient attention mechanism
-                (see https://pytorch.org/docs/2.2/generated/torch.nn.functional.scaled_dot_product_attention.html).
+            use_flash_attention: if True, dispatch attention through
+                ``torch.nn.functional.scaled_dot_product_attention``. PyTorch selects the backend;
+                the true flash kernel is used only when no attention bias is present. When combined
+                with ``rel_pos_embedding`` or ``causal``, PyTorch will fall back to the
+                memory-efficient or cuDNN SDPA backend.
         """
 
         super().__init__()
@@ -88,9 +91,6 @@ def __init__(
                 "to True. save_attn can only be used if use_flash_attention is False"
             )
 
-        if use_flash_attention and rel_pos_embedding is not None:
-            raise ValueError("rel_pos_embedding must be None if you are using flash_attention.")
-
         self.num_heads = num_heads
         self.hidden_input_size = hidden_input_size if hidden_input_size else hidden_size
         self.context_input_size = context_input_size if context_input_size else hidden_size
@@ -155,8 +155,31 @@ def forward(self, x: torch.Tensor, context: torch.Tensor | None = None):
             k = k.to(self.attention_dtype)
 
         if self.use_flash_attention:
+            # Additive bias path mirrors SABlock: null bias preserves the true
+            # flash kernel fast path; any of rel_pos_embedding / causal forces
+            # fallback to the efficient or cuDNN SDPA backend.
+            bias: torch.Tensor | None = None
+            lq, lk = q.shape[-2], k.shape[-2]
+
+            if self.rel_positional_embedding is not None:
+                zero_logits = torch.zeros(q.shape[0], self.num_heads, lq, lk, dtype=q.dtype, device=q.device)
+                bias = self.rel_positional_embedding(x, zero_logits, q)
+
+            is_causal_arg = self.causal
+            if self.causal and bias is not None:
+                causal_bias = torch.zeros(lq, lk, dtype=q.dtype, device=q.device)
+                causal_bias.masked_fill_(self.causal_mask[0, 0, :lq, :lk] == 0, float("-inf"))
+                bias = bias + causal_bias
+                is_causal_arg = False
+
             x = torch.nn.functional.scaled_dot_product_attention(
-                query=q, key=k, value=v, scale=self.scale, dropout_p=self.dropout_rate, is_causal=self.causal
+                query=q,
+                key=k,
+                value=v,
+                attn_mask=bias,
+                scale=self.scale,
+                dropout_p=self.dropout_rate,
+                is_causal=is_causal_arg,
             )
         else:
             att_mat = torch.einsum("blxd,blyd->blxy", q, k) * self.scale

monai/networks/blocks/selfattention.py

@@ -63,8 +63,11 @@ def __init__(
             attention_dtype: cast attention operations to this dtype.
             include_fc: whether to include the final linear layer. Default to True.
             use_combined_linear: whether to use a single linear layer for qkv projection, default to True.
-            use_flash_attention: if True, use Pytorch's inbuilt flash attention for a memory efficient attention mechanism
-                (see https://pytorch.org/docs/2.2/generated/torch.nn.functional.scaled_dot_product_attention.html).
+            use_flash_attention: if True, dispatch attention through
+                ``torch.nn.functional.scaled_dot_product_attention``. PyTorch selects the backend;
+                the true flash kernel is used only when no attention bias is present. When combined
+                with ``rel_pos_embedding``, ``causal``, or ``attn_mask``, PyTorch will fall back to
+                the memory-efficient or cuDNN SDPA backend.
 
         """
 
@@ -94,9 +97,6 @@ def __init__(
                 "to True. save_attn can only be used if use_flash_attention is False."
             )
 
-        if use_flash_attention and rel_pos_embedding is not None:
-            raise ValueError("rel_pos_embedding must be None if you are using flash_attention.")
-
         self.num_heads = num_heads
         self.hidden_input_size = hidden_input_size if hidden_input_size else hidden_size
         self.out_proj: nn.Linear | nn.Identity
@@ -174,14 +174,40 @@ def forward(self, x, attn_mask: torch.Tensor | None = None):
             k = k.to(self.attention_dtype)
 
         if self.use_flash_attention:
+            # Build an additive attention bias when we have to combine
+            # rel_pos_embedding, a causal mask, or a user attn_mask. A null bias
+            # preserves the no-mask fast path so PyTorch can still pick the true
+            # flash kernel when available.
+            bias: torch.Tensor | None = None
+            lq, lk = q.shape[-2], k.shape[-2]
+
+            if self.rel_positional_embedding is not None:
+                zero_logits = torch.zeros(q.shape[0], self.num_heads, lq, lk, dtype=q.dtype, device=q.device)
+                bias = self.rel_positional_embedding(x, zero_logits, q)
+
+            is_causal_arg = self.causal
+            if self.causal and (bias is not None or attn_mask is not None):
+                causal_bias = torch.zeros(lq, lk, dtype=q.dtype, device=q.device)
+                causal_bias.masked_fill_(self.causal_mask[0, 0, :lq, :lk] == 0, float("-inf"))
+                bias = causal_bias if bias is None else bias + causal_bias
+                is_causal_arg = False
+
+            if attn_mask is not None:
+                if self.causal:
+                    raise ValueError("Causal attention does not support attention masks.")
+                mask_bias = torch.zeros_like(attn_mask, dtype=q.dtype)
+                mask_bias.masked_fill_(attn_mask == 0, float("-inf"))
+                mask_bias = mask_bias.unsqueeze(1).unsqueeze(2)
+                bias = mask_bias if bias is None else bias + mask_bias
+
             x = F.scaled_dot_product_attention(
                 query=q,
                 key=k,
                 value=v,
-                attn_mask=attn_mask,
+                attn_mask=bias,
                 scale=self.scale,
                 dropout_p=self.dropout_rate,
-                is_causal=self.causal,
+                is_causal=is_causal_arg,
             )
         else:
             att_mat = torch.einsum("blxd,blyd->blxy", q, k) * self.scale

tests/networks/blocks/test_crossattention.py

@@ -30,7 +30,7 @@
     [
         {
             **{k: v for k, v in params.items() if k not in ["rel_pos_embedding_val"]},
-            "rel_pos_embedding": params["rel_pos_embedding_val"] if not params["use_flash_attention"] else None,
+            "rel_pos_embedding": params["rel_pos_embedding_val"],
         },
         (2, 512, params["hidden_size"]),
         (2, 512, params["hidden_size"]),
@@ -69,16 +69,28 @@ def test_save_attn_with_flash_attention(self):
                 hidden_size=128, num_heads=3, dropout_rate=0.1, use_flash_attention=True, save_attn=True
             )
 
+    @skipUnless(has_einops, "Requires einops")
     def test_rel_pos_embedding_with_flash_attention(self):
-        with self.assertRaises(ValueError):
-            CrossAttentionBlock(
-                hidden_size=128,
-                num_heads=3,
-                dropout_rate=0.1,
-                use_flash_attention=True,
-                save_attn=False,
-                rel_pos_embedding=RelPosEmbedding.DECOMPOSED,
-            )
+        # rel_pos_embedding combined with use_flash_attention now dispatches
+        # via SDPA with an additive bias. Must match the explicit path.
+        for input_size in [(16, 32), (8, 8, 8)]:
+            input_param = {
+                "hidden_size": 128,
+                "num_heads": 4,
+                "dropout_rate": 0.0,
+                "rel_pos_embedding": RelPosEmbedding.DECOMPOSED,
+                "input_size": input_size,
+            }
+            device = "cuda:0" if torch.cuda.is_available() else "cpu"
+            block_flash = CrossAttentionBlock(**input_param, use_flash_attention=True).to(device)
+            block_ref = CrossAttentionBlock(**input_param, use_flash_attention=False).to(device)
+            block_ref.load_state_dict(block_flash.state_dict())
+            seq_len = int(np.prod(input_size))
+            test_data = torch.randn(2, seq_len, 128).to(device)
+            with eval_mode(block_flash), eval_mode(block_ref):
+                out_flash = block_flash(test_data)
+                out_ref = block_ref(test_data)
+            assert_allclose(out_flash, out_ref, atol=1e-4)
 
     @skipUnless(has_einops, "Requires einops")
     def test_attention_dim_not_multiple_of_heads(self):

tests/networks/blocks/test_selfattention.py

@@ -43,7 +43,7 @@
                                     "input_size": input_size,
                                     "include_fc": include_fc,
                                     "use_combined_linear": use_combined_linear,
-                                    "use_flash_attention": True if rel_pos_embedding is None else False,
+                                    "use_flash_attention": True,
                                 },
                                 (2, 512, hidden_size),
                                 (2, 512, hidden_size),
@@ -67,16 +67,28 @@ def test_ill_arg(self):
         with self.assertRaises(ValueError):
             SABlock(hidden_size=620, num_heads=8, dropout_rate=0.4)
 
+    @skipUnless(has_einops, "Requires einops")
     def test_rel_pos_embedding_with_flash_attention(self):
-        with self.assertRaises(ValueError):
-            SABlock(
-                hidden_size=128,
-                num_heads=3,
-                dropout_rate=0.1,
-                use_flash_attention=True,
-                save_attn=False,
-                rel_pos_embedding=RelPosEmbedding.DECOMPOSED,
-            )
+        # rel_pos_embedding is now allowed with use_flash_attention; SDPA picks
+        # a fused backend that supports an additive attention bias. The two
+        # code paths must be numerically equivalent for the same weights.
+        for input_size in [(16, 32), (8, 8, 8)]:
+            input_param = {
+                "hidden_size": 128,
+                "num_heads": 4,
+                "dropout_rate": 0.0,
+                "rel_pos_embedding": RelPosEmbedding.DECOMPOSED,
+                "input_size": input_size,
+            }
+            device = "cuda:0" if torch.cuda.is_available() else "cpu"
+            block_flash = SABlock(**input_param, use_flash_attention=True).to(device)
+            block_ref = SABlock(**input_param, use_flash_attention=False).to(device)
+            block_ref.load_state_dict(block_flash.state_dict())
+            test_data = torch.randn(2, int(np.prod(input_size)), 128).to(device)
+            with eval_mode(block_flash), eval_mode(block_ref):
+                out_flash = block_flash(test_data)
+                out_ref = block_ref(test_data)
+            assert_allclose(out_flash, out_ref, atol=1e-4)
 
     def test_save_attn_with_flash_attention(self):
         with self.assertRaises(ValueError):