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Fix block size handling for cross attention in TPU flash attention #301

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entrpn merged 1 commit into from
Dec 30, 2025
Merged

Fix block size handling for cross attention in TPU flash attention #301

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14 changes: 6 additions & 8 deletions src/maxdiffusion/models/attention_flax.py
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Original file line number Diff line number Diff line change
Expand Up @@ -186,17 +186,19 @@ def _tpu_flash_attention(
kv_max_block_size = key.shape[1]
else:
kv_max_block_size = q_max_block_size
if flash_block_sizes:
# ensure that for cross attention we override the block sizes.
if flash_block_sizes and key.shape[1] == query.shape[1]:
block_sizes = flash_block_sizes
else:
block_size_q = flash_block_sizes.block_q if flash_block_sizes else q_max_block_size
block_sizes = splash_attention_kernel.BlockSizes(
block_q=min(q_max_block_size, query.shape[2]),
block_q=block_size_q,
block_kv_compute=min(kv_max_block_size, key.shape[2]),
block_kv=min(kv_max_block_size, key.shape[2]),
block_q_dkv=min(q_max_block_size, query.shape[2]),
block_q_dkv=block_size_q,
block_kv_dkv=min(kv_max_block_size, key.shape[2]),
block_kv_dkv_compute=min(kv_max_block_size, query.shape[2]),
block_q_dq=None if attention_kernel == "tokamax_flash" else block_sizes.block_q_dq,
block_q_dq=None if attention_kernel == "tokamax_flash" else block_size_q,
block_kv_dq=None if attention_kernel == "tokamax_flash" else min(kv_max_block_size, query.shape[2]),
use_fused_bwd_kernel=True if attention_kernel == "tokamax_flash" else False,
)
Expand All @@ -215,7 +217,6 @@ def _tpu_flash_attention(
check_rep=False,
)
def wrap_flash_attention(query, key, value):

uses_fused_kernel = block_sizes.use_fused_bwd_kernel
block_q_sizes = (
block_sizes.block_q,
Expand Down Expand Up @@ -1042,7 +1043,6 @@ def setup(self):
)

def __call__(self, hidden_states, encoder_hidden_states=None, attention_mask=None, image_rotary_emb=None):

qkv_proj = self.qkv(hidden_states)
B, L = hidden_states.shape[:2]
H, D, K = self.heads, qkv_proj.shape[-1] // (self.heads * 3), 3
Expand All @@ -1054,7 +1054,6 @@ def __call__(self, hidden_states, encoder_hidden_states=None, attention_mask=Non
key_proj = self.key_norm(key_proj)

if encoder_hidden_states is not None:

encoder_qkv_proj = self.encoder_qkv(encoder_hidden_states)
B, L = encoder_hidden_states.shape[:2]
H, D, K = self.heads, encoder_qkv_proj.shape[-1] // (self.heads * 3), 3
Expand Down Expand Up @@ -1148,7 +1147,6 @@ class FlaxAttention(nn.Module):
quant: Quant = None

def setup(self):

if self.attention_kernel == "flash" and self.mesh is None:
raise ValueError(f"The flash attention kernel requires a value for mesh, but mesh is {self.mesh}")
inner_dim = self.dim_head * self.heads
Expand Down