[Bugfix] Fix effective_mamba_size over-allocation (#20858)

Co-authored-by: Shangming Cai <csmthu@gmail.com>

python/sglang/srt/disaggregation/decode.py

@@ -195,9 +195,18 @@ class HybridMambaDecodeReqToTokenPool(HybridReqToTokenPool):
         self.mamba_ping_pong_track_buffer_size = 2 if enable_overlap_schedule else 1
         self.enable_mamba_extra_buffer = enable_mamba_extra_buffer
         self.enable_memory_saver = enable_memory_saver
-        effective_mamba_size = (
-            mamba_size if mamba_size is not None else size
-        ) + pre_alloc_size
+        if mamba_size is not None:
+            effective_mamba_size = min(mamba_size, size + pre_alloc_size)
+            if mamba_size > size + pre_alloc_size:
+                logger.warning(
+                    "mamba_size (%d) exceeds size + pre_alloc_size (%d), "
+                    "capping effective_mamba_size to %d",
+                    mamba_size,
+                    size + pre_alloc_size,
+                    effective_mamba_size,
+                )
+        else:
+            effective_mamba_size = size + pre_alloc_size
         self.start_layer = start_layer if start_layer is not None else 0
         self.layer_transfer_counter = None
         self._init_mamba_pool(