Split SWA leaf to one window on insert (#26919)

python/sglang/srt/mem_cache/unified_cache_components/swa_component.py

@@ -261,6 +261,32 @@ class SWAComponent(TreeComponent):
             node.component_data[self.component_type].value = swa_value
             self.cache.lru_lists[self.component_type].insert_mru(node)
             self.cache.component_evictable_size_[self.component_type] += len(swa_value)
+        else:
+            # Entire leaf is outside the SWA window — left as a tombstone.
+            return
+
+        self._maybe_split_leaf_for_swa_lock(node)
+
+    def _maybe_split_leaf_for_swa_lock(self, leaf: UnifiedTreeNode) -> None:
+        """Cap a fresh SWA leaf at one page-aligned window so locking it pins
+        only one window of SWA pool, not the whole (long chunked-prefill) leaf.
+        """
+        ct = self.component_type
+        cd = leaf.component_data[ct]
+        if leaf is self.cache.root_node or cd.value is None or cd.lock_ref > 0:
+            return
+
+        page_size = self.cache.page_size
+        # Smallest page-aligned size that still covers the sliding window.
+        tail_size = (self.sliding_window_size + page_size - 1) // page_size * page_size
+        leaf_len = len(leaf.key)
+        if leaf_len <= tail_size:
+            return
+        split_at = leaf_len - tail_size
+        if page_size > 1 and (split_at % page_size != 0 or leaf_len % page_size != 0):
+            return
+
+        self.cache._split_node(leaf.key, leaf, split_at)
 
     def redistribute_on_node_split(
         self, new_parent: UnifiedTreeNode, child: UnifiedTreeNode