Fix SWA eviction tombstoning the last leaf (#29860)

test/registered/unit/mem_cache/test_swa_eviction_boundary.py

@@ -7,7 +7,7 @@ handling for this, creating an incorrect non-tombstone node that caused
 inflated swa_evictable_size_, negative usage, and potential double-free.
 
 Two-sided fix:
-1. _evict_swa subtracts extra page_size (preventive).
+1. _evict_swa subtracts max(window, page) on the radix path (preventive).
 2. _insert_helper early-returns on case 3 (defensive).
 
 Tests use real tree/allocator/pool with mock Req/ScheduleBatch wrappers.
@@ -195,7 +195,7 @@ class TestSWAEvictionBoundary(unittest.TestCase):
     # -- Eviction formula: page_size == 1 --
 
     def test_formula_page_size_1(self):
-        """page_size=1: no floor alignment, -1 just means one less token evicted."""
+        """page_size=1: radix keeps max(window, page)=window, so the frontier is pre_len - window."""
         page_size, window = 1, 4
         tree, allocator, pool = _build_swa_tree(
             page_size=page_size, sliding_window_size=window
@@ -210,7 +210,9 @@ class TestSWAEvictionBoundary(unittest.TestCase):
             ScheduleBatch._evict_swa(batch, req, seq_len - 1)
 
             self.assertLess(req.swa_evicted_seqlen, seq_len)
-            self.assertEqual(req.swa_evicted_seqlen, max(0, seq_len - 1 - window - 1))
+            self.assertEqual(
+                req.swa_evicted_seqlen, max(0, seq_len - 1 - max(window, page_size))
+            )
 
             tree.cache_finished_req(req, is_insert=True)
             tree.sanity_check()
@@ -383,7 +385,9 @@ class TestSWAEvictionBoundary(unittest.TestCase):
             batch = _make_batch(tree, allocator, pool)
             ScheduleBatch._evict_swa(batch, req, seq_len - 1)
 
-            self.assertEqual(req.swa_evicted_seqlen, max(0, seq_len - 1 - window - 1))
+            self.assertEqual(
+                req.swa_evicted_seqlen, max(0, seq_len - 1 - max(window, page_size))
+            )
 
             tree.cache_finished_req(req, is_insert=True)
             tree.sanity_check()

test/registered/unit/mem_cache/test_unified_radix_cache_unittest.py

@@ -1977,7 +1977,7 @@ class UnifiedRadixCacheSuite:
         with envs.SGLANG_OPT_UNIFIED_CACHE_FREE_OUT_OF_WINDOW_SLOTS.override(True):
             tree.cache_unfinished_req(req)
 
-        cushion = self.cfg.sliding_window_size + self.cfg.page_size
+        cushion = max(self.cfg.sliding_window_size, self.cfg.page_size)
         expected_evicted = (pre_len - 1) - cushion
         self.assertEqual(
             req.swa_evicted_seqlen,