Fix SWA eviction tombstoning the last leaf (#29860)

python/sglang/srt/environ.py

@@ -881,7 +881,6 @@ class Envs:
     SGLANG_OPT_SWA_RADIX_CACHE_COMPACT = EnvBool(False)
     SGLANG_OPT_SWA_SPLIT_LEAF_ON_INSERT = EnvBool(False)
     SGLANG_OPT_SWA_RELEASE_LEAF_LOCK_AFTER_WINDOW = EnvBool(False)
-    SGLANG_OPT_SWA_EVICT_DROP_PAGE_MARGIN = EnvBool(False)
 
     # Unified radix cache
     SGLANG_OPT_UNIFIED_CACHE_FREE_OUT_OF_WINDOW_SLOTS = EnvBool(False)
@@ -1037,6 +1036,7 @@ def _convert_SGL_to_SGLANG():
         "SGLANG_ENABLE_TP_MEMORY_INBALANCE_CHECK",
     )
     _print_deprecated_env("SGLANG_PER_TOKEN_GROUP_QUANT_8BIT_V2")
+    _print_deprecated_env("SGLANG_OPT_SWA_EVICT_DROP_PAGE_MARGIN")
     _print_deprecated_env("SGLANG_ENABLE_THINKING", "SGLANG_DEFAULT_THINKING")
     _print_deprecated_env("SGLANG_REASONING_EFFORT", "SGLANG_DSV4_REASONING_EFFORT")
     _print_deprecated_env(

python/sglang/srt/managers/schedule_batch.py

@@ -2930,7 +2930,7 @@ class ScheduleBatch(ScheduleBatchDisaggregationDecodeMixin):
             page_size=self.tree_cache.page_size,
             req_to_token_pool=self.req_to_token_pool,
             token_to_kv_pool_allocator=self.token_to_kv_pool_allocator,
-            drop_page_margin=self.tree_cache.is_chunk_cache(),
+            is_chunk_cache=self.tree_cache.is_chunk_cache(),
         )
 
     def __str__(self):

python/sglang/srt/mem_cache/common.py

@@ -73,10 +73,8 @@ def free_swa_out_of_window_slots(
     page_size: int,
     req_to_token_pool: ReqToTokenPool,
     token_to_kv_pool_allocator: BaseTokenToKVPoolAllocator,
-    drop_page_margin: bool = False,
+    is_chunk_cache: bool = False,
 ) -> None:
-    from sglang.srt.environ import envs
-
     # For swa radix cache, we need to evict the tokens that are not in the tree cache and also not in the sliding window
     assert (
         req.cache_protected_len % page_size == 0
@@ -86,13 +84,16 @@ def free_swa_out_of_window_slots(
         evict_floor = -(-evict_floor // page_size) * page_size
     req.swa_evicted_seqlen = max(req.swa_evicted_seqlen, evict_floor)
 
-    # Subtract an extra page_size so the eviction frontier never reaches the
-    # radix tree insert boundary, keeping >=1 page of non-evicted SWA KV for the
-    # tree to store as a non-tombstone node (else leaf nodes tombstone -> SWA leak).
-    if drop_page_margin or envs.SGLANG_OPT_SWA_EVICT_DROP_PAGE_MARGIN.get():
+    if is_chunk_cache:
+        # Chunk cache builds no radix tree, so no tombstone-leaf concern; evict
+        # up to the window boundary (the trailing floor keeps it page-aligned).
         evict_threshold = pre_len - sliding_window_size
     else:
-        evict_threshold = pre_len - sliding_window_size - page_size
+        # Radix cache: keep max(window, page). The trailing floor page-aligns the
+        # frontier, and subtracting at least one page keeps it below the insert
+        # boundary (page_floor(seq_len)) so the last leaf is never all-tombstone.
+        # No extra page margin is needed.
+        evict_threshold = pre_len - max(sliding_window_size, page_size)
     new_swa_evicted_seqlen = max(
         req.swa_evicted_seqlen,
         evict_threshold,

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,