[Fix] Respect cache_protected_len in ChunkCache and disabled-radix release paths (#31662)

python/sglang/srt/mem_cache/chunk_cache.py

@@ -80,8 +80,9 @@ class ChunkCache(BasePrefixCache):
         self, req: Req, is_insert: bool = True, *, kv_len_to_handle: int
     ):
         # For decode server: if req.output_ids is empty, we want to free all req.origin_input_ids
+        # The protected prefix is not this req's to free.
         kv_indices = self.req_to_token_pool.req_to_token[
-            req.req_pool_idx, :kv_len_to_handle
+            req.req_pool_idx, req.cache_protected_len : kv_len_to_handle
         ]
         self.token_to_kv_pool_allocator.free(kv_indices)
 
@@ -146,9 +147,10 @@ class PureSWAChunkCache(SWAChunkCache):
     explicitly skip the range already freed by ``free_swa_out_of_window_slots``
     (a.k.a. _evict_swa) during decode.
 
-    ``req.swa_evict_floor`` only protects the prompt/image KV while the request
-    is active. ChunkCache does not retain finished prefixes, so the protected
-    prefix is released here when the request finishes.
+    ``req.swa_evict_floor`` shields the prompt/image KV from window eviction
+    only while the request is active, so that range IS released here on
+    finish. Distinct from the ``cache_protected_len`` prefix, which is owned
+    elsewhere and never freed by this path.
     """
 
     def cache_finished_req(
@@ -158,15 +160,19 @@ class PureSWAChunkCache(SWAChunkCache):
         kv_indices = self.req_to_token_pool.req_to_token[
             req.req_pool_idx, :kv_committed_len
         ]
+        # The cache_protected_len prefix is not this req's to free.
+        protected_len = req.cache_protected_len
         evict_floor = req.swa_evict_floor
         evicted_seqlen = req.kv.swa_evicted_seqlen
         if evicted_seqlen > evict_floor:
             parts = []
-            if evict_floor > 0:
-                parts.append(kv_indices[:evict_floor])
+            if evict_floor > protected_len:
+                parts.append(kv_indices[protected_len:evict_floor])
             if evicted_seqlen < kv_committed_len:
-                parts.append(kv_indices[evicted_seqlen:kv_committed_len])
+                parts.append(
+                    kv_indices[max(evicted_seqlen, protected_len) : kv_committed_len]
+                )
             if parts:
                 self.token_to_kv_pool_allocator.free(torch.cat(parts))
         else:
-            self.token_to_kv_pool_allocator.free(kv_indices)
+            self.token_to_kv_pool_allocator.free(kv_indices[protected_len:])

python/sglang/srt/mem_cache/radix_cache.py

@@ -443,8 +443,9 @@ class RadixCache(SessionRadixCacheMixin, KVCacheEventMixin, BasePrefixCache):
             is_insert = False
 
         if self.disable:
+            # The protected prefix is not this req's to free.
             kv_indices = self.req_to_token_pool.req_to_token[
-                req.req_pool_idx, :kv_len_to_handle
+                req.req_pool_idx, req.cache_protected_len : kv_len_to_handle
             ]
             self.token_to_kv_pool_allocator.free(kv_indices)
             return

test/registered/unit/mem_cache/test_pure_swa_chunk_cache.py

@@ -23,6 +23,7 @@ class _FakeAllocator:
 class _FakeReq:
     req_pool_idx = 0
     swa_evict_floor = 3
+    cache_protected_len = 0
     kv = SimpleNamespace(swa_evicted_seqlen=6)
 
     def pop_committed_kv_cache(self):
@@ -30,12 +31,16 @@ class _FakeReq:
 
 
 class TestPureSWAChunkCache(CustomTestCase):
-    def test_finished_req_skips_already_evicted_swa_range(self):
+    def _make_cache(self):
         cache = PureSWAChunkCache.__new__(PureSWAChunkCache)
         cache.req_to_token_pool = SimpleNamespace(
             req_to_token=torch.arange(10, dtype=torch.int64).unsqueeze(0)
         )
         cache.token_to_kv_pool_allocator = _FakeAllocator()
+        return cache
+
+    def test_finished_req_skips_already_evicted_swa_range(self):
+        cache = self._make_cache()
 
         cache.cache_finished_req(_FakeReq(), kv_len_to_handle=8)
 
@@ -43,6 +48,16 @@ class TestPureSWAChunkCache(CustomTestCase):
         freed = cache.token_to_kv_pool_allocator.freed[0]
         self.assertTrue(torch.equal(freed, torch.tensor([0, 1, 2, 6, 7])))
 
+    def test_finished_req_skips_protected_prefix(self):
+        cache = self._make_cache()
+        req = _FakeReq()
+        req.cache_protected_len = 2
+
+        cache.cache_finished_req(req, kv_len_to_handle=8)
+
+        freed = cache.token_to_kv_pool_allocator.freed[0]
+        self.assertTrue(torch.equal(freed, torch.tensor([2, 6, 7])))
+
 
 if __name__ == "__main__":
     unittest.main()