[HiCache] Clamp tombstoned SWA locs in UnifiedSWAKVPool translation (#35933)

python/sglang/srt/mem_cache/unified_memory_pool.py

@@ -1424,12 +1424,18 @@ class UnifiedSWAKVPool(SWAKVPool):
             "attach_allocators"
         )
         ps = self._swa_allocator.page_size
+        # Tombstone-safety clamp, matching MultiEndedAllocator.translate_kv_loc:
+        # a tombstoned v2p entry (-1) must not reach the caller as a negative
+        # loc. Clamp to 0 routes it to the reserved padding sink instead.
         if ps == 1:
-            return self._swa_allocator.virtual_to_physical[kv_indices].to(torch.int32)
-        virt_pages = kv_indices // ps
-        offsets = kv_indices % ps
-        swa_phys_pages = self._swa_allocator.virtual_to_physical[virt_pages]
-        return (swa_phys_pages * ps + offsets).to(torch.int32)
+            swa_locs = self._swa_allocator.virtual_to_physical[kv_indices]
+        else:
+            virt_pages = kv_indices // ps
+            offsets = kv_indices % ps
+            swa_phys_pages = self._swa_allocator.virtual_to_physical[virt_pages]
+            # Tombstoned page: -1 * ps + offset lands in [-ps, -1].
+            swa_locs = swa_phys_pages * ps + offsets
+        return swa_locs.clamp(min=0).to(torch.int32)
 
     def get_state_buf_infos(self):
         return self.swa_kv_pool.get_contiguous_buf_infos()