[HiCache] feat: truncate mamba prefetch length to available host KV size (#26945)

python/sglang/srt/mem_cache/hi_mamba_radix_cache.py

@@ -1735,6 +1735,16 @@ class HiMambaRadixCache(MambaRadixCache):
             prefetch_length,
             self.evict_host,
         )
+        if host_indices is None:
+            # truncate the prefetch length to the page-aligned available host size
+            available_size = self.cache_controller.mem_pool_host.available_size()
+            prefetch_length = available_size - (available_size % self.page_size)
+            if prefetch_length < self.prefetch_threshold:
+                self._release_host_node(last_host_node, release_mamba=False)
+                return
+            new_input_tokens = new_input_tokens[:prefetch_length]
+            host_indices = self.cache_controller.mem_pool_host.alloc(prefetch_length)
+
         if host_indices is None:
             self._release_host_node(last_host_node, release_mamba=False)
             return