diff --git a/pageserver/src/tenant/timeline/detach_ancestor.rs b/pageserver/src/tenant/timeline/detach_ancestor.rs
index add0857763..6ec479ae26 100644
--- a/pageserver/src/tenant/timeline/detach_ancestor.rs
+++ b/pageserver/src/tenant/timeline/detach_ancestor.rs
@@ -1280,6 +1280,7 @@ pub(super) async fn detach_and_reparent(
     }
 }
 
+/// Query against a locked `Tenant::timelines`.
 fn reparentable_timelines<'a, I>(
     timelines: I,
     detached: &'a Arc<Timeline>,