diff --git a/src/osd/OSD.cc b/src/osd/OSD.cc
index 6524e1e9a2d..dd72fbb29f9 100644
--- a/src/osd/OSD.cc
+++ b/src/osd/OSD.cc
@@ -8679,6 +8679,8 @@ void OSD::handle_pg_backfill_reserve(OpRequestRef op)
 	m->query_epoch,
 	PG::RemoteBackfillReserved()));
   } else if (m->type == MBackfillReserve::REJECT) {
+    // NOTE: this is replica -> primary "i reject your request"
+    //      and also primary -> replica "cancel my previously-granted request"
     evt = PG::CephPeeringEvtRef(
       new PG::CephPeeringEvt(
 	m->query_epoch,