diff --git a/src/backend/access/heap/heapam_handler.c b/src/backend/access/heap/heapam_handler.c
index 09a456e9966..cbef73e5d4b 100644
--- a/src/backend/access/heap/heapam_handler.c
+++ b/src/backend/access/heap/heapam_handler.c
@@ -837,7 +837,21 @@ heapam_relation_copy_for_cluster(Relation OldHeap, Relation NewHeap,
 		tuple = ExecFetchSlotHeapTuple(slot, false, NULL);
 		buf = hslot->buffer;
 
-		LockBuffer(buf, BUFFER_LOCK_SHARE);
+		/*
+		 * To be able to guarantee that we can set the hint bit, acquire an
+		 * exclusive lock on the old buffer. We need the hint bits, set in
+		 * heapam_relation_copy_for_cluster() -> HeapTupleSatisfiesVacuum(),
+		 * to be set, as otherwise reform_and_rewrite_tuple() ->
+		 * rewrite_heap_tuple() will get confused. Specifically,
+		 * rewrite_heap_tuple() checks for HEAP_XMAX_INVALID in the old tuple
+		 * to determine whether to check the old-to-new mapping hash table.
+		 *
+		 * It'd be better if we somehow could avoid setting hint bits on the
+		 * old page. One reason to use VACUUM FULL are very bloated tables -
+		 * rewriting most of the old table during VACUUM FULL doesn't exactly
+		 * help...
+		 */
+		LockBuffer(buf, BUFFER_LOCK_EXCLUSIVE);
 
 		switch (HeapTupleSatisfiesVacuum(tuple, OldestXmin, buf))
 		{
diff --git a/src/backend/access/heap/heapam_visibility.c b/src/backend/access/heap/heapam_visibility.c
index 05e70b7d92a..9a034d5c9e8 100644
--- a/src/backend/access/heap/heapam_visibility.c
+++ b/src/backend/access/heap/heapam_visibility.c
@@ -141,6 +141,13 @@ void
 HeapTupleSetHintBits(HeapTupleHeader tuple, Buffer buffer,
 					 uint16 infomask, TransactionId xid)
 {
+	/*
+	 * The uses from heapam.c rely on being able to perform the hint bit
+	 * updates, which can only be guaranteed if we are holding an exclusive
+	 * lock on the buffer - which all callers are doing.
+	 */
+	Assert(BufferIsLockedByMeInMode(buffer, BUFFER_LOCK_EXCLUSIVE));
+
 	SetHintBits(tuple, buffer, infomask, xid);
 }
 
diff --git a/src/backend/access/heap/rewriteheap.c b/src/backend/access/heap/rewriteheap.c
index bae3a2da77a..77fd48eb59e 100644
--- a/src/backend/access/heap/rewriteheap.c
+++ b/src/backend/access/heap/rewriteheap.c
@@ -382,6 +382,9 @@ rewrite_heap_tuple(RewriteState state,
 
 	/*
 	 * If the tuple has been updated, check the old-to-new mapping hash table.
+	 *
+	 * Note that this check relies on the HeapTupleSatisfiesVacuum() in
+	 * heapam_relation_copy_for_cluster() to have set hint bits.
 	 */
 	if (!((old_tuple->t_data->t_infomask & HEAP_XMAX_INVALID) ||
 		  HeapTupleHeaderIsOnlyLocked(old_tuple->t_data)) &&