diff --git a/src/backend/access/transam/subtrans.c b/src/backend/access/transam/subtrans.c
index dc9566fb51b..50bb1d8cfc5 100644
--- a/src/backend/access/transam/subtrans.c
+++ b/src/backend/access/transam/subtrans.c
@@ -311,7 +311,7 @@ StartupSUBTRANS(TransactionId oldestActiveXID)
 	FullTransactionId nextXid;
 	int64		startPage;
 	int64		endPage;
-	LWLock	   *prevlock;
+	LWLock	   *prevlock = NULL;
 	LWLock	   *lock;
 
 	/*
@@ -324,42 +324,27 @@ StartupSUBTRANS(TransactionId oldestActiveXID)
 	nextXid = TransamVariables->nextXid;
 	endPage = TransactionIdToPage(XidFromFullTransactionId(nextXid));
 
-	prevlock = SimpleLruGetBankLock(SubTransCtl, startPage);
-	LWLockAcquire(prevlock, LW_EXCLUSIVE);
-	while (startPage != endPage)
+	for (;;)
 	{
 		lock = SimpleLruGetBankLock(SubTransCtl, startPage);
-
-		/*
-		 * Check if we need to acquire the lock on the new bank then release
-		 * the lock on the old bank and acquire on the new bank.
-		 */
 		if (prevlock != lock)
 		{
-			LWLockRelease(prevlock);
+			if (prevlock)
+				LWLockRelease(prevlock);
 			LWLockAcquire(lock, LW_EXCLUSIVE);
 			prevlock = lock;
 		}
 
 		(void) ZeroSUBTRANSPage(startPage);
+		if (startPage == endPage)
+			break;
+
 		startPage++;
 		/* must account for wraparound */
 		if (startPage > TransactionIdToPage(MaxTransactionId))
 			startPage = 0;
 	}
 
-	lock = SimpleLruGetBankLock(SubTransCtl, startPage);
-
-	/*
-	 * Check if we need to acquire the lock on the new bank then release the
-	 * lock on the old bank and acquire on the new bank.
-	 */
-	if (prevlock != lock)
-	{
-		LWLockRelease(prevlock);
-		LWLockAcquire(lock, LW_EXCLUSIVE);
-	}
-	(void) ZeroSUBTRANSPage(startPage);
 	LWLockRelease(lock);
 }
 
diff --git a/src/test/recovery/t/009_twophase.pl b/src/test/recovery/t/009_twophase.pl
index a3cdcf32408..701f9cc20f8 100644
--- a/src/test/recovery/t/009_twophase.pl
+++ b/src/test/recovery/t/009_twophase.pl
@@ -45,6 +45,10 @@ $node_london->backup('london_backup');
 my $node_paris = PostgreSQL::Test::Cluster->new('paris');
 $node_paris->init_from_backup($node_london, 'london_backup',
 	has_streaming => 1);
+$node_paris->append_conf(
+	'postgresql.conf', qq(
+	subtransaction_buffers = 32
+));
 $node_paris->start;
 
 # Switch to synchronous replication in both directions
@@ -481,4 +485,42 @@ is( $psql_out,
 	qq{27|issued to paris},
 	"Check expected t_009_tbl2 data on standby");
 
+
+# Exercise the 2PC recovery code in StartupSUBTRANS, which is concerned with
+# ensuring that enough pg_subtrans pages exist on disk to cover the range of
+# prepared transactions at server start time.  There's not much we can verify
+# directly, but let's at least get the code to run.
+$cur_standby->stop();
+configure_and_reload($cur_primary, "synchronous_standby_names = ''");
+
+$cur_primary->safe_psql('postgres', "CHECKPOINT");
+
+my $start_lsn =
+  $cur_primary->safe_psql('postgres', 'select pg_current_wal_insert_lsn()');
+$cur_primary->safe_psql('postgres',
+	"CREATE TABLE test(); BEGIN; CREATE TABLE test1(); PREPARE TRANSACTION 'foo';"
+);
+my $osubtrans = $cur_primary->safe_psql('postgres',
+	"select 'pg_subtrans/'||f, s.size from pg_ls_dir('pg_subtrans') f, pg_stat_file('pg_subtrans/'||f) s"
+);
+$cur_primary->pgbench(
+	'--no-vacuum --client=5 --transactions=1000',
+	0,
+	[],
+	[],
+	'pgbench run to cause pg_subtrans traffic',
+	{
+		'009_twophase.pgb' => 'insert into test default values'
+	});
+# StartupSUBTRANS is exercised with a wide range of visible XIDs in this
+# stop/start sequence, because we left a prepared transaction open above.
+# Also, setting subtransaction_buffers to 32 above causes to switch SLRU
+# bank, for additional code coverage.
+$cur_primary->stop;
+$cur_primary->start;
+my $nsubtrans = $cur_primary->safe_psql('postgres',
+	"select 'pg_subtrans/'||f, s.size from pg_ls_dir('pg_subtrans') f, pg_stat_file('pg_subtrans/'||f) s"
+);
+isnt($osubtrans, $nsubtrans, "contents of pg_subtrans/ have changed");
+
 done_testing();