diff --git a/src/backend/replication/logical/decode.c b/src/backend/replication/logical/decode.c
index 4144a43afd..cfa955a679 100644
--- a/src/backend/replication/logical/decode.c
+++ b/src/backend/replication/logical/decode.c
@@ -618,9 +618,9 @@ logicalmsg_decode(LogicalDecodingContext *ctx, XLogRecordBuffer *buf)
 		return;
 
 	/*
-	 * We can also skip decoding when in 'fast_forward' mode. This check must
-	 * be last because we don't want to set that processing_required flag
-	 * unnecessarily.
+	 * We also skip decoding in 'fast_forward' mode. This check must be last
+	 * because we don't want to set the processing_required flag unless
+	 * we have a decodable message.
 	 */
 	if (ctx->fast_forward)
 	{
@@ -1307,8 +1307,8 @@ DecodeTXNNeedSkip(LogicalDecodingContext *ctx, XLogRecordBuffer *buf,
 		return true;
 
 	/*
-	 * We can also skip decoding when in 'fast_forward' mode. In passing set
-	 * the 'processing_required' flag to indicate, were it not for this mode,
+	 * We also skip decoding in 'fast_forward' mode. In passing set the
+	 * 'processing_required' flag to indicate, were it not for this mode,
 	 * processing *would* have been required.
 	 */
 	if (ctx->fast_forward)
diff --git a/src/backend/replication/logical/logical.c b/src/backend/replication/logical/logical.c
index 32869a75ab..e02cd0fa44 100644
--- a/src/backend/replication/logical/logical.c
+++ b/src/backend/replication/logical/logical.c
@@ -1953,9 +1953,9 @@ UpdateDecodingStats(LogicalDecodingContext *ctx)
 }
 
 /*
- * Read to end of WAL starting from the decoding slot's restart_lsn. Return
- * true if any meaningful/decodable WAL records are encountered, otherwise
- * false.
+ * Read up to the end of WAL starting from the decoding slot's restart_lsn.
+ * Return true if any meaningful/decodable WAL records are encountered,
+ * otherwise false.
  *
  * Although this function is currently used only during pg_upgrade, there are
  * no reasons to restrict it, so IsBinaryUpgrade is not checked here.
diff --git a/src/backend/utils/adt/pg_upgrade_support.c b/src/backend/utils/adt/pg_upgrade_support.c
index 2a831bc397..a3a8ade405 100644
--- a/src/backend/utils/adt/pg_upgrade_support.c
+++ b/src/backend/utils/adt/pg_upgrade_support.c
@@ -274,8 +274,8 @@ binary_upgrade_set_missing_value(PG_FUNCTION_ARGS)
  * Returns true if there are no decodable WAL records after the
  * confirmed_flush_lsn. Otherwise false.
  *
- * This is a special purpose function to ensure the given slot can be upgraded
- * without data loss.
+ * This is a special purpose function to ensure that the given slot can be
+ * upgraded without data loss.
  */
 Datum
 binary_upgrade_slot_has_pending_wal(PG_FUNCTION_ARGS)
@@ -294,16 +294,10 @@ binary_upgrade_slot_has_pending_wal(PG_FUNCTION_ARGS)
 
 	slot_name = PG_GETARG_NAME(0);
 
-	/*
-	 * Acquire the given slot. There should be no error because the caller has
-	 * already checked the slot exists.
-	 */
+	/* Acquire the given slot. */
 	ReplicationSlotAcquire(NameStr(*slot_name), true);
 
-	/*
-	 * It's caller's responsibility to check the health of the slot.  Upcoming
-	 * functions assume the restart_lsn points to a valid record.
-	 */
+	/* Slots must be valid as otherwise we won't be able to scan the WAL. */
 	Assert(MyReplicationSlot->data.invalidated == RS_INVAL_NONE);
 
 	end_of_wal = GetFlushRecPtr(NULL);
diff --git a/src/bin/pg_upgrade/check.c b/src/bin/pg_upgrade/check.c
index 123f47a81f..8f3f5585a4 100644
--- a/src/bin/pg_upgrade/check.c
+++ b/src/bin/pg_upgrade/check.c
@@ -1541,10 +1541,8 @@ check_new_cluster_logical_replication_slots(void)
 /*
  * check_old_cluster_for_valid_slots()
  *
- * Verify that all the logical slots are usable and have consumed all the WAL
- * before shutdown. The check has already been done in
- * get_old_cluster_logical_slot_infos(), so this function reads the result and
- * reports to the user.
+ * Verify that all the logical slots are valid and have consumed all the WAL
+ * before shutdown.
  */
 static void
 check_old_cluster_for_valid_slots(bool live_check)
@@ -1607,7 +1605,7 @@ check_old_cluster_for_valid_slots(bool live_check)
 		fclose(script);
 
 		pg_log(PG_REPORT, "fatal");
-		pg_fatal("Your installation contains logical replication slots that cannot be upgraded.\n"
+		pg_fatal("Your installation contains invalid logical replication slots.\n"
 				 "These slots can't be copied, so this cluster cannot be upgraded.\n"
 				 "Consider removing invalid slots and/or consuming the pending WAL if any,\n"
 				 "and then restart the upgrade.\n"
diff --git a/src/bin/pg_upgrade/info.c b/src/bin/pg_upgrade/info.c
index c56769fe54..5494e69227 100644
--- a/src/bin/pg_upgrade/info.c
+++ b/src/bin/pg_upgrade/info.c
@@ -651,8 +651,8 @@ get_old_cluster_logical_slot_infos(DbInfo *dbinfo, bool live_check)
 	/*
 	 * Fetch the logical replication slot information. The check whether the
 	 * slot is considered caught up is done by an upgrade function. This
-	 * regards the slot is caught up if any changes are not found while
-	 * decoding. See binary_upgrade_slot_has_pending_wal().
+	 * regards the slot as caught up if we don't find any decodable changes.
+	 * See binary_upgrade_slot_has_pending_wal().
 	 *
 	 * Note that we can't ensure whether the slot is caught up during
 	 * live_check as the new WAL records could be generated.
diff --git a/src/bin/pg_upgrade/pg_upgrade.c b/src/bin/pg_upgrade/pg_upgrade.c
index 7acdf31d02..3960af4036 100644
--- a/src/bin/pg_upgrade/pg_upgrade.c
+++ b/src/bin/pg_upgrade/pg_upgrade.c
@@ -190,7 +190,12 @@ main(int argc, char **argv)
 	check_ok();
 
 	/*
-	 * If the old cluster has logical slots, migrate them to a new cluster.
+	 * Migrate the logical slots to the new cluster.  Note that we need to do
+	 * this after resetting WAL because otherwise the required WAL would be
+	 * removed and slots would become unusable.  There is a possibility that
+	 * background processes might generate some WAL before we could create the
+	 * slots in the new cluster but we can ignore that WAL as that won't be
+	 * required downstream.
 	 */
 	if (count_old_cluster_logical_slots())
 	{
@@ -890,7 +895,6 @@ create_logical_replication_slots(void)
 		LogicalSlotInfoArr *slot_arr = &old_db->slot_arr;
 		PGconn	   *conn;
 		PQExpBuffer query;
-		char		log_file_name[MAXPGPATH];
 
 		/* Skip this database if there are no slots */
 		if (slot_arr->nslots == 0)
@@ -899,9 +903,6 @@ create_logical_replication_slots(void)
 		conn = connectToServer(&new_cluster, old_db->db_name);
 		query = createPQExpBuffer();
 
-		snprintf(log_file_name, sizeof(log_file_name),
-				 DB_DUMP_LOG_FILE_MASK, old_db->db_oid);
-
 		pg_log(PG_STATUS, "%s", old_db->db_name);
 
 		for (int slotnum = 0; slotnum < slot_arr->nslots; slotnum++)
diff --git a/src/include/replication/logical.h b/src/include/replication/logical.h
index 355247a58b..f8258d7c28 100644
--- a/src/include/replication/logical.h
+++ b/src/include/replication/logical.h
@@ -110,11 +110,7 @@ typedef struct LogicalDecodingContext
 	/* Are we processing the end LSN of a transaction? */
 	bool		end_xact;
 
-	/*
-	 * Did the logical decoding context require processing WALs?
-	 *
-	 * This flag is used only when in 'fast_forward' mode.
-	 */
+	/* Do we need to process any change in 'fast_forward' mode? */
 	bool		processing_required;
 } LogicalDecodingContext;