Just now I wanted to create a new post, and found there was a newer version of WP, and also there was a message said that the update automatically failed already, so need a manual update.
Such thing never happened before, so I knew this issue should be related with the php-fpm user. I searched and found maybe the directory wp-content/upgrade should be writable by php-fpm user, so I did it. And got a new message said that the unpack was OK, but could not replace some files.
Suddenly I realized such thing never happened in the past, so maybe I should solve this issue in another way, like changing the owner of php-fpm service.
Next I changed the owner and group to ngnix, and restarted it again, then upgraded the WP again, and this time all things went well.
