Changing One "If" To "While" Caused An Unexpected Shift In A Kernel Benchmark This Week

Several months back you may recall that Linux 5.9 kernel regression we noted that in turn was bisected to code introduced by Linus Torvalds around page lock fairness. That was ultimately worked out in time with allowing a control over the page lock (un)fairness to address the regressed workloads while being fair enough to satisfy his original change. But now this week for Linux 5.11, Linus Torvalds has again altered the behavior. It then ended up causing a PostgreSQL database server performance regression but fortunately any impact should be very minimal and hopefully not appearing in any real-world situation...