[PATCH 0/2] Alternative "pid_max" for 32-bit userspace

pid_max is sort of a legacy limit (its value and partially the concept
too, given the existence of pids cgroup controller).
It is tempting to make the pid_max value part of a pid namespace to
provide compat environment for 32-bit applications [1].  On the other
hand, it provides yet another mechanism for limitation of task count.
Even without namespacing of pid_max value, the configuration of
conscious limit is confusing for users [2].

This series builds upon the idea of restricting the number (amount) of
tasks by pids controller and ensuring that number (pid) never exceeds
the amount of tasks. This would not currently work out of the box
because next-fit pid allocation would continue to assign numbers (pids)
higher than the actual amount (there would be gaps in the lower range of
the interval). The patch 2/2 implements this idea by extending semantics
of ns_last_pid knob to allow first-fit numbering. (The implementation
has clumsy ifdefery, which can might be dropped since it's too
x86-centric.)

The patch 1/2 is a mere revert to simplify pid_max to one global limit
only.

(I pruned Cc: list from scripts/get_maintainer.pl for better focus, feel
free to bounce as necessary.)

[1] https://lore.kernel.org/r/20241122132459.135120-1-aleksandr.mikhalitsyn@canonical.com/ (local)
[2] https://lore.kernel.org/r/bnxhqrq7tip6jl2hu6jsvxxogdfii7ugmafbhgsogovrchxfyp@kagotkztqurt/ (local)

Michal Koutný (2):
  Revert "pid: allow pid_max to be set per pid namespace"
  pid: Optional first-fit pid allocation

 Documentation/admin-guide/sysctl/kernel.rst |   2 +
 include/linux/pid.h                         |   3 +
 include/linux/pid_namespace.h               |  11 +-
 kernel/pid.c                                | 137 +++-----------------
 kernel/pid_namespace.c                      |  71 +++++-----
 kernel/sysctl.c                             |   9 ++
 kernel/trace/pid_list.c                     |   2 +-
 kernel/trace/trace.h                        |   2 +
 kernel/trace/trace_sched_switch.c           |   2 +-
 9 files changed, 70 insertions(+), 169 deletions(-)


base-commit: 334426094588f8179fe175a09ecc887ff0c75758
-- 
2.48.1