Hey Linus, /* Summary */ The pid_max sysctl is a global value. For a long time the default value has been 65535 and during the pidfd dicussions Linus proposed to bump pid_max by default (cf. [1]). Based on this discussion systemd started bumping pid_max to 2^22. So all new systems now run with a very high pid_max limit with some distros having also backported that change. The decision to bump pid_max is obviously correct. It just doesn't make a lot of sense nowadays to enforce such a low pid number. There's sufficient tooling to make selecting specific processes without typing really large pid numbers available. In any case, there are workloads that have expections about how large pid numbers they accept. Either for historical reasons or architectural reasons. One concreate example is the 32-bit version of Android's bionic libc which requires pid numbers less than 65536. There are workloads where it is run in a 32-bit container on a 64-bit kernel. If the host has a pid_max value greater than 65535 the libc will abort thread creation because of size assumptions of pthread_mutex_t. That's a fairly specific use-case however, in general specific workloads that are moved into containers running on a host with a new kernel and a new systemd can run into issues with large pid_max values. Obviously making assumptions about the size of the allocated pid is suboptimal but we have userspace that does it. Of course, giving containers the ability to restrict the number of processes in their respective pid namespace indepent of the global limit through pid_max is something desirable in itself and comes in handy in general. Independent of motivating use-cases the existence of pid namespaces makes this also a good semantical extension and there have been prior proposals pushing in a similar direction. The trick here is to minimize the risk of regressions which I think is doable. The fact that pid namespaces are hierarchical will help us here. What we mostly care about is that when the host sets a low pid_max limit, say (crazy number) 100 that no descendant pid namespace can allocate a higher pid number in its namespace. Since pid allocation is hierarchial this can be ensured by checking each pid allocation against the pid namespace's pid_max limit. This means if the allocation in the descendant pid namespace succeeds, the ancestor pid namespace can reject it. If the ancestor pid namespace has a higher limit than the descendant pid namespace the descendant pid namespace will reject the pid allocation. The ancestor pid namespace will obviously not care about this. All in all this means pid_max continues to enforce a system wide limit on the number of processes but allows pid namespaces sufficient leeway in handling workloads with assumptions about pid values and allows containers to restrict the number of processes in a pid namespace through the pid_max interface. [1]: https://lore.kernel.org/linux-api/CAHk-=wiZ40LVjnXSi9iHLE_-ZBsWFGCgdmNiYZUXn1-V5YBg2g@xxxxxxxxxxxxxx /* Testing */ gcc version 14.2.0 (Debian 14.2.0-6) Debian clang version 16.0.6 (27+b1) No build failures or warnings were observed. /* Conflicts */ Merge conflicts with mainline ============================= No known conflicts. Merge conflicts with other trees ================================ This will have a merge conflict with vfs-6.14.mount pull request sent in https://lore.kernel.org/r/20250118-vfs-pidfs-5921bfa5632a@brauner and it can be resolved as follows: diff --cc kernel/pid.c index aa2a7d4da455,ce3e94e26a0f..000000000000 --- a/kernel/pid.c +++ b/kernel/pid.c @@@ -61,10 -60,13 +61,8 @@@ struct pid init_struct_pid = }, } }; - int pid_max = PID_MAX_DEFAULT; - - int pid_max_min = RESERVED_PIDS + 1; - int pid_max_max = PID_MAX_LIMIT; + static int pid_max_min = RESERVED_PIDS + 1; + static int pid_max_max = PID_MAX_LIMIT; -/* - * Pseudo filesystems start inode numbering after one. We use Reserved - * PIDs as a natural offset. - */ -static u64 pidfs_ino = RESERVED_PIDS; /* * PID-map pages start out as NULL, they get allocated upon The following changes since commit 40384c840ea1944d7c5a392e8975ed088ecf0b37: Linux 6.13-rc1 (2024-12-01 14:28:56 -0800) are available in the Git repository at: git@xxxxxxxxxxxxxxxxxxx:pub/scm/linux/kernel/git/vfs/vfs tags/kernel-6.14-rc1.pid for you to fetch changes up to c625aa276319f51e307ca10401baac4628bb25c2: Merge patch series "pid_namespace: namespacify sysctl kernel.pid_max" (2024-12-02 11:25:26 +0100) Please consider pulling these changes from the signed kernel-6.14-rc1.pid tag. Thanks! Christian ---------------------------------------------------------------- kernel-6.14-rc1.pid ---------------------------------------------------------------- Christian Brauner (3): pid: allow pid_max to be set per pid namespace tests/pid_namespace: add pid_max tests Merge patch series "pid_namespace: namespacify sysctl kernel.pid_max" include/linux/pid.h | 3 - include/linux/pid_namespace.h | 10 +- kernel/pid.c | 125 +++++++- kernel/pid_namespace.c | 43 ++- kernel/sysctl.c | 9 - kernel/trace/pid_list.c | 2 +- kernel/trace/trace.h | 2 - kernel/trace/trace_sched_switch.c | 2 +- tools/testing/selftests/pid_namespace/.gitignore | 1 + tools/testing/selftests/pid_namespace/Makefile | 2 +- tools/testing/selftests/pid_namespace/pid_max.c | 358 +++++++++++++++++++++++ 11 files changed, 521 insertions(+), 36 deletions(-) create mode 100644 tools/testing/selftests/pid_namespace/pid_max.c
