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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@mail.gmail.com - rebased from 5.14-rc1 - a few fixes (missing ns_free_inum on error path, missing initialization, etc) - permission check changes in pid_table_root_permissions - unsigned int pid_max -> int pid_max (keep pid_max type as it was) - add READ_ONCE in alloc_pid() as suggested by Christian - rebased from 6.7 and take into account: * sysctl: treewide: drop unused argument ctl_table_root::set_ownership(table) * sysctl: treewide: constify ctl_table_header::ctl_table_arg * pidfd: add pidfs * tracing: Move saved_cmdline code into trace_sched_switch.c Signed-off-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@canonical.com> Link: https://lore.kernel.org/r/20241122132459.135120-2-aleksandr.mikhalitsyn@canonical.com Signed-off-by: Christian Brauner <brauner@kernel.org>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.