Linux KVM is meant for VM guests and probably therefore was set to CONFIG_BASE_SMALL=1. The discussion "if this makes sense or not" is up to the kernel team but it seems this bug has got no attention by them yet :-/.
I was quickly assessing the case and found: - Linux-generic is on CONFIG_BASE_SMALL=0 in B/F/G/H as one would expect. - linux-kvm in B/F/G/H was all the time set to CONFIG_BASE_SMALL=0 - the option we talk about here is [1][2] - IMHO "speed loss for saving a few KB" does not sound right. IIRC linux-kvm was meant more for speed than density. We are virtual, but not as small as an embedded controller for example. - This goes back to [3] when linux-kvm was introduced - I didn't find a commit in [4] that listed any extended reasoning/discussion when enabling [5] it. IMHO I agree and this should be switched in the kernel config. I don't think that changing the pid-max was wrong. Even older Xenial linux-kvm running in 2020 can be considered bigger than what CONFIG_BASE_SMALL was meant for. But further actions are for the kernel Team to consider and make. Therefore I'm adding a src:linux task, subscribing some more people and pinging the kernel team internally to get some attention. [1]: https://mirrors.edge.kernel.org/pub/linux/kernel/people/akpm/patches/2.6/2.6.11-rc5/2.6.11-rc5-mm1/broken-out/base-small-introduce-the-config_base_small-flag.patch [2]: https://github.com/torvalds/linux/blob/master/init/Kconfig#L1518 [3]: https://kernel.ubuntu.com/~kernel-ppa/config/xenial/linux-kvm/4.4.0-1004.9/amd64-config.flavour.kvm [5]: https://git.launchpad.net/~canonical-kernel/ubuntu/+source/linux-kvm/tree/debian.kvm/config/config.common.ubuntu#n157 [4]: https://git.launchpad.net/~canonical-kernel/ubuntu/+source/linux-kvm ** Also affects: linux (Ubuntu) Importance: Undecided Status: New -- You received this bug notification because you are a member of Kernel Packages, which is subscribed to linux-kvm in Ubuntu. https://bugs.launchpad.net/bugs/1866149 Title: CONFIG_BASE_SMALL=1 restricts pid space, which conflicts with systemd default sysctl Status in linux package in Ubuntu: Incomplete Status in linux-kvm package in Ubuntu: Confirmed Bug description: I'm not completely sure which package to log this against. I'm running the kvm focal minimal cloud image from 20200302. I noticed on boot that there was an error complaining that systemd-systemctl couldn't update pid_max to the value it wanted: systemd-sysctl[117]: Couldn't write '4194304' to 'kernel/pid_max': Invalid argument Digging into it a bit more, this comes from /usr/lib/sysctl.d/50-pid-max.conf: # Bump the numeric PID range to its maximum of 2^22 (from the in-kernel default # of 2^16), to make PID collisions less likely. kernel.pid_max = 4194304 However, the linux-image-kvm kernel is compiled with CONFIG_BASE_SMALL=1 and this triggers the following code in include/linux/threads.h #define PID_MAX_LIMIT (CONFIG_BASE_SMALL ? PAGE_SIZE * 8 : \ (sizeof(long) > 4 ? 4 * 1024 * 1024 : PID_MAX_DEFAULT)) which means that if CONFIG_BASE_SMALL is set we get a maximum limit of PAGE_SIZE * 8, which on x86 would be 32768. As a workaround I can override it with a file in /etc/sysctl.d/ but this shouldn't be needed. I really don't know if CONFIG_BASE_SMALL makes any sense on x86 cloud images, they really aren't small machines in the scheme of things! Cheers David To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1866149/+subscriptions -- Mailing list: https://launchpad.net/~kernel-packages Post to : [email protected] Unsubscribe : https://launchpad.net/~kernel-packages More help : https://help.launchpad.net/ListHelp
