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Modern Versions of systemd Can Cause an Unmount Storm During Shutdowns

Accepted submission by canopic jug at 2020-05-10 04:42:45

System adminsitrator Chris Siebenmann has found a work-around to the out of memory lock up caused by recent systemd's unmount storms [].

One of my discoveries about Ubuntu 20.04 is that my test machine can trigger the kernel's out of memory killing during shutdown. My test virtual machine has 4 GB of RAM and 1 GB of swap, but it also has 347 NFS mounts, and after some investigation, what appears to be happening is that in the 20.04 version of systemd (systemd 245 plus whatever changes Ubuntu has made), systemd now seems to try to run umount for all of those filesystems all at once (which also starts a umount.nfs process for each one). On 20.04, this is apparently enough to OOM my test machine.

We've been here before and there is certainly more where that came from.

(2020) Linux Home Directory Management is About to Undergo Major Change []
(2019) System Down: A systemd-journald Exploit []
(2017) Savaged by Systemd []
(2017) Linux systemd Gives Root Privileges to Invalid Usernames []
(2016) Systemd Crashing Bug []
(2015) tmux Coders Asked to Add Special Code for systemd []
(2016) SystemD Mounts EFI pseudo-fs RW, Facilitates Permanently Bricking Laptops, Closes Bug Invalid []
(2015) A Technical Critique of Systemd []
(2014) Devuan Developers Can Be Reached Via []
(2014) Systemd-resolved Subject to Cache Poisoning []

Original Submission