So, in actuality, I think your assertion just taught us all something, because despite knowing that the OOM killer and that the Magic SysRq key[1] exists, I didn't know you could configure this as an input!
You could launch it as a systemd user target with OOMScoreAdjust=500 in the service section; weird and unconventional but wrapped in .desktop file it doesn't appear to be unwieldy.
It looks like it does, which depending on your goal is either helpful or part of the problem. By default processes should inherit their parent's oom_score_adj. If I exit out of firefox completely, then start it up (with no saved tabs), this is the behavior I see:
$ firefox-esr& PID=$!; choom -p $PID -n 42
[1] 105360
pid 105360's OOM score adjust value changed from 0 to 42
$ for p in $(ps --ppid $PID -opid --no-headers $PID); do printf "%3d" $(</proc/$p/oom_score_adj); ps -opid,comm --no-headers $p; done
0 105360 firefox-esr
0 105425 Socket Process
167 105451 Privileged Cont
0 105456 RDD Process
100 105495 WebExtensions
0 105524 Utility Process
233 105534 Web Content
233 105542 Web Content
233 105549 Web Content
See how each firefox process has a different oom_score_adj with Web Content being more likely to be killed than other processes (233), and none of them have the value that the process was started with (42). This is Firefox 140.11 ESR running on Debian 13.
Yes this would be nice. Or maybe the OOM system would have two other files, /sys/oom/kill_first and /sys/oom/kill_never which would solve the problem more directly for the majority of cases.
I should really send a patch rather than complaining ...
If it helps, I run ff in systemd-run with memory limits set -- that's usually enough to avoid the problem in the first place (ff does freeze when loading google spreadsheets or whatever heavy UI, so I also have a script to adjust /sys/fs/cgroup/user.slice/user-1000.slice/user@1000.service/app.slice/ff-*.scope/memory.max and memory.high at runtime... I should publish my $bindir someday)
You still need some way to make the kernel to send those signals to the processes of your choosing. If the kernel decides to send SIGLOWMEM to xlock instead of firefox, the xlock will get killed because it really doesn't have any memory it can give up.
SIGLOWMEM, if it existed, would have no reason to default to anything other than IGN.
It would also logically be sent to “every” process on the machine, with the subsystem probably having a heuristic to skip processes which were already signaled and have not had significant memory increases since. The goal of an early warning is to cooperatively release memory (and maybe abort memory intensive computations), the kernel already kills processes “at random”, it does not need a second way to do that.
So, in actuality, I think your assertion just taught us all something, because despite knowing that the OOM killer and that the Magic SysRq key[1] exists, I didn't know you could configure this as an input!
[1]: https://en.wikipedia.org/wiki/Magic_SysRq_key