vm_pageout_oom(): Allow the option (via sysctl) of panic'ing instead of killing a process when required.

[will]

Summary

This sysctl enables those who would rather panic than kill a process, when the system runs out of memory. Usually, this is most useful when the OOM condition is due to mismanagement of memory, on a system where the applications in question don't respond well to being killed.

In theory, if the system is properly managed, it shouldn't be possible to hit this condition. If it does, the panic can be more desirable for some users (since it can be a good means of finding the root cause) rather than killing the largest process and continuing on its merry way.

Test Plan

Install kernel, sysctl vm.panic_on_oom=1, run the system out of memory. Any further testing that could be done is contingent on the specific situation; this is primarily a development/debugging tool.

[kib]

There is also protect(1) tool, which uses procctl(PROC_SPROTECT) to ensure that some processes are immune to OOM. Still, I think that panic() option is useful in some environments.

[alc]

I see that you've already committed this, which is fine. For the record, I have no objections to this change.