Re: [PATCH] oom: split out forced OOM killer

[David Rientjes]

On Mon, 8 Jun 2015, Michal Hocko wrote:

> > This patch is not a functional change, so I don't interpret your feedback 
> > as any support of it being merged.
> 
> David, have you actually read the patch? The changelog is mentioning this:
> "
>     check_panic_on_oom on the other hand will work and that is kind of
>     unexpected because sysrq+f should be usable to kill a mem hog whether
>     the global OOM policy is to panic or not.
>     It also doesn't make much sense to panic the system when no task cannot
>     be killed because admin has a separate sysrq for that purpose.
> "
> and the patch exludes panic_on_oom from the sysrq path.
> 

Yes, and that's why I believe we should pursue that direction without the 
associated "cleanup" that adds 35 lines of code to supress a panic.  In 
other words, there's no reason to combine a patch that suppresses the 
panic even with panic_on_oom, which I support, and a "cleanup" that I 
believe just obfuscates the code.

It's a one-liner change: just test for force_kill and suppress the panic; 
we don't need 35 new lines that create even more unique entry paths.

> > That said, you raise an interesting point of whether sysrq+f should ever 
> > trigger a panic due to panic_on_oom.  The case can be made that it should 
> > ignore panic_on_oom and require the use of another sysrq to panic the 
> > machine instead.  Sysrq+f could then be used to oom kill a process, 
> > regardless of panic_on_oom, and the panic only occurs if userspace did not 
> > trigger the kill or the kill itself will fail.
> 
> Why would it panic the system if there is no killable task? Shoudln't
> be admin able to do additional steps after the explicit oom killer failed
> and only then panic by sysrq?
> 

Today it panics, I don't think it should panic when there are no killable 
processes because it's inherently racy with userspace.  It's similar to 
suppressing panic_on_oom for sysrq+f, but for a different reason, so it 
should probably be a separate patch with its own changelog (and update to 
documentation for both patches to make this explicit).
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