linux-kernel - Re: [PATCH 1/4] oom: Do not panic when OOM killer is sysrq triggered

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Date:	Fri, 10 Jul 2015 09:41:24 +0200
From:	Michal Hocko <mhocko@...nel.org>
To:	David Rientjes <rientjes@...gle.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Jakob Unterwurzacher <jakobunt@...il.com>, linux-mm@...ck.org,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/4] oom: Do not panic when OOM killer is sysrq triggered

On Thu 09-07-15 14:03:53, David Rientjes wrote:
> On Thu, 9 Jul 2015, Michal Hocko wrote:
> 
> > > the titles were wrong for patches 2 and 3, but it doesn't mean we need to 
> > > add hacks around the code before organizing this into struct oom_control 
> > 
> > It is much easier to backport _fixes_ into older kernels (and yes I do
> > care about that) if they do not depend on other cleanups. So I do not
> > understand your point here. Besides that the cleanup really didn't make
> > much change to the actuall fix because one way or another you still have
> > to add a simple condition to rule out a heuristic/configuration which
> > doesn't apply to sysrq+f path.
> > 
> > So I am really lost in your argumentation here.
> > 
> 
> This isn't a bugfix: sysrq+f has, at least for eight years, been able to 
> panic the kernel.

This is an unwanted behavior and that is why I call it a bug. The mere
fact that nobody has noticed because panic_on_oom is not used widely and
even less with sysrq+f has nothing to do with it.

> We're not fixing a bug, we're changing behavior.  It's 
> quite appropriate to reorganize code before a behavior change to make it 
> cleaner.
> 
> > > or completely pointless comments and printks that will fill the kernel 
> > > log.
> > 
> > Could you explain what is so pointless about a comment which clarifies
> > the fact which is not obviously visible from the current function?
> > 
> 
> It states the obvious, a kthread is not going to be oom killed for 
> oom_kill_allocating_task:

Sigh. The comment says that the force_kill path _runs_ from the kthread
context which is far from obvious in out_of_memory.

[...]

> > Also could you explain why the admin shouldn't get an information if
> > sysrq+f didn't kill anything because no eligible task has been found?
> 
> The kernel log is the only notification mechanism that we have of the 
> kernel killing a process, we want to avoid spamming it unnecessarily.  The 
> kernel log is not the appropriate place for your debugging information 
> that would only specify that yes, out_of_memory() was called, but there 
> was nothing actionable, especially when that trigger can be constantly 
> invoked by userspace once panicking is no longer possible.

So how would you find out that there is no oom killable task?
-- 
Michal Hocko
SUSE Labs
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