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

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Date:	Thu, 9 Jul 2015 14:03:53 -0700 (PDT)
From:	David Rientjes <rientjes@...gle.com>
To:	Michal Hocko <mhocko@...nel.org>
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, 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.  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: it's not only current->mm, but also 
oom_unkillable_task(), which quite explicitly checks for PF_KTHREAD.  I 
don't think any reader of this code will assume a kthread is going to be 
oom killed.

> 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.
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