linux-kernel - Re: [PATCH/RFC] mm, oom: Fix uninitialized ret in task_will_free

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Message-Id: <20160804144649.7ac4727ad0d93097c4055610@linux-foundation.org>
Date:	Thu, 4 Aug 2016 14:46:49 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
Cc:	Geert Uytterhoeven <geert@...ux-m68k.org>,
	Michal Hocko <mhocko@...e.com>,
	Oleg Nesterov <oleg@...hat.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH/RFC] mm, oom: Fix uninitialized ret in
 task_will_free_mem()

On Thu, 4 Aug 2016 21:28:13 +0900 Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp> wrote:

> > 
> > Fixes: 1af8bb43269563e4 ("mm, oom: fortify task_will_free_mem()")
> > Signed-off-by: Geert Uytterhoeven <geert@...ux-m68k.org>
> > ---
> > Untested. I'm not familiar with the code, hence the default value of
> > true was deducted from the logic in the loop (return false as soon as
> > __task_will_free_mem() has returned false).
> 
> I think ret = true is correct. Andrew, please send to linux.git.

task_will_free_mem() is too hard to understand.

We're examining task "A":

: 	for_each_process(p) {
: 		if (!process_shares_mm(p, mm))
: 			continue;
: 		if (same_thread_group(task, p))
: 			continue;

So here, we've found a process `p' which shares A's mm and which does
not share A's thread group.

: 		ret = __task_will_free_mem(p);

And here we check to see if killing `p' would free up memory.

: 		if (!ret)
: 			break;

If killing `p' will not free memory then give up the scan of all
processes because <reasons>, and we decide that killing `A' will
not free memory either, because some other task is holding onto
A's memory anyway.

: 	}

And if no task is found to be sharing A's mm while not sharing A's
thread group then fall through and decide to kill A.  In which case the
patch to return `true' is correct.

Correctish?  Maybe.  Can we please get some comments in there to
demystify the decision-making?