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From: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
To: Minchan Kim <minchan.kim@gmail.com>
Subject: Re: [PATCH 1/4] oom: move oom_adj to signal_struct
Cc: kosaki.motohiro@jp.fujitsu.com, LKML <linux-kernel@vger.kernel.org>,
       Paul Menage <menage@google.com>, David Rientjes <rientjes@google.com>,
       KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>,
       Rik van Riel <riel@redhat.com>,
       Andrew Morton <akpm@linux-foundation.org>,
       Linus Torvalds <torvalds@linux-foundation.org>,
       Oleg Nesterov <oleg@redhat.com>, linux-mm <linux-mm@kvack.org>
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Date: Wed,  5 Aug 2009 11:29:34 +0900 (JST)
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Hi

> Hi, Kosaki. 
> 
> I am so late to invole this thread. 
> But let me have a question. 
> 
> What's advantage of placing oom_adj in singal rather than task ?
> I mean task->oom_adj and task->signal->oom_adj ?
> 
> I am sorry if you already discussed it at last threads. 

Not sorry. that's very good question.

I'm trying to explain the detailed intention of commit 2ff05b2b4eac
(move oom_adj to mm_struct).

In 2.6.30, OOM logic callflow is here.

__out_of_memory
	select_bad_process		for each task
		badness			calculate badness of one task
	oom_kill_process		search child
		oom_kill_task		kill target task and mm shared tasks with it

example, process-A have two thread, thread-A and thread-B and it 
have very fat memory.
And, each thread have following likes oom property.

	thread-A: oom_adj = OOM_DISABLE, oom_score = 0
	thread-B: oom_adj = 0,           oom_score = very-high

Then, select_bad_process() select thread-B, but oom_kill_task refuse
kill the task because thread-A have OOM_DISABLE.
__out_of_memory() call select_bad_process() again. but select_bad_process()
select the same task. It mean kernel fall in the livelock.

The fact is, select_bad_process() must select killable task. otherwise
OOM logic go into livelock.

Is this enough explanation? thanks.