linux-kernel - Re: mm->oom_disable

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Message-ID: <alpine.DEB.2.00.1108291611070.32495@chino.kir.corp.google.com>
Date:	Mon, 29 Aug 2011 16:17:55 -0700 (PDT)
From:	David Rientjes <rientjes@...gle.com>
To:	Oleg Nesterov <oleg@...hat.com>, Ying Han <yinghan@...gle.com>
cc:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	roland@...k.frob.com, tj@...nel.org, dvlasenk@...hat.com,
	matt.fleming@...ux.intel.com, linux-kernel@...r.kernel.org,
	avagin@...nvz.org, fhrbata@...hat.com
Subject: Re: mm->oom_disable_count is broken

On Mon, 29 Aug 2011, Oleg Nesterov wrote:

> > IIRC, I did pointed out this issue. But nobody replied.
> > I think ->oom_disable_count is currently broken. but now I have no time to
> > audit this stuff. So, I'd suggest to revert this code if nobody don't fix it.
> 
> I tend to agree, of course we can fix oom_disable_count but I don't
> really understand why do we want it.
> 

I'd rather just remove it entirely, we'll have to ask it's author.  Ying, 
do you see a reason to keep oom_disable_count around?

The only thing that I can see it doing is preventing a thread that shares 
an ->mm with an unkillable thread from being killed itself since it won't 
lead to future memory freeing.  It prevents the second tasklist iteration 
after a task has been chosen to check if another thread sharing the memory 
cannot be killed.

I'd rather just kill the thread anyway because there's a chance that the 
OOM_DISABLE thread is waiting on it and may free its memory as well and 
there's no guarantee that when you set a thread to be OOM_DISABLE that all 
threads sharing the same memory are disabled as well.

> And. personally I dislike it because ->oom_disable_count is just another
> proof that ->oom_score_adj should be in ->mm, not per-process. IIRC,
> you already explained me why we can't do this, but - sorry - I forgot.
> May be something with vfork... Could you explain this again?
> 

I actually really wanted oom_score_adj to be in the ->mm, it would 
simplify a lot of the code :)  The problem was the inheritance property: 
we expect a job scheduler that is OOM_DISABLE to be able to vfork, change 
the oom_score_adj of the child, and then exec so that it is not oom 
disabled before starting to allocate memory.  If this were in the mm, then 
setting the oom_score_adj of the child prior to exec would change the job 
scheduler's oom score as well.
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