linux-kernel - Re: [patch 01/12] mm: oom_kill: remove unnecessary locking in oom

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Date:	Wed, 25 Mar 2015 17:51:31 -0700 (PDT)
From:	David Rientjes <rientjes@...gle.com>
To:	Johannes Weiner <hannes@...xchg.org>
cc:	linux-mm@...ck.org, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
	Huang Ying <ying.huang@...el.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Dave Chinner <david@...morbit.com>,
	Michal Hocko <mhocko@...e.cz>, Theodore Ts'o <tytso@....edu>
Subject: Re: [patch 01/12] mm: oom_kill: remove unnecessary locking in
 oom_enable()

On Wed, 25 Mar 2015, Johannes Weiner wrote:

> Setting oom_killer_disabled to false is atomic, there is no need for
> further synchronization with ongoing allocations trying to OOM-kill.
> 
> Signed-off-by: Johannes Weiner <hannes@...xchg.org>
> ---
>  mm/oom_kill.c | 2 --
>  1 file changed, 2 deletions(-)
> 
> diff --git a/mm/oom_kill.c b/mm/oom_kill.c
> index 2b665da1b3c9..73763e489e86 100644
> --- a/mm/oom_kill.c
> +++ b/mm/oom_kill.c
> @@ -488,9 +488,7 @@ bool oom_killer_disable(void)
>   */
>  void oom_killer_enable(void)
>  {
> -	down_write(&oom_sem);
>  	oom_killer_disabled = false;
> -	up_write(&oom_sem);
>  }
>  
>  #define K(x) ((x) << (PAGE_SHIFT-10))

I haven't looked through the new disable-oom-killer-for-pm patchset that 
was merged, but this oom_killer_disabled thing already looks improperly 
handled.  I think any correctness or cleanups in this area would be very 
helpful.

I think mark_tsk_oom_victim() in mem_cgroup_out_of_memory() is just 
luckily not racing with a call to oom_killer_enable() and triggering the 
WARN_ON(oom_killer_disabled) since there's no "oom_sem" held here, and 
it's an improper context based on the comment of mark_tsk_oom_victim().  
There might be something else that is intended but not implemented 
correctly that I'm unaware of, but I know of no reason why setting of 
oom_killer_disabled would need to take a semaphore?

I'm thinking it has something to do with the remainder of that comment, 
specifically the "never after oom has been disabled already."

Michal?
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