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

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Date:	Thu, 26 Mar 2015 14:18:39 +0100
From:	Michal Hocko <mhocko@...e.cz>
To:	David Rientjes <rientjes@...gle.com>
Cc:	Johannes Weiner <hannes@...xchg.org>, 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>,
	Theodore Ts'o <tytso@....edu>
Subject: Re: [patch 01/12] mm: oom_kill: remove unnecessary locking in
 oom_enable()

On Thu 26-03-15 12:51:40, Michal Hocko wrote:
> On Wed 25-03-15 17:51:31, David Rientjes wrote:
> > 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 
>                                     ^^^^^^^^^^
>                                     oom_killer_disable?
> 
> > 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().
> 
> OOM killer is disabled only _after_ all user tasks have been frozen. So
> we cannot get any page fault and a race. So the semaphore is not needed
> in this path although the comment says otherwise. I can add a comment
> clarifying this...

I am wrong here! pagefault_out_of_memory takes the lock and so the whole
mem_cgroup_out_of_memory is called under the same lock.
-- 
Michal Hocko
SUSE Labs
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