linux-kernel - Re: [PATCH 4.3-rc6] proc: fix oom_adj value read from /proc/<pid>/oom

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Date:	Wed, 21 Oct 2015 11:37:10 +0200
From:	Michal Hocko <mhocko@...nel.org>
To:	Hongjie Fang (方洪杰) 
	<Hongjie.Fang@...eadtrum.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	"gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
	David Rientjes <rientjes@...gle.com>
Subject: Re: [PATCH 4.3-rc6] proc: fix oom_adj value read from
 /proc/<pid>/oom_adj

[CC David as well]

The original patch has been posted here:
http://lkml.kernel.org/r/65a10261038346b1a778443fd15f0980%40SHMBX01.spreadtrum.com

On Tue 20-10-15 12:27:58, Eric W. Biederman wrote:
> "Hongjie Fang (方洪杰)" <Hongjie.Fang@...eadtrum.com> writes:
> 
> > The oom_adj's value reading through /proc/<pid>/oom_adj is different 
> > with the value written into /proc/<pid>/oom_adj.
> > Fix this by adding a adjustment factor.
> 
> *Scratches my head*
> 
> Won't changing the interpretation of what is written break existing
> userspace applications that write this value?

No, because they will see the same value they wrote. The current state
is broken because you get a different value than you wrote.

I am just wondering, how have you found this problem? Code review or
have you encountered a real failure because of this?

> Added a few more likely memory management suspects that might understand
> what is going on here.
> 
> Eric
> 
> >
> > Signed-off-by: Hongjie Fang <hongjie.fang@...eadtrum.com>
> > ---
> > diff --git a/fs/proc/base.c b/fs/proc/base.c
> > index b25eee4..1ea0589 100644
> > --- a/fs/proc/base.c
> > +++ b/fs/proc/base.c
> > @@ -1043,6 +1043,7 @@ static ssize_t oom_adj_write(struct file *file, const char __user *buf,
> >  	int oom_adj;
> >  	unsigned long flags;
> >  	int err;
> > +	int adjust;

This doesn't need the function visibility.

> >  
> >  	memset(buffer, 0, sizeof(buffer));
> >  	if (count > sizeof(buffer) - 1)
> > @@ -1084,8 +1085,10 @@ static ssize_t oom_adj_write(struct file *file, const char __user *buf,
> >  	 */
> >  	if (oom_adj == OOM_ADJUST_MAX)
> >  		oom_adj = OOM_SCORE_ADJ_MAX;
> > -	else
> > -		oom_adj = (oom_adj * OOM_SCORE_ADJ_MAX) / -OOM_DISABLE;
> > +	else{

space after else and checkpatch will probably complain about missing { }
for if...

Other than that the patch looks good to me. The changelog coul be
slightly improved as well.

> > +		adjust = oom_adj > 0 ? (-OOM_DISABLE-1) : -(-OOM_DISABLE-1);
> > +		oom_adj = (oom_adj * OOM_SCORE_ADJ_MAX + adjust) / -OOM_DISABLE;
> > +	}
> >  
> >  	if (oom_adj < task->signal->oom_score_adj &&
> >  	    !capable(CAP_SYS_RESOURCE)) {
> >
> > --
> 
> --
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-- 
Michal Hocko
SUSE Labs
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