linux-kernel - Re: [PATCHv2 4.3-rc6] proc: fix convert from oom_score_adj to oom

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Date:	Thu, 22 Oct 2015 11:47:52 +0200
From:	Michal Hocko <mhocko@...nel.org>
To:	Hongjie Fang (方洪杰) 
	<Hongjie.Fang@...eadtrum.com>
Cc:	David Rientjes <rientjes@...gle.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCHv2 4.3-rc6] proc: fix convert from oom_score_adj to oom_adj

On Thu 22-10-15 06:49:01, Hongjie Fang (方洪杰) wrote:
> 
> The oom_adj has been replaced by oom_score_adj in kernel,
> but the /proc/pid/oom_adj is provided for legacy purposes.
> When write/read a value into/from /proc/pid/oom_adj,
> there is a transformation between oom_adj and oom_score_adj.
> 
> After writing a new value into /proc/pid/oom_adj, then read it.
> The return value is a different value than you wrote.
> Fix this by adding a adjustment factor.

... when printing the value.

Your previous patch has changed the stored value while this one only
changes the presented value. The previous one was more correct IMO
but in reality the difference (+-1) in oom_score_adj should be hardly
noticeable and nobody has complained about the current scaling for years
so this approach is probably more conservative.

> Signed-off-by: Hongjie Fang <hongjie.fang@...eadtrum.com>

Acked-by: Michal Hocko <mhocko@...e.com>

> ---
> Encountered the problem when I changed a task's oom_adj on
> an Android smart phone. As follows,
> 1. # cat /proc/1450/oom_adj
>    15
> 2. # echo 10 > /proc/1450/oom_adj
> 3. # cat /proc/1450/oom_adj
>    9
> 
> 
> diff --git a/fs/proc/base.c b/fs/proc/base.c
> index b25eee4..2312e43 100644
> --- a/fs/proc/base.c
> +++ b/fs/proc/base.c
> @@ -1019,15 +1019,19 @@ static ssize_t oom_adj_read(struct file *file, char __user *buf, size_t count,
>  	int oom_adj = OOM_ADJUST_MIN;
>  	size_t len;
>  	unsigned long flags;
> +	int adjust;
>  
>  	if (!task)
>  		return -ESRCH;
>  	if (lock_task_sighand(task, &flags)) {
> -		if (task->signal->oom_score_adj == OOM_SCORE_ADJ_MAX)
> +		if (task->signal->oom_score_adj == OOM_SCORE_ADJ_MAX) {
>  			oom_adj = OOM_ADJUST_MAX;
> -		else
> -			oom_adj = (task->signal->oom_score_adj * -OOM_DISABLE) /
> +		} else {
> +			adjust = task->signal->oom_score_adj > 0 ?
> +				 (OOM_SCORE_ADJ_MAX-1) : -(OOM_SCORE_ADJ_MAX-1);
> +			oom_adj = (task->signal->oom_score_adj * -OOM_DISABLE + adjust) /
>  				  OOM_SCORE_ADJ_MAX;
> +		}
>  		unlock_task_sighand(task, &flags);
>  	}
>  	put_task_struct(task);
> --
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-- 
Michal Hocko
SUSE Labs
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