linux-kernel - Re: [patch] mm, oom: remove 3% bonus for CAP_SYS

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Date:   Tue, 13 Mar 2018 14:39:46 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     David Rientjes <rientjes@...gle.com>
Cc:     Gaurav Kohli <gkohli@...eaurora.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        kirill.shutemov@...ux.intel.com,
        Andrea Arcangeli <aarcange@...hat.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, linux-arm-msm@...r.kernel.org
Subject: Re: [patch] mm, oom: remove 3% bonus for CAP_SYS_ADMIN processes

On Wed 07-03-18 15:52:15, David Rientjes wrote:
> Since the 2.6 kernel, the oom killer has slightly biased away from 
> CAP_SYS_ADMIN processes by discounting some of its memory usage in 
> comparison to other processes.
> 
> This has always been implicit and nothing exactly relies on the behavior.
> 
> Gaurav notices that __task_cred() can dereference a potentially freed 
> pointer if the task under consideration is exiting because a reference to 
> the task_struct is not held.
> 
> Remove the CAP_SYS_ADMIN bias so that all processes are treated equally.
> 
> If any CAP_SYS_ADMIN process would like to be biased against, it is always 
> allowed to adjust /proc/pid/oom_score_adj.
> 
> Reported-by: Gaurav Kohli <gkohli@...eaurora.org>
> Signed-off-by: David Rientjes <rientjes@...gle.com>

This is simpler than playing reference counting tricks and whatnot.
Moreover I do agree that this heuristic is questionable on its own. The
bias is basically random and invisible to the userspace. We already have
a way to tune the same thing by oom_score_adj

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

> ---
>  mm/oom_kill.c | 7 -------
>  1 file changed, 7 deletions(-)
> 
> diff --git a/mm/oom_kill.c b/mm/oom_kill.c
> --- a/mm/oom_kill.c
> +++ b/mm/oom_kill.c
> @@ -224,13 +224,6 @@ unsigned long oom_badness(struct task_struct *p, struct mem_cgroup *memcg,
>  		mm_pgtables_bytes(p->mm) / PAGE_SIZE;
>  	task_unlock(p);
>  
> -	/*
> -	 * Root processes get 3% bonus, just like the __vm_enough_memory()
> -	 * implementation used by LSMs.
> -	 */
> -	if (has_capability_noaudit(p, CAP_SYS_ADMIN))
> -		points -= (points * 3) / 100;
> -
>  	/* Normalize to oom_score_adj units */
>  	adj *= totalpages / 1000;
>  	points += adj;

-- 
Michal Hocko
SUSE Labs