linux-kernel - Re: [PATCH -mm] mm/oom_kill: change oom_kill_process() to bump ->mm_count rather than ->mm

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Date:	Mon, 5 Oct 2015 19:36:01 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	David Rientjes <rientjes@...gle.com>,
	Kyle Walker <kwalker@...hat.com>,
	Michal Hocko <mhocko@...nel.org>,
	Stanislav Kozina <skozina@...hat.com>,
	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH -mm] mm/oom_kill: change oom_kill_process() to bump
	->mm_count rather than ->mm_users

On 10/05, Oleg Nesterov wrote:
>
> oom_kill_process() does atomic_inc(&mm->mm_users) to ensure that
> this ->mm can't go away and this is wrong, change it to rely on
> ->mm_count and mmdrop().
>
> Firstly, we do not want to delay exit_mmap/etc if the victim exits
> before we do mmput(), but this is minor.
>
> More importantly, we simply can not do mmput() in oom_kill_process(),
> this can deadlock. For example, suppose that access_process_vm(tsk)
> triggers OOM and oom-killer decides to kill this "tsk". If it exits
> and does mmput() before us, ksm_exit() called by us may want to want
> to take the same mmap_sem for writing.

Self nack to the changelog ;)

I still think the patch is fine, I'll resend it. But the changelog
is wrong, in the case above access_process_vm() adds another reference
to ->mm_users, so mmput() can never lead to mm_users == 0.


> Signed-off-by: Oleg Nesterov <oleg@...hat.com>
> ---
>  mm/oom_kill.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/oom_kill.c b/mm/oom_kill.c
> index 034d219..52abb78 100644
> --- a/mm/oom_kill.c
> +++ b/mm/oom_kill.c
> @@ -571,7 +571,7 @@ void oom_kill_process(struct oom_control *oc, struct task_struct *p,
>  
>  	/* Get a reference to safely compare mm after task_unlock(victim) */
>  	mm = victim->mm;
> -	atomic_inc(&mm->mm_users);
> +	atomic_inc(&mm->mm_count);
>  	/*
>  	 * We should send SIGKILL before setting TIF_MEMDIE in order to prevent
>  	 * the OOM victim from depleting the memory reserves from the user
> @@ -609,7 +609,7 @@ void oom_kill_process(struct oom_control *oc, struct task_struct *p,
>  	}
>  	rcu_read_unlock();
>  
> -	mmput(mm);
> +	mmdrop(mm);
>  	put_task_struct(victim);
>  }
>  #undef K
> -- 
> 2.4.3
> 

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