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Date:	Tue, 6 Oct 2015 18:56:12 +0200
From:	Michal Hocko <mhocko@...nel.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	David Rientjes <rientjes@...gle.com>,
	Kyle Walker <kwalker@...hat.com>,
	Stanislav Kozina <skozina@...hat.com>,
	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH -mm] mmoom-fix-potentially-killing-unrelated-process-fix

On Tue 06-10-15 18:28:04, 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 if (for example) the caller holds i_mmap_rwsem and
> mmput() actually leads to exit_mmap(); the victim can have this file
> mmaped and in this case unmap_vmas/free_pgtables paths will take the
> same lock for writing. And at least huge_pmd_share() does pmd_alloc()
> under i_mmap_rwsem because VM_HUGETLB memory is not reclaimable.

Ouch, I have completely missed this during review! Thanks for catching
this. On the second thought it is clear now. We really want to pin the
mm_struct not the address space.

> Signed-off-by: Oleg Nesterov <oleg@...hat.com>

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

-- 
Michal Hocko
SUSE Labs
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