linux-kernel - Re: [PATCH] mm, oom: allow oom reaper to race with exit

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Date:   Wed, 26 Jul 2017 07:45:57 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Andrea Arcangeli <aarcange@...hat.com>
Cc:     "Kirill A. Shutemov" <kirill@...temov.name>,
        Andrew Morton <akpm@...ux-foundation.org>,
        David Rientjes <rientjes@...gle.com>,
        Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
        Oleg Nesterov <oleg@...hat.com>,
        Hugh Dickins <hughd@...gle.com>, linux-mm@...ck.org,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm, oom: allow oom reaper to race with exit_mmap

On Tue 25-07-17 21:19:52, Andrea Arcangeli wrote:
> On Tue, Jul 25, 2017 at 06:04:00PM +0200, Michal Hocko wrote:
> > -	down_write(&mm->mmap_sem);
> > +	if (tsk_is_oom_victim(current))
> > +		down_write(&mm->mmap_sem);
> >  	free_pgtables(&tlb, vma, FIRST_USER_ADDRESS, USER_PGTABLES_CEILING);
> >  	tlb_finish_mmu(&tlb, 0, -1);
> >  
> > @@ -3012,7 +3014,8 @@ void exit_mmap(struct mm_struct *mm)
> >  	}
> >  	mm->mmap = NULL;
> >  	vm_unacct_memory(nr_accounted);
> > -	up_write(&mm->mmap_sem);
> > +	if (tsk_is_oom_victim(current))
> > +		up_write(&mm->mmap_sem);
> 
> How is this possibly safe? mark_oom_victim can run while exit_mmap is
> running.

I believe it cannot. We always call mark_oom_victim (on !current) with
task_lock held and check task->mm != NULL and we call do_exit->mmput after
mm is set to NULL under the same lock.
-- 
Michal Hocko
SUSE Labs