linux-kernel - Re: [RFC PATCH v2 3/3] mm, oom: hand over MMF_OOM_SKIP to exit path if it is guranteed to finish

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Date:   Tue, 30 Oct 2018 13:10:12 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
Cc:     linux-mm@...ck.org, Roman Gushchin <guro@...com>,
        David Rientjes <rientjes@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH v2 3/3] mm, oom: hand over MMF_OOM_SKIP to exit path
 if it is guranteed to finish

On Tue 30-10-18 21:02:40, Tetsuo Handa wrote:
> On 2018/10/30 20:39, Michal Hocko wrote:
> > On Tue 30-10-18 18:47:43, Tetsuo Handa wrote:
> >> On 2018/10/30 15:31, Michal Hocko wrote:
> >>> On Tue 30-10-18 13:45:22, Tetsuo Handa wrote:
> >>>> Michal Hocko wrote:
> >>>>> @@ -3156,6 +3166,13 @@ void exit_mmap(struct mm_struct *mm)
> >>>>>                 vma = remove_vma(vma);
> >>>>>         }
> >>>>>         vm_unacct_memory(nr_accounted);
> >>>>> +
> >>>>> +       /*
> >>>>> +        * Now that the full address space is torn down, make sure the
> >>>>> +        * OOM killer skips over this task
> >>>>> +        */
> >>>>> +       if (oom)
> >>>>> +               set_bit(MMF_OOM_SKIP, &mm->flags);
> >>>>>  }
> >>>>>
> >>>>>  /* Insert vm structure into process list sorted by address
> >>>>
> >>>> I don't like setting MMF_OOF_SKIP after remove_vma() loop. 50 users might
> >>>> call vma->vm_ops->close() from remove_vma(). Some of them are doing fs
> >>>> writeback, some of them might be doing GFP_KERNEL allocation from
> >>>> vma->vm_ops->open() with a lock also held by vma->vm_ops->close().
> >>>>
> >>>> I don't think that waiting for completion of remove_vma() loop is safe.
> >>>
> >>> What do you mean by 'safe' here?
> >>>
> >>
> >> safe = "Does not cause OOM lockup."
> >>
> >> remove_vma() is allowed to sleep, and some users might depend on memory
> >> allocation when the OOM killer is waiting for remove_vma() to complete.
> > 
> > But MMF_OOF_SKIP is set after we are done with remove_vma. In fact it is
> > the very last thing in exit_mmap. So I do not follow what you mean.
> > 
> 
> So what? Think the worst case. Quite obvious bug here.

I misunderstood your concern. oom_reaper would back off without
MMF_OOF_SKIP as well. You are right we cannot assume anything about
close callbacks so MMF_OOM_SKIP has to come before that. I will move it
behind the pagetable freeing.
-- 
Michal Hocko
SUSE Labs