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 12:39:15 +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 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.

-- 
Michal Hocko
SUSE Labs