linux-kernel - Re: Re: [PATCH] mm, oom: fix potential data corruption when oom

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Date:   Fri, 4 Aug 2017 11:16:29 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
Cc:     linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
        Wenwei Tao <wenwei.tww@...baba-inc.com>,
        Oleg Nesterov <oleg@...hat.com>,
        David Rientjes <rientjes@...gle.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: Re: [PATCH] mm, oom: fix potential data corruption when
 oom_reaper races with writer

On Fri 04-08-17 17:25:46, Tetsuo Handa wrote:
> Well, while lockdep warning is gone, this problem is remaining.
> 
> diff --git a/mm/memory.c b/mm/memory.c
> index edabf6f..1e06c29 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -3931,15 +3931,14 @@ int handle_mm_fault(struct vm_area_struct *vma, unsigned long address,
>         /*
>          * This mm has been already reaped by the oom reaper and so the
>          * refault cannot be trusted in general. Anonymous refaults would
> -        * lose data and give a zero page instead e.g. This is especially
> -        * problem for use_mm() because regular tasks will just die and
> -        * the corrupted data will not be visible anywhere while kthread
> -        * will outlive the oom victim and potentially propagate the date
> -        * further.
> +        * lose data and give a zero page instead e.g.
>          */
> -       if (unlikely((current->flags & PF_KTHREAD) && !(ret & VM_FAULT_ERROR)
> -                               && test_bit(MMF_UNSTABLE, &vma->vm_mm->flags)))
> +       if (unlikely(!(ret & VM_FAULT_ERROR)
> +                    && test_bit(MMF_UNSTABLE, &vma->vm_mm->flags))) {
> +               if (ret & VM_FAULT_RETRY)
> +                       down_read(&vma->vm_mm->mmap_sem);
>                 ret = VM_FAULT_SIGBUS;
> +       }
> 
>         return ret;
>  }

I have re-read your email again and I guess I misread previously. Are
you saying that the data corruption happens with the both patches
applied?

> 
> $ cat /tmp/file.* | od -b | head
> 0000000 377 377 377 377 377 377 377 377 377 377 377 377 377 377 377 377
> *
> 420330000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000
> *
> 420340000 377 377 377 377 377 377 377 377 377 377 377 377 377 377 377 377
> *
> 457330000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000
> *
> 457340000 377 377 377 377 377 377 377 377 377 377 377 377 377 377 377 377
> *
> 
> --
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-- 
Michal Hocko
SUSE Labs