Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Subject: Re: "reuse mergeable anon_vma as parent when fork" causes a crash on
 s390
To:     Wei Yang <richardw.yang@linux.intel.com>
Cc:     Shakeel Butt <shakeelb@google.com>, Qian Cai <cai@lca.pw>,
        Andrew Morton <akpm@linux-foundation.org>,
        Rik van Riel <riel@surriel.com>,
        Linux MM <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Heiko Carstens <heiko.carstens@de.ibm.com>,
        Vasily Gorbik <gor@linux.ibm.com>,
        Christian Borntraeger <borntraeger@de.ibm.com>,
        linux-s390@vger.kernel.org
References: <1570656570.5937.24.camel@lca.pw>
 <CALvZod4psOEyYwPOF1UcJoK96LbYBccYhsG0DrKD+CCf8Sc-Yg@mail.gmail.com>
 <20191010023601.GA4793@richard> <20191010031516.GA5060@richard>
From:   Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Message-ID: <8e0d9999-9ee3-78e5-2737-5a504243413c@yandex-team.ru>
Date:   Thu, 10 Oct 2019 11:29:44 +0300
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101
 Thunderbird/60.8.0
MIME-Version: 1.0
In-Reply-To: <20191010031516.GA5060@richard>
Content-Type: text/plain; charset=utf-8; format=flowed
Content-Language: en-CA
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On 10/10/2019 06.15, Wei Yang wrote:
> On Thu, Oct 10, 2019 at 10:36:01AM +0800, Wei Yang wrote:
>> Hi, Qian, Shakeel
>>
>> Thanks for testing.
>>
>> Sounds I missed some case to handle. anon_vma_clone() now would be called in
>> vma_adjust, which is a different case when it is introduced.
>>
> 
> Well, I have to correct my statement. The reason is we may did something more
> in anon_vma_clone().
> 
> Here is a quick fix, while I need to go through all the cases carefully.

Oops, I've overlooked this case too.

You have to check src->anon_vma
otherwise in  __split_vma or copy_vma dst could pick completely random anon_vma.

Also checking prev will not hurt, just to be sure.

So, something like this should work:

if (!dst->anon_vma && src->anon_vma &&
     prev && pprev && pprev->anon_vma == src->anon_vma)
       dst->anon_vma = prev->anon_vma;

> 
> diff --git a/mm/rmap.c b/mm/rmap.c
> index 12f6c3d7fd9d..2844f442208d 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -271,7 +271,7 @@ int anon_vma_clone(struct vm_area_struct *dst, struct vm_area_struct *src)
>           * 1. Parent has vm_prev, which implies we have vm_prev.
>           * 2. Parent and its vm_prev have the same anon_vma.
>           */
> -       if (pprev && pprev->anon_vma == src->anon_vma)
> +       if (!dst->anon_vma && pprev && pprev->anon_vma == src->anon_vma)
>                  dst->anon_vma = prev->anon_vma;
>   
>          list_for_each_entry_reverse(pavc, &src->anon_vma_chain, same_vma) {
> 
>> BTW, do you have the specific test case? So that I could verify my change. The
>> kernel build test doesn't trigger this.
>>
>> Thanks a lot :-)
>>
>> On Wed, Oct 09, 2019 at 03:21:11PM -0700, Shakeel Butt wrote:
>> -- 
>> Wei Yang
>> Help you, Help me
>