Re: "reuse mergeable anon_vma as parent when fork" causes a crash on s390

[Wei Yang]

On Thu, Oct 10, 2019 at 11:29:44AM +0300, Konstantin Khlebnikov wrote:
>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.
>

Per my understanding, the root cause is commit 7a3ef208e662 did a little
further for the reuse case.

We intend to reuse anon_vma on fork, but we also did this when we just want a
copy of src, like __split_vma(), copy_vma() and __vma_split(). This will lead
to a random anon_vma and not what we expect.

A fix patch is sent for this.

>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
>> 

-- 
Wei Yang
Help you, Help me