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

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Date:   Thu, 10 Oct 2019 11:15:16 +0800
From:   Wei Yang <richardw.yang@...ux.intel.com>
To:     Wei Yang <richardw.yang@...ux.intel.com>
Cc:     Shakeel Butt <shakeelb@...gle.com>, Qian Cai <cai@....pw>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Konstantin Khlebnikov <khlebnikov@...dex-team.ru>,
        Rik van Riel <riel@...riel.com>,
        Linux MM <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        linux-s390@...r.kernel.org
Subject: Re: "reuse mergeable anon_vma as parent when fork" causes a crash on
 s390

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.

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