linux-kernel - Re: [syzbot] [mm?] WARNING in page_add_anon

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Message-ID: <c5d93115-43c1-784b-8e21-70c3cc876778@redhat.com>
Date:   Mon, 18 Sep 2023 09:41:07 +0200
From:   David Hildenbrand <david@...hat.com>
To:     syzbot <syzbot+6e4f59235036c3c2e296@...kaller.appspotmail.com>,
        akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, syzkaller-bugs@...glegroups.com
Subject: Re: [syzbot] [mm?] WARNING in page_add_anon_rmap

On 18.09.23 05:57, syzbot wrote:
> Hello,
> 
> syzbot found the following issue on:
> 
> HEAD commit:    98897dc735cf Add linux-next specific files for 20230914
> git tree:       linux-next
> console+strace: https://syzkaller.appspot.com/x/log.txt?x=1214a152680000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=1502c503717ada5c
> dashboard link: https://syzkaller.appspot.com/bug?extid=6e4f59235036c3c2e296
> compiler:       gcc (Debian 12.2.0-14) 12.2.0, GNU ld (GNU Binutils for Debian) 2.40
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=15869ea0680000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=15f1b952680000
> 
> Downloadable assets:
> disk image: https://storage.googleapis.com/syzbot-assets/00e4c0af5a8a/disk-98897dc7.raw.xz
> vmlinux: https://storage.googleapis.com/syzbot-assets/7b54a00eee56/vmlinux-98897dc7.xz
> kernel image: https://storage.googleapis.com/syzbot-assets/100094353b8e/bzImage-98897dc7.xz
> 
> The issue was bisected to:
> 
> commit b8575fa4abaa1dee1a61f1f27a86a02757310a7e
> Author: David Hildenbrand <david@...hat.com>
> Date:   Wed Sep 13 12:51:12 2023 +0000
> 
>      mm/rmap: simplify PageAnonExclusive sanity checks when adding anon rmap
> 

Right, when PTE-mapping a THP we temporarily have an entire mapping and 
a sub-page mapping. It would even be clearer when passing RMAP_EXCLUSIVE 
from __split_huge_pmd_locked(), then setting it manually. But that's a 
different story; lets' fixup that sanity check first.

-- 
Cheers,

David / dhildenb