Re: [syzbot] [mm?] WARNING: bad unlock balance in do_wp_page

[Qi Zheng <qi.zheng@linux.dev>]

Hi Andrew,

On 4/26/26 6:49 PM, Andrew Morton wrote:
> On Sun, 26 Apr 2026 01:17:25 -0700 syzbot <syzbot+7d60b33a8a546263da7c@syzkaller.appspotmail.com> wrote:
> 
>> Hello,
>>
>> syzbot found the following issue on:
>>
>> HEAD commit:    6596a02b2078 Merge tag 'drm-next-2026-04-22' of https://gi..
>> git tree:       upstream
>> console output: https://syzkaller.appspot.com/x/log.txt?x=12483702580000
>> kernel config:  https://syzkaller.appspot.com/x/.config?x=24c8da4692f901cb
>> dashboard link: https://syzkaller.appspot.com/bug?extid=7d60b33a8a546263da7c
>> compiler:       gcc (Debian 14.2.0-19) 14.2.0, GNU ld (GNU Binutils for Debian) 2.44
>> userspace arch: i386
>>
>> Unfortunately, I don't have any reproducer for this issue yet.
> 
> argh, that dreaded sentence.
> 
> Thanks.
> 
> Something's definitely amiss.  This is at least the fifth report of
> rcu_read_lock() imbalance post-7.0.  Others:
> 
> https://lore.kernel.org/69eab803.a00a0220.17a17.004a.GAE@google.com
> https://lore.kernel.org/69eab803.a00a0220.17a17.004b.GAE@google.com
> https://lore.kernel.org/69eafb0e.a00a0220.9259.0031.GAE@google.com
> https://lore.kernel.org/69ebcbe2.a00a0220.7773.0005.GAE@google.com

All the kernel configs mentioned above include 'CONFIG_MEMCG_V1=y'.

Theoretically, a rebind_subsystems() can lead a rcu unbalance, see my
previous discussion with Shakeel for details:

https://lore.kernel.org/all/358c60e1-fa91-40a1-9e00-84c93340c04e@linux.dev/

However, in a production environment, this is practically impossible.
So Shakeel and I chose to wait for a reproducer at the time. :(

> 
> In some cases we released it too often, in other cases we failed to
> release it.
> 
> The first one is slightly more useful in that it tells us that the
> not-released rcu_read_lock() was taken in folio_lruvec_lock_irqsave().

I double-checked some callers of folio_lruvec_lock_irqsave() (such as
folios_put_refs()), but didn't find anything suspicious. :(

> 
> Muchun & Qi: you played with that rcu locking in 31b54a5e8916.  Can you
> please double-check that we didn't miss something?