Re: [syzbot] [cgroups?] [mm?] WARNING in memcg1_swapout

[Deepanshu Kartikey <kartikey406@gmail.com>]

On Sun, Jan 18, 2026 at 6:27 AM Andrew Morton <akpm@linux-foundation.org> wrote:
>
> On Sat, 17 Jan 2026 01:30:25 -0800 syzbot <syzbot+079a3b213add54dd18a7@syzkaller.appspotmail.com> wrote:
>
> > Hello,
> >
> > syzbot found the following issue on:
> >
> > HEAD commit:    0f853ca2a798 Add linux-next specific files for 20260113
> > git tree:       linux-next
> > console output: https://syzkaller.appspot.com/x/log.txt?x=14f7259a580000
> > kernel config:  https://syzkaller.appspot.com/x/.config?x=8d6e5303d96e21b5
> > dashboard link: https://syzkaller.appspot.com/bug?extid=079a3b213add54dd18a7
> > compiler:       Debian clang version 20.1.8 (++20250708063551+0c9f909b7976-1~exp1~20250708183702.136), Debian LLD 20.1.8
> > syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=167ef922580000
> > C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=16d295fa580000
> >
> > Downloadable assets:
> > disk image: https://storage.googleapis.com/syzbot-assets/480cd223f3f6/disk-0f853ca2.raw.xz
> > vmlinux: https://storage.googleapis.com/syzbot-assets/1ca2f0dbb7cc/vmlinux-0f853ca2.xz
> > kernel image: https://storage.googleapis.com/syzbot-assets/60a0fef5805b/bzImage-0f853ca2.xz
> >
> > IMPORTANT: if you fix the issue, please add the following tag to the commit:
> > Reported-by: syzbot+079a3b213add54dd18a7@syzkaller.appspotmail.com
>
> Thanks.
>
> > ------------[ cut here ]------------
> > WARNING: mm/memcontrol-v1.c:642 at memcg1_swapout+0x6c2/0x8d0 mm/memcontrol-v1.c:642, CPU#0: syz.4.233/6746
>
> That's
>
>         VM_WARN_ON_ONCE(oldid != 0);
>
> which was added by Deepanshu's "mm/swap_cgroup: fix kernel BUG in
> swap_cgroup_record".
>
> This patch has Fixes: 1a4e58cce84e ("mm: introduce MADV_PAGEOUT"),
> which is six years old.  For some reason it has no cc:stable.
>
> Deepanshu's patch has no reviews.
>
> So can I please do the memcg maintainer summoning dance here?  We have a
> repeatable BUG happening in mainline Linux.
>

Hi Andrew,

I checked the git blame output for commit 0f853ca2a798:

Line 763: memcg1_swapout(folio, swap);
Line 764: __swap_cache_del_folio(ci, folio, swap, shadow);
                    (d7a7b2f91f36b - Kairui Song, 2026-01-13 02:33:36 +0800)

Kairui's reordering patch appears to have been merged on Jan 13.
The syzbot report is also from Jan 13, likely from earlier in the
day before the reordering patch was merged.

So this report is from before the fix. The warning should not appear
in linux-next builds after Jan 13.

Thanks,

Deepanshu