From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-170.mta1.migadu.com (out-170.mta1.migadu.com [95.215.58.170]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 59A3F3EC2E9 for ; Tue, 14 Apr 2026 17:16:07 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=95.215.58.170 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1776186969; cv=none; b=eNZxTCMDgv3IaiX5ixTxSGIPOJSkPoHZiIXUU1iYTXI2ldTI+KYpyZsP7/8aMjKAeiG6qFv4r9CkN3Lm32o4bxOMzrKzgSMLpgu84W/HlRlPhR85g/bR+8fdIfq3GEeD+cA9Qc5COXVj8tQcRXdydn3K2kUu0cP5FdfmFIkfyNc= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1776186969; c=relaxed/simple; bh=+yOuWfkJMZGUjL1CffCk8qUOBnhgBBIANB0U4RMqN9E=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=RG/j6ISIaME0gNrwPHl5wZbVQZ5zcHCLEmmNsRQWpgNp3PG+3XsXdfgq3OJquqbnIiP4pZM1I5Zz5qJ0V8AxVEs3n12S+xU8GcuwDVSzZmzeeecMuKvvxtaDoMp63o0CfN6wv/2NfdgCAT3UGU7vGJquRc39yZ1aCfjUnHxrs2o= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev; spf=pass smtp.mailfrom=linux.dev; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b=lDiQZ3kP; arc=none smtp.client-ip=95.215.58.170 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.dev Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b="lDiQZ3kP" Date: Tue, 14 Apr 2026 10:15:50 -0700 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1776186955; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=sdGaJ1kfPvwvrDDRsofvgKCgSPPMcVc4q17G+sHRwG8=; b=lDiQZ3kP1q4ljLOtXGUeSiDgZVrlt+imZNexfUnsxU8YhLd0XOmGT3pF3uPDUsOcutbWli 5ZTtF5pRjNh1RX0UPL9rPW9f6duLdOvWhexGPRDTf58BbbvLhT6pW9XxzFkUcmLusLBSjg qZkttKYp+ZoKBxjOq9VWWerfHYKqDJ4= X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Shakeel Butt To: Qi Zheng Cc: syzbot , akpm@linux-foundation.org, cgroups@vger.kernel.org, hannes@cmpxchg.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, mhocko@kernel.org, muchun.song@linux.dev, roman.gushchin@linux.dev, syzkaller-bugs@googlegroups.com, zhengqi.arch@bytedance.com, yosry@kernel.org Subject: Re: [syzbot] [mm?] [cgroups?] WARNING: bad unlock balance in lruvec_stat_mod_folio Message-ID: References: <69d54494.050a0220.3030df.0002.GAE@google.com> <358c60e1-fa91-40a1-9e00-84c93340c04e@linux.dev> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <358c60e1-fa91-40a1-9e00-84c93340c04e@linux.dev> X-Migadu-Flow: FLOW_OUT On Tue, Apr 14, 2026 at 11:52:13AM +0800, Qi Zheng wrote: > Hi Shakeel, > > On 4/14/26 6:28 AM, Shakeel Butt wrote: > > +Qi & Yosry > > > > On Tue, Apr 07, 2026 at 10:53:24AM -0700, syzbot wrote: > > > Hello, > > > > > > syzbot found the following issue on: > > > > > > HEAD commit: cc13002a9f98 Add linux-next specific files for 20260402 > > > git tree: linux-next > > > console output: https://syzkaller.appspot.com/x/log.txt?x=10d8946a580000 > > > kernel config: https://syzkaller.appspot.com/x/.config?x=4e6c8be618ab359 > > > dashboard link: https://syzkaller.appspot.com/bug?extid=1a3353a77896e73a8f53 > > > compiler: Debian clang version 21.1.8 (++20251221033036+2078da43e25a-1~exp1~20251221153213.50), Debian LLD 21.1.8 > > > > > > Unfortunately, I don't have any reproducer for this issue yet. > > > > Let's wait for the reproducer. I can only think of cgroup_subsys_on_dfl() check > > returning different value in get_non_dying_memcg_start() and > > get_non_dying_memcg_end() to cause this uneven rcu unlock. However I can't think > > why and how that can happen. > > > > My AI bot told me that the cgroup_subsys_on_dfl_key can be dynamically > modified at runtime during a rebind: > > rebind_subsystems() > --> if (dst_root == &cgrp_dfl_root) { > static_branch_enable(cgroup_subsys_on_dfl_key[ssid]); > } else { > dcgrp->subtree_control |= 1 << ssid; > static_branch_disable(cgroup_subsys_on_dfl_key[ssid]); > } > > However, when I actually tested it, I hit the following error: > > mount: /tmp/cg-rb-repro: mount point is busy. > > Indeed, there are already many child cgroups under the cgroup v2 root > (the VM just booted): > > root@localhost:~# find /sys/fs/cgroup -mindepth 1 -maxdepth 2 -type d | head > -50 > /sys/fs/cgroup/sys-kernel-debug.mount > /sys/fs/cgroup/dev-mqueue.mount > /sys/fs/cgroup/user.slice > /sys/fs/cgroup/user.slice/user-0.slice > /sys/fs/cgroup/sys-kernel-tracing.mount > /sys/fs/cgroup/init.scope > /sys/fs/cgroup/system.slice > /sys/fs/cgroup/system.slice/systemd-networkd.service > /sys/fs/cgroup/system.slice/systemd-udevd.service > /sys/fs/cgroup/system.slice/system-serial\x2dgetty.slice > /sys/fs/cgroup/system.slice/wpa_supplicant.service > /sys/fs/cgroup/system.slice/system-modprobe.slice > /sys/fs/cgroup/system.slice/systemd-journald.service > /sys/fs/cgroup/system.slice/unattended-upgrades.service > /sys/fs/cgroup/system.slice/system-systemd\x2dgrowfs.slice > /sys/fs/cgroup/system.slice/ssh.service > /sys/fs/cgroup/system.slice/dhcpcd.service > /sys/fs/cgroup/system.slice/systemd-resolved.service > /sys/fs/cgroup/system.slice/dbus.service > /sys/fs/cgroup/system.slice/systemd-timesyncd.service > /sys/fs/cgroup/system.slice/system-getty.slice > /sys/fs/cgroup/system.slice/systemd-logind.service > /sys/fs/cgroup/dev-hugepages.mount > > So it seems impossible to rebind memory in a production environment > using systemd? > > Then I disabled systemd: > > set `init=/bin/bash` > > and found that I could successfully run the following commands: > > root@(none):/# mkdir -p /tmp/cg-rb-repro > root@(none):/# mount -t cgroup -o none,name=rb none /tmp/cg-rb-repro > root@(none):/# mount -t cgroup -o remount,memory none /tmp/cg-rb-repro > [ 65.903125][ T241] option changes via remount are deprecated (pid=241 > comm=mount) > root@(none):/# mount -t cgroup -o remount,name=rb none /tmp/cg-rb-repro > [ 73.405829][ T242] option changes via remount are deprecated (pid=242 > comm=mount) > root@(none):/# umount /tmp/cg-rb-repro > > So it seems this race condition does exist. Should we fix it? This only succeeded because there weren't any active cgroups. Were you able to trigger the warning as well. If not, I think we should just wait for reproducer from syzbot before doing anything.