Re: [syzbot] [cgroups?] BUG: sleeping function called from invalid context in cgroup_rstat_flush

[Johannes Weiner <hannes@cmpxchg.org>]

On Thu, Jun 27, 2024 at 07:03:21AM -0700, syzbot wrote:
> Hello,
> 
> syzbot found the following issue on:
> 
> HEAD commit:    7c16f0a4ed1c Merge tag 'i2c-for-6.10-rc5' of git://git.ker..
> git tree:       upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=1511528e980000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=12f98862a3c0c799
> dashboard link: https://syzkaller.appspot.com/bug?extid=b7f13b2d0cc156edf61a
> compiler:       Debian clang version 15.0.6, GNU ld (GNU Binutils for Debian) 2.40
> 
> Unfortunately, I don't have any reproducer for this issue yet.
> 
> Downloadable assets:
> disk image: https://storage.googleapis.com/syzbot-assets/50560e9024e5/disk-7c16f0a4.raw.xz
> vmlinux: https://storage.googleapis.com/syzbot-assets/080c27daee72/vmlinux-7c16f0a4.xz
> kernel image: https://storage.googleapis.com/syzbot-assets/c528e0da4544/bzImage-7c16f0a4.xz
> 
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+b7f13b2d0cc156edf61a@syzkaller.appspotmail.com
> 
> BUG: sleeping function called from invalid context at kernel/cgroup/rstat.c:351
> in_atomic(): 0, irqs_disabled(): 0, non_block: 0, pid: 17332, name: syz-executor.4
> preempt_count: 0, expected: 0
> RCU nest depth: 1, expected: 0
> 1 lock held by syz-executor.4/17332:
>  #0: ffffffff8e333fa0 (rcu_read_lock){....}-{1:2}, at: rcu_lock_acquire include/linux/rcupdate.h:329 [inline]
>  #0: ffffffff8e333fa0 (rcu_read_lock){....}-{1:2}, at: rcu_read_lock include/linux/rcupdate.h:781 [inline]
>  #0: ffffffff8e333fa0 (rcu_read_lock){....}-{1:2}, at: filemap_cachestat mm/filemap.c:4251 [inline]
>  #0: ffffffff8e333fa0 (rcu_read_lock){....}-{1:2}, at: __do_sys_cachestat mm/filemap.c:4407 [inline]
>  #0: ffffffff8e333fa0 (rcu_read_lock){....}-{1:2}, at: __se_sys_cachestat+0x3ee/0xbb0 mm/filemap.c:4372
> CPU: 1 PID: 17332 Comm: syz-executor.4 Not tainted 6.10.0-rc4-syzkaller-00330-g7c16f0a4ed1c #0
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 06/07/2024
> Call Trace:
>  <TASK>
>  __dump_stack lib/dump_stack.c:88 [inline]
>  dump_stack_lvl+0x241/0x360 lib/dump_stack.c:114
>  __might_resched+0x5d4/0x780 kernel/sched/core.c:10196
>  cgroup_rstat_flush+0x1e/0x50 kernel/cgroup/rstat.c:351
>  workingset_test_recent+0x48a/0xa90 mm/workingset.c:473
>  filemap_cachestat mm/filemap.c:4314 [inline]
>  __do_sys_cachestat mm/filemap.c:4407 [inline]
>  __se_sys_cachestat+0x795/0xbb0 mm/filemap.c:4372
>  do_syscall_x64 arch/x86/entry/common.c:52 [inline]
>  do_syscall_64+0xf3/0x230 arch/x86/entry/common.c:83
>  entry_SYSCALL_64_after_hwframe+0x77/0x7f

Ok yeah, cachestat() holds the rcu read lock, so
workingset_test_recent() can't do a sleepable rstat flush.

I think the easiest fix would be to flush rstat from the root down
(NULL) in filemap_cachestat(), before the rcu section, and add a flag
to workingset_test_recent() to forego it. Nhat?