linux-kernel - Re: [syzbot] possible deadlock in start_this

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Date:   Fri, 15 Jul 2022 01:39:08 +0000
From:   Shakeel Butt <shakeelb@...gle.com>
To:     Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
Cc:     Jan Kara <jack@...e.cz>, linux-mm@...ck.org, jack@...e.com,
        linux-ext4@...r.kernel.org, linux-kernel@...r.kernel.org,
        syzkaller-bugs@...glegroups.com, tytso@....edu,
        linux-fsdevel@...r.kernel.org
Subject: Re: [syzbot] possible deadlock in start_this_handle (3)

On Fri, Jul 15, 2022 at 07:24:55AM +0900, Tetsuo Handa wrote:
> On 2022/07/14 23:18, Jan Kara wrote:
> > Hello,
> > 
> > so this lockdep report looks real but is more related to OOM handling than
> > to ext4 as such. The immediate problem I can see is that
> > mem_cgroup_print_oom_meminfo() which is called under oom_lock calls
> > memory_stat_format() which does GFP_KERNEL allocations to allocate buffers
> > for dumping of MM statistics. This creates oom_lock -> fs reclaim
> > dependency and because OOM can be hit (and thus oom_lock acquired) in
> > practically any allocation (regardless of GFP_NOFS) this has a potential of
> > creating real deadlock cycles.
> > 
> > So should mem_cgroup_print_oom_meminfo() be using
> > memalloc_nofs_save/restore() to avoid such deadlocks? Or perhaps someone
> > sees another solution? Generally allocating memory to report OOM looks a
> > bit dangerous to me ;).

mem_cgroup_print_oom_meminfo() is called only for memcg OOMs. So, the
situaion would be dangerous only if the system is also OOM at that time.

> > 
> > 								Honza
> 
> I think mem_cgroup_print_oom_meminfo() should use GFP_ATOMIC, for it will fall into
> infinite loop if kmalloc(GFP_NOFS) under oom_lock reached __alloc_pages_may_oom() path.

I would prefer GFP_NOWAIT. This is printing info for memcg OOMs and if
the system is low on memory then memcg OOMs has lower importance than
the system state.