linux-ext4 - Re: [syzbot] possible deadlock in start_this

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Date:   Fri, 15 Jul 2022 07:24:55 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To:     Jan Kara <jack@...e.cz>, linux-mm@...ck.org
Cc:     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 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 ;).
> 
> 								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.