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From:   Thomas Gleixner <tglx@linutronix.de>
To:     Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>,
        Andrew Morton <akpm@linux-foundation.org>
Cc:     syzbot <syzbot+fe0c72f0ccbb93786380@syzkaller.appspotmail.com>,
        syzkaller-bugs@googlegroups.com, Ingo Molnar <mingo@elte.hu>,
        linux-kernel@vger.kernel.org, linux-mm <linux-mm@kvack.org>
Subject: Re: [PATCH] debugobject: don't wake up kswapd from fill_pool()
In-Reply-To: <39c79d27-73ea-06a8-62fe-2b64d0fd8db5@I-love.SAKURA.ne.jp>
References: <000000000000008ddb05fb5e2576@google.com>
 <6577e1fa-b6ee-f2be-2414-a2b51b1c5e30@I-love.SAKURA.ne.jp>
 <20230511204458.819f9009d2ef8b46cc163191@linux-foundation.org>
 <d642e597-cf7d-b410-16ce-22dff483fd8e@I-love.SAKURA.ne.jp>
 <87v8gxbthf.ffs@tglx>
 <39c79d27-73ea-06a8-62fe-2b64d0fd8db5@I-love.SAKURA.ne.jp>
Date:   Fri, 12 May 2023 20:07:56 +0200
Message-ID: <87bkipbeyr.ffs@tglx>
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On Fri, May 12 2023 at 22:09, Tetsuo Handa wrote:
> On 2023/05/12 21:54, Thomas Gleixner wrote:
>> On Fri, May 12 2023 at 19:57, Tetsuo Handa wrote:
>>> On 2023/05/12 12:44, Andrew Morton wrote:
>>>> On Thu, 11 May 2023 22:47:32 +0900 Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp> wrote:
>>>>
>>>>> syzbot is reporting lockdep warning in fill_pool(), for GFP_ATOMIC is
>>>>> (__GFP_HIGH | __GFP_KSWAPD_RECLAIM) which wakes up kswapd.
>>>>> Since fill_pool() might be called with arbitrary locks held,
>>>>> fill_pool() should not assume that holding pgdat->kswapd_wait is safe.
>> 
>> https://lore.kernel.org/lkml/871qjldbes.ffs@tglx/
>
> .config says IS_ENABLED(CONFIG_PREEMPT_RT) == false, and lockdep says about
> base->lock => pgdat->kswapd_wait => p->pi_lock => rq->__lock => base->lock
> dependency but does not say about db->lock.
>
> How can your patch fix this problem?

It's described in the changelog, no?

The main change is to make the refill invocation conditional when the
lookup fails. That's how that code has been from day one.

The patch which closed the race recently wreckaged those refill
oportunities and the fix for that introduced this problem.

Thanks,

        tglx