linux-kernel - Re: [RFC PATCH] bfq: fix waker

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Date:   Thu, 3 Nov 2022 11:51:15 +0800
From:   Yu Kuai <yukuai1@...weicloud.com>
To:     Khazhy Kumykov <khazhy@...omium.org>,
        Yu Kuai <yukuai1@...weicloud.com>
Cc:     Paolo Valente <paolo.valente@...aro.org>,
        Jens Axboe <axboe@...nel.dk>, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, "yukuai (C)" <yukuai3@...wei.com>,
        Jan Kara <jack@...e.cz>
Subject: Re: [RFC PATCH] bfq: fix waker_bfqq inconsistency crash

Hi,

在 2022/11/03 11:05, Khazhy Kumykov 写道:
> On Wed, Nov 2, 2022 at 7:56 PM Yu Kuai <yukuai1@...weicloud.com> wrote:
>>
>> Hi,
>>
>> 在 2022/11/03 9:39, Khazhismel Kumykov 写道:
>>> This fixes crashes in bfq_add_bfqq_busy due to waker_bfqq being NULL,
>>> but woken_list_node still being hashed. This would happen when
>>> bfq_init_rq() expects a brand new allocated queue to be returned from
>>
>>   From what I see, bfqq->waker_bfqq is updated in bfq_init_rq() only if
>> 'new_queue' is false, but if 'new_queue' is false, the returned 'bfqq'
>> from bfq_get_bfqq_handle_split() will never be oom_bfqq, so I'm confused
>> here...
> There's two calls for bfq_get_bfqq_handle_split in this function - the
> second one is after the check you mentioned, and is the problematic
> one.
Yes, thanks for the explanation. Now I understand how the problem
triggers.

>>
>>> bfq_get_bfqq_handle_split() and unconditionally updates waker_bfqq
>>> without resetting woken_list_node. Since we can always return oom_bfqq
>>> when attempting to allocate, we cannot assume waker_bfqq starts as NULL.
>>> We must either reset woken_list_node, or avoid setting woken_list at all
>>> for oom_bfqq - opt to do the former.
>>
>> Once oom_bfqq is used, I think the io is treated as issued from root
>> group. Hence I don't think it's necessary to set woken_list or
>> waker_bfqq for oom_bfqq.
> Ack, I was wondering what's right here since, evidently, *someone* had
> already set oom_bfqq->waker_bfqq to *something* (although... maybe it
> was an earlier init_rq). But maybe it's better to do nothing if we
> *know* it's oom_bfqq.

I need to have a check how oom_bfqq get involved with waker_bfqq, and
then see if it's reasonable.

Probably Jan and Paolo will have better view on this.

Thanks,
Kuai
> 
> Is it a correct interpretation here that setting waker_bfqq won't
> accomplish anything anyways on oom_bfqq, so better off not?