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From:   Khazhy Kumykov <khazhy@chromium.org>
Date:   Wed, 2 Nov 2022 20:05:08 -0700
Message-ID: <CACGdZYJ0WH+Y9sdchXy30UVTQgPCEo=fW+W9atZh1Ki7Ov4_Gw@mail.gmail.com>
Subject: Re: [RFC PATCH] bfq: fix waker_bfqq inconsistency crash
To:     Yu Kuai <yukuai1@huaweicloud.com>
Cc:     Paolo Valente <paolo.valente@linaro.org>,
        Jens Axboe <axboe@kernel.dk>, linux-block@vger.kernel.org,
        linux-kernel@vger.kernel.org, "yukuai (C)" <yukuai3@huawei.com>
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On Wed, Nov 2, 2022 at 7:56 PM Yu Kuai <yukuai1@huaweicloud.com> wrote:
>
> Hi,
>
> =E5=9C=A8 2022/11/03 9:39, Khazhismel Kumykov =E5=86=99=E9=81=93:
> > 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.
>
> > 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 al=
l
> > 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.

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

>
> Thanks,
> Kuai