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Subject: Re: [RFC PATCH] bfq: fix waker_bfqq inconsistency crash
To:     Khazhismel Kumykov <khazhy@chromium.org>,
        Paolo Valente <paolo.valente@linaro.org>,
        Jens Axboe <axboe@kernel.dk>
Cc:     linux-block@vger.kernel.org, linux-kernel@vger.kernel.org,
        Khazhismel Kumykov <khazhy@google.com>,
        "yukuai (C)" <yukuai3@huawei.com>
References: <20221103013937.603626-1-khazhy@google.com>
From:   Yu Kuai <yukuai1@huaweicloud.com>
Message-ID: <3c0df3fa-8731-5863-ccc5-f2e60601dbf9@huaweicloud.com>
Date:   Thu, 3 Nov 2022 10:55:37 +0800
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Hi,

?? 2022/11/03 9:39, Khazhismel Kumykov д??:
> 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...

> 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.

Thanks,
Kuai
> 
> Crashes would have a stacktrace like:
> [160595.656560]  bfq_add_bfqq_busy+0x110/0x1ec
> [160595.661142]  bfq_add_request+0x6bc/0x980
> [160595.666602]  bfq_insert_request+0x8ec/0x1240
> [160595.671762]  bfq_insert_requests+0x58/0x9c
> [160595.676420]  blk_mq_sched_insert_request+0x11c/0x198
> [160595.682107]  blk_mq_submit_bio+0x270/0x62c
> [160595.686759]  __submit_bio_noacct_mq+0xec/0x178
> [160595.691926]  submit_bio+0x120/0x184
> [160595.695990]  ext4_mpage_readpages+0x77c/0x7c8
> [160595.701026]  ext4_readpage+0x60/0xb0
> [160595.705158]  filemap_read_page+0x54/0x114
> [160595.711961]  filemap_fault+0x228/0x5f4
> [160595.716272]  do_read_fault+0xe0/0x1f0
> [160595.720487]  do_fault+0x40/0x1c8
> 
> Tested by injecting random failures into bfq_get_queue, crashes go away
> completely.
> 
> Fixes: 8ef3fc3a043c ("block, bfq: make shared queues inherit wakers")
> Signed-off-by: Khazhismel Kumykov <khazhy@google.com>
> ---
> RFC mainly because it's not clear to me the best policy here - but the
> patch is tested and fixes a real crash we started seeing in 5.15
> 
> This is following up my ramble over at
> https://lore.kernel.org/lkml/CACGdZYLMnfcqwbAXDx+x9vUOMn2cz55oc+8WySBS3J2Xd_q7Lg@mail.gmail.com/
> 
>   block/bfq-iosched.c | 5 +++++
>   1 file changed, 5 insertions(+)
> 
> diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
> index 7ea427817f7f..5d2861119d20 100644
> --- a/block/bfq-iosched.c
> +++ b/block/bfq-iosched.c
> @@ -6793,7 +6793,12 @@ static struct bfq_queue *bfq_init_rq(struct request *rq)
>   				 * reset. So insert new_bfqq into the
>   				 * woken_list of the waker. See
>   				 * bfq_check_waker for details.
> +				 *
> +				 * Also, if we got oom_bfqq, we must check if
> +				 * it's already in a woken_list
>   				 */
> +				if (unlikely(!hlist_unhashed(&bfqq->woken_list_node)))
> +					hlist_del_init(&bfqq->woken_list_node);
>   				if (bfqq->waker_bfqq)
>   					hlist_add_head(&bfqq->woken_list_node,
>   						       &bfqq->waker_bfqq->woken_list);
>