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Date:   Mon, 10 Apr 2023 15:20:06 -0700
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Subject: Re: [PATCH v2 0/3] blk-cgroup: some cleanup
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From:   Bart Van Assche <bvanassche@acm.org>
To:     Yu Kuai <yukuai1@huaweicloud.com>,
        Chengming Zhou <zhouchengming@bytedance.com>, axboe@kernel.dk,
        tj@kernel.org
Cc:     paolo.valente@linaro.org, josef@toxicpanda.com,
        linux-block@vger.kernel.org, cgroups@vger.kernel.org,
        linux-kernel@vger.kernel.org, "yukuai (C)" <yukuai3@huawei.com>
References: <20230406145050.49914-1-zhouchengming@bytedance.com>
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 <d23a6891-96f0-2ee0-70c7-869123d85362@huaweicloud.com>
 <8651fa1f-7f50-03c6-cd85-e61aa44d4927@acm.org>
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On 4/10/23 11:47, Bart Van Assche wrote:
> On 4/9/23 18:57, Yu Kuai wrote:
>> Hi, Bart
>>
>> 在 2023/04/08 2:41, Bart Van Assche 写道:
>>> On 4/6/23 07:50, Chengming Zhou wrote:
>>>> These are some cleanup patches of blk-cgroup. Thanks for review.
>>>
>>> With these patches applied, my kernel test VM crashes during boot. 
>>> The following crash disappears if I revert these patches:
>>>
>>> BUG: KASAN: null-ptr-deref in bio_associate_blkg_from_css+0x83/0x240
>>> Read of size 8 at addr 0000000000000518 by task blkid/5885
>>> Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 
>>> 1.16.0-debian-1.16.0-5 04/01/2014
>>> Call Trace:
>>>   dump_stack_lvl+0x4a/0x80
>>>   print_report+0x21e/0x260
>>>   kasan_report+0xc2/0xf0
>>>   __asan_load8+0x69/0x90
>>>   bio_associate_blkg_from_css+0x83/0x240
>>>   bfq_bio_bfqg+0xce/0x120 [bfq]
>>>   bfq_bic_update_cgroup+0x2f/0x3c0 [bfq]
>>>   bfq_init_rq+0x1e8/0xb10 [bfq]
>>>   bfq_insert_request.isra.0+0xa3/0x420 [bfq]
>>>   bfq_insert_requests+0xca/0xf0 [bfq]
>>>   blk_mq_dispatch_rq_list+0x4c0/0xb00
>>
>> I found this call trace quite weird, I can't figure out how
>> bfq_insert_requests can be called from blk_mq_dispatch_rq_list,
>> can you show the add2line result?
> 
> Hi Kuai,
> 
> Thanks for having taken a look. I ran my tests with this patch series on 
> top of Jens' for-next branch: "[PATCH v2 00/12] Submit zoned writes in 
> order" 
> (https://lore.kernel.org/linux-block/20230407235822.1672286-1-bvanassche@acm.org/T/#m4c8c7ca5a5627510dc1709847b11589e8791b6b6). I will take a closer look and see which of these two patch series needs to be adjusted.

(replying to my own e-mail)

I think I found the root cause: bio->bi_bdev is NULL for pass-through 
requests and BFQ doesn't like it that bio->bi_bdev is NULL. I will make 
sure that pass-through requests are not submitted to any I/O scheduler.

Thanks,

Bart.