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Subject: Re: bcache: oops when writing to writeback_percent without a cache
 device
From:   Coly Li <colyli@suse.de>
To:     =?UTF-8?Q?Bj=c3=b8rn_Forsman?= <bjorn.forsman@gmail.com>
Cc:     Kent Overstreet <kent.overstreet@gmail.com>,
        linux-bcache@vger.kernel.org, LKML <linux-kernel@vger.kernel.org>
References: <CAEYzJUH1L5qyWKN3_s4Sz81frho6nKB9bkyDoGxXCvLNO484ew@mail.gmail.com>
 <6823d3ab-5f93-da74-0dbc-19bdb7be6907@suse.de>
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Date:   Tue, 4 Jun 2019 23:41:43 +0800
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On 2019/6/4 10:59 下午, Coly Li wrote:
> On 2019/6/4 7:00 下午, Bjørn Forsman wrote:
>> Hi all,
>>
>> I get a kernel oops from bcache when writing to
>> /sys/block/bcache0/bcache/writeback_percent and there is no attached
>> cache device. See the oops itself below my signature.
>>
>> This is on Linux 4.19.46. I looked in git and see many commits to
>> bcache lately, but none seem to address this particular issue.
>>
>> Background: I'm writing to .../writeback_percent with
>> systemd-tmpfiles. I'd rather not replace it with a script that figures
>> out whether or not the kernel will oops if writing to the sysfs file
>> -- the kernel should not oops in the first place.
> 
> Hi Bjorn,
> 
> Thank you for the reporting. I believe this is a case we missed in
> testings. When a bcache device is not attached, it does not make sense
> to update the writeback rate in period by the changing of writeback_percent.
> 
> I will post a patch for your testing soon.

Hi Bjorn,

Could you please to try this patch ? Hope it may help a bit.

Thanks.

Coly Li


>>
>> Jun 04 12:35:42 kernel: BUG: unable to handle kernel NULL pointer
>> dereference at 0000000000000340
>> Jun 04 12:35:42 kernel: PGD 0 P4D 0
>> Jun 04 12:35:42 kernel: Oops: 0000 [#1] SMP PTI
>> Jun 04 12:35:42 kernel: CPU: 6 PID: 20266 Comm: kworker/6:220 Not
>> tainted 4.19.46 #1-NixOS
>> Jun 04 12:35:42 kernel: Hardware name: To Be Filled By O.E.M. To Be
>> Filled By O.E.M./X99 Extreme4/3.1, BIOS P3.60 04/06/2018
>> Jun 04 12:35:42 kernel: Workqueue: events update_writeback_rate [bcache]
>> Jun 04 12:35:42 kernel: RIP: 0010:update_writeback_rate+0x2f/0x2e0 [bcache]
[snipped]

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