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From:   Cong Wang <xiyou.wangcong@gmail.com>
Date:   Tue, 20 Apr 2021 12:53:00 -0700
Message-ID: <CAM_iQpVsAhsG_SLDSgTkZ0RjvCzhu2d5Nt5SQ=NoVa0UYYMoug@mail.gmail.com>
Subject: Re: [Patch] blk-wbt: fix a divide-by-zero error in rwb_arm_timer()
To:     linux-block@vger.kernel.org
Cc:     LKML <linux-kernel@vger.kernel.org>,
        Cong Wang <cong.wang@bytedance.com>,
        Jens Axboe <axboe@kernel.dk>,
        Fam Zheng <fam.zheng@bytedance.com>,
        Xiongchun Duan <duanxiongchun@bytedance.com>
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On Sat, Apr 17, 2021 at 9:41 PM Cong Wang <xiyou.wangcong@gmail.com> wrote:
>
> From: Cong Wang <cong.wang@bytedance.com>
>
> We hit a divide error in rwb_arm_timer() and crash dump shows
> rqd->scale_step is 16777215 (0xffffff in hex), so the expression
> "(rqd->scale_step + 1) << 8)" is 0x100000000, which is just beyond
> 32-bit integer range, hence it is truncated to 0 and int_sqrt(0)
> returns 0 too, so we end up passing 0 as a divisor to div_u64().
>

Never mind. rqd->scale_step should be capped by
rq_depth_scale_down(), so should never be so large. In the old
calc_wb_limits() implementation, rwb->wb_max was set to zero
accidentally.

Thanks.