linux-kernel - Re: undefined shift in wb_update_dirty

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Date:	Mon, 7 Dec 2015 09:26:21 -0500
From:	Tejun Heo <tj@...nel.org>
To:	Andrey Ryabinin <aryabinin@...n.com>
Cc:	Wu Fengguang <fengguang.wu@...el.com>,
	Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
	LKML <linux-kernel@...r.kernel.org>,
	Dmitry Vyukov <dvyukov@...gle.com>,
	Sasha Levin <sasha.levin@...cle.com>
Subject: Re: undefined shift in wb_update_dirty_ratelimit()

Hello, Andrey.

On Mon, Dec 07, 2015 at 05:17:06PM +0300, Andrey Ryabinin wrote:
> I've hit undefined shift in wb_update_dirty_ratelimit() which does some
> mysterious 'step' calculations:
> 
> 	/*
> 	 * Don't pursue 100% rate matching. It's impossible since the balanced
> 	 * rate itself is constantly fluctuating. So decrease the track speed
> 	 * when it gets close to the target. Helps eliminate pointless tremors.
> 	 */
> 	step >>= dirty_ratelimit / (2 * step + 1);
> 
> 
> dirty_ratelimit = INIT_BW and step = 0 results in this:
> 
> [ 5006.957366] ================================================================================
> [ 5006.957798] UBSAN: Undefined behaviour in ../mm/page-writeback.c:1286:7
> [ 5006.958091] shift exponent 25600 is too large for 64-bit type 'long unsigned int'

We prolly should do sth like

	shift = dirty_ratelimit / (2 * step = 1);
	if (shift < BITS_PER_LONG) {
		step = (step >> shift) + 7 / 8;
	} else {
		step = 0;
	}

Thanks.

-- 
tejun
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