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Date: Mon, 7 Dec 2015 09:26:21 -0500
From: Tejun Heo <tj@kernel.org>
To: Andrey Ryabinin <aryabinin@odin.com>
Cc: Wu Fengguang <fengguang.wu@intel.com>,
        Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org,
        LKML <linux-kernel@vger.kernel.org>,
        Dmitry Vyukov <dvyukov@google.com>,
        Sasha Levin <sasha.levin@oracle.com>
Subject: Re: undefined shift in wb_update_dirty_ratelimit()
Message-ID: <20151207142621.GA7012@mtj.duckdns.org>
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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