linux-kernel - Re: A regression in recent 3.2 kernel: bdi_dirty

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Message-ID: <20120109040445.GB31049@localhost>
Date:	Mon, 9 Jan 2012 12:04:45 +0800
From:	Wu Fengguang <wfg@...ux.intel.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Илья Тумайкин 
	<librarian_rus@...oo.com>, LKML <linux-kernel@...r.kernel.org>,
	linux-fsdevel@...r.kernel.org
Subject: Re: A regression in recent 3.2 kernel: bdi_dirty_limit() divide
 error

On Sun, Jan 08, 2012 at 11:19:14AM +0100, Peter Zijlstra wrote:

> But even then, we limit PROP_MAX_SHIFT to 3*BITS_PER_LONG/4, I don't
> think that could ever overflow.

do_div() only uses the lower 32 bit value of the 64 bit denominator,
which may happen to be 0.

The denominator is not really 64 bit, but limited by PROP_MAX_SHIFT =
48 bit, however that upper limit looks not enough.

Ilya, would you help try the attached patch instead?

Thanks,
Fengguang

View attachment "proportion-max-shift.patch" of type "text/x-diff" (756 bytes)