From mboxrd@z Thu Jan 1 00:00:00 1970 From: Wu Fengguang Subject: Re: A regression in recent 3.2 kernel: bdi_dirty_limit() divide error Date: Mon, 9 Jan 2012 12:04:45 +0800 Message-ID: <20120109040445.GB31049@localhost> References: <1325884395.57034.YahooMailClassic@web161605.mail.bf1.yahoo.com> <20120107145645.GA4997@localhost> <1325954125.2442.27.camel@twins> <20120108023305.GA5074@localhost> <1326017954.2442.35.camel@twins> Mime-Version: 1.0 Content-Type: multipart/mixed; boundary="tjCHc7DPkfUGtrlw" Cc: =?utf-8?B?0JjQu9GM0Y8g0KLRg9C80LDQudC60LjQvQ==?= , LKML , linux-fsdevel@vger.kernel.org To: Peter Zijlstra Return-path: Content-Disposition: inline In-Reply-To: <1326017954.2442.35.camel@twins> Sender: linux-kernel-owner@vger.kernel.org List-Id: linux-fsdevel.vger.kernel.org --tjCHc7DPkfUGtrlw Content-Type: text/plain; charset=us-ascii Content-Disposition: inline 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 --tjCHc7DPkfUGtrlw Content-Type: text/x-diff; charset=us-ascii Content-Disposition: attachment; filename="proportion-max-shift.patch" Subject: Date: Mon Jan 09 11:53:50 CST 2012 Signed-off-by: Wu Fengguang --- include/linux/proportions.h | 4 ++++ 1 file changed, 4 insertions(+) --- linux.orig/include/linux/proportions.h 2012-01-09 11:51:19.000000000 +0800 +++ linux/include/linux/proportions.h 2012-01-09 11:53:47.000000000 +0800 @@ -81,7 +81,11 @@ void prop_inc_percpu(struct prop_descrip * Limit the time part in order to ensure there are some bits left for the * cycle counter and fraction multiply. */ +#if BITS_PER_LONG == 32 #define PROP_MAX_SHIFT (3*BITS_PER_LONG/4) +#else +#define PROP_MAX_SHIFT (BITS_PER_LONG/2) +#endif #define PROP_FRAC_SHIFT (BITS_PER_LONG - PROP_MAX_SHIFT - 1) #define PROP_FRAC_BASE (1UL << PROP_FRAC_SHIFT) --tjCHc7DPkfUGtrlw--