From mboxrd@z Thu Jan 1 00:00:00 1970 From: Wu Fengguang Subject: [PATCH] lib: proportion: lower PROP_MAX_SHIFT to 32 on 64-bit kernel Date: Tue, 10 Jan 2012 22:54:55 +0800 Message-ID: <20120110145455.GA8999@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> <20120109045515.GB3976@localhost> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: =?utf-8?B?0JjQu9GM0Y8g0KLRg9C80LDQudC60LjQvQ==?= , LKML , linux-fsdevel@vger.kernel.org, Jan Kara To: Peter Zijlstra Return-path: Content-Disposition: inline In-Reply-To: <20120109045515.GB3976@localhost> Sender: linux-kernel-owner@vger.kernel.org List-Id: linux-fsdevel.vger.kernel.org PROP_MAX_SHIFT should be set to <=32 on 64-bit box. This fixes two bugs in the below lines of bdi_dirty_limit(): bdi_dirty *= numerator; do_div(bdi_dirty, denominator); 1) divide error: do_div() only uses the lower 32 bit of the denominator, which may trimmed to be 0 when PROP_MAX_SHIFT > 32. 2) overflow: (bdi_dirty * numerator) could easily overflow if numerator used up to 48 bits, leaving only 16 bits to bdi_dirty Tested-by: Ilya Tumaykin 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)