Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933314Ab3GRRVP (ORCPT ); Thu, 18 Jul 2013 13:21:15 -0400 Received: from mail-ee0-f54.google.com ([74.125.83.54]:53296 "EHLO mail-ee0-f54.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932540Ab3GRRVN (ORCPT ); Thu, 18 Jul 2013 13:21:13 -0400 MIME-Version: 1.0 In-Reply-To: <20968.8776.742486.828059@quad.stoffel.home> References: <1374166572-7988-1-git-send-email-uulinux@gmail.com> <20968.8776.742486.828059@quad.stoffel.home> Date: Fri, 19 Jul 2013 01:21:12 +0800 Message-ID: Subject: Re: [PATCH] mm: negative left shift count when PAGE_SHIFT > 20 From: Jerry To: John Stoffel Cc: Andrew Morton , zhuwei.lu@archermind.com, tianfu.huang@archermind.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org Content-Type: text/plain; charset=ISO-8859-1 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2297 Lines: 56 > Jerry> When PAGE_SHIFT > 20, the result of "20 - PAGE_SHIFT" is negative. The > Jerry> calculating here will generate an unexpected result. In addition, if > Jerry> PAGE_SHIFT > 20, The memory size represented by numentries was already > Jerry> integral multiple of 1MB. > > Why this magic number of 20? Please explain it better and replace it > was a #define that means something here. Because 2^20 = 1MB. The intention of previous code is "/* round applicable memory size up to nearest megabyte */". > > Jerry> Signed-off-by: Jerry > Jerry> --- > Jerry> mm/page_alloc.c | 8 +++++--- > Jerry> 1 file changed, 5 insertions(+), 3 deletions(-) > > Jerry> diff --git a/mm/page_alloc.c b/mm/page_alloc.c > Jerry> index b100255..cd41797 100644 > Jerry> --- a/mm/page_alloc.c > Jerry> +++ b/mm/page_alloc.c > Jerry> @@ -5745,9 +5745,11 @@ void *__init alloc_large_system_hash(const char *tablename, > Jerry> if (!numentries) { > Jerry> /* round applicable memory size up to nearest megabyte */ > Jerry> numentries = nr_kernel_pages; > Jerry> - numentries += (1UL << (20 - PAGE_SHIFT)) - 1; > Jerry> - numentries >>= 20 - PAGE_SHIFT; > Jerry> - numentries <<= 20 - PAGE_SHIFT; > Jerry> + if (20 > PAGE_SHIFT) { > Jerry> + numentries += (1UL << (20 - PAGE_SHIFT)) - 1; > Jerry> + numentries >>= 20 - PAGE_SHIFT; > Jerry> + numentries <<= 20 - PAGE_SHIFT; > Jerry> + } > > Jerry> /* limit to 1 bucket per 2^scale bytes of low memory */ > Jerry> if (scale > PAGE_SHIFT) > Jerry> -- > Jerry> 1.8.1.5 > > Jerry> -- > Jerry> To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > Jerry> the body of a message to majordomo@vger.kernel.org > Jerry> More majordomo info at http://vger.kernel.org/majordomo-info.html > Jerry> Please read the FAQ at http://www.tux.org/lkml/ -- I love linux!!! -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/