Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1759261Ab3GRR0E (ORCPT ); Thu, 18 Jul 2013 13:26:04 -0400 Received: from Mycroft.westnet.com ([216.187.52.7]:46479 "EHLO mycroft.westnet.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1758781Ab3GRR0B (ORCPT ); Thu, 18 Jul 2013 13:26:01 -0400 X-Greylist: delayed 751 seconds by postgrey-1.27 at vger.kernel.org; Thu, 18 Jul 2013 13:26:00 EDT MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit Message-ID: <20968.8776.742486.828059@quad.stoffel.home> Date: Thu, 18 Jul 2013 13:13:44 -0400 From: "John Stoffel" To: Jerry Cc: akpm@linux-foundation.org, zhuwei.lu@archermind.com, tianfu.huang@archermind.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] mm: negative left shift count when PAGE_SHIFT > 20 In-Reply-To: <1374166572-7988-1-git-send-email-uulinux@gmail.com> References: <1374166572-7988-1-git-send-email-uulinux@gmail.com> X-Mailer: VM 8.1.2 under 23.2.1 (x86_64-pc-linux-gnu) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1892 Lines: 47 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. 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/ -- 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/