Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752404Ab3GSV5c (ORCPT ); Fri, 19 Jul 2013 17:57:32 -0400 Received: from mail.linuxfoundation.org ([140.211.169.12]:52104 "EHLO mail.linuxfoundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751768Ab3GSV5b (ORCPT ); Fri, 19 Jul 2013 17:57:31 -0400 Date: Fri, 19 Jul 2013 14:57:29 -0700 From: Andrew Morton To: Jerry Cc: 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 Message-Id: <20130719145729.840eeae88fad89d2c6915163@linux-foundation.org> In-Reply-To: References: <1374166572-7988-1-git-send-email-uulinux@gmail.com> <20130718143928.4f9b45807956e2fdb1ee3a22@linux-foundation.org> X-Mailer: Sylpheed 3.2.0beta5 (GTK+ 2.24.10; x86_64-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1705 Lines: 49 On Fri, 19 Jul 2013 07:47:02 +0800 Jerry wrote: > 2013/7/19 Andrew Morton : > > On Fri, 19 Jul 2013 00:56:12 +0800 Jerry wrote: > > > >> When PAGE_SHIFT > 20, the result of "20 - PAGE_SHIFT" is negative. The > >> calculating here will generate an unexpected result. In addition, if > >> PAGE_SHIFT > 20, The memory size represented by numentries was already > >> integral multiple of 1MB. > >> > > > > If you tell me that you have a machine which has PAGE_SIZE=2MB and this > > was the only problem which prevented Linux from running on that machine > > then I'll apply the patch ;) > > > > Hi Morton: > I just "grep -rn "#define\s\+PAGE_SHIFT" arch/", and find the > PAGE_SHIFT in some architecture is very big. > such as the following in "arch/hexagon/include/asm/page.h" > .... > #ifdef CONFIG_PAGE_SIZE_256KB > #define PAGE_SHIFT 18 > #define HEXAGON_L1_PTE_SIZE __HVM_PDE_S_256KB > #endif > > #ifdef CONFIG_PAGE_SIZE_1MB > #define PAGE_SHIFT 20 > #define HEXAGON_L1_PTE_SIZE __HVM_PDE_S_1MB > #endif > ..... Good heavens. > In my patch, I think compiler would optimize "if (20 > PAGE_SIZE)", it > won't generate any machine instruction. Just a guarantee. Well the existing code is a bit silly looking. Why can't we just do /* round applicable memory size up to nearest megabyte */ if (PAGE_SHIFT < 20) numentries = round_up(nr_kernel_pages, (1 << 20)/PAGE_SIZE); or similar? -- 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/