Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S934876Ab3GRXrF (ORCPT ); Thu, 18 Jul 2013 19:47:05 -0400 Received: from mail-oa0-f54.google.com ([209.85.219.54]:54020 "EHLO mail-oa0-f54.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S934780Ab3GRXrE (ORCPT ); Thu, 18 Jul 2013 19:47:04 -0400 MIME-Version: 1.0 In-Reply-To: <20130718143928.4f9b45807956e2fdb1ee3a22@linux-foundation.org> References: <1374166572-7988-1-git-send-email-uulinux@gmail.com> <20130718143928.4f9b45807956e2fdb1ee3a22@linux-foundation.org> Date: Fri, 19 Jul 2013 07:47:02 +0800 Message-ID: Subject: Re: [PATCH] mm: negative left shift count when PAGE_SHIFT > 20 From: Jerry To: Andrew Morton Cc: 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: 1625 Lines: 45 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 ..... Maybe the day of "A 2MB page" is not far. :-) I know it is just a latent issue. Even if it won't generate a error when PAGE_SIZE == 20, the calculating here is not necessary. In my mind, compiler would optimize the calculating at that situation. But it is a little tricky. In my patch, I think compiler would optimize "if (20 > PAGE_SIZE)", it won't generate any machine instruction. Just a guarantee. -- 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/