Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754677Ab3GTRRB (ORCPT ); Sat, 20 Jul 2013 13:17:01 -0400 Received: from mail-ea0-f181.google.com ([209.85.215.181]:62309 "EHLO mail-ea0-f181.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754470Ab3GTRRA (ORCPT ); Sat, 20 Jul 2013 13:17:00 -0400 MIME-Version: 1.0 In-Reply-To: <20130719145729.840eeae88fad89d2c6915163@linux-foundation.org> References: <1374166572-7988-1-git-send-email-uulinux@gmail.com> <20130718143928.4f9b45807956e2fdb1ee3a22@linux-foundation.org> <20130719145729.840eeae88fad89d2c6915163@linux-foundation.org> Date: Sun, 21 Jul 2013 01:16:59 +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: 1922 Lines: 55 2013/7/20 Andrew Morton : > 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? Great. I have adjusted these code lines, and sent the latest one. -- 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/