From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758936AbXFGL4B (ORCPT ); Thu, 7 Jun 2007 07:56:01 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1752612AbXFGLzy (ORCPT ); Thu, 7 Jun 2007 07:55:54 -0400 Received: from nic.NetDirect.CA ([216.16.235.2]:43158 "EHLO rubicon.netdirect.ca" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750870AbXFGLzx (ORCPT ); Thu, 7 Jun 2007 07:55:53 -0400 X-Originating-Ip: 72.143.66.27 Date: Thu, 7 Jun 2007 07:53:08 -0400 (EDT) From: "Robert P. J. Day" X-X-Sender: rpjday@localhost.localdomain To: Satyam Sharma cc: Nick Piggin , Linux Kernel Mailing List , Ralf Baechle Subject: Re: why does the macro "ZERO_PAGE" take an argument? In-Reply-To: Message-ID: References: <4667EC18.4080904@yahoo.com.au> MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII X-Net-Direct-Inc-MailScanner-Information: Please contact the ISP for more information X-Net-Direct-Inc-MailScanner: Found to be clean X-Net-Direct-Inc-MailScanner-SpamCheck: not spam, SpamAssassin (not cached, score=-16.8, required 5, autolearn=not spam, ALL_TRUSTED -1.80, BAYES_00 -15.00, INIT_RECVD_OUR_AUTH -20.00, RCVD_IN_SORBS_DUL 20.00) X-Net-Direct-Inc-MailScanner-From: rpjday@mindspring.com Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org On Thu, 7 Jun 2007, Satyam Sharma wrote: > On 6/7/07, Robert P. J. Day wrote: > > On Thu, 7 Jun 2007, Nick Piggin wrote: > > > > > Robert P. J. Day wrote: > > > > probably making a fool of myself here, but what is the purpose of > > > > that single argument to the macro "ZERO_PAGE"? > > > > > > > > $ grep -r "define ZERO_PAGE" include > > > > include/asm-frv/pgtable.h:#define ZERO_PAGE(vaddr) ({ BUG(); NULL; > > }) > > > > include/asm-frv/pgtable.h:#define ZERO_PAGE(vaddr) > > > > virt_to_page(empty_zero_page) > > > > include/asm-v850/pgtable.h:#define ZERO_PAGE(vaddr) ((void > > *)0x87654321) > > > > include/asm-mips/pgtable.h:#define ZERO_PAGE(vaddr) \ > > > > include/asm-blackfin/pgtable.h:#define ZERO_PAGE(vaddr) > > (virt_to_page(0)) > > > > include/asm-parisc/pgtable.h:#define ZERO_PAGE(vaddr) > > > > (virt_to_page(empty_zero_page)) > > > > include/asm-alpha/pgtable.h:#define ZERO_PAGE(vaddr) > > > > (virt_to_page(ZERO_PGE)) > > > > ... > > > > > > > > AFAICT, there are no definitions of that macro that actually use > > > > that argument. is that some kind of historical cruft? > > > > > > MIPS? > > > > argh. that would be the *one* definition whose output got chopped > > because of line continuation, and it would be only one that actually > > uses the argument: > > > > #define ZERO_PAGE(vaddr) \ > > (virt_to_page((void *)(empty_zero_page + (((unsigned long)(vaddr)) & > > zero_page_mask)))) > > > > > > but it still leaves the question -- if ZERO_PAGE is meant to represent > > a single, global shared page that is always zero, why would it *ever* > > need to take an argument? and what's so special about MIPS that it > > differs from all the rest? > > The comment above empty_zero_page and zero_page_mask > declarations at arch/mips/mm/init.c:508 sheds light on this ... well, it *sort of* does. at line 64 of that file: /* * We have up to 8 empty zeroed pages so we can map one of the right colour * when needed. This is necessary only on R4000 / R4400 SC and MC versions * where we have to avoid VCED / VECI exceptions for good performance at * any price. Since page is never written to after the initialization we * don't have to care about aliases on other CPUs. */ although it's not clear where in the source tree are the invocations that would actually make a difference to a MIPS system, which is why i've CC'ed ralf on this. i'm sure he can clear this up. :-) rday -- ======================================================================== Robert P. J. Day Linux Consulting, Training and Annoying Kernel Pedantry Waterloo, Ontario, CANADA http://fsdev.net/wiki/index.php?title=Main_Page ========================================================================