From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754470Ab3CRTBo (ORCPT ); Mon, 18 Mar 2013 15:01:44 -0400 Received: from terminus.zytor.com ([198.137.202.10]:38415 "EHLO mail.zytor.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753170Ab3CRTBn (ORCPT ); Mon, 18 Mar 2013 15:01:43 -0400 Message-ID: <51476402.7050102@zytor.com> Date: Mon, 18 Mar 2013 11:59:14 -0700 From: "H. Peter Anvin" User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20130311 Thunderbird/17.0.4 MIME-Version: 1.0 To: Yinghai Lu CC: Lin Feng , akpm@linux-foundation.org, linux-mm@kvack.org, x86@kernel.org, linux-kernel@vger.kernel.org, tglx@linutronix.de, mingo@redhat.com, penberg@kernel.org, jacob.shin@amd.com Subject: Re: [PATCH] x86: mm: accurate the comments for STEP_SIZE_SHIFT macro References: <1363602068-11924-1-git-send-email-linfeng@cn.fujitsu.com> In-Reply-To: X-Enigmail-Version: 1.5.1 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 03/18/2013 11:53 AM, Yinghai Lu wrote: > On Mon, Mar 18, 2013 at 3:21 AM, Lin Feng wrote: >> For x86 PUD_SHIFT is 30 and PMD_SHIFT is 21, so the consequence of >> (PUD_SHIFT-PMD_SHIFT)/2 is 4. Update the comments to the code. >> >> Cc: Yinghai Lu >> Signed-off-by: Lin Feng >> --- >> arch/x86/mm/init.c | 2 +- >> 1 file changed, 1 insertion(+), 1 deletion(-) >> >> diff --git a/arch/x86/mm/init.c b/arch/x86/mm/init.c >> index 59b7fc4..637a95b 100644 >> --- a/arch/x86/mm/init.c >> +++ b/arch/x86/mm/init.c >> @@ -389,7 +389,7 @@ static unsigned long __init init_range_memory_mapping( >> return mapped_ram_size; >> } >> >> -/* (PUD_SHIFT-PMD_SHIFT)/2 */ >> +/* (PUD_SHIFT-PMD_SHIFT)/2+1 */ >> #define STEP_SIZE_SHIFT 5 >> void __init init_mem_mapping(void) >> { > > 9/2=4.5, so it becomes 5. > No, it doesn't. This is C, not elementary school Now I'm really bothered. The comment doesn't say *why* (PUD_SHIFT-PMD_SHIFT)/2 or any other variant is correct, furthermore I suspect that the +1 is misplaced. However, what is really needed is: 1. Someone needs to explain what the logic should be and why, and 2. replace the macro with a symbolic macro, not with a constant and a comment explaining, incorrectly, how that value was derived. -hpa