From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx174.postini.com [74.125.245.174]) by kanga.kvack.org (Postfix) with SMTP id 3FDD66B0005 for ; Mon, 18 Mar 2013 14:59:29 -0400 (EDT) Message-ID: <51476402.7050102@zytor.com> Date: Mon, 18 Mar 2013 11:59:14 -0700 From: "H. Peter Anvin" MIME-Version: 1.0 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: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: 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 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 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org