From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-ed1-f71.google.com (mail-ed1-f71.google.com [209.85.208.71]) by kanga.kvack.org (Postfix) with ESMTP id 690CD8E004D for ; Tue, 11 Dec 2018 05:19:08 -0500 (EST) Received: by mail-ed1-f71.google.com with SMTP id w15so6709861edl.21 for ; Tue, 11 Dec 2018 02:19:08 -0800 (PST) Received: from mail-sor-f65.google.com (mail-sor-f65.google.com. [209.85.220.65]) by mx.google.com with SMTPS id x22sor7651392eda.18.2018.12.11.02.19.07 for (Google Transport Security); Tue, 11 Dec 2018 02:19:07 -0800 (PST) Date: Tue, 11 Dec 2018 10:19:05 +0000 From: Wei Yang Subject: Re: [PATCH] mm, sparse: remove check with __highest_present_section_nr in for_each_present_section_nr() Message-ID: <20181211101905.xczl6bndmrqwukni@master> Reply-To: Wei Yang References: <20181211035128.43256-1-richard.weiyang@gmail.com> <20181211094441.GD1286@dhcp22.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20181211094441.GD1286@dhcp22.suse.cz> Sender: owner-linux-mm@kvack.org List-ID: To: Michal Hocko Cc: Wei Yang , linux-mm@kvack.org, akpm@linux-foundation.org, osalvador@suse.de On Tue, Dec 11, 2018 at 10:44:41AM +0100, Michal Hocko wrote: >On Tue 11-12-18 11:51:28, Wei Yang wrote: >> A valid present section number is in [0, __highest_present_section_nr]. >> And the return value of next_present_section_nr() meets this >> requirement. This means it is not necessary to check it with >> __highest_present_section_nr again in for_each_present_section_nr(). >> >> Since we pass an unsigned long *section_nr* to >> for_each_present_section_nr(), we need to cast it to int before >> comparing. > >Why do we want this patch? Is it an improvement? If yes, it is >performance visible change or does it make the code easier to maintain? > Michal I know you concern, maintainance is a very critical part of review. >To me at least the later seems dubious to be honest because it adds a >non-obvious dependency of the terminal condition to the >next_present_section_nr implementation and that might turn out error >prone. > While I think the original code is not that clear about the syntax. When we look at the next_present_section_nr(section_nr), the return value falls into two categories: -1 : no more present section after section_nr other: the next present section number after section_nr Based on this syntax, the iteration could be simpler to terminate when the return value is less than 0. This is what the patch tries to do. Maybe I could do more to help the maintainance: * add some comment about the return value of next_present_section_nr * terminate the loop when section_nr == -1 Hope this would help a little. -- Wei Yang Help you, Help me