From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pg1-f200.google.com (mail-pg1-f200.google.com [209.85.215.200]) by kanga.kvack.org (Postfix) with ESMTP id 069ED6B0003 for ; Thu, 18 Oct 2018 12:30:43 -0400 (EDT) Received: by mail-pg1-f200.google.com with SMTP id u43-v6so14843215pgn.4 for ; Thu, 18 Oct 2018 09:30:42 -0700 (PDT) Received: from mx1.suse.de (mx2.suse.de. [195.135.220.15]) by mx.google.com with ESMTPS id w19-v6si14758128pgf.197.2018.10.18.09.30.41 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Thu, 18 Oct 2018 09:30:41 -0700 (PDT) Date: Thu, 18 Oct 2018 18:30:39 +0200 From: Michal Hocko Subject: Re: [PATCH] mm: get pfn by page_to_pfn() instead of save in page->private Message-ID: <20181018163039.GF18839@dhcp22.suse.cz> References: <20181018130429.37837-1-richard.weiyang@gmail.com> <20181018131504.GC18839@dhcp22.suse.cz> <20181018141008.lcyttmp7bb42uigi@master> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20181018141008.lcyttmp7bb42uigi@master> Sender: owner-linux-mm@kvack.org List-ID: To: Wei Yang Cc: akpm@linux-foundation.org, mgorman@techsingularity.net, linux-mm@kvack.org On Thu 18-10-18 14:10:08, Wei Yang wrote: > On Thu, Oct 18, 2018 at 03:15:04PM +0200, Michal Hocko wrote: > >On Thu 18-10-18 21:04:29, Wei Yang wrote: > >> This is not necessary to save the pfn to page->private. > >> > >> The pfn could be retrieved by page_to_pfn() directly. > > > >Yes it can, but a cursory look at the commit which has introduced this > >suggests that this is a micro-optimization. Mel would know more of > >course. There are some memory models where page_to_pfn is close to free. > > > >If that is the case I am not really sure it is measurable or worth it. > >In any case any change to this code should have a proper justification. > >In other words, is this change really needed? Does it help in any > >aspect? Possibly readability? The only thing I can guess from this > >changelog is that you read the code and stumble over this. If that is > >the case I would recommend asking author for the motivation and > >potentially add a comment to explain it better rather than shoot a patch > >rightaway. > > > > Your are right. I am really willing to understand why we want to use > this mechanisum. I am happy to hear that. > So the correct procedure is to send a mail to the mail list to query the > reason? It is certainly better to ask a question than send a patch without a proper justification. I would also encourage to use git blame to see which patch has introduced the specific piece of code. Many times it helps to understand the motivation. I would also encourage to go back to the mailing list archives and the associate discussion to the specific patch. In many cases there is Link: tag which can help you to find the respective discussion. Thanks! -- Michal Hocko SUSE Labs