From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-15.2 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,NICE_REPLY_A,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED, USER_AGENT_SANE_1 autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id A5AD4C433ED for ; Fri, 23 Apr 2021 14:58:18 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 6CB7260724 for ; Fri, 23 Apr 2021 14:58:18 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S236700AbhDWO6x (ORCPT ); Fri, 23 Apr 2021 10:58:53 -0400 Received: from mx2.suse.de ([195.135.220.15]:36382 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229454AbhDWO6w (ORCPT ); Fri, 23 Apr 2021 10:58:52 -0400 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id 500AFAF17; Fri, 23 Apr 2021 14:58:15 +0000 (UTC) Subject: Re: [PATCH 4/6] mm: Make compound_head const-preserving To: "Matthew Wilcox (Oracle)" , linux-mm@kvack.org Cc: Andrew Morton , linux-kernel@vger.kernel.org References: <20210416231531.2521383-1-willy@infradead.org> <20210416231531.2521383-5-willy@infradead.org> From: Vlastimil Babka Message-ID: <228b6174-5fa7-23d8-5a17-f85f6725068b@suse.cz> Date: Fri, 23 Apr 2021 16:58:14 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.9.1 MIME-Version: 1.0 In-Reply-To: <20210416231531.2521383-5-willy@infradead.org> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 4/17/21 1:15 AM, Matthew Wilcox (Oracle) wrote: > If you pass a const pointer to compound_head(), you get a const pointer > back; if you pass a mutable pointer, you get a mutable pointer back. Hmm, nice trick. > Also remove an unnecessary forward definition of struct page; we're about > to dereference page->compound_head, so it must already have been defined. Right. > Signed-off-by: Matthew Wilcox (Oracle) Reviewed-by: Vlastimil Babka > --- > include/linux/page-flags.h | 10 +++++----- > 1 file changed, 5 insertions(+), 5 deletions(-) > > diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h > index 04a34c08e0a6..d8e26243db25 100644 > --- a/include/linux/page-flags.h > +++ b/include/linux/page-flags.h > @@ -177,17 +177,17 @@ enum pageflags { > > #ifndef __GENERATING_BOUNDS_H > > -struct page; /* forward declaration */ > - > -static inline struct page *compound_head(struct page *page) > +static inline unsigned long _compound_head(const struct page *page) > { > unsigned long head = READ_ONCE(page->compound_head); > > if (unlikely(head & 1)) > - return (struct page *) (head - 1); > - return page; > + return head - 1; > + return (unsigned long)page; > } > > +#define compound_head(page) ((typeof(page))_compound_head(page)) > + > static __always_inline int PageTail(struct page *page) > { > return READ_ONCE(page->compound_head) & 1; >