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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id A172CC433EF for ; Wed, 15 Jun 2022 07:45:16 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S245326AbiFOHoR (ORCPT ); Wed, 15 Jun 2022 03:44:17 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:52188 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1346795AbiFOHoK (ORCPT ); Wed, 15 Jun 2022 03:44:10 -0400 Received: from szxga08-in.huawei.com (szxga08-in.huawei.com [45.249.212.255]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 5CFD63FDA5 for ; Wed, 15 Jun 2022 00:44:09 -0700 (PDT) Received: from canpemm500002.china.huawei.com (unknown [172.30.72.54]) by szxga08-in.huawei.com (SkyGuard) with ESMTP id 4LNHM54571z1K9tw; Wed, 15 Jun 2022 15:42:09 +0800 (CST) Received: from [10.174.177.76] (10.174.177.76) by canpemm500002.china.huawei.com (7.192.104.244) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) id 15.1.2375.24; Wed, 15 Jun 2022 15:44:06 +0800 Subject: Re: [PATCH] mm/page_alloc: make calling prep_compound_head more reliable To: Matthew Wilcox CC: Joao Martins , Andrew Morton , , , Dan Williams References: <20220607144157.36411-1-linmiaohe@huawei.com> <20220607113257.84b1bdd993f19be26b8c4944@linux-foundation.org> <65e5da9c-32d1-17d7-d8c6-96cbfac23fec@oracle.com> <4a30f026-789a-9235-2fbd-f553e4d7b45d@huawei.com> From: Miaohe Lin Message-ID: <40a07ce5-414a-a3b8-53ee-6c348635f03a@huawei.com> Date: Wed, 15 Jun 2022 15:44:06 +0800 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:78.0) Gecko/20100101 Thunderbird/78.6.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset="utf-8" Content-Language: en-US Content-Transfer-Encoding: 7bit X-Originating-IP: [10.174.177.76] X-ClientProxiedBy: dggems703-chm.china.huawei.com (10.3.19.180) To canpemm500002.china.huawei.com (7.192.104.244) X-CFilter-Loop: Reflected Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2022/6/14 21:13, Matthew Wilcox wrote: > On Wed, Jun 08, 2022 at 08:17:35PM +0800, Miaohe Lin wrote: >> +++ b/mm/page_alloc.c >> @@ -6771,13 +6771,18 @@ static void __ref memmap_init_compound(struct page *head, >> set_page_count(page, 0); >> >> /* >> - * The first tail page stores compound_mapcount_ptr() and >> - * compound_order() and the second tail page stores >> - * compound_pincount_ptr(). Call prep_compound_head() after >> - * the first and second tail pages have been initialized to >> - * not have the data overwritten. >> + * The first tail page stores compound_mapcount_ptr(), >> + * compound_order() and compound_pincount_ptr(). Call >> + * prep_compound_head() after the first tail page have >> + * been initialized to not have the data overwritten. >> + * >> + * Note the idea to make this right after we initialize >> + * the offending tail pages is trying to take advantage >> + * of the likelihood of those tail struct pages being >> + * cached given that we will read them right after in >> + * prep_compound_head(). > > It's not that we'll read them again, it's that the cacheline will still > be in cache, and therefore dirty. Thanks for pointing this out. > > Honestly, I don't think we need this extra explanation in a comment. > Just change the first paragraph to reflect reality and leave it at that. Will do it in next version if prep_compound_head is not moved outside loop. > >> */ >> - if (pfn == head_pfn + 2) >> + if (unlikely(pfn == head_pfn + 1)) > > We definitely don't need the unlikely here. Could you please give me a more detailed explanation? IIUC, the above if condition will only meet at a probability of 1/512. So unlikely tells the compiler to do some optimization around it. Or am I miss something? Thanks! > >> prep_compound_head(head, order); >> } >> } >> >> Or am I miss something? >> >> Thanks! >> >>> . >>> >> > > . >