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 kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by smtp.lore.kernel.org (Postfix) with ESMTP id DC050C433EF for ; Wed, 15 Jun 2022 07:44:13 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 29E576B0071; Wed, 15 Jun 2022 03:44:13 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 24E036B0072; Wed, 15 Jun 2022 03:44:13 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 115E36B0073; Wed, 15 Jun 2022 03:44:13 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0010.hostedemail.com [216.40.44.10]) by kanga.kvack.org (Postfix) with ESMTP id F34256B0071 for ; Wed, 15 Jun 2022 03:44:12 -0400 (EDT) Received: from smtpin26.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay07.hostedemail.com (Postfix) with ESMTP id C8EB220347 for ; Wed, 15 Jun 2022 07:44:12 +0000 (UTC) X-FDA: 79579681944.26.6750388 Received: from szxga08-in.huawei.com (szxga08-in.huawei.com [45.249.212.255]) by imf21.hostedemail.com (Postfix) with ESMTP id 6D2411C0099 for ; Wed, 15 Jun 2022 07:44:11 +0000 (UTC) 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 ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1655279052; a=rsa-sha256; cv=none; b=tljFsvvc+xhb9iESI93Lhji7oW+SzET2XmTPM9H/CkC4b9Hw+Brs00qhGYjB1YsypVJCYY sNKHw8BISPv+C2qN94mqhr9ma0jtWEymdofQ2fSo1LDR5tkzMy4W4Fumdk8jK66vW6j057 pEAp3nZBM+cN3VobQ4Cus/x+OP8BGJA= ARC-Authentication-Results: i=1; imf21.hostedemail.com; dkim=none; spf=pass (imf21.hostedemail.com: domain of linmiaohe@huawei.com designates 45.249.212.255 as permitted sender) smtp.mailfrom=linmiaohe@huawei.com; dmarc=pass (policy=quarantine) header.from=huawei.com ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1655279052; h=from:from:sender:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=ZCL82q67gjBW0gvSYtYX3h4iDXdKgU7ABo2H5IKhKCQ=; b=SQOxGH1ia6/E8UH+gum+BAj1WaCJon7F7H9Ww7RQ8A/n0b0jvU2tCQFXe+lkfpnmwSobRS 0HRpZaKLFA89QiKPh5eN8md0RCr3vYVwIforBsHrXwGB0J9lbM5J6f0JKeS2h9FohzG5Np tKlGWg1s9Hc0mWlmJCxwe2TVJjfObkE= Authentication-Results: imf21.hostedemail.com; dkim=none; spf=pass (imf21.hostedemail.com: domain of linmiaohe@huawei.com designates 45.249.212.255 as permitted sender) smtp.mailfrom=linmiaohe@huawei.com; dmarc=pass (policy=quarantine) header.from=huawei.com X-Rspam-User: X-Stat-Signature: bo5u3ffcow65xgqxiofz4mdx6u51qfcu X-Rspamd-Queue-Id: 6D2411C0099 X-Rspamd-Server: rspam08 X-HE-Tag: 1655279051-741960 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: 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! >> >>> . >>> >> > > . >