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 519D1C433EF for ; Tue, 15 Feb 2022 03:14:18 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S233764AbiBODOY (ORCPT ); Mon, 14 Feb 2022 22:14:24 -0500 Received: from mxb-00190b01.gslb.pphosted.com ([23.128.96.19]:36180 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229545AbiBODOV (ORCPT ); Mon, 14 Feb 2022 22:14:21 -0500 Received: from szxga08-in.huawei.com (szxga08-in.huawei.com [45.249.212.255]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 0E763AC910 for ; Mon, 14 Feb 2022 19:14:11 -0800 (PST) Received: from canpemm500002.china.huawei.com (unknown [172.30.72.53]) by szxga08-in.huawei.com (SkyGuard) with ESMTP id 4JyR0D5TqHz1FDBC; Tue, 15 Feb 2022 11:09:48 +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 15.1.2308.21; Tue, 15 Feb 2022 11:14:08 +0800 Subject: Re: [PATCH 4/8] mm/memory-failure.c: remove unneeded orig_head To: Naoya Horiguchi CC: , , , References: <20220210141733.1908-1-linmiaohe@huawei.com> <20220210141733.1908-5-linmiaohe@huawei.com> <20220214145019.GD2624914@u2004> From: Miaohe Lin Message-ID: Date: Tue, 15 Feb 2022 11:14:07 +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: <20220214145019.GD2624914@u2004> Content-Type: text/plain; charset="utf-8" Content-Language: en-US Content-Transfer-Encoding: 7bit X-Originating-IP: [10.174.177.76] X-ClientProxiedBy: dggems702-chm.china.huawei.com (10.3.19.179) 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 2022/2/14 22:50, Naoya Horiguchi wrote: > On Thu, Feb 10, 2022 at 10:17:29PM +0800, Miaohe Lin wrote: >> orig_head is used to check whether the page have changed compound pages >> during the locking. But it's always equal to hpage. So we can use hpage >> directly and remove this redundant one. >> >> Signed-off-by: Miaohe Lin >> --- >> mm/memory-failure.c | 5 ++--- >> 1 file changed, 2 insertions(+), 3 deletions(-) >> >> diff --git a/mm/memory-failure.c b/mm/memory-failure.c >> index 2dd7f35ee65a..4370c2f407c5 100644 >> --- a/mm/memory-failure.c >> +++ b/mm/memory-failure.c >> @@ -1691,7 +1691,6 @@ int memory_failure(unsigned long pfn, int flags) >> { >> struct page *p; >> struct page *hpage; >> - struct page *orig_head; >> struct dev_pagemap *pgmap; >> int res = 0; >> unsigned long page_flags; >> @@ -1737,7 +1736,7 @@ int memory_failure(unsigned long pfn, int flags) >> goto unlock_mutex; >> } >> >> - orig_head = hpage = compound_head(p); >> + hpage = compound_head(p); >> num_poisoned_pages_inc(); >> >> /* >> @@ -1821,7 +1820,7 @@ int memory_failure(unsigned long pfn, int flags) >> * The page could have changed compound pages during the locking. >> * If this happens just bail out. >> */ >> - if (PageCompound(p) && compound_head(p) != orig_head) { >> + if (PageCompound(p) && compound_head(p) != hpage) { > > I think that this if-check was intended to detect the case that page p > belongs to a thp when memory_failure() is called and belongs to a compound > page in different size (like slab or some driver page) after the thp is > split. But your suggestion makes me aware that the page p could be embedded > on a thp again after thp split. I think this might be rare, but if it IIUC, this page can't be embedded on a thp again after thp split because memory_failure hold an __extra__ page refcnt. I think there exist below race windows: memory_failure orig_head = hpage = compound_head(p); -- page is Non-compound yet < -- Page becomes compound page, like thp, slab, some driver page and even hugetlb page -- > get_hwpoison_page failed to split thp page, as hpage is Non-compound ... lock_page > happens the current if-check (or suggested one) cannot detect it. > So I feel that simply dropping compound_head() check might be better? > > - if (PageCompound(p) && compound_head(p) != orig_head) { > + if (PageCompound(p)) { However this change could also catch the above race correctly. In fact, we can't handle compound page here. But is it enough to just return -EBUSY here as it's really rare or we should do more things (like split thp, retry if in PageHuge case)? > > This should ensure the assumption (mentioned in 8/8 patch) more that > the error page never belongs to compound page after taking page lock. Agree, this really helps. >> Thanks, > Naoya Horiguchi > . > Many thanks.