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 39292C0015E for ; Thu, 10 Aug 2023 01:49:22 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232082AbjHJBtV (ORCPT ); Wed, 9 Aug 2023 21:49:21 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:59336 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231989AbjHJBtT (ORCPT ); Wed, 9 Aug 2023 21:49:19 -0400 Received: from szxga03-in.huawei.com (szxga03-in.huawei.com [45.249.212.189]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id D425810DA for ; Wed, 9 Aug 2023 18:49:17 -0700 (PDT) Received: from dggpemm100001.china.huawei.com (unknown [172.30.72.57]) by szxga03-in.huawei.com (SkyGuard) with ESMTP id 4RLqXK0Sj2zkXwR; Thu, 10 Aug 2023 09:46:25 +0800 (CST) Received: from [10.174.177.243] (10.174.177.243) by dggpemm100001.china.huawei.com (7.185.36.93) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) id 15.1.2507.27; Thu, 10 Aug 2023 09:49:15 +0800 Message-ID: <2da95492-079b-43b1-a950-d290984a21c0@huawei.com> Date: Thu, 10 Aug 2023 09:49:14 +0800 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 1/4] mm: migrate: use a folio in add_page_for_migration() Content-Language: en-US To: Mike Kravetz CC: Zi Yan , Naoya Horiguchi , Matthew Wilcox , Andrew Morton , , , Huang Ying , David Hildenbrand References: <001ee9b0-ea25-a896-e3ae-9a9b05a46546@huawei.com> <5BBFF5D3-3416-4C0E-9FDD-655661657D67@nvidia.com> <20230809205316.GA3537@monkey> <20230809224424.GB3537@monkey> From: Kefeng Wang In-Reply-To: <20230809224424.GB3537@monkey> Content-Type: text/plain; charset="UTF-8"; format=flowed Content-Transfer-Encoding: 8bit X-Originating-IP: [10.174.177.243] X-ClientProxiedBy: dggems703-chm.china.huawei.com (10.3.19.180) To dggpemm100001.china.huawei.com (7.185.36.93) X-CFilter-Loop: Reflected Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2023/8/10 6:44, Mike Kravetz wrote: > On 08/09/23 13:53, Mike Kravetz wrote: >> On 08/09/23 20:37, Kefeng Wang wrote: >>>> >>>> Cc Mike to help us clarify the expected behavior of hugetlb. >>>> >>>> Hi Mike, what is the expected behavior, if a user tries to use move_pages() >>>> to migrate a non head page of a hugetlb page? >>> >>> Could you give some advise, thanks >>> >> >> Sorry, I was away for a while. >> >> It seems unfortunate that move_pages says the passed user addresses >> should be aligned to page boundaries. However, IIUC this is not checked >> or enforced. Otherwise, passing a hugetlb page should return the same >> error. >> >> One thought would be that hugetlb mappings should behave the same >> non-hugetlb mappings. If passed the address of a hugetlb tail page, align >> the address to a hugetlb boundary and migrate the page. This changes the >> existing behavior. However, it would be hard to imagine anyone depending >> on this. >> >> After taking a closer look at the add_page_for_migration(), it seems to >> just ignore passed tail pages and do nothing for such passed addresses. >> Correct? Or, am I missing something? Perhaps that is behavior we want/ >> need to preserve? > > My mistake, status -EACCES is returned when passing a tail page of a > hugetlb page. > As mentioned in previous mail， before e66f17ff7177 ("mm/hugetlb: take page table lock in follow_huge_pmd()") in v4.0, follow_page() will return NULL on tail page for Huagetlb page, so move_pages() will return -ENOENT errno, but after that commit, -EACCES is returned. Meanwhile, the behavior of THP/HUGETLB is different, the whole THP will be migrated on a tail page, but HUGETLB will return -EACCES(after v4.0) or -ENOENT(before v4.0) on tail page. > Back to the question of 'What is the expected behavior if a tail page is > passed?'. I do not think we have defined an expected behavior. If > anything is 'expected' I would say it is -EACCES as returned today. > My question is, Should we keep seem behavior between HUGETLB and THP, or only change the errno from -EACCES to -ENOENT/-EBUSY. I would like to drop PageHead() check for Hugetlb to keep seem behavior, which will keep seem error code if isolate fail or success on head/tail page. Thanks. > BTW - hugetlb pages not migrated due to passing a tail page does not > seem to contribute to a 'Positive return value' indicating the number of > non-migrated pages.