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 7F89CC41513 for ; Wed, 16 Aug 2023 00:51:08 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S240904AbjHPAuh (ORCPT ); Tue, 15 Aug 2023 20:50:37 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:33302 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S240899AbjHPAuT (ORCPT ); Tue, 15 Aug 2023 20:50:19 -0400 Received: from szxga01-in.huawei.com (szxga01-in.huawei.com [45.249.212.187]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 80C6F2122 for ; Tue, 15 Aug 2023 17:50:15 -0700 (PDT) Received: from dggpemm100001.china.huawei.com (unknown [172.30.72.55]) by szxga01-in.huawei.com (SkyGuard) with ESMTP id 4RQTz91PzDzrS81; Wed, 16 Aug 2023 08:48:53 +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 15.1.2507.31; Wed, 16 Aug 2023 08:50:13 +0800 Message-ID: <2a834c28-2122-482a-9db6-8b8572932b10@huawei.com> Date: Wed, 16 Aug 2023 08:50:12 +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 , "Huang, Ying" CC: Zi Yan , Naoya Horiguchi , Matthew Wilcox , Andrew Morton , , , David Hildenbrand References: <5BBFF5D3-3416-4C0E-9FDD-655661657D67@nvidia.com> <20230809205316.GA3537@monkey> <20230809224424.GB3537@monkey> <2da95492-079b-43b1-a950-d290984a21c0@huawei.com> <20230810162920.GA4734@monkey> <87wmxx7y9j.fsf@yhuang6-desk2.ccr.corp.intel.com> <20230815211226.GA4150@monkey> From: Kefeng Wang In-Reply-To: <20230815211226.GA4150@monkey> Content-Type: text/plain; charset="UTF-8"; format=flowed Content-Transfer-Encoding: 8bit X-Originating-IP: [10.174.177.243] X-ClientProxiedBy: dggems701-chm.china.huawei.com (10.3.19.178) 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 2023/8/16 5:12, Mike Kravetz wrote: > On 08/15/23 11:58, Huang, Ying wrote: >> Mike Kravetz writes: >> >>> On 08/10/23 09:49, Kefeng Wang wrote: >>>> >>>> >>>> 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. >>> >>> Just to be clear. When you say "keep seem behavior between HUGETLB and THP", >>> are you saying that you would like hugetlb to perform migration of the entire >>> hugetlb page if a tail page is passed? >>> >>> IMO, this would be ideal as it would mean that hugetlb and THP behave the same >>> when passed the address of a tail page. The fewer places where hugetlb >>> behavior diverges, the better. However, this does change behavior. >> >> A separate patch will be needed for behavior change. >> > > Correct. > > Since the goal of this series is to convert to folios, we should maintain the > existing behavior and errno (-EACCES). In a subsequent patch, we can > change behavior. > > That would be my suggestion. Thanks all, will following the suggestion and re-post.