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 8790EC0015E for ; Wed, 9 Aug 2023 12:37:24 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232132AbjHIMhW (ORCPT ); Wed, 9 Aug 2023 08:37:22 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:42084 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229996AbjHIMhV (ORCPT ); Wed, 9 Aug 2023 08:37:21 -0400 Received: from szxga01-in.huawei.com (szxga01-in.huawei.com [45.249.212.187]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id E529E10F3 for ; Wed, 9 Aug 2023 05:37:19 -0700 (PDT) Received: from dggpemm100001.china.huawei.com (unknown [172.30.72.55]) by szxga01-in.huawei.com (SkyGuard) with ESMTP id 4RLV0N4qggzmXSV; Wed, 9 Aug 2023 20:36:04 +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; Wed, 9 Aug 2023 20:37:16 +0800 Message-ID: Date: Wed, 9 Aug 2023 20:37:15 +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: Zi Yan , Mike Kravetz CC: Naoya Horiguchi , Matthew Wilcox , Andrew Morton , , , Huang Ying , David Hildenbrand , Mike Kravetz References: <20230802095346.87449-1-wangkefeng.wang@huawei.com> <20230802095346.87449-2-wangkefeng.wang@huawei.com> <001ee9b0-ea25-a896-e3ae-9a9b05a46546@huawei.com> <5BBFF5D3-3416-4C0E-9FDD-655661657D67@nvidia.com> From: Kefeng Wang In-Reply-To: <5BBFF5D3-3416-4C0E-9FDD-655661657D67@nvidia.com> Content-Type: text/plain; charset="UTF-8"; format=flowed Content-Transfer-Encoding: 8bit X-Originating-IP: [10.174.177.243] X-ClientProxiedBy: dggems704-chm.china.huawei.com (10.3.19.181) 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 Hi Mike On 2023/8/8 2:45, Zi Yan wrote: > On 7 Aug 2023, at 8:20, Kefeng Wang wrote: > >> Hi Zi Yan and Matthew and Naoya, >> >> On 2023/8/4 13:54, Kefeng Wang wrote: >>> >>> >>> On 2023/8/4 10:42, Zi Yan wrote: >>>> On 3 Aug 2023, at 21:45, Kefeng Wang wrote: >>>> >>>>> On 2023/8/3 20:30, Matthew Wilcox wrote: >>>>>> On Thu, Aug 03, 2023 at 03:13:21PM +0800, Kefeng Wang wrote: >>>>>>> >> >> ... >> >>>>> >>>>> >>>>>    if (PageHuge(page))  // page must be a hugetlb page >>>>>     if (PageHead(page)) // page must be a head page, not tail >>>>>               isolate_hugetlb() // isolate the hugetlb page if head >>>>> >>>>> After using folio, >>>>> >>>>>    if (folio_test_hugetlb(folio)) // only check folio is hugetlb or not >>>>> >>>>> I don't check the page is head or not, since the follow_page could >>>>> return a sub-page, so the check PageHead need be retained, right? >>>> >>>> Right. It will prevent the kernel from trying to isolate the same hugetlb page >>>> twice when two pages are in the same hugetlb folio. But looking at the >>>> code, if you try to isolate an already-isolated hugetlb folio, isolate_hugetlb() >>>> would return false, no error would show up. But it changes err value >>>> from -EACCES to -EBUSY and user will see a different page status than before. >>> >> >> 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, >> and move_pages() will return -ENOENT errno,but after that commit, >> -EACCES is returned, which not match the manual, >> >>> >>> When check man[1], the current -EACCES is not right, -EBUSY is not >>> precise but more suitable for this scenario, >>> >>>      -EACCES >>>               The page is mapped by multiple processes and can be moved >>>               only if MPOL_MF_MOVE_ALL is specified. >>> >>>      -EBUSY The page is currently busy and cannot be moved.  Try again >>>               later.  This occurs if a page is undergoing I/O or another >>>               kernel subsystem is holding a reference to the page. >>>     -ENOENT >>>               The page is not present. >>> >>>> >>>> I wonder why we do not have follow_folio() and returns -ENOENT error pointer >>>> when addr points to a non head page. It would make this patch more folio if >>>> follow_folio() can be used in place of follow_page(). One caveat is that >>>> user will see -ENOENT instead of -EACCES after this change. >>>> >>> >>> -ENOENT is ok, but maybe the man need to be updated too. >> >> According to above analysis, -ENOENT is suitable when introduce the >> follow_folio(), but when THP migrate support is introduced by >> e8db67eb0ded ("mm: migrate: move_pages() supports thp migration") in >> v4.14, the tail page will be turned into head page and return -EBUSY, >> >> So should we unify errno(maybe use -ENOENT) about the tail page? >> >> >>> >>> >>> >>> [1] https://man7.org/linux/man-pages/man2/move_pages.2.html > > I think so. I think -EBUSY is more reasonable for tail pages. But there is > some subtle difference between THP and hugetlb from current code: > > For THP, compound_head() is used to get the head page for isolation, this means > if user specifies a tail page address in move_pages(), the whole THP can be > migrated. > > For hugetlb, only if user specifies the head page address of a hugetlb page, > the hugetlb page will be migrated. Otherwise, an error would show up. > > 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 > > -- > Best Regards, > Yan, Zi