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 32FCBEB64DD for ; Mon, 7 Aug 2023 12:21:10 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232250AbjHGMVI (ORCPT ); Mon, 7 Aug 2023 08:21:08 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:51956 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233396AbjHGMVE (ORCPT ); Mon, 7 Aug 2023 08:21:04 -0400 Received: from szxga03-in.huawei.com (szxga03-in.huawei.com [45.249.212.189]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id DC71710E2 for ; Mon, 7 Aug 2023 05:20:43 -0700 (PDT) Received: from dggpemm100001.china.huawei.com (unknown [172.30.72.53]) by szxga03-in.huawei.com (SkyGuard) with ESMTP id 4RKFgr319fz1Z1Yh; Mon, 7 Aug 2023 20:17:28 +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; Mon, 7 Aug 2023 20:20:15 +0800 Message-ID: Date: Mon, 7 Aug 2023 20:20:15 +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 From: Kefeng Wang To: Zi Yan , Naoya Horiguchi CC: Matthew Wilcox , Andrew Morton , , , Huang Ying , David Hildenbrand References: <20230802095346.87449-1-wangkefeng.wang@huawei.com> <20230802095346.87449-2-wangkefeng.wang@huawei.com> <001ee9b0-ea25-a896-e3ae-9a9b05a46546@huawei.com> In-Reply-To: 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 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