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 43186C04A94 for ; Fri, 4 Aug 2023 05:54:24 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232329AbjHDFyW (ORCPT ); Fri, 4 Aug 2023 01:54:22 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:53046 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231553AbjHDFyS (ORCPT ); Fri, 4 Aug 2023 01:54:18 -0400 Received: from szxga08-in.huawei.com (szxga08-in.huawei.com [45.249.212.255]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id F1B853A90 for ; Thu, 3 Aug 2023 22:54:15 -0700 (PDT) Received: from dggpemm100001.china.huawei.com (unknown [172.30.72.55]) by szxga08-in.huawei.com (SkyGuard) with ESMTP id 4RHFHm5Ztbz1KCDj; Fri, 4 Aug 2023 13:53:08 +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; Fri, 4 Aug 2023 13:54:12 +0800 Message-ID: Date: Fri, 4 Aug 2023 13:54:12 +0800 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:102.0) Gecko/20100101 Thunderbird/102.10.1 Subject: Re: [PATCH 1/4] mm: migrate: use a folio in add_page_for_migration() Content-Language: en-US To: Zi Yan 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> From: Kefeng Wang In-Reply-To: Content-Type: text/plain; charset="UTF-8"; format=flowed Content-Transfer-Encoding: 7bit X-Originating-IP: [10.174.177.243] X-ClientProxiedBy: dggems702-chm.china.huawei.com (10.3.19.179) 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 2023/8/4 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: >>>> >>>> >>>> On 2023/8/2 20:21, Matthew Wilcox wrote: >>>>> On Wed, Aug 02, 2023 at 05:53:43PM +0800, Kefeng Wang wrote: >>>>>> err = -EACCES; >>>>>> - if (page_mapcount(page) > 1 && !migrate_all) >>>>>> - goto out_putpage; >>>>>> + if (folio_estimated_sharers(folio) > 1 && !migrate_all) >>>>>> + goto out_putfolio; >>>>> >>>>> I do not think this is the correct change. Maybe leave this line >>>>> alone. >>>> >>>> Ok, I am aware of the discussion about this in other mail, will not >>>> change it(also the next two patch about this function), or wait the >>>> new work of David. >>>>> >>>>>> - if (PageHuge(page)) { >>>>>> - if (PageHead(page)) { >>>>>> - isolated = isolate_hugetlb(page_folio(page), pagelist); >>>>>> + if (folio_test_hugetlb(folio)) { >>>>>> + if (folio_test_large(folio)) { >>>>> >>>>> This makes no sense when you read it. All hugetlb folios are large, >>>>> by definition. Think about what this code used to do, and what it >>>>> should be changed to. >>>> >>>> hugetlb folio is self large folio, will drop redundant check >>> >>> No, that's not the difference. Keep thinking about it. This is not >>> a mechanical translation! >> >> >> 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. 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. [1] https://man7.org/linux/man-pages/man2/move_pages.2.html > > -- > Best Regards, > Yan, Zi