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 X-Spam-Level: X-Spam-Status: No, score=-2.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,UNPARSEABLE_RELAY,USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id B0E93C43331 for ; Tue, 12 Nov 2019 00:02:40 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 907172184C for ; Tue, 12 Nov 2019 00:02:40 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726924AbfKLACh (ORCPT ); Mon, 11 Nov 2019 19:02:37 -0500 Received: from out30-43.freemail.mail.aliyun.com ([115.124.30.43]:40947 "EHLO out30-43.freemail.mail.aliyun.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726845AbfKLACh (ORCPT ); Mon, 11 Nov 2019 19:02:37 -0500 X-Alimail-AntiSpam: AC=PASS;BC=-1|-1;BR=01201311R711e4;CH=green;DM=||false|;FP=0|-1|-1|-1|0|-1|-1|-1;HT=e01e07487;MF=yang.shi@linux.alibaba.com;NM=1;PH=DS;RN=6;SR=0;TI=SMTPD_---0ThqC6Nj_1573516950; Received: from US-143344MP.local(mailfrom:yang.shi@linux.alibaba.com fp:SMTPD_---0ThqC6Nj_1573516950) by smtp.aliyun-inc.com(127.0.0.1); Tue, 12 Nov 2019 08:02:33 +0800 Subject: Re: [PATCH] mm: migrate: handle freed page at the first place To: Andrew Morton Cc: mhocko@suse.com, mgorman@techsingularity.net, vbabka@suse.cz, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <1573510165-113395-1-git-send-email-yang.shi@linux.alibaba.com> <20191111151801.fb206aa84cc8ab3059994f7e@linux-foundation.org> From: Yang Shi Message-ID: Date: Mon, 11 Nov 2019 16:02:30 -0800 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.12; rv:52.0) Gecko/20100101 Thunderbird/52.7.0 MIME-Version: 1.0 In-Reply-To: <20191111151801.fb206aa84cc8ab3059994f7e@linux-foundation.org> Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 7bit Content-Language: en-US Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 11/11/19 3:18 PM, Andrew Morton wrote: > On Tue, 12 Nov 2019 06:09:25 +0800 Yang Shi wrote: > >> When doing migration if the freed page is met, we just return without >> migrating it since it is pointless to migrate a freed page. But, the >> current code did two things before handling freed page: >> >> 1. Return -ENOMEM if the page is THP and THP migration is not supported. >> 2. Allocate target page unconditionally. >> >> Both makes not too much sense. If we handle freed page at the first place >> we don't have to worry about allocating/freeing target page and split >> THP at all. >> >> For example (worst case) if we are trying to migrate a freed THP without >> THP migration supported, the migrate_pages() would just split the THP then >> retry to migrate base pages one by one by pointless allocating and freeing >> pages, this is just waste of time. >> >> I didn't run into any actual problem with the current code (or I may >> just not notice it yet), it was found by visual inspection. >> >> >> --- a/mm/migrate.c >> +++ b/mm/migrate.c >> @@ -1170,13 +1170,6 @@ static ICE_noinline int unmap_and_move(new_page_t get_new_page, >> int rc = MIGRATEPAGE_SUCCESS; >> struct page *newpage; >> >> - if (!thp_migration_supported() && PageTransHuge(page)) >> - return -ENOMEM; >> - >> - newpage = get_new_page(page, private); >> - if (!newpage) >> - return -ENOMEM; >> - >> if (page_count(page) == 1) { > Is it possible to have (!thp_migration_supported() && > PageTransHuge(page) && page_count(page) == 1)? If so, isn't this new > behviour? IMHO it should be possible on some architectures, i.e. aarch64, with anonymous THP. I just saw PowerPC and x86_64 have CONFIG_ARCH_ENABLE_THP_MIGRATION selected. I'm not quite sure if I miss something. It should be not new behavior since migrate_pages() should just split the THP then retry with base pages one by one. Even though it returns -EBUSY due to THP split failure in the current code, the behavior sounds problematic. We should not return errno for a freed page, right? > >> /* page was freed from under us. So we are done. */ >> ClearPageActive(page); >> @@ -1187,13 +1180,16 @@ static ICE_noinline int unmap_and_move(new_page_t get_new_page, >> __ClearPageIsolated(page); >> unlock_page(page); >> } >> - if (put_new_page) >> - put_new_page(newpage, private); >> - else >> - put_page(newpage); >> goto out; >> } >> >> + if (!thp_migration_supported() && PageTransHuge(page)) >> + return -ENOMEM; >> + >> + newpage = get_new_page(page, private); >> + if (!newpage) >> + return -ENOMEM; >> + >> rc = __unmap_and_move(page, newpage, force, mode); >> if (rc == MIGRATEPAGE_SUCCESS) >> set_page_owner_migrate_reason(newpage, reason);