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 AA76EC433FE for ; Fri, 21 Oct 2022 02:52:12 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229835AbiJUCwL (ORCPT ); Thu, 20 Oct 2022 22:52:11 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:50456 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229449AbiJUCwI (ORCPT ); Thu, 20 Oct 2022 22:52:08 -0400 Received: from out30-45.freemail.mail.aliyun.com (out30-45.freemail.mail.aliyun.com [115.124.30.45]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id EC8D81956CB for ; Thu, 20 Oct 2022 19:52:07 -0700 (PDT) X-Alimail-AntiSpam: AC=PASS;BC=-1|-1;BR=01201311R351e4;CH=green;DM=||false|;DS=||;FP=0|-1|-1|-1|0|-1|-1|-1;HT=ay29a033018046049;MF=baolin.wang@linux.alibaba.com;NM=1;PH=DS;RN=9;SR=0;TI=SMTPD_---0VShO9Hv_1666320722; Received: from 30.97.48.58(mailfrom:baolin.wang@linux.alibaba.com fp:SMTPD_---0VShO9Hv_1666320722) by smtp.aliyun-inc.com; Fri, 21 Oct 2022 10:52:04 +0800 Message-ID: <0855246a-8425-2aca-1d67-305d6866ed17@linux.alibaba.com> Date: Fri, 21 Oct 2022 10:51:58 +0800 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:102.0) Gecko/20100101 Thunderbird/102.3.0 Subject: Re: [PATCH 1/2] mm: gup: Re-pin pages in case of trying several times to migrate To: Alistair Popple Cc: "Huang, Ying" , akpm@linux-foundation.org, david@redhat.com, ziy@nvidia.com, shy828301@gmail.com, jingshan@linux.alibaba.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <87r0z2nc6j.fsf@yhuang6-desk2.ccr.corp.intel.com> <87o7u6soip.fsf@nvidia.com> From: Baolin Wang In-Reply-To: <87o7u6soip.fsf@nvidia.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 10/20/2022 7:43 PM, Alistair Popple wrote: > > Baolin Wang writes: > >> On 10/20/2022 4:15 PM, Huang, Ying wrote: >>> Baolin Wang writes: >>> >>>> The migrate_pages() will return the number of {normal page, THP, hugetlb} >>>> that were not migrated, or an error code. That means it can still return >>>> the number of failure count, though the pages have been migrated >>>> successfully with several times re-try. >>> If my understanding were correct, if pages are migrated successfully >>> after several times re-tries, the return value will be 0. There's one >>> possibility for migrate_pages() to return non-zero but all pages are >>> migrated. That is, when THP is split and all subpages are migrated >>> successfully. >> >> Yeah, that's the case I tested. Thanks for pointing out. I'll re-write my >> incorrect commit message next time. > > This is confusing to me. So users of move_page() will see an > unsuccessful migration even when all subpages were migrated? Seems like Yes. > we should fix the return code of migrate_pages() for this case where all > subpages were successfully migrated. After more investigation, some other callers will also check the return value to see of all pages are migrated successfully. So yes, I will change the return value in migrate_pages() to fix this issue for all callers like you and Ying suggested. Thanks. >>>> So we should not use the return value of migrate_pages() to determin >>>> if there are pages are failed to migrate. Instead we can validate the >>>> 'movable_page_list' to see if there are pages remained in the list, >>>> which are failed to migrate. That can mitigate the failure of longterm >>>> pinning. >>> Another choice is to use a special return value for split THP + success >>> migration. But I'm fine to use list_empty(return_pages). >> >> OK. Using list_empty(return_pages) looks more simple. >> >>> >>>> Signed-off-by: Baolin Wang >>>> --- >>>> mm/gup.c | 7 ++++--- >>>> 1 file changed, 4 insertions(+), 3 deletions(-) >>>> >>>> diff --git a/mm/gup.c b/mm/gup.c >>>> index 5182aba..bd8cfcd 100644 >>>> --- a/mm/gup.c >>>> +++ b/mm/gup.c >>>> @@ -1914,9 +1914,10 @@ static int migrate_longterm_unpinnable_pages( >>>> .gfp_mask = GFP_USER | __GFP_NOWARN, >>>> }; >>>> - if (migrate_pages(movable_page_list, alloc_migration_target, >>>> - NULL, (unsigned long)&mtc, MIGRATE_SYNC, >>>> - MR_LONGTERM_PIN, NULL)) { >>>> + ret = migrate_pages(movable_page_list, alloc_migration_target, >>>> + NULL, (unsigned long)&mtc, MIGRATE_SYNC, >>>> + MR_LONGTERM_PIN, NULL); >>>> + if (ret < 0 || !list_empty(movable_page_list)) { >>> It seems that !list_empty() is sufficient here. >> >> OK. Drop the 'ret < 0' >> >>>> ret = -ENOMEM; >>> Why change the error code? I don't think it's a good idea to do that. >> >> The GUP need a -errno for failure or partial success when migration, and we can >> not return the number of pages failed to migrate. So returning -ENOMEM seems >> suitable for both cases? > > Seem reasonable to me. migrate_pages() might return -EAGAIN which would > cause everything to be re-pinned and tried again which is not what you > want here. See the comment at the start of > check_and_migrate_movable_pages().