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=-8.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_1 autolearn=unavailable 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 F248BC33C9E for ; Fri, 17 Jan 2020 23:48:49 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id CF3F621D56 for ; Fri, 17 Jan 2020 23:48:49 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1730315AbgAQXst (ORCPT ); Fri, 17 Jan 2020 18:48:49 -0500 Received: from mga06.intel.com ([134.134.136.31]:11481 "EHLO mga06.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1730117AbgAQXss (ORCPT ); Fri, 17 Jan 2020 18:48:48 -0500 X-Amp-Result: UNKNOWN X-Amp-Original-Verdict: FILE UNKNOWN X-Amp-File-Uploaded: False Received: from fmsmga005.fm.intel.com ([10.253.24.32]) by orsmga104.jf.intel.com with ESMTP/TLS/DHE-RSA-AES256-GCM-SHA384; 17 Jan 2020 15:48:22 -0800 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.70,332,1574150400"; d="scan'208";a="424651931" Received: from richard.sh.intel.com (HELO localhost) ([10.239.159.54]) by fmsmga005.fm.intel.com with ESMTP; 17 Jan 2020 15:48:18 -0800 Date: Sat, 18 Jan 2020 07:48:29 +0800 From: Wei Yang To: Yang Shi Cc: Wei Yang , Andrew Morton , Linux MM , Linux Kernel Mailing List Subject: Re: [PATCH] mm/migrate.c: also overwrite error when it is bigger than zero Message-ID: <20200117234829.GA2844@richard> Reply-To: Wei Yang References: <20200117074534.25324-1-richardw.yang@linux.intel.com> <20200117222740.GB29229@richard> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.9.4 (2018-02-28) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Jan 17, 2020 at 03:30:18PM -0800, Yang Shi wrote: >On Fri, Jan 17, 2020 at 2:27 PM Wei Yang wrote: >> >> On Fri, Jan 17, 2020 at 03:45:34PM +0800, Wei Yang wrote: >> >If we get here after successfully adding page to list, err would be >> >the number of pages in the list. >> > >> >Current code has two problems: >> > >> > * on success, 0 is not returned >> > * on error, the real error code is not returned >> > >> >> Well, this breaks the user interface. User would receive 1 even the migration >> succeed. >> >> The change is introduced by e0153fc2c760 ("mm: move_pages: return valid node >> id in status if the page is already on the target node"). > >Yes, it may return a value which is > 0. But, it seems do_pages_move() >could return > 0 value even before this commit. > >For example, if I read the code correctly, it would do: > >If we already have some pages on the queue then >add_page_for_migration() return error, then do_move_pages_to_node() is >called, but it may return > 0 value (the number of pages that were >*not* migrated by migrate_pages()), then the code flow would just jump >to "out" and return the value. And, it may happen to be 1. > This is another point I think current code is not working well. And actually, the behavior is not well defined or our kernel is broken for a while. When you look at the man page, it says: RETURN VALUE On success move_pages() returns zero. On error, it returns -1, and sets errno to indicate the error So per my understanding, the design is to return -1 on error instead of the pages not managed to move. For the user interface, if original code check 0 for success, your change breaks it. Because your code would return 1 instead of 0. Suppose most user just read the man page for programming instead of reading the kernel source code. I believe we need to fix it. Not sure how to include some user interface related developer to look into this issue. Hope this thread may catch their eyes. >I'm not sure if it breaks the user interface since the behavior has >been existed for years, and it looks nobody complains about it. Maybe >glibc helps hide it or people just care if it is 0 and the status. > >> >> >Signed-off-by: Wei Yang >> >--- >> > mm/migrate.c | 2 +- >> > 1 file changed, 1 insertion(+), 1 deletion(-) >> > >> >diff --git a/mm/migrate.c b/mm/migrate.c >> >index 557da996b936..c3ef70de5876 100644 >> >--- a/mm/migrate.c >> >+++ b/mm/migrate.c >> >@@ -1677,7 +1677,7 @@ static int do_pages_move(struct mm_struct *mm, nodemask_t task_nodes, >> > err1 = do_move_pages_to_node(mm, &pagelist, current_node); >> > if (!err1) >> > err1 = store_status(status, start, current_node, i - start); >> >- if (!err) >> >+ if (err >= 0) >> > err = err1; >> > out: >> > return err; >> >-- >> >2.17.1 >> >> -- >> Wei Yang >> Help you, Help me >> -- Wei Yang Help you, Help me