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,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 C56C0C33CAA for ; Thu, 23 Jan 2020 22:57:05 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id 8C38E20718 for ; Thu, 23 Jan 2020 22:57:05 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 8C38E20718 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=linux.intel.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id 126C56B028F; Thu, 23 Jan 2020 17:57:05 -0500 (EST) Received: by kanga.kvack.org (Postfix, from userid 40) id 0D68B6B0290; Thu, 23 Jan 2020 17:57:05 -0500 (EST) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id F2E486B0291; Thu, 23 Jan 2020 17:57:04 -0500 (EST) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0143.hostedemail.com [216.40.44.143]) by kanga.kvack.org (Postfix) with ESMTP id DD8D66B028F for ; Thu, 23 Jan 2020 17:57:04 -0500 (EST) Received: from smtpin15.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay03.hostedemail.com (Postfix) with SMTP id 81CC78245578 for ; Thu, 23 Jan 2020 22:57:04 +0000 (UTC) X-FDA: 76410411168.15.feet49_2096838bdfe28 X-HE-Tag: feet49_2096838bdfe28 X-Filterd-Recvd-Size: 4735 Received: from mga03.intel.com (mga03.intel.com [134.134.136.65]) by imf30.hostedemail.com (Postfix) with ESMTP for ; Thu, 23 Jan 2020 22:57:03 +0000 (UTC) X-Amp-Result: UNKNOWN X-Amp-Original-Verdict: FILE UNKNOWN X-Amp-File-Uploaded: False Received: from orsmga002.jf.intel.com ([10.7.209.21]) by orsmga103.jf.intel.com with ESMTP/TLS/DHE-RSA-AES256-GCM-SHA384; 23 Jan 2020 14:56:37 -0800 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.70,355,1574150400"; d="scan'208";a="245541197" Received: from richard.sh.intel.com (HELO localhost) ([10.239.159.54]) by orsmga002.jf.intel.com with ESMTP; 23 Jan 2020 14:56:35 -0800 Date: Fri, 24 Jan 2020 06:56:47 +0800 From: Wei Yang To: Michal Hocko Cc: Wei Yang , Yang Shi , akpm@linux-foundation.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org Subject: Re: [v2 PATCH] mm: move_pages: report the number of non-attempted pages Message-ID: <20200123225647.GB29851@richard> Reply-To: Wei Yang References: <1579736331-85494-1-git-send-email-yang.shi@linux.alibaba.com> <20200123032736.GA22196@richard> <20200123085526.GH29276@dhcp22.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200123085526.GH29276@dhcp22.suse.cz> User-Agent: Mutt/1.9.4 (2018-02-28) X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Thu, Jan 23, 2020 at 09:55:26AM +0100, Michal Hocko wrote: >On Thu 23-01-20 11:27:36, Wei Yang wrote: >> On Thu, Jan 23, 2020 at 07:38:51AM +0800, Yang Shi wrote: >> >Since commit a49bd4d71637 ("mm, numa: rework do_pages_move"), >> >the semantic of move_pages() was changed to return the number of >> >non-migrated pages (failed to migration) and the call would be aborted >> >immediately if migrate_pages() returns positive value. But it didn't >> >report the number of pages that we even haven't attempted to migrate. >> >So, fix it by including non-attempted pages in the return value. >> > >> >> First, we want to change the semantic of move_pages(2). The return value >> indicates the number of pages we didn't managed to migrate? >> >> Second, the return value from migrate_pages() doesn't mean the number of pages >> we failed to migrate. For example, one -ENOMEM is returned on the first page, >> migrate_pages() would return 1. But actually, no page successfully migrated. > >ENOMEM is considered a permanent failure and as such it is returned by >migrate pages (see goto out). > >> Third, even the migrate_pages() return the exact non-migrate page, we are not >> sure those non-migrated pages are at the tail of the list. Because in the last >> case in migrate_pages(), it just remove the page from list. It could be a page >> in the middle of the list. Then, in userspace, how the return value be >> leveraged to determine the valid status? Any page in the list could be the >> victim. > >Yes, I was wrong when stating that the caller would know better which >status to check. I misremembered the original patch as it was quite some >time ago. While storing the error code would be possible after some >massaging of migrate_pages is this really something we deeply care >about. The caller can achieve the same by initializing the status array >to a non-node number - e.g. -1 - and check based on that. > So for a user, the best practice is to initialize the status array to -1 and check each status to see whether the page is migrated successfully? Then do we need to return the number of non-migrated page? What benefit could user get from the number. How about just return an error code to indicate the failure? I may miss some point, would you mind giving me a hint? >This system call has quite a complex semantic and I am not 100% sure >what is the right thing to do here. Maybe we do want to continue and try >to migrate as much as possible on non-fatal migration failures and >accumulate the number of failed pages while doing so. > >The main problem is that we can have an academic discussion but >the primary question is what do actual users want. A lack of real >bug reports suggests that nobody has actually noticed this. So I >would rather keep returning the correct number of non-migrated >pages. Why? Because new users could have started depending on it. It >is not all that unlikely that the current implementation would just >work for them because they are migrating a set of pages on to the same >node so the batch would be a single list throughout the whole given >page set. >-- >Michal Hocko >SUSE Labs -- Wei Yang Help you, Help me