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=-2.3 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 EEB66C2D0CE for ; Fri, 24 Jan 2020 23:19:19 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id AE6182075D for ; Fri, 24 Jan 2020 23:19:19 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org AE6182075D 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 4A3E86B0003; Fri, 24 Jan 2020 18:19:19 -0500 (EST) Received: by kanga.kvack.org (Postfix, from userid 40) id 453A86B0006; Fri, 24 Jan 2020 18:19:19 -0500 (EST) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 343036B0007; Fri, 24 Jan 2020 18:19:19 -0500 (EST) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0110.hostedemail.com [216.40.44.110]) by kanga.kvack.org (Postfix) with ESMTP id 1B9EB6B0003 for ; Fri, 24 Jan 2020 18:19:19 -0500 (EST) Received: from smtpin04.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay05.hostedemail.com (Postfix) with SMTP id C21EB181AEF21 for ; Fri, 24 Jan 2020 23:19:18 +0000 (UTC) X-FDA: 76414095996.04.cause26_8b682f69baa37 X-HE-Tag: cause26_8b682f69baa37 X-Filterd-Recvd-Size: 7875 Received: from mga11.intel.com (mga11.intel.com [192.55.52.93]) by imf25.hostedemail.com (Postfix) with ESMTP for ; Fri, 24 Jan 2020 23:19:17 +0000 (UTC) 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 fmsmga102.fm.intel.com with ESMTP/TLS/DHE-RSA-AES256-GCM-SHA384; 24 Jan 2020 15:19:15 -0800 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.70,359,1574150400"; d="scan'208";a="426771863" Received: from richard.sh.intel.com (HELO localhost) ([10.239.159.54]) by fmsmga005.fm.intel.com with ESMTP; 24 Jan 2020 15:19:14 -0800 Date: Sat, 25 Jan 2020 07:19:26 +0800 From: Wei Yang To: Michal Hocko Cc: Wei Yang , g@richard.suse.de, 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: <20200124231926.GA16638@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> <20200123225647.GB29851@richard> <20200124064649.GM29276@dhcp22.suse.cz> <20200124152642.GB12509@richard> <20200124154015.GW29276@dhcp22.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200124154015.GW29276@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 Fri, Jan 24, 2020 at 04:40:15PM +0100, Michal Hocko wrote: >On Fri 24-01-20 23:26:42, Wei Yang wrote: >> On Fri, Jan 24, 2020 at 07:46:49AM +0100, Michal Hocko wrote: >> >On Fri 24-01-20 06:56:47, Wei Yang wrote: >> >> 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? >> > >> >Yes IMO. Just consider -errno return value. You have no way to find out >> >which pages have been migrated until we reached that error. The >> >possitive return value would fall into the same case. >> > >> >> 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 is certainly possible. We can return -EAGAIN if some pages couldn't >> >be migrated because they are pinned. But please read my previous email >> >to the very end for arguments why this might cause more problems than it >> >actually solves. >> > >> >> Let me put your comment here: >> >> 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. >> >> Your idea is to preserve current semantic, return non-migrated pages number to >> userspace. >> >> And the reason is: >> >> 1. Users have started depending on it. >> 2. No real bug reported yet. >> 3. User always migrate page to the same node. (If my understanding is >> correct) >> >> I think this gets some reason, since we want to minimize the impact to >> userland. >> >> While let's see what user probably use this syscall. Since from the man page, >> we never told the return value could be positive, the number of non-migrated >> pages, user would think only 0 means a successful migration and all other >> cases are failure. Then user probably handle negative and positive return >> value the same way, like (!err). >> >> If my guess is true, return a negative error value for this case could >> minimize the impact to userland here. >> 1. Preserve the semantic of move_pages(2): 0 means success, negative means >> some error and needs extra handling. >> 2. Trivial change to the man page. >> 3. Suppose no change to users. > >Do you have any actual proposal we can discuss? I suspect we are going >in circles here. Sure both ways are possible. The disucssion we are >having here is which behavior makes more sense. The interface is and has >been in the past very awkward. Some corner cases have been fixed some >new created. While I am not happy about the later we should finally land >with some decision. Ok, I found myself may miss some mechanism about the err reporting from kernel to userland. If do_pages_move() returns a negative err, the value would be set into errno and actually user just see a return value of -1? So userland just see two types of return value if kernel comply with man page: 0 : success -1: failure, with reason set into errno Is my understanding correct? I tried to read the syscall path, but not find how the negative value is set into errno. Since our kernel already return a positive value one migration failure, so the exact return value from move_pages() syscall is: > 0: number of non-migrate pages 0 : success -1 : failure, with reason set into errno Since everything looks good to userland now, we just extend the semantic of move_pages() to make positive return value an explicit error case. Is my understanding correct here? If this is the case, I agree with this fix. It looks the minimal change to current real world. >-- >Michal Hocko >SUSE Labs -- Wei Yang Help you, Help me