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=-1.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS,UNPARSEABLE_RELAY 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 05C2BC10F00 for ; Thu, 21 Mar 2019 23:25:34 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id D19EA21902 for ; Thu, 21 Mar 2019 23:25:33 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727386AbfCUXZc (ORCPT ); Thu, 21 Mar 2019 19:25:32 -0400 Received: from out30-57.freemail.mail.aliyun.com ([115.124.30.57]:49098 "EHLO out30-57.freemail.mail.aliyun.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726695AbfCUXZc (ORCPT ); Thu, 21 Mar 2019 19:25:32 -0400 X-Alimail-AntiSpam: AC=PASS;BC=-1|-1;BR=01201311R171e4;CH=green;DM=||false|;FP=0|-1|-1|-1|0|-1|-1|-1;HT=e01f04446;MF=yang.shi@linux.alibaba.com;NM=1;PH=DS;RN=6;SR=0;TI=SMTPD_---0TNJJX9z_1553210725; Received: from US-143344MP.local(mailfrom:yang.shi@linux.alibaba.com fp:SMTPD_---0TNJJX9z_1553210725) by smtp.aliyun-inc.com(127.0.0.1); Fri, 22 Mar 2019 07:25:27 +0800 Subject: Re: [RFC PATCH] mm: mempolicy: remove MPOL_MF_LAZY To: Michal Hocko Cc: mgorman@techsingularity.net, vbabka@suse.cz, akpm@linux-foundation.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <1553041659-46787-1-git-send-email-yang.shi@linux.alibaba.com> <20190321145745.GS8696@dhcp22.suse.cz> <75059b39-dbc4-3649-3e6b-7bdf282e3f53@linux.alibaba.com> <20190321165112.GU8696@dhcp22.suse.cz> <60ef6b4a-4f24-567f-af2f-50d97a2672d6@linux.alibaba.com> <20190321194539.GY8696@dhcp22.suse.cz> From: Yang Shi Message-ID: <2cbd2f5c-4cb9-457a-6a0a-8ae99ca5eb6e@linux.alibaba.com> Date: Thu, 21 Mar 2019 16:25:24 -0700 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: <20190321194539.GY8696@dhcp22.suse.cz> Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit Content-Language: en-US Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 3/21/19 12:45 PM, Michal Hocko wrote: > On Thu 21-03-19 10:25:08, Yang Shi wrote: >> >> On 3/21/19 9:51 AM, Michal Hocko wrote: >>> On Thu 21-03-19 09:21:39, Yang Shi wrote: >>>> On 3/21/19 7:57 AM, Michal Hocko wrote: >>>>> On Wed 20-03-19 08:27:39, Yang Shi wrote: >>>>>> MPOL_MF_LAZY was added by commit b24f53a0bea3 ("mm: mempolicy: Add >>>>>> MPOL_MF_LAZY"), then it was disabled by commit a720094ded8c ("mm: >>>>>> mempolicy: Hide MPOL_NOOP and MPOL_MF_LAZY from userspace for now") >>>>>> right away in 2012. So, it is never ever exported to userspace. >>>>>> >>>>>> And, it looks nobody is interested in revisiting it since it was >>>>>> disabled 7 years ago. So, it sounds pointless to still keep it around. >>>>> The above changelog owes us a lot of explanation about why this is >>>>> safe and backward compatible. I am also not sure you can change >>>>> MPOL_MF_INTERNAL because somebody still might use the flag from >>>>> userspace and we want to guarantee it will have the exact same semantic. >>>> Since MPOL_MF_LAZY is never exported to userspace (Mel helped to confirm >>>> this in the other thread), so I'm supposed it should be safe and backward >>>> compatible to userspace. >>> You didn't get my point. The flag is exported to the userspace and >>> nothing in the syscall entry path checks and masks it. So we really have >>> to preserve the semantic of the flag bit for ever. >> Thanks, I see you point. Yes, it is exported to userspace in some sense >> since it is in uapi header. But, it is never documented and MPOL_MF_VALID >> excludes it. mbind() does check and mask it. It would return -EINVAL if >> MPOL_MF_LAZY or any other undefined/invalid flag is set. See the below code >> snippet from do_mbind(): >> >> ... >> #define MPOL_MF_VALID    (MPOL_MF_STRICT   |     \ >>              MPOL_MF_MOVE     |     \ >>              MPOL_MF_MOVE_ALL) >> >> if (flags & ~(unsigned long)MPOL_MF_VALID) >>         return -EINVAL; >> >> So, I don't think any application would really use the flag for mbind() >> unless it is aimed to test the -EINVAL. If just test program, it should be >> not considered as a regression. > I have overlook that MPOL_MF_VALID doesn't include MPOL_MF_LAZY. Anyway, > my argument still holds that the bit has to be reserved for ever because > it used to be valid at some point of time and not returning EINVAL could > imply you are running on the kernel which supports the flag. I'd say it is not valid since very beginning. MPOL_MF_LAZY was added by commit b24f53a0bea3 ("mm: mempolicy: Add MPOL_MF_LAZY"), then it was hidden by commit a720094ded8c ("mm: mempolicy: Hide MPOL_NOOP and MPOL_MF_LAZY from userspace for now"). And, git describe --contains shows: US-143344MP:linux yang.s$ git describe --contains b24f53a0bea3 v3.8-rc1~92^2~27 US-143344MP:linux yang.s$ git describe --contains a720094ded8c v3.8-rc1~92^2~25 This is why I thought it is never ever exported to userspace. > >>>> I'm also not sure if anyone use MPOL_MF_INTERNAL or not and how they use it >>>> in their applications, but how about keeping it unchanged? >>> You really have to. Because it is an offset of other MPLO flags for >>> internal usage. >>> >>> That being said. Considering that we really have to preserve >>> MPOL_MF_LAZY value (we cannot even rename it because it is in uapi >>> headers and we do not want to break compilation). What is the point of >>> this change? Why is it an improvement? Yes, nobody is probably using >>> this because this is not respected in anything but the preferred mem >>> policy. At least that is the case from my quick glance. I might be still >>> wrong as it is quite easy to overlook all the consequences. So the risk >>> is non trivial while the benefit is not really clear to me. If you see >>> one, _document_ it. "Mel said it is not in use" is not a justification, >>> with all due respect. >> As I elaborated above, mbind() syscall does check it and treat it as an >> invalid flag. MPOL_PREFERRED doesn't use it either, but just use MPOL_F_MOF >> directly. > As Mel already pointed out. This doesn't really sound like a sound > argument. Say we would remove those few lines of code and preserve the > flag for future reservation of the flag bit. I would bet my head that it > will not be long before somebody just goes and clean it up and remove > because the flag is unused. So you would have to put a note explaining > why this has to be preserved. Maybe the current code is better to > document that. It would be much more sound to remove the code if it was > causing a measurable overhead or a maintenance burden. Is any of that > the case? As what I found out, I just thought it may be dead code, if so why not remove it otherwise we may have to keep maintaining the unused code. Thanks, Yang >