From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-it0-f70.google.com (mail-it0-f70.google.com [209.85.214.70]) by kanga.kvack.org (Postfix) with ESMTP id 383F26B0069 for ; Mon, 19 Sep 2016 17:39:31 -0400 (EDT) Received: by mail-it0-f70.google.com with SMTP id e20so2933799itc.3 for ; Mon, 19 Sep 2016 14:39:31 -0700 (PDT) Received: from out28-2.mail.aliyun.com (out28-2.mail.aliyun.com. [115.124.28.2]) by mx.google.com with ESMTP id q144si28215072itc.110.2016.09.19.14.39.29 for ; Mon, 19 Sep 2016 14:39:30 -0700 (PDT) Message-ID: <57E05CD2.5090408@emindsoft.com.cn> Date: Tue, 20 Sep 2016 05:46:58 +0800 From: Chen Gang MIME-Version: 1.0 Subject: Re: [PATCH] mm: migrate: Return false instead of -EAGAIN for dummy functions References: <1474096836-31045-1-git-send-email-chengang@emindsoft.com.cn> <20160917154659.GA29145@dhcp22.suse.cz> In-Reply-To: <20160917154659.GA29145@dhcp22.suse.cz> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Sender: owner-linux-mm@kvack.org List-ID: To: Michal Hocko Cc: akpm@linux-foundation.org, minchan@kernel.org, vbabka@suse.cz, mgorman@techsingularity.net, gi-oh.kim@profitbricks.com, opensource.ganesh@gmail.com, hughd@google.com, kirill.shutemov@linux.intel.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, Chen Gang On 9/17/16 23:46, Michal Hocko wrote: > On Sat 17-09-16 15:20:36, chengang@emindsoft.com.cn wrote: > >> Also change their related pure Boolean function numamigrate_isolate_page. > > this is not true. Just look at the current usage > > migrated = migrate_misplaced_page(page, vma, target_nid); > if (migrated) { > page_nid = target_nid; > flags |= TNF_MIGRATED; > } else > flags |= TNF_MIGRATE_FAIL; > > and now take your change which changes -EAGAIN into false. See the > difference? Now I didn't even try to understand why > CONFIG_NUMA_BALANCING=n pretends a success but then in order to keep the > current semantic your patch should return true in that path. So NAK from > me until you either explain why this is OK or change it. > For me, it really need return false: - For real implementation, when do nothing, it will return false. - I assume that the input page already is in a node (although maybe my assumption incorrect), and migrate to the same node. When the real implementation fails (e.g. -EAGAIN 10 times), it still returns false. - Original dummy implementation always return -EAGAIN, And -EAGAIN in real implementation will trigger returning false, after 10 times. - After grep TNF_MIGRATE_FAIL and TNF_MIGRATED, we only use them in task_numa_fault in kernel/sched/fair.c for numa_pages_migrated and numa_faults_locality, I guess they are only used for statistics. So for me the dummy implementation need return false instead of -EAGAIN. > But to be honest I am not keen of this int -> bool changes much. > Especially if they are bringing a risk of subtle behavior change like > this patch. And without a good changelog explaining why this makes > sense. > If our original implementation already used bool, our this issue (return -EAGAIN) would be avoided (compiler would help us to find this issue). Thanks. -- Chen Gang (e??a??) Managing Natural Environments is the Duty of Human Beings. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org