From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wj0-f200.google.com (mail-wj0-f200.google.com [209.85.210.200]) by kanga.kvack.org (Postfix) with ESMTP id 4E0926B0253 for ; Fri, 2 Dec 2016 09:44:43 -0500 (EST) Received: by mail-wj0-f200.google.com with SMTP id o2so38338622wje.5 for ; Fri, 02 Dec 2016 06:44:43 -0800 (PST) Received: from mail-wj0-f196.google.com (mail-wj0-f196.google.com. [209.85.210.196]) by mx.google.com with ESMTPS id i125si3321859wmg.121.2016.12.02.06.44.42 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Fri, 02 Dec 2016 06:44:42 -0800 (PST) Received: by mail-wj0-f196.google.com with SMTP id xy5so30520746wjc.1 for ; Fri, 02 Dec 2016 06:44:42 -0800 (PST) Date: Fri, 2 Dec 2016 15:44:40 +0100 From: Michal Hocko Subject: Re: [PATCH v2] zswap: only use CPU notifier when HOTPLUG_CPU=y Message-ID: <20161202144440.GQ6830@dhcp22.suse.cz> References: <1480540516-6458-1-git-send-email-yuzhao@google.com> <20161202134604.GA6837@dhcp22.suse.cz> <20161202143848.GP6830@dhcp22.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20161202143848.GP6830@dhcp22.suse.cz> Sender: owner-linux-mm@kvack.org List-ID: To: Dan Streetman Cc: Yu Zhao , Seth Jennings , Linux-MM , linux-kernel On Fri 02-12-16 15:38:48, Michal Hocko wrote: > On Fri 02-12-16 09:24:35, Dan Streetman wrote: > > On Fri, Dec 2, 2016 at 8:46 AM, Michal Hocko wrote: > > > On Wed 30-11-16 13:15:16, Yu Zhao wrote: > > >> __unregister_cpu_notifier() only removes registered notifier from its > > >> linked list when CPU hotplug is configured. If we free registered CPU > > >> notifier when HOTPLUG_CPU=n, we corrupt the linked list. > > >> > > >> To fix the problem, we can either use a static CPU notifier that walks > > >> through each pool or just simply disable CPU notifier when CPU hotplug > > >> is not configured (which is perfectly safe because the code in question > > >> is called after all possible CPUs are online and will remain online > > >> until power off). > > >> > > >> v2: #ifdef for cpu_notifier_register_done during cleanup. > > > > > > this ifedfery is just ugly as hell. I am also wondering whether it is > > > really needed. __register_cpu_notifier and __unregister_cpu_notifier are > > > noops for CONFIG_HOTPLUG_CPU=n. So what's exactly that is broken here? > > > > hmm, that's interesting, __unregister_cpu_notifier is always a noop if > > HOTPLUG_CPU=n, but __register_cpu_notifier is only a noop if > > HOTPLUG_CPU=n *and* MODULE. If !MODULE, __register_cpu_notifier does > > OK, I've missed the MODULE part > > > actually register! This was added by commit > > 47e627bc8c9a70392d2049e6af5bd55fae61fe53 ('hotplug: Allow modules to > > use the cpu hotplug notifiers even if !CONFIG_HOTPLUG_CPU') and looks > > like it's to allow built-ins to register so they can notice during > > boot when cpus are initialized. > > I cannot say I wound understand the motivation but that is not really > all that important. > > > IMHO, that is the real problem - sure, without HOTPLUG_CPU, nobody > > should ever get a notification that a cpu is dying, but that doesn't > > mean builtins that register notifiers will never unregister their > > notifiers and then free them. > > Yes that is true. That suggests that __unregister_cpu_notifier should > the the symmetric thing to the __register_cpu_notifier for > CONFIG_MODULE, right? I meant the following. Completely untested --- diff --git a/include/linux/cpu.h b/include/linux/cpu.h index 797d9c8e9a1b..8d7b473426af 100644 --- a/include/linux/cpu.h +++ b/include/linux/cpu.h @@ -120,6 +120,7 @@ extern void __unregister_cpu_notifier(struct notifier_block *nb); #ifndef MODULE extern int register_cpu_notifier(struct notifier_block *nb); extern int __register_cpu_notifier(struct notifier_block *nb); +extern void __unregister_cpu_notifier(struct notifier_block *nb); #else static inline int register_cpu_notifier(struct notifier_block *nb) { -- Michal Hocko SUSE Labs -- 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