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=-4.0 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS 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 989B4C433E1 for ; Thu, 13 Aug 2020 17:12:17 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 7E9CF20715 for ; Thu, 13 Aug 2020 17:12:17 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726600AbgHMRMP (ORCPT ); Thu, 13 Aug 2020 13:12:15 -0400 Received: from mx2.suse.de ([195.135.220.15]:60930 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726174AbgHMRMO (ORCPT ); Thu, 13 Aug 2020 13:12:14 -0400 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id D9100AFBB; Thu, 13 Aug 2020 17:12:34 +0000 (UTC) Date: Thu, 13 Aug 2020 19:12:11 +0200 From: Michal Hocko To: "Paul E. McKenney" Cc: Thomas Gleixner , Uladzislau Rezki , LKML , RCU , linux-mm@kvack.org, Andrew Morton , Vlastimil Babka , Matthew Wilcox , "Theodore Y . Ts'o" , Joel Fernandes , Sebastian Andrzej Siewior , Oleksiy Avramchenko , Peter Zijlstra Subject: Re: [RFC-PATCH 1/2] mm: Add __GFP_NO_LOCKS flag Message-ID: <20200813171211.GT9477@dhcp22.suse.cz> References: <20200813095840.GA25268@pc636> <874kp6llzb.fsf@nanos.tec.linutronix.de> <20200813133308.GK9477@dhcp22.suse.cz> <87sgcqty0e.fsf@nanos.tec.linutronix.de> <20200813145335.GN9477@dhcp22.suse.cz> <20200813154159.GR4295@paulmck-ThinkPad-P72> <20200813155412.GP9477@dhcp22.suse.cz> <20200813160442.GV4295@paulmck-ThinkPad-P72> <20200813161357.GQ9477@dhcp22.suse.cz> <20200813162904.GX4295@paulmck-ThinkPad-P72> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200813162904.GX4295@paulmck-ThinkPad-P72> Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu 13-08-20 09:29:04, Paul E. McKenney wrote: > On Thu, Aug 13, 2020 at 06:13:57PM +0200, Michal Hocko wrote: > > On Thu 13-08-20 09:04:42, Paul E. McKenney wrote: > > > On Thu, Aug 13, 2020 at 05:54:12PM +0200, Michal Hocko wrote: > > [...] > > > > If the whole bailout is guarded by CONFIG_PREEMPT_RT specific atomicity > > > > check then there is no functional problem - GFP_RT_SAFE would still be > > > > GFP_NOWAIT so functional wise the allocator will still do the right > > > > thing. > > > > > > Perhaps it was just me getting confused, early hour Pacific Time and > > > whatever other excuses might apply. But I thought that you still had > > > an objection to GFP_RT_SAFE based on changes in allocator semantics for > > > other users. > > > > There is still that problem with lockdep complaining about raw->regular > > spinlock on !PREEMPT_RT that would need to get resolved somehow. Thomas > > is not really keen on adding some lockdep annotation mechanism and > > unfortunatelly I do not have a different idea how to get rid of those. > > OK. So the current situation requires a choice between these these > alternatives, each of which has shortcomings that have been mentioned > earlier in this thread: > > 1. Prohibit invoking allocators from raw atomic context, such > as when holding a raw spinlock. > > 2. Adding a GFP_ flag. Which would implemente a completely new level atomic allocation for all preemption models > > 3. Reusing existing GFP_ flags/values/whatever to communicate > the raw-context information that was to be communicated by > the new GFP_ flag. this would have to be RT specific to not change the semantic for existing users. In other words make NOWAIT semantic working for RT atomic contexts. > > 4. Making lockdep forgive acquiring spinlocks while holding > raw spinlocks, but only in CONFIG_PREEMPT_NONE=y kernels. and this would have to go along with 3 to remove false positives on !RT. -- Michal Hocko SUSE Labs