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=-8.6 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 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 C1E0DC47425 for ; Wed, 30 Sep 2020 12:44:17 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 7D51E20789 for ; Wed, 30 Sep 2020 12:44:17 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=suse.com header.i=@suse.com header.b="VZ+48TXP" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729969AbgI3MoQ (ORCPT ); Wed, 30 Sep 2020 08:44:16 -0400 Received: from mx2.suse.de ([195.135.220.15]:49692 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725771AbgI3MoP (ORCPT ); Wed, 30 Sep 2020 08:44:15 -0400 X-Virus-Scanned: by amavisd-new at test-mx.suse.de DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.com; s=susede1; t=1601469853; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=ua5GAxIwg4VuIAgWqlzBE5mMAJGN8QYCNlhi6+jB07I=; b=VZ+48TXP34SnwePm3NIE9dkLCd9xjRcOcr8NJqcyoTSAEj4AQkBL1EUGt0bSwrpeZZb7dB rFc5a3wxIqgPjT9iV6+wJyMaQK7jrf9xq+un3/4/B6ouWYz6N7AVPgDTTkcC8ec9UKM2pj tAgDxSlTD+Yp0J9ADZjmEtXuMwvj/HY= Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id 808FBAC3C; Wed, 30 Sep 2020 12:44:13 +0000 (UTC) Date: Wed, 30 Sep 2020 14:44:13 +0200 From: Michal Hocko To: Uladzislau Rezki Cc: Mel Gorman , "Paul E. McKenney" , LKML , RCU , linux-mm@kvack.org, Andrew Morton , Peter Zijlstra , Vlastimil Babka , Thomas Gleixner , "Theodore Y . Ts'o" , Joel Fernandes , Sebastian Andrzej Siewior , Oleksiy Avramchenko , Mel Gorman Subject: Re: [RFC-PATCH 2/4] mm: Add __rcu_alloc_page_lockless() func. Message-ID: <20200930124413.GU2277@dhcp22.suse.cz> References: <20200923103706.GJ3179@techsingularity.net> <20200923154105.GO29330@paulmck-ThinkPad-P72> <20200923232251.GK3179@techsingularity.net> <20200924081614.GA14819@pc636> <20200925080503.GC3389@dhcp22.suse.cz> <20200925153129.GB25350@pc636> <20200925154741.GI3389@dhcp22.suse.cz> <20200929162514.GA8768@pc636> <20200930092732.GP2277@dhcp22.suse.cz> <20200930123535.GB18005@pc636> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200930123535.GB18005@pc636> User-Agent: Mutt/1.10.1 (2018-07-13) Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed 30-09-20 14:35:35, Uladzislau Rezki wrote: > On Wed, Sep 30, 2020 at 11:27:32AM +0200, Michal Hocko wrote: > > On Tue 29-09-20 18:25:14, Uladzislau Rezki wrote: > > > > > I look at it in scope of GFP_ATOMIC/GFP_NOWAIT issues, i.e. inability > > > > > to provide a memory service for contexts which are not allowed to > > > > > sleep, RCU is part of them. Both flags used to provide such ability > > > > > before but not anymore. > > > > > > > > > > Do you agree with it? > > > > > > > > Yes this sucks. But this is something that we likely really want to live > > > > with. We have to explicitly _document_ that really atomic contexts in RT > > > > cannot use the allocator. From the past discussions we've had this is > > > > likely the most reasonable way forward because we do not really want to > > > > encourage anybody to do something like that and there should be ways > > > > around that. The same is btw. true also for !RT. The allocator is not > > > > NMI safe and while we should be able to make it compatible I am not > > > > convinced we really want to. > > > > > > > > Would something like this be helpful wrt documentation? > > > > > > > > diff --git a/include/linux/gfp.h b/include/linux/gfp.h > > > > index 67a0774e080b..9fcd47606493 100644 > > > > --- a/include/linux/gfp.h > > > > +++ b/include/linux/gfp.h > > > > @@ -238,7 +238,9 @@ struct vm_area_struct; > > > > * %__GFP_FOO flags as necessary. > > > > * > > > > * %GFP_ATOMIC users can not sleep and need the allocation to succeed. A lower > > > > - * watermark is applied to allow access to "atomic reserves" > > > > + * watermark is applied to allow access to "atomic reserves". > > > > + * The current implementation doesn't support NMI and other non-preemptive context > > > > + * (e.g. raw_spin_lock). > > > > * > > > > * %GFP_KERNEL is typical for kernel-internal allocations. The caller requires > > > > * %ZONE_NORMAL or a lower zone for direct access but can direct reclaim. > > > > > > > To me it is clear. But also above conflicting statement: > > > > > > > > > %GFP_ATOMIC users can not sleep and need the allocation to succeed. A %lower > > > > > > > > > should be rephrased, IMHO. > > > > Any suggestions? Or more specifics about which part is conflicting? It > > tries to say that there is a higher demand to succeed even though the > > context cannot sleep to take active measures to achieve that. So the > > only way to achieve that is to break the watermakrs to a certain degree > > which is making them more "higher class" than other allocations. > > > Michal, i had only one concern about it. It says that %GFP_ATOMIC users > can not sleep, i.e. callers know that they are in atomic, thus no any > sleeping, but the chose they make will force them to sleep. I am not sure I follow you here. Do you mean they will be forced to sleep with PREEMPT_RT? -- Michal Hocko SUSE Labs