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[23.128.96.18]) by mx.google.com with ESMTP id qq22si799458ejb.281.2020.09.30.02.28.51; Wed, 30 Sep 2020 02:29:15 -0700 (PDT) Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18; Authentication-Results: mx.google.com; dkim=pass header.i=@suse.com header.s=susede1 header.b=vUcdbq2Q; spf=pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) smtp.mailfrom=linux-kernel-owner@vger.kernel.org; dmarc=pass (p=NONE sp=NONE dis=NONE) header.from=suse.com Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728797AbgI3J1k (ORCPT + 99 others); Wed, 30 Sep 2020 05:27:40 -0400 Received: from mx2.suse.de ([195.135.220.15]:58478 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725823AbgI3J1k (ORCPT ); Wed, 30 Sep 2020 05:27:40 -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=1601458057; 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=qbc8B6vU59oEG9W3JMzEwnRNloOF/cIBuPfCLo3VkAE=; b=vUcdbq2QIKk5C/VB0n77/zrg1g7OE2ROYhA+Wb4h9EX9/s20mWtx4DVMGhtFUbS6uBzKWd FrGJCAsVIT+v+Aj659DEKVq45+wMd+mRgy/xjte/xNJBdW7Loyo4elhjeGNqytBkQ3CfFA KrJB2dONzz0Epcmz0/GFnVP3ZN2e89o= Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id AB788ACDF; Wed, 30 Sep 2020 09:27:37 +0000 (UTC) Date: Wed, 30 Sep 2020 11:27:32 +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: <20200930092732.GP2277@dhcp22.suse.cz> References: <20200922075002.GU12990@dhcp22.suse.cz> <20200922131257.GA29241@pc636> <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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200929162514.GA8768@pc636> User-Agent: Mutt/1.10.1 (2018-07-13) Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 Hocko SUSE Labs