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[23.128.96.18]) by mx.google.com with ESMTP id a8si1579919ejs.101.2020.09.30.09.47.35; Wed, 30 Sep 2020 09:47:58 -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=kUh2mB6O; 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 S1731342AbgI3QqE (ORCPT + 99 others); Wed, 30 Sep 2020 12:46:04 -0400 Received: from mx2.suse.de ([195.135.220.15]:44410 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728729AbgI3QqD (ORCPT ); Wed, 30 Sep 2020 12:46:03 -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=1601484361; 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=92hdBQ0+yqLjn7t4Piw/YoIQ6f47TF4BG37Q/DtfEDs=; b=kUh2mB6Ow8VCZ29mrkt+i1nOawaa3g5RaWz0wGyZWRw+C1sBG/mhUaxse5Lay4CSYnOh/H p00+DHKNHxKOfRQtQLOjY0UXTuD8KZAddjxcF10N9hA4uhCTRlpFhq9e8tzFWZNXTcZaQF KHV/tFY9wdnjhfQJR5R9KDYv9cVgdWI= Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id 77B19AC8C; Wed, 30 Sep 2020 16:46:01 +0000 (UTC) Date: Wed, 30 Sep 2020 18:46:00 +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: <20200930164600.GW2277@dhcp22.suse.cz> References: <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> <20200930124413.GU2277@dhcp22.suse.cz> <20200930133954.GA19235@pc636> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200930133954.GA19235@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 15:39:54, Uladzislau Rezki wrote: > On Wed, Sep 30, 2020 at 02:44:13PM +0200, Michal Hocko wrote: > > 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? > > > Exactly :) We can make that more specific once RT patchset is merged. As of now this is not the thing in the Linus tree. I believe there will be more to clarify about atomic contexts in the RT tree as it means something else than people are used to think. -- Michal Hocko SUSE Labs