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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id F1103C433EF for ; Tue, 3 May 2022 16:04:22 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S239101AbiECQHx (ORCPT ); Tue, 3 May 2022 12:07:53 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:49434 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S238959AbiECQHw (ORCPT ); Tue, 3 May 2022 12:07:52 -0400 Received: from smtp-out2.suse.de (smtp-out2.suse.de [195.135.220.29]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 0E624193C1 for ; Tue, 3 May 2022 09:04:18 -0700 (PDT) Received: from relay2.suse.de (relay2.suse.de [149.44.160.134]) by smtp-out2.suse.de (Postfix) with ESMTP id B23861F74D; Tue, 3 May 2022 16:04:16 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.com; s=susede1; t=1651593856; h=from:from:reply-to: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=gg2obvCP4uGseNM46DcVy8f8HF/W6yCP38YvJGSTCWY=; b=WXl7GwcbpZMOrdvBuHPm05ZIolsnfWDVcyxt2pJJi0NW3TbhVom0xDX1WuJcSWvlq7cthf HuG8d5ra9EZ6ufcEt75ipkmTdfbDXvguUtiKRCMACmKu2b/d7XPJpKtX09Cmxt/tKazh8q 0ucBzIOCAnyRPUM0uDZsr6B1ghqIOZk= Received: from suse.cz (unknown [10.100.201.86]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by relay2.suse.de (Postfix) with ESMTPS id DAB3B2C145; Tue, 3 May 2022 16:04:15 +0000 (UTC) Date: Tue, 3 May 2022 18:04:13 +0200 From: Michal Hocko To: "Paul E. McKenney" Cc: liam.howlett@oracle.com, willy@infradead.org, walken.cr@gmail.com, hannes@cmpxchg.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: Memory allocation on speculative fastpaths Message-ID: References: <20220503155913.GA1187610@paulmck-ThinkPad-P17-Gen-1> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20220503155913.GA1187610@paulmck-ThinkPad-P17-Gen-1> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue 03-05-22 08:59:13, Paul E. McKenney wrote: > Hello! > > Just following up from off-list discussions yesterday. > > The requirements to allocate on an RCU-protected speculative fastpath > seem to be as follows: > > 1. Never sleep. > 2. Never reclaim. > 3. Leave emergency pools alone. > > Any others? > > If those rules suffice, and if my understanding of the GFP flags is > correct (ha!!!), then the following GFP flags should cover this: > > __GFP_NOMEMALLOC | __GFP_NOWARN GFP_NOWAIT | __GFP_NOMEMALLOC | __GFP_NOWARN > Or is this just a fancy way of always returning NULL or some such? ;-) It could fail quite easily. We would also want to guarantee (by documenting I guess) that the page allocator never does anything that would depend or invoke rcu_synchronize or something like that. I believe this is the case currently. -- Michal Hocko SUSE Labs