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=-4.1 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,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 5EA7EC433E3 for ; Thu, 13 Aug 2020 14:35:03 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 3862B2078D for ; Thu, 13 Aug 2020 14:35:03 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=linutronix.de header.i=@linutronix.de header.b="vAXzAv9Y"; dkim=permerror (0-bit key) header.d=linutronix.de header.i=@linutronix.de header.b="FuOoN+kM" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726499AbgHMOfC (ORCPT ); Thu, 13 Aug 2020 10:35:02 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:43260 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726082AbgHMOfB (ORCPT ); Thu, 13 Aug 2020 10:35:01 -0400 Received: from galois.linutronix.de (Galois.linutronix.de [IPv6:2a0a:51c0:0:12e:550::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id EA4E7C061757; Thu, 13 Aug 2020 07:35:00 -0700 (PDT) From: Thomas Gleixner DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020; t=1597329298; 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=cGR1gww3S6soUuuDFLRPw7CKS7tkn5nwoE4l+V3J1G4=; b=vAXzAv9YzNbly1PaIVpyuI+K246OfGFWqG7IuB4rUQ+51Vbt5vZAnF5eXBeGjAdYj2J+CC yvuCm4AQ3zq4RGDkIibLJ6ZVUmgGt8sneNEhdXuiy19fgK18dyKM+TL4cdkErPiEdZBh3a arcZw7ZiyEqHzEAmYHdp2wlHQdjXdf61LzyzYKh56zDv7+c4+h2nXVLwRBnJz4TjTptznm t5Y3DMV0TfVkCEC42mx9FZ9fQiFeJDUndPOyvI7Omdtsg1GOo7OlV0YdtByHjdPw0B36/4 0q1EfBcGI+jZttPQtIP33n1IUxUrcJB/vp7jHnMUDUZhf8tLzz3f8z3RcNz1fA== DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020e; t=1597329298; 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=cGR1gww3S6soUuuDFLRPw7CKS7tkn5nwoE4l+V3J1G4=; b=FuOoN+kM/a9ynXkbNB0B9O0qiEp+LMDng3MIJ2Xg1K4hdRvDuZOO5I+StbrJAM+I+as7p/ RWwJ1PQYXIxFbRBQ== To: Michal Hocko Cc: Uladzislau Rezki , paulmck@kernel.org, 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 In-Reply-To: <20200813133308.GK9477@dhcp22.suse.cz> References: <20200811210931.GZ4295@paulmck-ThinkPad-P72> <874kp87mca.fsf@nanos.tec.linutronix.de> <20200813075027.GD9477@dhcp22.suse.cz> <20200813095840.GA25268@pc636> <874kp6llzb.fsf@nanos.tec.linutronix.de> <20200813133308.GK9477@dhcp22.suse.cz> Date: Thu, 13 Aug 2020 16:34:57 +0200 Message-ID: <87sgcqty0e.fsf@nanos.tec.linutronix.de> MIME-Version: 1.0 Content-Type: text/plain Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Michal Hocko writes: > On Thu 13-08-20 15:22:00, Thomas Gleixner wrote: >> It basically requires to convert the wait queue to something else. Is >> the waitqueue strict single waiter? > > I would have to double check. From what I remember only kswapd should > ever sleep on it. That would make it trivial as we could simply switch it over to rcu_wait. >> So that should be: >> >> if (!preemptible() && gfp == GFP_RT_NOWAIT) >> >> which is limiting the damage to those callers which hand in >> GFP_RT_NOWAIT. >> >> lockdep will yell at invocations with gfp != GFP_RT_NOWAIT when it hits >> zone->lock in the wrong context. And we want to know about that so we >> can look at the caller and figure out how to solve it. > > Yes, that would have to somehow need to annotate the zone_lock to be ok > in those paths so that lockdep doesn't complain. That opens the worst of all cans of worms. If we start this here then Joe programmer and his dog will use these lockdep annotation to evade warnings and when exposed to RT it will fall apart in pieces. Just that at that point Joe programmer moved on to something else and the usual suspects can mop up the pieces. We've seen that all over the place and some people even disable lockdep temporarily because annotations don't help. PeterZ might have opinions about that too I suspect. Really, if your primary lockless caches are empty then any allocation which comes from deep atomic context should simply always fail. Being stuck in an interrupt handler or even deeper for 200+ microseconds waiting for zone lock is just bonkers IMO. Thanks, tglx