From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751393AbdJBMpo (ORCPT ); Mon, 2 Oct 2017 08:45:44 -0400 Received: from mx1.redhat.com ([209.132.183.28]:28572 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751167AbdJBMpm (ORCPT ); Mon, 2 Oct 2017 08:45:42 -0400 DMARC-Filter: OpenDMARC Filter v1.3.2 mx1.redhat.com 5A7CB61475 Authentication-Results: ext-mx10.extmail.prod.ext.phx2.redhat.com; dmarc=none (p=none dis=none) header.from=redhat.com Authentication-Results: ext-mx10.extmail.prod.ext.phx2.redhat.com; spf=fail smtp.mailfrom=pbonzini@redhat.com Subject: Re: [PATCH] kvm/x86: Handle async PF in RCU read-side critical sections To: paulmck@linux.vnet.ibm.com, Boqun Feng Cc: linux-kernel@vger.kernel.org, kvm@vger.kernel.org, Peter Zijlstra , =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= , Thomas Gleixner , Ingo Molnar , "H. Peter Anvin" , x86@kernel.org References: <20170929110148.3467-1-boqun.feng@gmail.com> <95ecfeff-093d-4438-dbcb-ecc81abca993@redhat.com> <20170929164339.GC3521@linux.vnet.ibm.com> <20170929234156.5e5oucxsqhxoqcml@tardis> <20170930171515.GK3521@linux.vnet.ibm.com> From: Paolo Bonzini Message-ID: <42a732c2-e644-99dc-0fa0-81ebc919251c@redhat.com> Date: Mon, 2 Oct 2017 14:45:34 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.3.0 MIME-Version: 1.0 In-Reply-To: <20170930171515.GK3521@linux.vnet.ibm.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.39]); Mon, 02 Oct 2017 12:45:42 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 30/09/2017 19:15, Paul E. McKenney wrote: > On Sat, Sep 30, 2017 at 07:41:56AM +0800, Boqun Feng wrote: >> On Fri, Sep 29, 2017 at 04:43:39PM +0000, Paul E. McKenney wrote: >>> Not to be repetitive, but if the schedule() is on the guest, this change >>> really does silently break up an RCU read-side critical section on >>> guests built with PREEMPT=n. (Yes, they were already being broken, >>> but it would be good to avoid this breakage in PREEMPT=n as well as >>> in PREEMPT=y.) Yes, you're right. It's pretty surprising that it's never been reported. >> Then probably adding !IS_ENABLED(CONFIG_PREEMPT) as one of the reason we >> choose the halt path? Like: >> >> n.halted = is_idle_task(current) || preempt_count() > 1 || >> !IS_ENABLED(CONFIG_PREEMPT) || rcu_preempt_depth(); >> >> >> But I think async PF could also happen while a user program is running? >> Then maybe add a second parameter @user for kvm_async_pf_task_wait(), >> like: >> >> kvm_async_pf_task_wait((u32)read_cr2(), user_mode(regs)); >> >> and the halt condition becomes: >> >> n.halted = is_idle_task(current) || preempt_count() > 1 || >> (!IS_ENABLED(CONFIG_PREEMPT) && !user) || rcu_preempt_depth(); >> >> Thoughts? > > This looks to me like it would cover it. If !PREEMPT interrupt from > kernel, we halt, which would prevent the sleep. > > I take it that we get unhalted when the host gets things patched up? Yes. You get another page fault (this time it's a "page ready" page fault rather than a "page not present" one), which has the side effecting of ending the halt. Paolo >> A side thing is being broken already for PREEMPT=n means we maybe fail >> to detect this in rcutorture? Then should we add a config with >> KVM_GUEST=y and try to run some memory consuming things(e.g. stress >> --vm) in the rcutorture kvm script simultaneously? Paolo, do you have >> any test workload that could trigger async PF quickly? > > I do not believe that have seen this in rcutorture, but I always run in > a guest OS on a large-memory system (well, by my old-fashioned standards, > anyway) that would be quite unlikely to evict a guest OS's pages. Plus > I tend to run on shared systems, and deliberately running them out of > memory would not be particularly friendly to others using those systems. > > I -do- run background scripts that are intended to force the host OS to > preempt the guest OSes frequently, but I don't believe that this would > cause that bug. > > But it seems like it would make more sense to add this sort of thing to > whatever KVM tests there are for host-side eviction of guest pages.