From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751720AbdJBNxO (ORCPT ); Mon, 2 Oct 2017 09:53:14 -0400 Received: from mx0b-001b2d01.pphosted.com ([148.163.158.5]:57110 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1750957AbdJBNxM (ORCPT ); Mon, 2 Oct 2017 09:53:12 -0400 Date: Mon, 2 Oct 2017 06:53:06 -0700 From: "Paul E. McKenney" To: Paolo Bonzini Cc: Boqun Feng , linux-kernel@vger.kernel.org, kvm@vger.kernel.org, Peter Zijlstra , Radim =?utf-8?B?S3LEjW3DocWZ?= , Thomas Gleixner , Ingo Molnar , "H. Peter Anvin" , x86@kernel.org Subject: Re: [PATCH] kvm/x86: Handle async PF in RCU read-side critical sections Reply-To: paulmck@linux.vnet.ibm.com 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> <42a732c2-e644-99dc-0fa0-81ebc919251c@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <42a732c2-e644-99dc-0fa0-81ebc919251c@redhat.com> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 17100213-0040-0000-0000-000003ABA199 X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00007828; HX=3.00000241; KW=3.00000007; PH=3.00000004; SC=3.00000233; SDB=6.00925465; UDB=6.00465434; IPR=6.00705635; BA=6.00005616; NDR=6.00000001; ZLA=6.00000005; ZF=6.00000009; ZB=6.00000000; ZP=6.00000000; ZH=6.00000000; ZU=6.00000002; MB=3.00017360; XFM=3.00000015; UTC=2017-10-02 13:53:09 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 17100213-0041-0000-0000-000007A0AF92 Message-Id: <20171002135306.GQ3521@linux.vnet.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10432:,, definitions=2017-10-02_03:,, signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 spamscore=0 suspectscore=0 malwarescore=0 phishscore=0 adultscore=0 bulkscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1707230000 definitions=main-1710020201 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Oct 02, 2017 at 02:45:34PM +0200, Paolo Bonzini wrote: > 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. It would look like random memory corruption in the guest, so it might well have been encountered. Though you have to get a page fault in just the wrong place and an update has to happen just at that time, so perhaps low probability. Still, good to fix. > >> 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. Got it, thank you! Thanx, Paul > 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. >