From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932497AbbFROrp (ORCPT ); Thu, 18 Jun 2015 10:47:45 -0400 Received: from mx1.redhat.com ([209.132.183.28]:41716 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932152AbbFROrg (ORCPT ); Thu, 18 Jun 2015 10:47:36 -0400 Date: Thu, 18 Jun 2015 16:47:33 +0200 From: "Michael S. Tsirkin" To: Paolo Bonzini Cc: Igor Mammedov , linux-kernel@vger.kernel.org, kvm@vger.kernel.org, andrey@xdel.ru Subject: Re: [PATCH 3/5] vhost: support upto 509 memory regions Message-ID: <20150618164559-mutt-send-email-mst@redhat.com> References: <20150617173736-mutt-send-email-mst@redhat.com> <20150617180921.7972345d@igors-macbook-pro.local> <20150617182917-mutt-send-email-mst@redhat.com> <20150618111224.0aa6dff7@nial.brq.redhat.com> <20150618112004-mutt-send-email-mst@redhat.com> <20150618133912.63636a47@nial.brq.redhat.com> <20150618134040-mutt-send-email-mst@redhat.com> <5582B088.1090207@redhat.com> <20150618142455-mutt-send-email-mst@redhat.com> <5582CBA6.5070105@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <5582CBA6.5070105@redhat.com> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Jun 18, 2015 at 03:46:14PM +0200, Paolo Bonzini wrote: > > > On 18/06/2015 15:19, Michael S. Tsirkin wrote: > > On Thu, Jun 18, 2015 at 01:50:32PM +0200, Paolo Bonzini wrote: > >> > >> > >> On 18/06/2015 13:41, Michael S. Tsirkin wrote: > >>> On Thu, Jun 18, 2015 at 01:39:12PM +0200, Igor Mammedov wrote: > >>>> Lets leave decision upto users instead of making them live with > >>>> crashing guests. > >>> > >>> Come on, let's fix it in userspace. > >> > >> It's not trivial to fix it in userspace. Since QEMU uses RCU there > >> isn't a single memory map to use for a linear gpa->hva map. > > > > Could you elaborate? > > > > I'm confused by this mention of RCU. > > You use RCU for accesses to the memory map, correct? > > So memory map itself is a write side operation, as such all you need to > > do is take some kind of lock to prevent conflicting with other memory > > maps, do rcu sync under this lock. > > You're right, the problem isn't directly related to RCU. RCU would be > easy to handle by using synchronize_rcu instead of call_rcu. While I > identified an RCU-related problem with Igor's patches, it's much more > entrenched. > > RAM can be used by asynchronous operations while the VM runs, between > address_space_map and address_space_unmap. It is possible and common to > have a quiescent state between the map and unmap, and a memory map > change can happen in the middle of this. Normally this is not a > problem, because changes to the memory map do not make the hva go away > (memory regions are reference counted). Right, so you want mmap(MAP_NORESERVE) when that reference count becomes 0. > However, with Igor's patches a memory_region_del_subregion will cause a > mmap(MAP_NORESERVE), which _does_ have the effect of making the hva go away. > > I guess one way to do it would be to alias the same page in two places, > one for use by vhost and one for use by everything else. However, the > kernel does not provide the means to do this kind of aliasing for > anonymous mmaps. > > Paolo Basically pages go away on munmap, so won't simple lock munmap mmap(MAP_NORESERVE) unlock do the trick? -- MST