From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754363AbbFSH5n (ORCPT ); Fri, 19 Jun 2015 03:57:43 -0400 Received: from mx1.redhat.com ([209.132.183.28]:46605 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754185AbbFSH51 (ORCPT ); Fri, 19 Jun 2015 03:57:27 -0400 Message-ID: <5583CB62.6030405@redhat.com> Date: Fri, 19 Jun 2015 09:57:22 +0200 From: Paolo Bonzini User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.6.0 MIME-Version: 1.0 To: "Michael S. Tsirkin" 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 References: <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> <20150618164559-mutt-send-email-mst@redhat.com> <5582EBA6.1080607@redhat.com> <20150619095515-mutt-send-email-mst@redhat.com> In-Reply-To: <20150619095515-mutt-send-email-mst@redhat.com> Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 19/06/2015 09:56, Michael S. Tsirkin wrote: > On Thu, Jun 18, 2015 at 06:02:46PM +0200, Paolo Bonzini wrote: >> >> >> On 18/06/2015 16:47, Michael S. Tsirkin wrote: >>>> 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. >>> >>> Basically pages go away on munmap, so won't simple >>> lock >>> munmap >>> mmap(MAP_NORESERVE) >>> unlock >>> do the trick? >> >> Not sure I follow. Here we have this: >> >> VCPU 1 VCPU 2 I/O worker >> ---------------------------------------------------------------------------------------- >> take big QEMU lock >> p = address_space_map(hva, len) >> pass I/O request to worker thread >> read(fd, p, len) >> release big QEMU lock >> >> memory_region_del_subregion >> mmap(MAP_NORESERVE) >> >> read returns EFAULT > > Why doesn't it EFAULT without mmap(MAP_NORESERVE)? > Doesn't memory_region_del_subregion free the memory? No, only destruction of the memory region frees it. address_space_map takes a reference to the memory region and address_space_unmap releases it. Paolo >> wake up VCPU 1 >> take big QEMU lock >> EFAULT? What's that? >> >> In another scenario you are less lucky: the memory accesses >> between address_space_map/unmap aren't done in the kernel and >> you get a plain old SIGSEGV. >> >> This is not something that you can fix with a lock. The very >> purpose of the map/unmap API is to do stuff asynchronously while >> the lock is released. >> >> Thanks, >> >> Paolo