From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754567AbcCAQOJ (ORCPT ); Tue, 1 Mar 2016 11:14:09 -0500 Received: from mx1.redhat.com ([209.132.183.28]:60987 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753734AbcCAQOI (ORCPT ); Tue, 1 Mar 2016 11:14:08 -0500 Date: Tue, 1 Mar 2016 18:14:02 +0200 From: "Michael S. Tsirkin" To: Michal Hocko Cc: Vladimir Davydov , Andrew Morton , Tetsuo Handa , David Rientjes , linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] exit: clear TIF_MEMDIE after exit_task_work Message-ID: <20160301181136-mutt-send-email-mst@redhat.com> References: <1456765329-14890-1-git-send-email-vdavydov@virtuozzo.com> <20160301155212.GJ9461@dhcp22.suse.cz> <20160301175431-mutt-send-email-mst@redhat.com> <20160301160813.GM9461@dhcp22.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20160301160813.GM9461@dhcp22.suse.cz> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Mar 01, 2016 at 05:08:13PM +0100, Michal Hocko wrote: > On Tue 01-03-16 17:57:04, Michael S. Tsirkin wrote: > > On Tue, Mar 01, 2016 at 04:52:12PM +0100, Michal Hocko wrote: > > > [CCing vhost-net maintainer] > > > > > > On Mon 29-02-16 20:02:09, Vladimir Davydov wrote: > > > > An mm_struct may be pinned by a file. An example is vhost-net device > > > > created by a qemu/kvm (see vhost_net_ioctl -> vhost_net_set_owner -> > > > > vhost_dev_set_owner). > > > > > > The more I think about that the more I am wondering whether this is > > > actually OK and correct. Why does the driver have to pin the address > > > space? Nothing really prevents from parallel tearing down of the address > > > space anyway so the code cannot expect all the vmas to stay. Would it be > > > enough to pin the mm_struct only? > > > > I'll need to research this. It's a fact that as long as the > > device is not stopped, vhost can attempt to access > > the address space. > > But does it expect any specific parts of the address space to be mapped? > E.g. proc needs to keep the mm allocated as well for some files but it > doesn't pin the address space (mm_users) but rather mm_count (see > proc_mem_open). As I said, I need to research this. > > > I am not sure I understand the code properly but what prevents from > > > the situation when a VHOST_SET_OWNER caller dies without calling > > > VHOST_RESET_OWNER and so the mm would be pinned indefinitely? > > > > > > [Keeping the reset of the email for reference] > > > > We have: > > > > static const struct file_operations vhost_net_fops = { > > .owner = THIS_MODULE, > > .release = vhost_net_release, > > ... > > }; > > > > When caller dies and after fds are closed, > > vhost_net_release calls vhost_dev_cleanup and that > > drops the mm reference. > > Can another process have the device open as well and prevent from > destruction? Yes. > -- > Michal Hocko > SUSE Labs