linux-kernel - Re: [PATCH] exit: clear TIF_MEMDIE after exit_task

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Date:	Tue, 1 Mar 2016 18:14:02 +0200
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Michal Hocko <mhocko@...nel.org>
Cc:	Vladimir Davydov <vdavydov@...tuozzo.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
	David Rientjes <rientjes@...gle.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] exit: clear TIF_MEMDIE after exit_task_work

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