Re: [RFC PATCH 5/6] vhost, mm: make sure that oom_reaper doesn't reap memory read by vhost

[Oleg Nesterov <oleg@redhat.com>]

On 07/11, Michal Hocko wrote:
>
> On Fri 08-07-16 14:29:48, Oleg Nesterov wrote:
> > On 07/07, Michael S. Tsirkin wrote:
> > >
> > > On Thu, Jul 07, 2016 at 10:28:12AM +0200, Michal Hocko wrote:
> > > >
> > > > Just to make sure we are all at the same page. I guess the scenario is
> > > > as follows. The owner of the mm has ring and other statefull information
> > > > in the private memory but consumers living with their own mm consume
> > > > some data from a shared memory segments (e.g. files). The worker would
> > > > misinterpret statefull information (zeros rather than the original
> > > > content) and would copy invalid/corrupted data to the consumer. Am I
> > > > correct?
> > >
> > > Exactly.
> >
> > Michael, let me ask again.
> >
> > But what if we simply kill the owner of this mm?
>
> I might be wrong here but the mm owner doesn't really matter AFAIU. It
> is the holder of the file descriptor for the "device" who control all
> the actions, no? The fact that it hijacked the mm along the way is hiden
> from users. If you kill the owner but pass the fd somewhere else then
> the mm will live as long as the fd.

Of course. I meant that qemu/guest won't update that statefull info in its
anonymous memory after we kill it. And I have no idea if it is fine or not.

As I said, I do not even know what drivers/vhost actually does, and probably
that is why I do not understand why this particular problem (bogus zeroes in
anonymous memory) is worse than other problems we can't avoid anyway when we
kill the victim and this affects other applications.

> > If yes, note that this means that any process which can do VHOST_SET_OWNER becomes
> > "oom-unkillable" to some degree, and this doesn't look right. It can spawn another
> > CLONE_FILES process and this will block fops->release() which (iiuc) should stop
> > the kernel thread which pins the memory hog's memory.
>
> I believe this is indeed possible. It can even pass the fd to a
> different process and keep it alive, hidden from the oom killer causing
> other processes to be killed.

Yes, so I think we should unmap the memory even if it is used by kthread.

Oleg.

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