Re: [PATCH 09/10] vhost, mm: make sure that oom_reaper doesn't reap memory read by vhost

[Michal Hocko <mhocko@kernel.org>]

On Fri 29-07-16 20:57:44, Michael S. Tsirkin wrote:
> On Fri, Jul 29, 2016 at 03:35:29PM +0200, Michal Hocko wrote:
> > On Fri 29-07-16 16:14:10, Michael S. Tsirkin wrote:
> > > On Fri, Jul 29, 2016 at 08:04:22AM +0200, Michal Hocko wrote:
> > > > On Thu 28-07-16 23:41:53, Michael S. Tsirkin wrote:
> > > > > On Thu, Jul 28, 2016 at 09:42:33PM +0200, Michal Hocko wrote:
> > [...]
> > > > > > and the reader would hit a page fault
> > > > > > +	 * if it stumbled over a reaped memory.
> > > > > 
> > > > > This last point I don't get. flag read could bypass data read
> > > > > if that happens data read could happen after unmap
> > > > > yes it might get a PF but you handle that, correct?
> > > > 
> > > > The point I've tried to make is that if the reader really page faults
> > > > then get_user will imply the full barrier already. If get_user didn't
> > > > page fault then the state of the flag is not really important because
> > > > the reaper shouldn't have touched it. Does it make more sense now or
> > > > I've missed your question?
> > > 
> > > Can task flag read happen before the get_user pagefault?
> > 
> > Do you mean?
> > 
> > get_user_mm()
> >   temp = false <- test_bit(MMF_UNSTABLE, &mm->flags)
> >   ret = __get_user(x, ptr)
> >   #PF
> >   if (!ret && temp) # misses the flag
> > 
> > The code is basically doing
> > 
> >   if (!__get_user() && test_bit(MMF_UNSTABLE, &mm->flags))
> > 
> > so test_bit part of the conditional cannot be evaluated before
> > __get_user() part is done. Compiler cannot reorder two depending
> > subconditions AFAIK.
> 
> But maybe the CPU can.

Are you sure? How does that differ from
	if (ptr && ptr->something)
construct?

Let's CC Paul. Just to describe the situation. We have the following
situation:

#define __get_user_mm(mm, x, ptr)				\
({								\
	int ___gu_err = __get_user(x, ptr);			\
	if (!___gu_err && test_bit(MMF_UNSTABLE, &mm->flags))	\
		___gu_err = -EFAULT;				\
	___gu_err;						\
})

and the oom reaper doing:

	set_bit(MMF_UNSTABLE, &mm->flags);

	for (vma = mm->mmap ; vma; vma = vma->vm_next) {
		unmap_page_range

I assume that write memory barrier between set_bit and unmap_page_range
is not really needed because unmapping should already imply the memory
barrier. A read memory barrier between __get_user and test_bit shouldn't
be really needed because we can tolerate a stale value if __get_user
didn't #PF because we haven't unmapped that address obviously. If we
unmapped it then __get_user would #PF and that should imply a full
memory barrier as well. Now the question is whether a CPU can speculate
and read the flag before we issue the #PF.
-- 
Michal Hocko
SUSE Labs

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