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

[Oleg Nesterov <oleg@redhat.com>]

On 08/12, Michal Hocko wrote:
>
> > 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.

Well, I leave this to Paul, but...

I think it is not needed because we can rely on pte locking. We do
not care if anything is re-ordered UNLESS __get_user() above actually
triggers a fault and re-populates the page which was already unmapped
by __oom_reap_task(), and in the latter case __get_user_mm() can't
miss MMF_UNSTABLE simply because __get_user() and unmap_page_range()
need to lock/unlock the same ptlock_ptr().

So we only need the compiler barrier to ensure that __get_user_mm()
won't read MMF_UNSTABLE before __get_user(). But since __get_user()
is function, it is not needed too.

There is a more interesting case when another 3rd thread can trigger
a fault and populate this page before __get_user_mm() calls _get_user().
But even in this case I think we are fine.


Whats really interesting is that I still fail to understand do we really
need this hack, iiuc you are not sure too, and Michael didn't bother to
explain why a bogus zero from anon memory is worse than other problems
caused by SIGKKILL from oom-kill.c.

Oleg.

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>