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.
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