Race condition observed between page migration and page fault handling on arm64 machines

[David Hildenbrand]

>>> To dampen the tradeoff, we could do this in shmem_fault() instead? But
>>> then, this would mean that we do this in all
>>>
>>> kinds of vma->vm_ops->fault, only when we discover another reference
>>> count race condition :) Doing this in do_fault()
>>>
>>> should solve this once and for all. In fact, do_pte_missing() may call
>>> do_anonymous_page() or do_fault(), and I just
>>>
>>> noticed that the former already checks this using vmf_pte_changed().
>>
>> What I am still missing is why this is (a) arm64 only; and (b) if this
>> is something we should really worry about. There are other reasons
>> (e.g., speculative references) why migration could temporarily fail,
>> does it happen that often that it is really something we have to worry
>> about?
> 
> 
> (a) See discussion at [1]; I guess it passes on x86, which is quite
> strange since the race is clearly arch-independent.

Yes, I think this is what we have to understand. Is the race simply less 
likely to trigger on x86?

I would assume that it would trigger on any arch.

I just ran it on a x86 VM with 2 NUMA nodes and it also seems to work here.

Is this maybe related to deferred flushing? Such that the other CPU will 
by accident just observe the !pte_none a little less likely?

But arm64 also usually defers flushes, right? At least unless 
ARM64_WORKAROUND_REPEAT_TLBI is around. With that we never do deferred 
flushes.

> 
> (b) On my machine, on an average in under 10 iterations of move_pages(),
> it fails, which seems problematic to

Yes, it's a big difference compared to what I encounter.

-- 
Cheers,

David / dhildenb