Re: [RFC PATCH v1 3/4] mm/memory: Use ptep_get_lockless_norecency() for orig_pte

[David Hildenbrand <david@redhat.com>]

On 27.03.24 10:51, Ryan Roberts wrote:
> On 26/03/2024 17:58, David Hildenbrand wrote:
>>>>>>
>>>>>> vmf->orig_pte = ptep_get_lockless_norecency(vmf->pte)
>>>>>> /* not dirty */
>>>>>>
>>>>>> /* Now, thread 2 ends up setting the PTE dirty under PT lock. */
>>>
>>> Ahh, this comment about thread 2 is not referring to the code immediately below
>>> it. It all makes much more sense now. :)
>>
>> Sorry :)
>>
>>>
>>>>>>
>>>>>> spin_lock(vmf->ptl);
>>>>>> entry = vmf->orig_pte;
>>>>>> if (unlikely(!pte_same(ptep_get(vmf->pte), entry))) {
>>>>>>        ...
>>>>>> }
>>>>>> ...
>>>>>> entry = pte_mkyoung(entry);
>>>>>
>>>>> Do you mean pte_mkdirty() here? You're talking about dirty everywhere else.
>>>>
>>>> No, that is just thread 1 seeing "oh, nothing to do" and then goes ahead and
>>>> unconditionally does that in handle_pte_fault().
>>>>
>>>>>
>>>>>> if (ptep_set_access_flags(vmf->vma, ...)
>>>>>> ...
>>>>>> pte_unmap_unlock(vmf->pte, vmf->ptl);
>>>>>>
>>>>>>
>>>>>> Generic ptep_set_access_flags() will do another pte_same() check and realize
>>>>>> "hey, there was a change!" let's update the PTE!
>>>>>>
>>>>>> set_pte_at(vma->vm_mm, address, ptep, entry);
>>>>>
>>>>> This is called from the generic ptep_set_access_flags() in your example, right?
>>>>>
>>>>
>>>> Yes.
>>>>
>>>>>>
>>>>>> would overwrite the dirty bit set by thread 2.
>>>>>
>>>>> I'm not really sure what you are getting at... Is your concern that there is a
>>>>> race where the page could become dirty in the meantime and it now gets lost? I
>>>>> think that's why arm64 overrides ptep_set_access_flags(); since the hw can
>>>>> update access/dirty we have to deal with the races.
>>>>
>>>> My concern is that your patch can in subtle ways lead to use losing PTE dirty
>>>> bits on architectures that don't have the HW-managed dirty bit. They do exist ;)
>>>
>>> But I think the example you give can already happen today? Thread 1 reads
>>> orig_pte = ptep_get_lockless(). So that's already racy, if thread 2 is going to
>>> set dirty just after the get, then thread 1 is going to set the PTE back to (a
>>> modified version of) orig_pte. Isn't it already broken?
>>
>> No, because the pte_same() check under PTL would have detected it, and we would
>> have backed out. And I think the problem comes to live when we convert
>> pte_same()->pte_same_norecency(), because we fail to protect PTE access/dirty
>> changes that happend under PTL from another thread.
> 
> Ahh yep. Got it. I absolutely knew that you would be correct, but I still walked
> right into it!
> 
> I think one could argue that the generic ptep_set_access_flags() is not
> implementing its own spec:
> 
> "
> ... Only sets the access flags (dirty, accessed), as well as write permission.
> Furthermore, we know it always gets set to a "more permissive" setting ...
> "
> 
> Surely it should be folding the access and dirty bits from *ptep into entry if
> they are set?

Likely yes. Unless it's also used to clear access/dirty (don't think so, 
and would not be documented).

But the simplification made sense for now, because you previously 
checked that pte_same(), and nobody can modify it concurrently.

> 
> Regardless, I think this example proves that its fragile and subtle. I'm not
> really sure how to fix it more generally/robustly. Any thoughts? If not perhaps
> we are better off keeping ptep_get_lockless() around and only using
> ptep_get_lockless_norecency() for the really obviously correct cases?

Maybe one of the "sources of problems" is that we have a 
ptep_get_lockless_norecency() call followed by a pte_same() check, like 
done here.

Not the source of all problems I believe, though ...

-- 
Cheers,

David / dhildenb


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