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

[Ryan Roberts]

On 26/03/2024 17:02, David Hildenbrand wrote:
> On 15.02.24 13:17, Ryan Roberts wrote:
>> Let's convert handle_pte_fault()'s use of ptep_get_lockless() to
>> ptep_get_lockless_norecency() to save orig_pte.
>>
>> There are a number of places that follow this model:
>>
>>      orig_pte = ptep_get_lockless(ptep)
>>      ...
>>      <lock>
>>      if (!pte_same(orig_pte, ptep_get(ptep)))
>>              // RACE!
>>      ...
>>      <unlock>
>>
>> So we need to be careful to convert all of those to use
>> pte_same_norecency() so that the access and dirty bits are excluded from
>> the comparison.
>>
>> Additionally there are a couple of places that genuinely rely on the
>> access and dirty bits of orig_pte, but with some careful refactoring, we
>> can use ptep_get() once we are holding the lock to achieve equivalent
>> logic.
> 
> We really should document that changed behavior somewhere where it can be easily
> found: that orig_pte might have incomplete/stale accessed/dirty information.

I could add it to the orig_pte definition in the `struct vm_fault`?

> 
> 
>> @@ -5343,7 +5356,7 @@ static vm_fault_t handle_pte_fault(struct vm_fault *vmf)
>>                            vmf->address, &vmf->ptl);
>>           if (unlikely(!vmf->pte))
>>               return 0;
>> -        vmf->orig_pte = ptep_get_lockless(vmf->pte);
>> +        vmf->orig_pte = ptep_get_lockless_norecency(vmf->pte);
>>           vmf->flags |= FAULT_FLAG_ORIG_PTE_VALID;
>>
>>           if (pte_none(vmf->orig_pte)) {
>> @@ -5363,7 +5376,7 @@ static vm_fault_t handle_pte_fault(struct vm_fault *vmf)
>>
>>       spin_lock(vmf->ptl);
>>       entry = vmf->orig_pte;
>> -    if (unlikely(!pte_same(ptep_get(vmf->pte), entry))) {
>> +    if (unlikely(!pte_same_norecency(ptep_get(vmf->pte), entry))) {
>>           update_mmu_tlb(vmf->vma, vmf->address, vmf->pte);
>>           goto unlock;
> 
> I was wondering about the following:
> 
> Assume the PTE is not dirty.
> 
> Thread 1 does

Sorry not sure what threads have to do with this? How is the vmf shared between
threads? What have I misunderstood...

> 
> vmf->orig_pte = ptep_get_lockless_norecency(vmf->pte)
> /* not dirty */
> 
> /* Now, thread 2 ends up setting the PTE dirty under PT lock. */
> 
> 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.

> 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?

> 
> 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.