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

[David Hildenbrand]

> > > > 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.

But could be I am missing something :)

> > Arm64 should be fine in that regard.
>
> There is plenty of arm64 HW that doesn't do HW access/dirty update. But our
> ptep_set_access_flags() can always deal with a racing update, even if that
> update originates from SW.
>
> Why do I have the feeling you're about to explain (very patiently) exactly why
> I'm wrong?... :)

heh ... or you'll tell me (vary patiently) why I am wrong :)

--
Cheers,

David / dhildenb