Re: [RFC PATCH v1 0/4] Reduce cost of ptep_get_lockless on arm64

[David Hildenbrand]

On 26.03.24 18:32, Ryan Roberts wrote:
> On 26/03/2024 17:04, David Hildenbrand wrote:
> > > > > > Likely, we just want to read "the real deal" on both sides of the pte_same()
> > > > > > handling.
> > > > >
> > > > > Sorry I'm not sure I understand? You mean read the full pte including
> > > > > access/dirty? That's the same as dropping the patch, right? Of course if we do
> > > > > that, we still have to keep pte_get_lockless() around for this case. In an
> > > > > ideal
> > > > > world we would convert everything over to ptep_get_lockless_norecency() and
> > > > > delete ptep_get_lockless() to remove the ugliness from arm64.
> > > >
> > > > Yes, agreed. Patch #3 does not look too crazy and it wouldn't really affect any
> > > > architecture.
> > > >
> > > > I do wonder if pte_same_norecency() should be defined per architecture and the
> > > > default would be pte_same(). So we could avoid the mkold etc on all other
> > > > architectures.
> > >
> > > Wouldn't that break it's semantics? The "norecency" of
> > > ptep_get_lockless_norecency() means "recency information in the returned pte may
> > > be incorrect". But the "norecency" of pte_same_norecency() means "ignore the
> > > access and dirty bits when you do the comparison".
> >
> > My idea was that ptep_get_lockless_norecency() would return the actual result on
> > these architectures. So e.g., on x86, there would be no actual change in
> > generated code.
>
> I think this is a bad plan... You'll end up with subtle differences between
> architectures.
>
> > But yes, the documentation of these functions would have to be improved.
> >
> > Now I wonder if ptep_get_lockless_norecency() should actively clear
> > dirty/accessed bits to more easily find any actual issues where the bits still
> > matter ...
>
> I did a version that took that approach. Decided it was not as good as this way
> though. Now for the life of me, I can't remember my reasoning.

Maybe because there are some code paths that check accessed/dirty 
without "correctness" implications? For example, if the PTE is already 
dirty, no need to set it dirty etc?

--
Cheers,

David / dhildenb