On 26/03/2024 17:39, David Hildenbrand wrote: > 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? I think I decided I was penalizing the architectures that don't care because all their ptep_get_norecency() and ptep_get_lockless_norecency() need to explicitly clear access/dirty. And I would have needed ptep_get_norecency() from day 1 so that I could feed its result into pte_same().
