On 27/03/2024 09:34, David Hildenbrand wrote: > On 26.03.24 18:51, Ryan Roberts wrote: >> 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(). > > True. With ptep_get_norecency() you're also penalizing other architectures. > Therefore my original thought about making the behavior arch-specific, but the > arch has to make sure to get the combination of > ptep_get_lockless_norecency()+ptep_same_norecency() is right. > > So if an arch decide to ignore bits in ptep_get_lockless_norecency(), it must > make sure to also ignore them in ptep_same_norecency(), and must be able to > handle access/dirty bit changes differently. > > Maybe one could have one variant for "hw-managed access/dirty" vs. "sw managed > accessed or dirty". Only the former would end up ignoring stuff here, the latter > would not. > > But again, just some random thoughts how this affects other architectures and > how we could avoid it. The issue I describe in patch #3 would be gone if > ptep_same_norecency() would just do a ptep_same() check on other architectures > -- and would make it easier to sell :) Perhaps - let me chew on that for a bit. It doesn't feel as easy as you suggest to me. But I can't put my finger on why...
