On 12/02/2024 16:24, David Hildenbrand wrote: > On 12.02.24 16:34, Ryan Roberts wrote: >> On 12/02/2024 15:26, David Hildenbrand wrote: >>> On 12.02.24 15:45, Ryan Roberts wrote: >>>> On 12/02/2024 13:54, David Hildenbrand wrote: >>>>>>> If so, I wonder if we could instead do that comparison modulo the >>>>>>> access/dirty >>>>>>> bits, >>>>>> >>>>>> I think that would work - but will need to think a bit more on it. >>>>>> >>>>>>> and leave ptep_get_lockless() only reading a single entry? >>>>>> >>>>>> I think we will need to do something a bit less fragile. ptep_get() does >>>>>> collect >>>>>> the access/dirty bits so its confusing if ptep_get_lockless() doesn't >>>>>> IMHO. So >>>>>> we will likely want to rename the function and make its documentation >>>>>> explicit >>>>>> that it does not return those bits. >>>>>> >>>>>> ptep_get_lockless_noyoungdirty()? yuk... Any ideas? >>>>>> >>>>>> Of course if I could convince you the current implementation is safe, I >>>>>> might be >>>>>> able to sidestep this optimization until a later date? >>>>> >>>>> As discussed (and pointed out abive), there might be quite some callsites >>>>> where >>>>> we don't really care about uptodate accessed/dirty bits -- where ptep_get() is >>>>> used nowadays. >>>>> >>>>> One way to approach that I had in mind was having an explicit interface: >>>>> >>>>> ptep_get() >>>>> ptep_get_uptodate() >>>>> ptep_get_lockless() >>>>> ptep_get_lockless_uptodate() >>>> >>>> Yes, I like the direction of this. I guess we anticipate that call sites >>>> requiring the "_uptodate" variant will be the minority so it makes sense to use >>>> the current names for the "_not_uptodate" variants? But to do a slow migration, >>>> it might be better/safer to have the weaker variant use the new name - that >>>> would allow us to downgrade one at a time? >>> >>> Yes, I was primarily struggling with names. Likely it makes sense to either have >>> two completely new function names, or use the new name only for the "faster but >>> less precise" variant. >>> >>>> >>>>> >>>>> Especially the last one might not be needed. >>>> I've done a scan through the code and agree with Mark's original conclusions. >>>> Additionally, huge_pte_alloc() (which isn't used for arm64) doesn't rely on >>>> access/dirty info. So I think I could migrate everything to the weaker variant >>>> fairly easily. >>>> >>>>> >>>>> Futher, "uptodate" might not be the best choice because of PageUptodate() and >>>>> friends. But it's better than "youngdirty"/"noyoungdirty" IMHO. >>>> >>>> Certainly agree with "noyoungdirty" being a horrible name. How about "_sync" / >>>> "_nosync"? >>> >>> I could live with >>> >>> ptep_get_sync() >>> ptep_get_nosync() >>> >>> with proper documentation :) >> >> but could you live with: >> >> ptep_get() >> ptep_get_nosync() >> ptep_get_lockless_nosync() >> >> ? >> >> So leave the "slower, more precise" version with the existing name. > > Sure. > I'm just implementing this (as a separate RFC), and had an alternative idea for naming/semantics: ptep_get() ptep_get_norecency() ptep_get_lockless() ptep_get_lockless_norecency() The "_norecency" versions explicitly clear the access/dirty bits. This is useful for the "compare to original pte to check we are not racing" pattern: pte = ptep_get_lockless_norecency(ptep) ... <lock> if (!pte_same(pte, ptep_get_norecency(ptep))) // RACE! ... <unlock> With the "_nosync" semantic, the access/dirty bits may or may not be set, so the user has to explicitly clear them to do the comparison. (although I considered a pte_same_nosync() that would clear the bits for you - but that name is pretty naff). Although the _norecency semantic requires always explicitly clearing the bits, so may be infinitesimally slower, it gives a very clear expectation that the access/dirty bits are always clear and I think that's conveyed well in the name too. Thoughts?
