On 26/03/2024 16:34, David Hildenbrand wrote: > On 26.03.24 17:31, Ryan Roberts wrote: >> On 26/03/2024 16:17, David Hildenbrand wrote: >>> On 15.02.24 13:17, Ryan Roberts wrote: >>>> This is an RFC for a series that aims to reduce the cost and complexity of >>>> ptep_get_lockless() for arm64 when supporting transparent contpte mappings [1]. >>>> The approach came from discussion with Mark and David [2]. >>>> >>>> It introduces a new helper, ptep_get_lockless_norecency(), which allows the >>>> access and dirty bits in the returned pte to be incorrect. This relaxation >>>> permits arm64's implementation to just read the single target pte, and avoids >>>> having to iterate over the full contpte block to gather the access and dirty >>>> bits, for the contpte case. >>>> >>>> It turns out that none of the call sites using ptep_get_lockless() require >>>> accurate access and dirty bit information, so we can also convert those sites. >>>> Although a couple of places need care (see patches 2 and 3). >>>> >>>> Arguably patch 3 is a bit fragile, given the wide accessibility of >>>> vmf->orig_pte. So it might make sense to drop this patch and stick to using >>>> ptep_get_lockless() in the page fault path. I'm keen to hear opinions. >>> >>> Yes. Especially as we have these pte_same() checks that might just fail now >>> because of wrong accessed/dirty bits? >> >> Which pte_same() checks are you referring to? I've changed them all to >> pte_same_norecency() which ignores the access/dirty bits when doing the >> comparison. > > I'm reading the patches just now. So I stumbled over that just after I wrote > that, so I was missing that part from the description here. > >> >>> >>> 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". I think you could only do the optimization you describe if you required that pte_same_norecency() would only be given values returned by ptep_get_lockless_norecency() (or ptep_get_norecency()). Even then, its not quite the same; if a page is accessed between gets one will return true and the other false.
