Messages in this thread | | | Date | Tue, 26 Mar 2024 17:34:43 +0100 | Subject | Re: [RFC PATCH v1 0/4] Reduce cost of ptep_get_lockless on arm64 | From | David Hildenbrand <> |
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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.
-- Cheers,
David / dhildenb
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