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?
--
Cheers,
David / dhildenb
