Re: [RFC PATCH v1 0/4] Reduce cost of ptep_get_lockless on arm64

[David Hildenbrand]

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