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

[Ryan Roberts <ryan.roberts@arm.com>]

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.