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Date: Mon, 12 Feb 2024 14:54:27 +0100
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Subject: Re: [PATCH v5 19/25] arm64/mm: Wire up PTE_CONT for user mappings
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To: Ryan Roberts <ryan.roberts@arm.com>, Mark Rutland <mark.rutland@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>, Will Deacon <will@kernel.org>,
 Ard Biesheuvel <ardb@kernel.org>, Marc Zyngier <maz@kernel.org>,
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From: David Hildenbrand <david@redhat.com>
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>> If so, I wonder if we could instead do that comparison modulo the access/dirty
>> bits,
> 
> I think that would work - but will need to think a bit more on it.
> 
>> and leave ptep_get_lockless() only reading a single entry?
> 
> I think we will need to do something a bit less fragile. ptep_get() does collect
> the access/dirty bits so its confusing if ptep_get_lockless() doesn't IMHO. So
> we will likely want to rename the function and make its documentation explicit
> that it does not return those bits.
> 
> ptep_get_lockless_noyoungdirty()? yuk... Any ideas?
> 
> Of course if I could convince you the current implementation is safe, I might be
> able to sidestep this optimization until a later date?

As discussed (and pointed out abive), there might be quite some 
callsites where we don't really care about uptodate accessed/dirty bits 
-- where ptep_get() is used nowadays.

One way to approach that I had in mind was having an explicit interface:

ptep_get()
ptep_get_uptodate()
ptep_get_lockless()
ptep_get_lockless_uptodate()

Especially the last one might not be needed.

Futher, "uptodate" might not be the best choice because of 
PageUptodate() and friends. But it's better than 
"youngdirty"/"noyoungdirty" IMHO.

Of course, any such changes require care and are better done one step at 
at time separately.

-- 
Cheers,

David / dhildenb