Re: [PATCH v6 13/18] arm64/mm: Implement new wrprotect_ptes() batch API

[Catalin Marinas]

On Thu, Feb 15, 2024 at 10:32:00AM +0000, Ryan Roberts wrote:
> Optimize the contpte implementation to fix some of the fork performance
> regression introduced by the initial contpte commit. Subsequent patches
> will solve it entirely.
> 
> During fork(), any private memory in the parent must be write-protected.
> Previously this was done 1 PTE at a time. But the core-mm supports
> batched wrprotect via the new wrprotect_ptes() API. So let's implement
> that API and for fully covered contpte mappings, we no longer need to
> unfold the contpte. This has 2 benefits:
> 
>   - reduced unfolding, reduces the number of tlbis that must be issued.
>   - The memory remains contpte-mapped ("folded") in the parent, so it
>     continues to benefit from the more efficient use of the TLB after
>     the fork.
> 
> The optimization to wrprotect a whole contpte block without unfolding is
> possible thanks to the tightening of the Arm ARM in respect to the
> definition and behaviour when 'Misprogramming the Contiguous bit'. See
> section D21194 at https://developer.arm.com/documentation/102105/ja-07/
> 
> Tested-by: John Hubbard <jhubbard@xxxxxxxxxx>
> Signed-off-by: Ryan Roberts <ryan.roberts@xxxxxxx>

Acked-by: Catalin Marinas <catalin.marinas@xxxxxxx>