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@nvidia.com>
Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>

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