Re: [PATCH v6 16/18] arm64/mm: Implement pte_batch_hint()

[Catalin Marinas]

On Thu, Feb 15, 2024 at 10:32:03AM +0000, Ryan Roberts wrote:
> When core code iterates over a range of ptes and calls ptep_get() for
> each of them, if the range happens to cover contpte mappings, the number
> of pte reads becomes amplified by a factor of the number of PTEs in a
> contpte block. This is because for each call to ptep_get(), the
> implementation must read all of the ptes in the contpte block to which
> it belongs to gather the access and dirty bits.
> 
> This causes a hotspot for fork(), as well as operations that unmap
> memory such as munmap(), exit and madvise(MADV_DONTNEED). Fortunately we
> can fix this by implementing pte_batch_hint() which allows their
> iterators to skip getting the contpte tail ptes when gathering the batch
> of ptes to operate on. This results in the number of PTE reads returning
> to 1 per pte.
> 
> Acked-by: Mark Rutland <mark.rutland@xxxxxxx>
> Reviewed-by: David Hildenbrand <david@xxxxxxxxxx>
> Tested-by: John Hubbard <jhubbard@xxxxxxxxxx>
> Signed-off-by: Ryan Roberts <ryan.roberts@xxxxxxx>

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