[PATCH v2] mm: arm64: document why pte is not advanced in contpte_ptep_set_access_flags()

[David Hildenbrand]

On 05.09.24 10:11, Barry Song wrote:
> From: Barry Song <v-songbaohua at oppo.com>
> 
> According to David and Ryan, there isn't a bug here, even though we
> don't advance the PTE entry, because __ptep_set_access_flags() only
> uses the access flags from the entry.
> 
> However, we always check pte_same(pte, entry) using the first entry
> in __ptep_set_access_flags(). This means that the checks from 1 to
> nr - 1 are not comparing the same PTE indexes (thus, they always
> return false), which can be a bit confusing. To clarify the code, let's
> add some comments.
> 
> Reviewed-by: Ryan Roberts <ryan.roberts at arm.com>
> Signed-off-by: Barry Song <v-songbaohua at oppo.com>
> Cc: Ard Biesheuvel <ardb at kernel.org>
> Cc: John Hubbard <jhubbard at nvidia.com>
> Cc: Mark Rutland <mark.rutland at arm.com>
> Cc: Catalin Marinas <catalin.marinas at arm.com>
> Cc: David Hildenbrand <david at redhat.com>
> Cc: Will Deacon <will at kernel.org>
> ---
>   -v2:
>   * collect Ryan's rb, thanks!
>   * doc why but not advance entry
>   * refine changelog and subject
> 
>   -v1:
>   https://lore.kernel.org/linux-mm/20240831083537.62111-1-21cnbao@gmail.com/
> 
>   arch/arm64/mm/contpte.c | 6 ++++++
>   1 file changed, 6 insertions(+)
> 
> diff --git a/arch/arm64/mm/contpte.c b/arch/arm64/mm/contpte.c
> index a3edced29ac1..55107d27d3f8 100644
> --- a/arch/arm64/mm/contpte.c
> +++ b/arch/arm64/mm/contpte.c
> @@ -421,6 +421,12 @@ int contpte_ptep_set_access_flags(struct vm_area_struct *vma,
>   		ptep = contpte_align_down(ptep);
>   		start_addr = addr = ALIGN_DOWN(addr, CONT_PTE_SIZE);
>   
> +		/*
> +		 * We are not advancing entry because __ptep_set_access_flags()
> +		 * only consumes access flags from entry. And since we have checked
> +		 * for the whole contpte block and returned early, pte_same()
> +		 * within __ptep_set_access_flags() is likely false.
> +		 */
>   		for (i = 0; i < CONT_PTES; i++, ptep++, addr += PAGE_SIZE)
>   			__ptep_set_access_flags(vma, addr, ptep, entry, 0);
>   

Reviewed-by: David Hildenbrand <david at redhat.com>

-- 
Cheers,

David / dhildenb