Re: [PATCH] arm64/hugetlb: Simplify the huge_ptep_set_access_flags()

[Will Deacon <will@kernel.org>]

On Wed, May 25, 2022 at 06:31:09PM +0800, Baolin Wang wrote:
> After commit bc5dfb4fd7bd ("arm64/hugetlb: Implement arm64 specific
> huge_ptep_get()"), the arm64 specific huge_ptep_get() will always
> consider the subpages' dirty and young state for CONT-PTE/PMD hugetlb,
> so there is no need to check them again when setting the access flags
> for CONT-PTE/PMD hugetlb in huge_ptep_set_access_flags().
> 
> Meanwhile this also fixes an issue when users want to make the CONT-PTE/PMD
> hugetlb's pte entry old, which will be failed to make the pte entry old
> since the original code will always consider the subpages' young state
> if the subpages' young state is set. For example, we will make the
> CONT-PTE/PMD hugetlb pte entry old in DAMON to monitoring the accesses,
> but we'll failed to monitoring the actual accesses of the CONT-PTE/PMD
> hugetlb page, due to we can not make its pte old.
> 
> Thus remove the code considering the subpages' dirty and young state in
> huge_ptep_set_access_flags() to fix this issue and simplify the function.
> 
> Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
> ---
>  arch/arm64/mm/hugetlbpage.c | 10 +---------
>  1 file changed, 1 insertion(+), 9 deletions(-)
> 
> diff --git a/arch/arm64/mm/hugetlbpage.c b/arch/arm64/mm/hugetlbpage.c
> index e2a5ec9..5c703aa 100644
> --- a/arch/arm64/mm/hugetlbpage.c
> +++ b/arch/arm64/mm/hugetlbpage.c
> @@ -448,7 +448,6 @@ int huge_ptep_set_access_flags(struct vm_area_struct *vma,
>  	size_t pgsize = 0;
>  	unsigned long pfn = pte_pfn(pte), dpfn;
>  	pgprot_t hugeprot;
> -	pte_t orig_pte;
>  
>  	if (!pte_cont(pte))
>  		return ptep_set_access_flags(vma, addr, ptep, pte, dirty);
> @@ -459,14 +458,7 @@ int huge_ptep_set_access_flags(struct vm_area_struct *vma,
>  	if (!__cont_access_flags_changed(ptep, pte, ncontig))
>  		return 0;
>  
> -	orig_pte = get_clear_contig(vma->vm_mm, addr, ptep, pgsize, ncontig);
> -
> -	/* Make sure we don't lose the dirty or young state */
> -	if (pte_dirty(orig_pte))
> -		pte = pte_mkdirty(pte);
> -
> -	if (pte_young(orig_pte))
> -		pte = pte_mkyoung(pte);
> +	clear_flush(vma->vm_mm, addr, ptep, pgsize, ncontig);

I don't understand what this clear_flush() call is doing here; notably, it
includes TLB invalidation which we don't have for the non-cont case.

Why isn't huge_ptep_set_access_flags() just a loop around
ptep_set_access_flags() if huge_ptep_get() is taking care of collapsing the
dirty/young state?

Will