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

[Catalin Marinas <catalin.marinas@arm.com>]

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.

The ptep_set_access_flags() semantics (non-huge) never clear the access
flag, so mkold is not allowed. I think damon_hugetlb_mkold() is wrong in
assuming that huge_ptep_set_access_flags() allows a young->old huge pte
transition.

-- 
Catalin