linux-kernel - Re: [PATCH] arm64/hugetlb: Simplify the huge_ptep_set_access

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Message-ID: <Yqy97gXI4Nqb7dYo@arm.com>
Date:   Fri, 17 Jun 2022 18:46:22 +0100
From:   Catalin Marinas <catalin.marinas@....com>
To:     Baolin Wang <baolin.wang@...ux.alibaba.com>
Cc:     will@...nel.org, mike.kravetz@...cle.com, songmuchun@...edance.com,
        anshuman.khandual@....com, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH] arm64/hugetlb: Simplify the huge_ptep_set_access_flags()

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