linux-kernel - Re: [PATCH] mm/damon: Validate if the pmd entry is present before accessing

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Date:   Thu, 18 Aug 2022 10:41:10 +0800
From:   Muchun Song <muchun.song@...ux.dev>
To:     Baolin Wang <baolin.wang@...ux.alibaba.com>
Cc:     sj@...nel.org, Andrew Morton <akpm@...ux-foundation.org>,
        damon@...ts.linux.dev, Linux MM <linux-mm@...ck.org>,
        linux-kernel@...r.kernel.org,
        Mike Kravetz <mike.kravetz@...cle.com>
Subject: Re: [PATCH] mm/damon: Validate if the pmd entry is present before
 accessing



> On Aug 17, 2022, at 14:21, Baolin Wang <baolin.wang@...ux.alibaba.com> wrote:
> 
> The pmd_huge() is used to validate if the pmd entry is mapped by a huge
> page, also including the case of non-present (migration or hwpoisoned)
> pmd entry on arm64 or x86 architectures. Thus we should validate if it
> is present before making the pmd entry old or getting young state,
> otherwise we can not get the correct corresponding page.
> 
> Signed-off-by: Baolin Wang <baolin.wang@...ux.alibaba.com>
> ---
> mm/damon/vaddr.c | 10 ++++++++++
> 1 file changed, 10 insertions(+)
> 
> diff --git a/mm/damon/vaddr.c b/mm/damon/vaddr.c
> index 3c7b9d6..1d16c6c 100644
> --- a/mm/damon/vaddr.c
> +++ b/mm/damon/vaddr.c
> @@ -304,6 +304,11 @@ static int damon_mkold_pmd_entry(pmd_t *pmd, unsigned long addr,
> 
> 	if (pmd_huge(*pmd)) {
> 		ptl = pmd_lock(walk->mm, pmd);
> +		if (!pmd_present(*pmd)) {

Unluckily, we should use pte_present here. See commit c9d398fa23788. We can use
huge_ptep_get() to get a hugetlb pte, so it’s better to put the check after
pmd_huge.

Cc Mike to make sure I am not missing something.

Muchun,
Thanks.

> +			spin_unlock(ptl);
> +			return 0;
> +		}
> +
> 		if (pmd_huge(*pmd)) {
> 			damon_pmdp_mkold(pmd, walk->mm, addr);
> 			spin_unlock(ptl);
> @@ -431,6 +436,11 @@ static int damon_young_pmd_entry(pmd_t *pmd, unsigned long addr,
> #ifdef CONFIG_TRANSPARENT_HUGEPAGE
> 	if (pmd_huge(*pmd)) {
> 		ptl = pmd_lock(walk->mm, pmd);
> +		if (!pmd_present(*pmd)) {
> +			spin_unlock(ptl);
> +			return 0;
> +		}
> +
> 		if (!pmd_huge(*pmd)) {
> 			spin_unlock(ptl);
> 			goto regular_page;
> -- 
> 1.8.3.1
> 
>