linux-kernel - Re: [PATCH v2 2/5] mm: damon must atomically clear young on ptes and pmds

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Message-ID: <CAOUHufYXuH+mUNAtiRbAqqKTKO-nGR248Z2MVgGzMJqYu8yawQ@mail.gmail.com>
Date:   Thu, 18 May 2023 17:19:48 -0600
From:   Yu Zhao <yuzhao@...gle.com>
To:     Ryan Roberts <ryan.roberts@....com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        SeongJae Park <sj@...nel.org>,
        Christoph Hellwig <hch@...radead.org>,
        "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Lorenzo Stoakes <lstoakes@...il.com>,
        Uladzislau Rezki <urezki@...il.com>, Zi Yan <ziy@...dia.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        damon@...ts.linux.dev
Subject: Re: [PATCH v2 2/5] mm: damon must atomically clear young on ptes and pmds

On Thu, May 18, 2023 at 5:07 AM Ryan Roberts <ryan.roberts@....com> wrote:
>
> It is racy to non-atomically read a pte, then clear the young bit, then
> write it back as this could discard dirty information. Further, it is
> bad practice to directly set a pte entry within a table. Instead
> clearing young must go through the arch-provided helper,
> ptep_test_and_clear_young() to ensure it is modified atomically and to
> give the arch code visibility and allow it to check (and potentially
> modify) the operation.
>
> Fixes: 46c3a0accdc4 ("mm/damon/vaddr: separate commonly usable functions")

This should be a separate patch, since it's independent from what the
series tries to do.

And that patch should cc stable, since it fixes user data corruption.

> Signed-off-by: Ryan Roberts <ryan.roberts@....com>
> Reviewed-by: Zi Yan <ziy@...dia.com>
> ---
>  mm/damon/ops-common.c | 16 ++++++----------
>  mm/damon/ops-common.h |  4 ++--
>  mm/damon/paddr.c      |  4 ++--
>  mm/damon/vaddr.c      |  4 ++--
>  4 files changed, 12 insertions(+), 16 deletions(-)
>
> diff --git a/mm/damon/ops-common.c b/mm/damon/ops-common.c
> index cc63cf953636..acc264b97903 100644
> --- a/mm/damon/ops-common.c
> +++ b/mm/damon/ops-common.c
> @@ -37,7 +37,7 @@ struct folio *damon_get_folio(unsigned long pfn)
>         return folio;
>  }
>
> -void damon_ptep_mkold(pte_t *pte, struct mm_struct *mm, unsigned long addr)
> +void damon_ptep_mkold(pte_t *pte, struct vm_area_struct *vma, unsigned long addr)
>  {
>         bool referenced = false;
>         struct folio *folio = damon_get_folio(pte_pfn(*pte));
> @@ -45,13 +45,11 @@ void damon_ptep_mkold(pte_t *pte, struct mm_struct *mm, unsigned long addr)
>         if (!folio)
>                 return;
>
> -       if (pte_young(*pte)) {
> +       if (ptep_test_and_clear_young(vma, addr, pte))
>                 referenced = true;
> -               *pte = pte_mkold(*pte);
> -       }
>
>  #ifdef CONFIG_MMU_NOTIFIER
> -       if (mmu_notifier_clear_young(mm, addr, addr + PAGE_SIZE))
> +       if (mmu_notifier_clear_young(vma->vm_mm, addr, addr + PAGE_SIZE))
>                 referenced = true;
>  #endif /* CONFIG_MMU_NOTIFIER */

Use ptep_clear_young_notify(). Similar below.