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

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Message-Id: <20230519195435.90394-1-sj@kernel.org>
Date:   Fri, 19 May 2023 19:54:35 +0000
From:   SeongJae Park <sj@...nel.org>
To:     Ryan Roberts <ryan.roberts@....com>
Cc:     Yu Zhao <yuzhao@...gle.com>,
        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 Fri, 19 May 2023 10:02:48 +0100 Ryan Roberts <ryan.roberts@....com> wrote:

> On 19/05/2023 00:19, Yu Zhao wrote:
> > 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.
> 
> Fair point. The first 3 patches are fixes for issues I found during the
> conversion. So if you're ok with it, I'll split into 2 series; the first 3
> patches in the first, and the conversion to ptep_deref() (or ptep_get(), as per
> your comments in the other thread) in the second. I guess the whole first series
> should go to stable.
> 
> Let me know if you disagree.
> 
> > 
> >> 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.
> 
> This looks sensible but I'd like clarification from SeongJae: Are you happy for
> me to do this refactoring as part of the patch?

Yes, I would be happy for that :)  Nevertheless, because that's a refactoring
rather than a fix, I'd be happier if you could do that with a separate patch.


Thanks,
SJ

> 
> Thanks,
> Ryan
>