Message-ID: <CAHVXubiBS-m91bpx-NbKdkcSk3HdStTxY8QLVTKxxozL2Od8Qw@mail.gmail.com>
Date: Mon, 24 Feb 2025 12:27:03 +0100
From: Alexandre Ghiti <alexghiti@...osinc.com>
To: Ryan Roberts <ryan.roberts@....com>
Cc: Catalin Marinas <catalin.marinas@....com>, Will Deacon <will@...nel.org>, 
	Huacai Chen <chenhuacai@...nel.org>, WANG Xuerui <kernel@...0n.name>, 
	Thomas Bogendoerfer <tsbogend@...ha.franken.de>, 
	"James E.J. Bottomley" <James.Bottomley@...senpartnership.com>, Helge Deller <deller@....de>, 
	Madhavan Srinivasan <maddy@...ux.ibm.com>, Michael Ellerman <mpe@...erman.id.au>, 
	Nicholas Piggin <npiggin@...il.com>, Christophe Leroy <christophe.leroy@...roup.eu>, 
	Naveen N Rao <naveen@...nel.org>, Paul Walmsley <paul.walmsley@...ive.com>, 
	Palmer Dabbelt <palmer@...belt.com>, Albert Ou <aou@...s.berkeley.edu>, 
	Heiko Carstens <hca@...ux.ibm.com>, Vasily Gorbik <gor@...ux.ibm.com>, 
	Alexander Gordeev <agordeev@...ux.ibm.com>, Christian Borntraeger <borntraeger@...ux.ibm.com>, 
	Sven Schnelle <svens@...ux.ibm.com>, Gerald Schaefer <gerald.schaefer@...ux.ibm.com>, 
	"David S. Miller" <davem@...emloft.net>, Andreas Larsson <andreas@...sler.com>, Arnd Bergmann <arnd@...db.de>, 
	Muchun Song <muchun.song@...ux.dev>, Andrew Morton <akpm@...ux-foundation.org>, 
	Uladzislau Rezki <urezki@...il.com>, Christoph Hellwig <hch@...radead.org>, 
	David Hildenbrand <david@...hat.com>, "Matthew Wilcox (Oracle)" <willy@...radead.org>, 
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	linux-arm-kernel@...ts.infradead.org, linux-mm@...ck.org, 
	linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH v2 2/4] arm64: hugetlb: Fix huge_ptep_get_and_clear() for
 non-present ptes

Hi Ryan,

On Mon, Feb 17, 2025 at 3:04 PM Ryan Roberts <ryan.roberts@....com> wrote:
>
> arm64 supports multiple huge_pte sizes. Some of the sizes are covered by
> a single pte entry at a particular level (PMD_SIZE, PUD_SIZE), and some
> are covered by multiple ptes at a particular level (CONT_PTE_SIZE,
> CONT_PMD_SIZE). So the function has to figure out the size from the
> huge_pte pointer. This was previously done by walking the pgtable to
> determine the level and by using the PTE_CONT bit to determine the
> number of ptes at the level.
>
> But the PTE_CONT bit is only valid when the pte is present. For
> non-present pte values (e.g. markers, migration entries), the previous
> implementation was therefore erroniously determining the size. There is
> at least one known caller in core-mm, move_huge_pte(), which may call
> huge_ptep_get_and_clear() for a non-present pte. So we must be robust to
> this case. Additionally the "regular" ptep_get_and_clear() is robust to
> being called for non-present ptes so it makes sense to follow the
> behaviour.
>
> Fix this by using the new sz parameter which is now provided to the
> function. Additionally when clearing each pte in a contig range, don't
> gather the access and dirty bits if the pte is not present.
>
> An alternative approach that would not require API changes would be to
> store the PTE_CONT bit in a spare bit in the swap entry pte for the
> non-present case. But it felt cleaner to follow other APIs' lead and
> just pass in the size.
>
> As an aside, PTE_CONT is bit 52, which corresponds to bit 40 in the swap
> entry offset field (layout of non-present pte). Since hugetlb is never
> swapped to disk, this field will only be populated for markers, which
> always set this bit to 0 and hwpoison swap entries, which set the offset
> field to a PFN; So it would only ever be 1 for a 52-bit PVA system where
> memory in that high half was poisoned (I think!). So in practice, this
> bit would almost always be zero for non-present ptes and we would only
> clear the first entry if it was actually a contiguous block. That's
> probably a less severe symptom than if it was always interpretted as 1
> and cleared out potentially-present neighboring PTEs.
>
> Cc: stable@...r.kernel.org
> Fixes: 66b3923a1a0f ("arm64: hugetlb: add support for PTE contiguous bit")
> Signed-off-by: Ryan Roberts <ryan.roberts@....com>
> ---
>  arch/arm64/mm/hugetlbpage.c | 40 ++++++++++++++++---------------------
>  1 file changed, 17 insertions(+), 23 deletions(-)
>
> diff --git a/arch/arm64/mm/hugetlbpage.c b/arch/arm64/mm/hugetlbpage.c
> index 06db4649af91..614b2feddba2 100644
> --- a/arch/arm64/mm/hugetlbpage.c
> +++ b/arch/arm64/mm/hugetlbpage.c
> @@ -163,24 +163,23 @@ static pte_t get_clear_contig(struct mm_struct *mm,
>                              unsigned long pgsize,
>                              unsigned long ncontig)
>  {
> -       pte_t orig_pte = __ptep_get(ptep);
> -       unsigned long i;
> -
> -       for (i = 0; i < ncontig; i++, addr += pgsize, ptep++) {
> -               pte_t pte = __ptep_get_and_clear(mm, addr, ptep);
> -
> -               /*
> -                * If HW_AFDBM is enabled, then the HW could turn on
> -                * the dirty or accessed bit for any page in the set,
> -                * so check them all.
> -                */
> -               if (pte_dirty(pte))
> -                       orig_pte = pte_mkdirty(orig_pte);
> -
> -               if (pte_young(pte))
> -                       orig_pte = pte_mkyoung(orig_pte);
> +       pte_t pte, tmp_pte;
> +       bool present;
> +
> +       pte = __ptep_get_and_clear(mm, addr, ptep);
> +       present = pte_present(pte);
> +       while (--ncontig) {
> +               ptep++;
> +               addr += pgsize;
> +               tmp_pte = __ptep_get_and_clear(mm, addr, ptep);
> +               if (present) {
> +                       if (pte_dirty(tmp_pte))
> +                               pte = pte_mkdirty(pte);
> +                       if (pte_young(tmp_pte))
> +                               pte = pte_mkyoung(pte);
> +               }
>         }
> -       return orig_pte;
> +       return pte;
>  }
>
>  static pte_t get_clear_contig_flush(struct mm_struct *mm,
> @@ -401,13 +400,8 @@ pte_t huge_ptep_get_and_clear(struct mm_struct *mm, unsigned long addr,
>  {
>         int ncontig;
>         size_t pgsize;
> -       pte_t orig_pte = __ptep_get(ptep);
> -
> -       if (!pte_cont(orig_pte))
> -               return __ptep_get_and_clear(mm, addr, ptep);
> -
> -       ncontig = find_num_contig(mm, addr, ptep, &pgsize);
>
> +       ncontig = num_contig_ptes(sz, &pgsize);
>         return get_clear_contig(mm, addr, ptep, pgsize, ncontig);
>  }
>
> --
> 2.43.0
>

Thanks for ccing me on this fix, we have the same issue (in
huge_pte_clear() too) in riscv. I'll come up with a fix and wait for
you guys to merge this.

That only comforts my idea to merge both riscv and arm64 implementations :)

Thanks again,

Alex

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