Message-ID: <20250221153156.GC20567@willie-the-truck>
Date: Fri, 21 Feb 2025 15:31:56 +0000
From: Will Deacon <will@...nel.org>
To: Ryan Roberts <ryan.roberts@....com>
Cc: Catalin Marinas <catalin.marinas@....com>,
	Huacai Chen <chenhuacai@...nel.org>,
	WANG Xuerui <kernel@...0n.name>,
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	linux-arm-kernel@...ts.infradead.org, linux-mm@...ck.org,
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Subject: Re: [PATCH v2 2/4] arm64: hugetlb: Fix huge_ptep_get_and_clear() for
 non-present ptes

On Mon, Feb 17, 2025 at 02:04:15PM +0000, Ryan Roberts 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);
> +		}
>  	}

nit: With the loop now structured like this, we really can't handle
num_contig_ptes() returning 0 if it gets an unknown size. Granted, that
really shouldn't happen, but perhaps it would be better to add a 'default'
case with a WARN() to num_contig_ptes() and then add an early return here?

Will

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