Re: [PATCH v2 2/4] arm64: hugetlb: Fix huge_ptep_get_and_clear() for non-present ptes

[Anshuman Khandual <anshuman.khandual@arm.com>]

On 2/17/25 19:34, 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@vger.kernel.org
> Fixes: 66b3923a1a0f ("arm64: hugetlb: add support for PTE contiguous bit")
> Signed-off-by: Ryan Roberts <ryan.roberts@arm.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);

pte_present() may not be evaluated for standard huge pages at [PMD|PUD]_SIZE
e.g when ncontig = 1 in the argument.

> +	while (--ncontig) {

Should this be converted into a for loop instead just to be in sync with other
similar iterators in this file.

for (i = 1; i < ncontig; i++, addr += pgsize, ptep++)
{
	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);
	}
}

> +		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);
>  }
>