Re: [PATCH v2 3/6] mm/rmap: use huge_ptep_get() in try_to_migrate_one()

["David Hildenbrand (Arm)" <david@kernel.org>]

On 7/2/26 07:13, Dev Jain wrote:
> try_to_migrate_one() is used by folio migration to replace a present
> mapping with a migration entry. For hugetlb folios, page_vma_mapped_walk()
> returns the pte pointer to the hugetlb folio in pvmw.pte, but the code
> reads the huge pte entry with ptep_get().
> 
> On arches which provide their own huge_ptep_get() to dereference a huge
> pte pointer, accessing via ptep_get() would cause pte_pfn(), pte_present()
> etc to misbehave.
> 
> It is not clear whether this has a trivially visible effect to userspace.
> 
> Use huge_ptep_get() to dereference a huge pte pointer.
> 
> Commit a98a2f0c8ce1 copied the bug from try_to_unmap_one into
> try_to_migrate_one.
> 
> Fixes: a98a2f0c8ce1 ("mm/rmap: split migration into its own function")
> Cc: stable@vger.kernel.org
> Acked-by: Muchun Song <muchun.song@linux.dev>
> Signed-off-by: Dev Jain <dev.jain@arm.com>
> ---
>  mm/rmap.c | 16 ++++++++++------
>  1 file changed, 10 insertions(+), 6 deletions(-)
> 
> diff --git a/mm/rmap.c b/mm/rmap.c
> index aa8a254efaecc..abc3a44baaa3d 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -2505,11 +2505,16 @@ static bool try_to_migrate_one(struct folio *folio, struct vm_area_struct *vma,
>  		/* Unexpected PMD-mapped THP? */
>  		VM_BUG_ON_FOLIO(!pvmw.pte, folio);
>  
> -		/*
> -		 * Handle PFN swap PTEs, such as device-exclusive ones, that
> -		 * actually map pages.
> -		 */
> -		pteval = ptep_get(pvmw.pte);
> +		address = pvmw.address;
> +		if (folio_test_hugetlb(folio)) {
> +			pteval = huge_ptep_get(mm, address, pvmw.pte);
> +		} else {
> +			/*
> +			 * Handle PFN swap PTEs, such as device-exclusive ones,
> +			 * that actually map pages.
> +			 */

Again, the comment is related to the pte_present() handling, so best to move it
down there.

Apart from that LGTM.

-- 
Cheers,

David