Re: [PATCH v2 2/6] mm/rmap: use huge_ptep_get() in try_to_unmap_one()

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

On 7/2/26 07:13, Dev Jain wrote:
> try_to_unmap_one() handles hugetlb folios when memory failure needs
> to replace a poisoned hugetlb mapping with a hwpoison entry. In that
> case page_vma_mapped_walk() returns the pte pointer to the hugetlb folio
> in pvmw.pte, but the code reads it 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.
> 
> Just use huge_ptep_get() for dereferencing a huge pte pointer.
> 
> Fixes: c7ab0d2fdc84 ("mm: convert try_to_unmap_one() to use page_vma_mapped_walk()")
> Cc: stable@vger.kernel.org
> Reported-by: David Hildenbrand <david@kernel.org>
> Signed-off-by: Dev Jain <dev.jain@arm.com>
> ---
>  include/linux/hugetlb.h |  3 +++
>  mm/rmap.c               | 16 ++++++++++------
>  2 files changed, 13 insertions(+), 6 deletions(-)
> 
> diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
> index 2abaf99321e90..fdb7bdf7645c5 100644
> --- a/include/linux/hugetlb.h
> +++ b/include/linux/hugetlb.h
> @@ -1261,6 +1261,9 @@ static inline void hugetlb_count_sub(long l, struct mm_struct *mm)
>  {
>  }
>  
> +pte_t huge_ptep_get(struct mm_struct *mm, unsigned long addr,
> +		    pte_t *ptep);

Two tabs, or just in a single line.

If others prefer a stub, I don't care. This here is shortest to let the linker
bail out.

> +
>  static inline pte_t huge_ptep_clear_flush(struct vm_area_struct *vma,
>  					  unsigned long addr, pte_t *ptep)
>  {
> diff --git a/mm/rmap.c b/mm/rmap.c
> index 1c77d5dc06e9f..aa8a254efaecc 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -2095,11 +2095,16 @@ static bool try_to_unmap_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.
> -		 */

That comment now actually belongs above the pte_present() check below.


-- 
Cheers,

David