Re: [PATCH v1 1/4] mm/hugetlb: fix hugetlb_pmd_shared()

[Lorenzo Stoakes]

On Fri, Dec 05, 2025 at 10:35:55PM +0100, David Hildenbrand (Red Hat) wrote:
> We switched from (wrongly) using the page count to an independent
> shared count. Now, shared page tables have a refcount of 1 (excluding
> speculative references) and instead use ptdesc->pt_share_count to
> identify sharing.
>
> We didn't convert hugetlb_pmd_shared(), so right now, we would never
> detect a shared PMD table as such, because sharing/unsharing no longer
> touches the refcount of a PMD table.
>
> Page migration, like mbind() or migrate_pages() would allow for migrating
> folios mapped into such shared PMD tables, even though the folios are
> not exclusive. In smaps we would account them as "private" although they
> are "shared", and we would be wrongly setting the PM_MMAP_EXCLUSIVE in the
> pagemap interface.

Yikes this seems pretty serious!!

How did we not pick up on this before...

>
> Fix it by properly using ptdesc_pmd_is_shared() in hugetlb_pmd_shared().
>
> Fixes: 59d9094df3d7 ("mm: hugetlb: independent PMD page table shared count")
> Cc: <stable@xxxxxxxxxxxxxxx>
> Cc: Liu Shixin <liushixin2@xxxxxxxxxx>
> Signed-off-by: David Hildenbrand (Red Hat) <david@xxxxxxxxxx>

Esp. given Lance's testing... LGTM so:

Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>

> ---
>  include/linux/hugetlb.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
> index 019a1c5281e4e..03c8725efa289 100644
> --- a/include/linux/hugetlb.h
> +++ b/include/linux/hugetlb.h
> @@ -1326,7 +1326,7 @@ static inline __init void hugetlb_cma_reserve(int order)
>  #ifdef CONFIG_HUGETLB_PMD_PAGE_TABLE_SHARING
>  static inline bool hugetlb_pmd_shared(pte_t *pte)
>  {
> -	return page_count(virt_to_page(pte)) > 1;
> +	return ptdesc_pmd_is_shared(virt_to_ptdesc(pte));
>  }
>  #else
>  static inline bool hugetlb_pmd_shared(pte_t *pte)
> --
> 2.52.0
>