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

[Lance Yang <ioworker0@gmail.com>]

From: Lance Yang <lance.yang@linux.dev>


On Fri,  5 Dec 2025 22:35:55 +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.
> 
> 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@vger.kernel.org>
> Cc: Liu Shixin <liushixin2@huawei.com>
> Signed-off-by: David Hildenbrand (Red Hat) <david@kernel.org>
> ---

Tested on x86 with two independent processes sharing a 1GiB hugetlbfs file
(aligned a 1GiB boundary).

Before the fix, even though PMD sharing worked (pt_share_count=1),
hugetlb_pmd_shared() returned false because page_count() was still 1,
causing smaps to report it as "Private" and pagemap to set it
PM_MMAP_EXCLUSIVE incorrectly :(

After the fix, hugetlb_pmd_shared() correctly detects the sharing, smaps
reports it as "Shared", and PM_MMAP_EXCLUSIVE is cleared ;)

Tested-by: Lance Yang <lance.yang@linux.dev>

Cheers!

>  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)