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

[Harry Yoo]

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.
> 
> 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>
> ---

Oops, didn't notice there's still missing conversion!

Looks good to me,
Reviewed-by: Harry Yoo <harry.yoo@xxxxxxxxxx>

-- 
Cheers,
Harry / Hyeonggon