Re: [PATCH v2 2/3] mm/huge_memory: Prevent huge zeropage refcount corruption in PMD move

[Lorenzo Stoakes <lorenzo.stoakes@oracle.com>]

On Thu, Feb 26, 2026 at 10:16:47PM +0800, Chris Down wrote:
> After commit d82d09e48219 ("mm/huge_memory: mark PMD mappings of the
> huge zero folio special"), moved huge zero PMDs must remain special so
> vm_normal_page_pmd() continues to treat them as special mappings.
>
> move_pages_huge_pmd() currently reconstructs the destination PMD in the
> huge zero page branch, which drops PMD state such as pmd_special() on
> architectures with CONFIG_ARCH_HAS_PTE_SPECIAL. As a result,
> vm_normal_page_pmd() can treat the moved huge zero PMD as a normal page
> and corrupt its refcount.
>
> Instead of reconstructing the PMD from the folio, derive the destination
> entry from src_pmdval after pmdp_huge_clear_flush(), then handle the PMD
> metadata the same way move_huge_pmd() does for moved entries by marking
> it soft-dirty and clearing uffd-wp.
>
> Fixes: d82d09e48219 ("mm/huge_memory: mark PMD mappings of the huge zero folio special")
> Cc: stable@vger.kernel.org
> Signed-off-by: Chris Down <chris@chrisdown.name>
> ---
>  mm/huge_memory.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index fed57951a7cd..8166b5e871ad 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -2794,7 +2794,8 @@ int move_pages_huge_pmd(struct mm_struct *mm, pmd_t *dst_pmd, pmd_t *src_pmd, pm
>  		_dst_pmd = pmd_mkwrite(pmd_mkdirty(_dst_pmd), dst_vma);
>  	} else {
>  		src_pmdval = pmdp_huge_clear_flush(src_vma, src_addr, src_pmd);
> -		_dst_pmd = folio_mk_pmd(page_folio(src_page), dst_vma->vm_page_prot);
> +		_dst_pmd = move_soft_dirty_pmd(src_pmdval);
> +		_dst_pmd = clear_uffd_wp_pmd(_dst_pmd);

I'm confused as to what's going on here, it seems like the 2/3 is simply
updating the 1/3 with a different fixes?

I agree with David that just moving it is probably completely fine, so I think
this should be the only actual patch you need, and you can just Fixes:
e3981db444a0 with it? Then make this a v3 series with 2 patches this + the test
right (but maybe best not backport the test :)?

>  	}
>  	set_pmd_at(mm, dst_addr, dst_pmd, _dst_pmd);
>
> --
> 2.51.2
>
>
>