Re: [PATCH 1/2] mm/huge_memory: Fix use of NULL folio in move_pages_huge_pmd()

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

On 2/18/26 04:45, Chris Down wrote:
> move_pages_huge_pmd() handles UFFDIO_MOVE for both normal THPs and huge
> zero pages. For the huge zero page path, src_folio is explicitly set to
> NULL (used as a sentinel to skip folio operations like lock and rmap).
> 
> In the huge zero page branch, src_folio is NULL, so folio_mk_pmd(NULL,
> pgprot) passes NULL through folio_pfn() and page_to_pfn(). With
> SPARSEMEM_VMEMMAP this silently produces a bogus PFN, installing a PMD
> pointing to non-existent physical memory. On other memory models it
> is a NULL dereference.
> 
> Use page_folio(src_page) to obtain the valid huge zero folio from the
> page, which was obtained from pmd_page() and remains valid throughout.
> 
> Fixes: e3981db444a0 ("mm: add folio_mk_pmd()")
> Cc: stable@vger.kernel.org
> Signed-off-by: Chris Down <chris@chrisdown.name>
> ---
>   mm/huge_memory.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 44ff8a648afd..fed57951a7cd 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -2794,7 +2794,7 @@ 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(src_folio, dst_vma->vm_page_prot);
> +		_dst_pmd = folio_mk_pmd(page_folio(src_page), dst_vma->vm_page_prot);
>   	}
>   	set_pmd_at(mm, dst_addr, dst_pmd, _dst_pmd);
>   

God this code is horrible.

Why can't we simply move the PMD like move_huge_pmd() would do and make 
this code less error prone?

	_dst_pmd = src_pmdval;

According to validate_move_areas(), the permissions of both VMAs are equal.


We might have to handle it just like move_huge_pmd():

	_dst_pmd = move_soft_dirty_pmd(src_pmdval);

To also set the softdirty bit.

Then, we would want to just clear uffd.

	_dst_pmd = move_soft_dirty_pmd(src_pmdval);
	_dst_pmd = clear_uffd_wp_pmd(_dst_pmd);

-- 
Cheers,

David