Re: [PATCH v2 1/1] mm/rmap: fix soft-dirty and uffd-wp bit loss when remapping zero-filled mTHP subpage to shared zeropage

[Dev Jain <dev.jain@arm.com>]

On 30/09/25 10:03 am, Lance Yang wrote:
> From: Lance Yang <lance.yang@linux.dev>
>
> When splitting an mTHP and replacing a zero-filled subpage with the shared
> zeropage, try_to_map_unused_to_zeropage() currently drops several important
> PTE bits.
>
> For userspace tools like CRIU, which rely on the soft-dirty mechanism for
> incremental snapshots, losing the soft-dirty bit means modified pages are
> missed, leading to inconsistent memory state after restore.
>
> As pointed out by David, the more critical uffd-wp bit is also dropped.
> This breaks the userfaultfd write-protection mechanism, causing writes
> to be silently missed by monitoring applications, which can lead to data
> corruption.
>
> Preserve both the soft-dirty and uffd-wp bits from the old PTE when
> creating the new zeropage mapping to ensure they are correctly tracked.
>
> Cc: <stable@vger.kernel.org>
> Fixes: b1f202060afe ("mm: remap unused subpages to shared zeropage when splitting isolated thp")
> Suggested-by: David Hildenbrand <david@redhat.com>
> Suggested-by: Dev Jain <dev.jain@arm.com>
> Acked-by: David Hildenbrand <david@redhat.com>
> Signed-off-by: Lance Yang <lance.yang@linux.dev>
> ---
> v1 -> v2:
>   - Avoid calling ptep_get() multiple times (per Dev)
>   - Double-check the uffd-wp bit (per David)
>   - Collect Acked-by from David - thanks!
>   - https://lore.kernel.org/linux-mm/20250928044855.76359-1-lance.yang@linux.dev/
>
>   mm/migrate.c | 9 ++++++++-
>   1 file changed, 8 insertions(+), 1 deletion(-)
>
> diff --git a/mm/migrate.c b/mm/migrate.c
> index ce83c2c3c287..50aa91d9ab4e 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -300,13 +300,14 @@ static bool try_to_map_unused_to_zeropage(struct page_vma_mapped_walk *pvmw,
>   					  unsigned long idx)
>   {
>   	struct page *page = folio_page(folio, idx);
> +	pte_t oldpte = ptep_get(pvmw->pte);

What I meant to say was, you can pass oldpte from remove_migration_pte to this
function. Basically define old_pte = ptep_get(pvmw.pte) in the declarations of
the start of the while block in remove_migration_pte and remove the existing
one. That will ensure ptep_get() gets called only once per iteration.

>   	pte_t newpte;
>   
>   	if (PageCompound(page))
>   		return false;
>   	VM_BUG_ON_PAGE(!PageAnon(page), page);
>   	VM_BUG_ON_PAGE(!PageLocked(page), page);
> -	VM_BUG_ON_PAGE(pte_present(ptep_get(pvmw->pte)), page);
> +	VM_BUG_ON_PAGE(pte_present(oldpte), page);
>   
>   	if (folio_test_mlocked(folio) || (pvmw->vma->vm_flags & VM_LOCKED) ||
>   	    mm_forbids_zeropage(pvmw->vma->vm_mm))
> @@ -322,6 +323,12 @@ static bool try_to_map_unused_to_zeropage(struct page_vma_mapped_walk *pvmw,
>   
>   	newpte = pte_mkspecial(pfn_pte(my_zero_pfn(pvmw->address),
>   					pvmw->vma->vm_page_prot));
> +
> +	if (pte_swp_soft_dirty(oldpte))
> +		newpte = pte_mksoft_dirty(newpte);
> +	if (pte_swp_uffd_wp(oldpte))
> +		newpte = pte_mkuffd_wp(newpte);
> +
>   	set_pte_at(pvmw->vma->vm_mm, pvmw->address, pvmw->pte, newpte);
>   
>   	dec_mm_counter(pvmw->vma->vm_mm, mm_counter(folio));