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

[David Hildenbrand <david@redhat.com>]

On 28.09.25 06:48, 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 the soft-dirty
> bit.
> 
> For userspace tools like CRIU, which rely on the soft-dirty mechanism for
> incremental snapshots, losing this bit means modified pages are missed,
> leading to inconsistent memory state after restore.
> 
> Preserve the soft-dirty bit from the old PTE when creating the zeropage
> mapping to ensure modified pages are correctly tracked.
> 
> Cc: <stable@vger.kernel.org>
> Fixes: b1f202060afe ("mm: remap unused subpages to shared zeropage when splitting isolated thp")
> Signed-off-by: Lance Yang <lance.yang@linux.dev>
> ---
>   mm/migrate.c | 4 ++++
>   1 file changed, 4 insertions(+)
> 
> diff --git a/mm/migrate.c b/mm/migrate.c
> index ce83c2c3c287..bf364ba07a3f 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -322,6 +322,10 @@ 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(ptep_get(pvmw->pte)))
> +		newpte = pte_mksoft_dirty(newpte);
> +
>   	set_pte_at(pvmw->vma->vm_mm, pvmw->address, pvmw->pte, newpte);
>   
>   	dec_mm_counter(pvmw->vma->vm_mm, mm_counter(folio));

It's interesting that there isn't a single occurrence of the stof-dirty 
flag in khugepaged code. I guess it all works because we do the

	_pmd = maybe_pmd_mkwrite(pmd_mkdirty(_pmd), vma);

and the pmd_mkdirty() will imply marking it soft-dirty.

Now to the problem at hand: I don't think this is particularly 
problematic in the common case: if the page is zero, it likely was never 
written to (that's what the unerused shrinker is targeted at), so the 
soft-dirty setting on the PMD is actually just an over-indication for 
this page.

For example, when we just install the shared zeropage directly in 
do_anonymous_page(), we obviously also don't set it dirty/soft-dirty.

Now, one could argue that if the content was changed from non-zero to 
zero, it ould actually be soft-dirty.

Long-story short: I don't think this matters much in practice, but it's 
an easy fix.

As said by dev, please avoid double ptep_get() if possible.

Acked-by: David Hildenbrand <david@redhat.com>


@Lance, can you double-check that the uffd-wp bit is handled correctly? 
I strongly assume we lose that as well here.

-- 
Cheers

David / dhildenb