Re: [PATCH RFC] mm: entirely reuse the whole anon mTHP in do_wp_page

[David Hildenbrand]

> +		idx = folio_page_idx(folio, vmf->page);
> +		folio_start = address - idx * PAGE_SIZE;
> +		folio_end = folio_start + nr * PAGE_SIZE;
> +
> +		if (unlikely(folio_start < max(address & PMD_MASK, vma->vm_start)))
> +			return false;
> +		if (unlikely(folio_end > pmd_addr_end(address, vma->vm_end)))
> +			return false;
> +		folio_ptep = vmf->pte - idx;
> +		folio_pte = ptep_get(folio_ptep);
> +		if (!pte_present(folio_pte) || pte_pfn(folio_pte) != folio_pfn(folio))
> +			return false;
> +		if (folio_pte_batch(folio, folio_start, folio_ptep, folio_pte, nr, 0,
> +				NULL, NULL, NULL) != nr)
> +			return false;
> +		if (folio_mapcount(folio) != nr)
> +			return false;

BTW, you're not checking against the refcount (and it's all a bit racy 
on concurrent unmapping!). So you're re-introducing the vmsplice 
child->parent attak.

--
Cheers,

David / dhildenb