Re: [PATCH v3 5/9] mm/rmap: batch unmap folios belonging to uffd-wp VMAs

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

On 5/6/26 11:45, Dev Jain wrote:
> Commit a67fe41e214f ("mm: rmap: support batched unmapping for file large folios")
> extended batched unmapping for file folios. That also required making
> install_uffd_wp_ptes_if_needed() support batching, but that was left
> out for the time being, and correctness was maintained by stopping
> batching in case the VMA the folio belongs to is marked uffd-wp.
> 
> Now that we have a batched version called install_uffd_wp_ptes_if_needed,
> simply call that. folio_unmap_pte_batch() ensures that the original state
> of the ptes is either all uffd or all non-uffd, so we maintain
> correctness.
> 
> If uffd-wp bit is there, we have the following transitions of ptes
> after unmapping:
> 
> 1) anon folio: present -> uffd-wp swap
> 2) file folio: present -> uffd-wp marker
> 
> We must ensure that these ptes are not reprocessed by the while loop -
> if the batch length is less than the number of pages in the folio, then
> we must skip over this batch.
> 
> The page_vma_mapped_walk API ensures this - check_pte() will return true
> only if any of [pvmw->pfn, pvmw->pfn + nr_pages) is mapped by the pte.
> There is no pfn underlying either a uffd-wp swap pte or a uffd-wp marker
> pte, so check_pte returns false and we keep skipping until we hit a
> present entry, which is where we want to batch from next.

Right. Or a non-present ZONE_DEVICE entry. Your patch shouldn't change anything
about that.

> 
> Signed-off-by: Dev Jain <dev.jain@arm.com>
> ---
>  mm/rmap.c | 5 +----
>  1 file changed, 1 insertion(+), 4 deletions(-)
> 
> diff --git a/mm/rmap.c b/mm/rmap.c
> index b17dce752a1ea..25813e3605991 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -1965,9 +1965,6 @@ static inline unsigned int folio_unmap_pte_batch(struct folio *folio,
>  	if (pte_unused(pte))
>  		return 1;
>  
> -	if (userfaultfd_wp(vma))
> -		return 1;
> -
>  	/*
>  	 * If unmap fails, we need to restore the ptes. To avoid accidentally
>  	 * upgrading write permissions for ptes that were not originally
> @@ -2266,7 +2263,7 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
>  		 * we may want to replace a none pte with a marker pte if
>  		 * it's file-backed, so we don't lose the tracking info.
>  		 */
> -		install_uffd_wp_ptes_if_needed(vma, address, pvmw.pte, pteval, 1);
> +		install_uffd_wp_ptes_if_needed(vma, address, pvmw.pte, pteval, nr_pages);
>  
>  		/* Update high watermark before we lower rss */
>  		update_hiwater_rss(mm);

Yes, that should do the trick. folio_pte_batch_flags() will still respect the
uffd-wp bit.

Acked-by: David Hildenbrand (Arm) <david@kernel.org>

-- 
Cheers,

David