Re: [PATCH mm-unstable] mm: Fix uffd-wp bit loss when batching file folio unmapping

[Lorenzo Stoakes <lorenzo.stoakes@oracle.com>]

On Fri, Jan 16, 2026 at 01:57:21PM +0530, Dev Jain wrote:
> The recently added file folio unmap batching support forgets to update
> pte_install_uffd_wp_if_needed(), which still updates a single pte.
> We end up jumping to the end of the folio in page_vma_mapped_walk(), thus
> setting the uffd-wp marker only on a single pte in the batch. Fix this by
> passing nr_pages into the function, and set the uffd-wp marker on all ptes.
>
> Note that, since the nr_pages passed to this function is always derived by
> some sort of batching, it is guaranteed that the set of old ptevals of the
> batch have uffd-wp bit on all ptes or no ptes, therefore it is safe to derive
> the value of the local variable "arm_uffd_pte" from only the particular
> pteval passed to this function, but apply the result on all ptes of the batch.
>
> Use set_pte_at() in a loop to set the markers - we cannot use set_ptes()
> as that will increment the PFN, but we don't have any PFN to update here.
>
> The userspace visible effect of the bug is inaccuracy observed by workloads
> relying on uffd-wp regions to install their own pages.
>
> Fixes: 8798e255b5ec ("mm: rmap: support batched unmapping for file large folios")

Hmm this patch isn't upstream yet, so wouldn't this patch be better as a comment
replying to the series so it can be respun?

You only do a fixes tag for either upstream things that obviously have to stay
in place or perhaps things in mm-stable immediately prior to merge window that
can't be adjusted.

> Signed-off-by: Dev Jain <dev.jain@arm.com>

Let's replace this with a comment on the series at [0] please.

[0]: https://lore.kernel.org/linux-mm/cover.1766631066.git.baolin.wang@linux.alibaba.com/

Also the original series really shouldn't go in without signoff from me/David.

Thanks, Lorenzo

> ---
> Patch applies on mm-unstable, commit f8ed52ac0cfb.
>
> I observed this bug during code inspection, but it turns out that the uffd-wp-mremap
> selftest will skip some tests with a bogus complain that "MADV_PAGEOUT didn't work,
> is swap enabled?" even when swap is enabled. It first sets the region uffd-wp,
> then swaps it out, then checks through pagemap whether it got swapped out. For
> file folios, this check makes no sense since the ptes are simply cleared, but in
> this particular case, because of uffd-wp preservation, we need to store PTE_MARKER_UFFD_WP,
> which is stored as a swap entry, that is why the test works out on a non-buggy kernel.
>
>  include/linux/mm_inline.h |  7 ++++---
>  mm/memory.c               | 14 +-------------
>  mm/rmap.c                 |  2 +-
>  3 files changed, 6 insertions(+), 17 deletions(-)
>
> diff --git a/include/linux/mm_inline.h b/include/linux/mm_inline.h
> index fa2d6ba811b5..adec1dcb8793 100644
> --- a/include/linux/mm_inline.h
> +++ b/include/linux/mm_inline.h
> @@ -568,7 +568,7 @@ static inline pte_marker copy_pte_marker(
>   */
>  static inline bool
>  pte_install_uffd_wp_if_needed(struct vm_area_struct *vma, unsigned long addr,
> -			      pte_t *pte, pte_t pteval)
> +			      pte_t *pte, pte_t pteval, unsigned int nr_pages)
>  {
>  	bool arm_uffd_pte = false;
>
> @@ -599,8 +599,9 @@ pte_install_uffd_wp_if_needed(struct vm_area_struct *vma, unsigned long addr,
>  		arm_uffd_pte = true;
>
>  	if (unlikely(arm_uffd_pte)) {
> -		set_pte_at(vma->vm_mm, addr, pte,
> -			   make_pte_marker(PTE_MARKER_UFFD_WP));
> +		for (int i = 0; i < nr_pages; ++i, ++pte, addr += PAGE_SIZE)
> +			set_pte_at(vma->vm_mm, addr, pte,
> +				make_pte_marker(PTE_MARKER_UFFD_WP));
>  		return true;
>  	}
>
> diff --git a/mm/memory.c b/mm/memory.c
> index 6b22dd72ebc8..35ac86d29e77 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -1588,8 +1588,6 @@ zap_install_uffd_wp_if_needed(struct vm_area_struct *vma,
>  			      unsigned long addr, pte_t *pte, int nr,
>  			      struct zap_details *details, pte_t pteval)
>  {
> -	bool was_installed = false;
> -
>  	if (!uffd_supports_wp_marker())
>  		return false;
>
> @@ -1600,17 +1598,7 @@ zap_install_uffd_wp_if_needed(struct vm_area_struct *vma,
>  	if (zap_drop_markers(details))
>  		return false;
>
> -	for (;;) {
> -		/* the PFN in the PTE is irrelevant. */
> -		if (pte_install_uffd_wp_if_needed(vma, addr, pte, pteval))
> -			was_installed = true;
> -		if (--nr == 0)
> -			break;
> -		pte++;
> -		addr += PAGE_SIZE;
> -	}
> -
> -	return was_installed;
> +	return pte_install_uffd_wp_if_needed(vma, addr, pte, pteval, nr);
>  }
>
>  static __always_inline void zap_present_folio_ptes(struct mmu_gather *tlb,
> diff --git a/mm/rmap.c b/mm/rmap.c
> index f13480cb9f2e..d6ca002bf79c 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -2171,7 +2171,7 @@ 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.
>  		 */
> -		pte_install_uffd_wp_if_needed(vma, address, pvmw.pte, pteval);
> +		pte_install_uffd_wp_if_needed(vma, address, pvmw.pte, pteval, nr_pages);
>
>  		/* Update high watermark before we lower rss */
>  		update_hiwater_rss(mm);
> --
> 2.34.1
>