Re: [PATCH] mm: rmap: skip batched unmapping for UFFD vmas

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

On 1/16/26 17:26, Baolin Wang wrote:
> As Dev reported[1], it's not ready to support batched unmapping for uffd case.
> Let's still fallback to per-page unmapping for the uffd case.
> 
> [1] https://lore.kernel.org/linux-mm/20260116082721.275178-1-dev.jain@arm.com/
> Reported-by: Dev Jain <dev.jain@arm.com>
> Suggested-by: Barry Song <baohua@kernel.org>
> Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
> ---
>   mm/rmap.c | 3 +++
>   1 file changed, 3 insertions(+)
> 
> diff --git a/mm/rmap.c b/mm/rmap.c
> index f13480cb9f2e..172643092dcf 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -1953,6 +1953,9 @@ static inline unsigned int folio_unmap_pte_batch(struct folio *folio,
>   	if (pte_unused(pte))
>   		return 1;
>   
> +	if (userfaultfd_wp(vma))
> +		return 1;
> +

Interesting. I was just wondering why we didn't run into that with lazyfree folios.

Staring at pte_install_uffd_wp_if_needed(), we never set the marker for
anonymous VMAs.

So, yeah, if one sets lazyfree on a uffd-wp PTE, the uffd-wp bit will just get
zapped alongside. Just like MADV_DONTNEED.


I'm fine with that temporary fix. But I guess the non-hacky way to handle this would be:


 From 53d016d6e6f624425dbdbc2fb1dec7c91fbef15c Mon Sep 17 00:00:00 2001
From: "David Hildenbrand (Arm)" <david@kernel.org>
Date: Mon, 9 Feb 2026 10:52:59 +0100
Subject: [PATCH] tmp

Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>
---
  include/linux/mm_inline.h | 15 ++++++---------
  mm/memory.c               | 21 +--------------------
  mm/rmap.c                 |  2 +-
  3 files changed, 8 insertions(+), 30 deletions(-)

diff --git a/include/linux/mm_inline.h b/include/linux/mm_inline.h
index fa2d6ba811b5..8a9a2c5f5ee3 100644
--- a/include/linux/mm_inline.h
+++ b/include/linux/mm_inline.h
@@ -566,9 +566,8 @@ static inline pte_marker copy_pte_marker(
   *
   * Returns true if an uffd-wp pte was installed, false otherwise.
   */
-static inline bool
-pte_install_uffd_wp_if_needed(struct vm_area_struct *vma, unsigned long addr,
-			      pte_t *pte, pte_t pteval)
+static inline bool install_uffd_wp_ptes_if_needed(struct vm_area_struct *vma,
+		unsigned long addr, pte_t *pte, unsigned int nr, pte_t pteval)
  {
  	bool arm_uffd_pte = false;
  
@@ -598,13 +597,11 @@ pte_install_uffd_wp_if_needed(struct vm_area_struct *vma, unsigned long addr,
  	if (unlikely(pte_swp_uffd_wp_any(pteval)))
  		arm_uffd_pte = true;
  
-	if (unlikely(arm_uffd_pte)) {
-		set_pte_at(vma->vm_mm, addr, pte,
-			   make_pte_marker(PTE_MARKER_UFFD_WP));
-		return true;
-	}
+	if (likely(!arm_uffd_pte))
+		return false;
  
-	return false;
+	set_ptes(vma->vm_mm, addr, pte, make_pte_marker(PTE_MARKER_UFFD_WP), nr);
+	return true;
  }
  
  static inline bool vma_has_recency(const struct vm_area_struct *vma)
diff --git a/mm/memory.c b/mm/memory.c
index da360a6eb8a4..0a87d02a9a69 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -1592,29 +1592,10 @@ 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;
-
-	/* Zap on anonymous always means dropping everything */
-	if (vma_is_anonymous(vma))
-		return false;
-
  	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 install_uffd_wp_ptes_if_needed(vma, addr, pte, nr, pteval);
  }
  
  static __always_inline void zap_present_folio_ptes(struct mmu_gather *tlb,
diff --git a/mm/rmap.c b/mm/rmap.c
index 7b9879ef442d..f71aacf35925 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -2061,7 +2061,7 @@ 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);
+		install_uffd_wp_ptes_if_needed(vma, address, pvmw.pte, nr_pages, pteval);
  
  		/* Update high watermark before we lower rss */
  		update_hiwater_rss(mm);
-- 
2.43.0



Does somebody have time to look into that? We should also adjust the doc of pte_install_uffd_wp_if_needed()
and turn it into some proper kerneldoc.

-- 
Cheers,

David