[PATCH 2/3] mm/memory: batch set uffd-wp markers during zapping

[Dev Jain]

Enable batch setting of uffd-wp ptes.

The code paths passing nr > 1 to zap_install_uffd_wp_if_needed() produce
that nr through either folio_pte_batch or swap_pte_batch, therefore
batching is correct:

1) all ptes belong to the same type of VMA (anonymous or non-anonymous,
   wp-armed or non-wp-armed)

2) all ptes being marked with uffd-wp or all being not marked (same is the
   case with the pte_swp_uffd_wp_any check)

3) uffd_supports_wp_marker() is independent of the function parameters

Note that we will have to use set_pte_at() in a loop instead of set_ptes()
since the latter cannot handle present->non-present conversion for
nr_pages > 1.

Rename the function to cond_install_uffd_wp_ptes.

Signed-off-by: Dev Jain <dev.jain@xxxxxxx>
---
To handle nonpresent->nonpresent transition in the ptes, we can have a
set_nonpresent_ptes() (in my unmap series) : if !softleaf_has_pfn(), use
set the same pte value to all ptep's in the patch. if softleaf_has_pfn(),
then add a softleaf_next_pfn() to construct the next softleaf, and
pte_next_softleaf() to call softleaf_next_pfn() and preserve the
wp bit, s-d bit, etc from the previous pte.

 include/linux/mm.h |  6 +++--
 mm/memory.c        | 64 +++++++++++++++++-----------------------------
 mm/rmap.c          |  2 +-
 3 files changed, 29 insertions(+), 43 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index a71341c44655e..94e0a92bc70b1 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -5406,6 +5406,8 @@ void map_anon_folio_pte_nopf(struct folio *folio, pte_t *pte,
 		struct vm_area_struct *vma, unsigned long addr,
 		bool uffd_wp);
 
-bool pte_install_uffd_wp_if_needed(struct vm_area_struct *vma,
-		unsigned long addr, pte_t *ptep, pte_t pte);
+bool cond_install_uffd_wp_ptes(struct vm_area_struct *vma, unsigned long addr,
+			       pte_t *ptep, pte_t pte,
+			       unsigned long nr_ptes);
+
 #endif /* _LINUX_MM_H */
diff --git a/mm/memory.c b/mm/memory.c
index 98b3ace15cef2..5d2b567b383d4 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -1676,27 +1676,29 @@ static inline bool zap_drop_markers(struct zap_details *details)
 }
 
 /**
- * pte_install_uffd_wp_if_needed - install uffd-wp marker after clearing a PTE
- * @vma: The VMA the page is mapped into.
- * @addr: Address the page is mapped at.
- * @ptep: Page table pointer for this entry.
+ * cond_install_uffd_wp_ptes - install uffd-wp markers after clearing PTEs
+ * @vma: The VMA the pages are mapped into.
+ * @addr: Address the first page of this batch is mapped at.
+ * @ptep: Page table pointer for the first entry of this batch.
  * @pte: Old value of the entry pointed to by @ptep.
+ * @nr_ptes: Number of entries to install.
  *
- * If the PTE was write-protected by uffd-wp in any form, arm a special PTE
- * to replace a none PTE. NOTE! This should only be called when the PTE is
- * already cleared so we will never accidentally replace something valuable.
- * Meanwhile none PTEs also mean we are not demoting the PTE so a TLB flush is
- * not needed. E.g., when the PTE was cleared, the caller should have taken care
- * of the TLB flush.
+ * If the PTEs were write-protected by uffd-wp in any form, arm special
+ * PTEs to replace none PTEs. NOTE! This should only be called when the PTEs
+ * are already cleared so we will never accidentally replace something
+ * valuable. Meanwhile none PTEs also mean we are not demoting the PTEs so a
+ * TLB flush is not needed. E.g., when PTEs were cleared, the caller should
+ * have taken care of the TLB flush.
  *
- * Must be called with the page table lock held so that no thread will see the
- * none PTE, and if they see it, they'll fault and serialize at the page table
- * lock.
+ * Must be called with the page table lock held so that no thread will see
+ * the none PTEs, and if they see them, they'll fault and serialize at the
+ * page table lock.
  *
- * Returns true if an uffd-wp PTE was installed, false otherwise.
+ * Returns true if uffd-wp PTEs were installed, false otherwise.
  */
-bool pte_install_uffd_wp_if_needed(struct vm_area_struct *vma,
-				   unsigned long addr, pte_t *ptep, pte_t pte)
+bool cond_install_uffd_wp_ptes(struct vm_area_struct *vma,
+		unsigned long addr, pte_t *ptep, pte_t pte,
+		unsigned long nr_ptes)
 {
 	bool arm_uffd_pte = false;
 
@@ -1726,13 +1728,14 @@ bool pte_install_uffd_wp_if_needed(struct vm_area_struct *vma,
 	if (unlikely(pte_swp_uffd_any(pte)))
 		arm_uffd_pte = true;
 
-	if (unlikely(arm_uffd_pte)) {
+	if (likely(!arm_uffd_pte))
+		return false;
+
+	for (unsigned long i = 0; i < nr_ptes; ++i, ++ptep, addr += PAGE_SIZE)
 		set_pte_at(vma->vm_mm, addr, ptep,
 			   make_pte_marker(PTE_MARKER_UFFD_WP));
-		return true;
-	}
 
-	return false;
+	return true;
 }
 
 /*
@@ -1746,29 +1749,10 @@ 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 cond_install_uffd_wp_ptes(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 ad820fe86f7d8..2f938d0ac6953 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -2345,7 +2345,7 @@ 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);
+		cond_install_uffd_wp_ptes(vma, address, pvmw.pte, pteval, 1);
 
 		/* Update high watermark before we lower rss */
 		update_hiwater_rss(mm);
-- 
2.43.0