mm/ksm: convert break_ksm() from walk_page_range_vma() to folio_walk

Let's simplify by reusing folio_walk.  Keep the existing behavior by
handling migration entries and zeropages.

Link: https://lkml.kernel.org/r/20240802155524.517137-12-david@redhat.comSigned-off-by: David Hildenbrand <david@redhat.com>
Cc: Alexander Gordeev <agordeev@linux.ibm.com>
Cc: Christian Borntraeger <borntraeger@linux.ibm.com>
Cc: Claudio Imbrenda <imbrenda@linux.ibm.com>
Cc: Gerald Schaefer <gerald.schaefer@linux.ibm.com>
Cc: Heiko Carstens <hca@linux.ibm.com>
Cc: Janosch Frank <frankja@linux.ibm.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Sven Schnelle <svens@linux.ibm.com>
Cc: Vasily Gorbik <gor@linux.ibm.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Zi Yan <ziy@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/ksm.c

@@ -608,47 +608,6 @@ static inline bool ksm_test_exit(struct mm_struct *mm)
 	return atomic_read(&mm->mm_users) == 0;
 }
 
-static int break_ksm_pmd_entry(pmd_t *pmd, unsigned long addr, unsigned long next,
-			struct mm_walk *walk)
-{
-	struct page *page = NULL;
-	spinlock_t *ptl;
-	pte_t *pte;
-	pte_t ptent;
-	int ret;
-
-	pte = pte_offset_map_lock(walk->mm, pmd, addr, &ptl);
-	if (!pte)
-		return 0;
-	ptent = ptep_get(pte);
-	if (pte_present(ptent)) {
-		page = vm_normal_page(walk->vma, addr, ptent);
-	} else if (!pte_none(ptent)) {
-		swp_entry_t entry = pte_to_swp_entry(ptent);
-
-		/*
-		 * As KSM pages remain KSM pages until freed, no need to wait
-		 * here for migration to end.
-		 */
-		if (is_migration_entry(entry))
-			page = pfn_swap_entry_to_page(entry);
-	}
-	/* return 1 if the page is an normal ksm page or KSM-placed zero page */
-	ret = (page && PageKsm(page)) || is_ksm_zero_pte(ptent);
-	pte_unmap_unlock(pte, ptl);
-	return ret;
-}
-
-static const struct mm_walk_ops break_ksm_ops = {
-	.pmd_entry = break_ksm_pmd_entry,
-	.walk_lock = PGWALK_RDLOCK,
-};
-
-static const struct mm_walk_ops break_ksm_lock_vma_ops = {
-	.pmd_entry = break_ksm_pmd_entry,
-	.walk_lock = PGWALK_WRLOCK,
-};
-
 /*
  * We use break_ksm to break COW on a ksm page by triggering unsharing,
  * such that the ksm page will get replaced by an exclusive anonymous page.
@@ -665,16 +624,26 @@ static const struct mm_walk_ops break_ksm_lock_vma_ops = {
 static int break_ksm(struct vm_area_struct *vma, unsigned long addr, bool lock_vma)
 {
 	vm_fault_t ret = 0;
-	const struct mm_walk_ops *ops = lock_vma ?
-				&break_ksm_lock_vma_ops : &break_ksm_ops;
+
+	if (lock_vma)
+		vma_start_write(vma);
 
 	do {
-		int ksm_page;
+		bool ksm_page = false;
+		struct folio_walk fw;
+		struct folio *folio;
 
 		cond_resched();
-		ksm_page = walk_page_range_vma(vma, addr, addr + 1, ops, NULL);
-		if (WARN_ON_ONCE(ksm_page < 0))
-			return ksm_page;
+		folio = folio_walk_start(&fw, vma, addr,
+					 FW_MIGRATION | FW_ZEROPAGE);
+		if (folio) {
+			/* Small folio implies FW_LEVEL_PTE. */
+			if (!folio_test_large(folio) &&
+			    (folio_test_ksm(folio) || is_ksm_zero_pte(fw.pte)))
+				ksm_page = true;
+			folio_walk_end(&fw, vma);
+		}
+
 		if (!ksm_page)
 			return 0;
 		ret = handle_mm_fault(vma, addr,