[PATCH v4 1/3] Revert "mm/ksm: convert break_ksm() from walk_page_range_vma() to folio_walk"

[Pedro Demarchi Gomes <pedrodemargomes@gmail.com>]

This reverts commit e317a8d8b4f600fc7ec9725e26417030ee594f52 and changes
function break_ksm_pmd_entry() to use folios.

This reverts break_ksm() to use walk_page_range_vma() instead of
folio_walk_start().
Change break_ksm_pmd_entry() to call is_ksm_zero_pte() only if we know
the folio is present, and also rename variable ret to found.
This will make it easier to later modify break_ksm() to perform a proper
range walk.

Suggested-by: David Hildenbrand (Red Hat) <david@kernel.org>
Acked-by: David Hildenbrand (Red Hat) <david@kernel.org>
Signed-off-by: Pedro Demarchi Gomes <pedrodemargomes@gmail.com>
---
 mm/ksm.c | 64 ++++++++++++++++++++++++++++++++++++++++++--------------
 1 file changed, 48 insertions(+), 16 deletions(-)

diff --git a/mm/ksm.c b/mm/ksm.c
index 4f672f4f2140..9f74baf01e46 100644
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -607,6 +607,48 @@ 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 folio *folio = NULL;
+	spinlock_t *ptl;
+	pte_t *pte;
+	pte_t ptent;
+	int found;
+
+	pte = pte_offset_map_lock(walk->mm, pmd, addr, &ptl);
+	if (!pte)
+		return 0;
+	ptent = ptep_get(pte);
+	if (pte_present(ptent)) {
+		folio = vm_normal_folio(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))
+			folio = pfn_swap_entry_folio(entry);
+	}
+	/* return 1 if the page is an normal ksm page or KSM-placed zero page */
+	found = (folio && folio_test_ksm(folio)) ||
+		(pte_present(ptent) && is_ksm_zero_pte(ptent));
+	pte_unmap_unlock(pte, ptl);
+	return found;
+}
+
+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.
@@ -623,26 +665,16 @@ static inline bool ksm_test_exit(struct mm_struct *mm)
 static int break_ksm(struct vm_area_struct *vma, unsigned long addr, bool lock_vma)
 {
 	vm_fault_t ret = 0;
-
-	if (lock_vma)
-		vma_start_write(vma);
+	const struct mm_walk_ops *ops = lock_vma ?
+				&break_ksm_lock_vma_ops : &break_ksm_ops;
 
 	do {
-		bool ksm_page = false;
-		struct folio_walk fw;
-		struct folio *folio;
+		int ksm_page;
 
 		cond_resched();
-		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);
-		}
-
+		ksm_page = walk_page_range_vma(vma, addr, addr + 1, ops, NULL);
+		if (WARN_ON_ONCE(ksm_page < 0))
+			return ksm_page;
 		if (!ksm_page)
 			return 0;
 		ret = handle_mm_fault(vma, addr,
-- 
2.43.0