[PATCH v2 2/2] mm/ksm: add initialization helpers for scan_get_next_rmap_item()

["Xu Xin (vivo)" <xu.xin@linux.dev>]

Extract the new scan cycle check into scan_should_start_new_cycle()
and add the helper scan_get_first_mm() that advances the scan
cursor to the first mm_slot.

This keeps the exact execution order intact while making
scan_get_next_rmap_item() cleaner and easier to follow.

No functional change intended.

Signed-off-by: Xu Xin (vivo) <xu.xin@linux.dev>
Reviewed-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
---
Changes in v2:
- Rename scan_begin_new_cycle() to scan_should_start_new_cycle() and keep
  it simple (David).
- Keep lru_add_drain_all() in scan_get_next_rmap_item() (David).
- Use scoped_guard(spinlock, &ksm_mmlist_lock) in scan_get_first_mm()
  (Lorenzo).
- Keep Lorenzo's Reviewed-by tag as the changes are minor and follow the
  suggested cleanups (please let me know if you prefer otherwise).

 mm/ksm.c | 44 +++++++++++++++++++++++++++++++-------------
 1 file changed, 31 insertions(+), 13 deletions(-)

diff --git a/mm/ksm.c b/mm/ksm.c
index 8db6743ece6d..337fcc9c6322 100644
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -2638,6 +2638,34 @@ static void ksm_reset_unstable_trees(void)
 		root_unstable_tree[nid] = RB_ROOT;
 }
 
+static bool scan_should_start_new_cycle(void)
+{
+	/* Check if the cursor points to the head. */
+	return ksm_scan.mm_slot == &ksm_mm_head;
+}
+
+static struct ksm_mm_slot *scan_get_first_mm(void)
+{
+	struct ksm_mm_slot *mm_slot = &ksm_mm_head;
+	struct mm_slot *slot;
+
+	scoped_guard(spinlock, &ksm_mmlist_lock) {
+		slot = list_entry(mm_slot->slot.mm_node.next,
+				  struct mm_slot, mm_node);
+		mm_slot = mm_slot_entry(slot, struct ksm_mm_slot, slot);
+		ksm_scan.mm_slot = mm_slot;
+	}
+
+	/*
+	 * Although we tested list_empty() above, a racing __ksm_exit
+	 * of the last mm on the list may have removed it since then.
+	 */
+	if (mm_slot == &ksm_mm_head)
+		return NULL;
+
+	return mm_slot;
+}
+
 static struct ksm_rmap_item *scan_get_next_rmap_item(struct page **page)
 {
 	struct mm_struct *mm;
@@ -2650,8 +2678,7 @@ static struct ksm_rmap_item *scan_get_next_rmap_item(struct page **page)
 	if (list_empty(&ksm_mm_head.slot.mm_node))
 		return NULL;
 
-	mm_slot = ksm_scan.mm_slot;
-	if (mm_slot == &ksm_mm_head) {
+	if (scan_should_start_new_cycle()) {
 		advisor_start_scan();
 		trace_ksm_start_scan(ksm_scan.seqnr, ksm_rmap_items);
 
@@ -2670,17 +2697,8 @@ static struct ksm_rmap_item *scan_get_next_rmap_item(struct page **page)
 		ksm_prune_migrate_nodes();
 		ksm_reset_unstable_trees();
 
-		spin_lock(&ksm_mmlist_lock);
-		slot = list_entry(mm_slot->slot.mm_node.next,
-				  struct mm_slot, mm_node);
-		mm_slot = mm_slot_entry(slot, struct ksm_mm_slot, slot);
-		ksm_scan.mm_slot = mm_slot;
-		spin_unlock(&ksm_mmlist_lock);
-		/*
-		 * Although we tested list_empty() above, a racing __ksm_exit
-		 * of the last mm on the list may have removed it since then.
-		 */
-		if (mm_slot == &ksm_mm_head)
+		mm_slot = scan_get_first_mm();
+		if (!mm_slot)
 			return NULL;
 next_mm:
 		ksm_scan.address = 0;
-- 
2.34.1