[PATCH v2 0/2] mm/ksm: clean up scan_get_next_rmap_item() initialization

[PATCH v2 0/2] mm/ksm: clean up scan_get_next_rmap_item() initialization

[Xu Xin (vivo)]

Currently, scan_get_next_rmap_item() has grown quite large and complex.
When introducing new features, such as the only-merge-zero-page mode we
are currently working on, we don't want to add more complexity to it
and don't want to touch rmap_item structures to avoid unnecessary overhead,
so we need to extract some common logic.

With these changes, the entry of scan_get_next_rmap_item() becomes much
cleaner and easier to follow, instead of being cluttered with unrelated
boilerplate such as stale stable_node pruning and cycle initialization.

Let's start by cleaning up these two parts at the beginning (where I think
abstraction is necessary), and we can continue cleaning up further if
needed later.

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).
- Collect Reviewed-by and Acked-by tags from v1.

Xu Xin (vivo) (2):
  mm/ksm: extract helper functions for scan cycle cleanup
  mm/ksm: add initialization helpers for scan_get_next_rmap_item()

 mm/ksm.c | 98 +++++++++++++++++++++++++++++++++++---------------------
 1 file changed, 62 insertions(+), 36 deletions(-)

-- 
2.34.1

[PATCH v2 1/2] mm/ksm: extract helper functions for scan cycle cleanup

[Xu Xin (vivo)]

Since scan_get_next_rmap_item() is getting too large, extract two new
helper functions, ksm_prune_migrate_nodes() and ksm_reset_unstable_trees(),
to reduce indentation levels, isolate local variables, and improve the
readability of the main scan loop.

No functional change intended.

Signed-off-by: Xu Xin (vivo) <xu.xin@linux.dev>
Reviewed-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
---
 mm/ksm.c | 54 +++++++++++++++++++++++++++++++-----------------------
 1 file changed, 31 insertions(+), 23 deletions(-)

diff --git a/mm/ksm.c b/mm/ksm.c
index 5e24b5ca8dc8..8db6743ece6d 100644
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -2609,6 +2609,35 @@ static struct mm_walk_ops ksm_next_page_ops = {
 	.walk_lock = PGWALK_RDLOCK,
 };
 
+static void ksm_prune_migrate_nodes(void)
+{
+	struct ksm_stable_node *stable_node, *next;
+	struct folio *folio;
+
+	if (ksm_merge_across_nodes)
+		return;
+
+	/*
+	 * Whereas stale stable_nodes on the stable_tree itself
+	 * get pruned in the regular course of stable_tree_search(),
+	 * those moved out to the migrate_nodes list can accumulate:
+	 * so prune them once before each full scan.
+	 */
+	list_for_each_entry_safe(stable_node, next, &migrate_nodes, list) {
+		folio = ksm_get_folio(stable_node, KSM_GET_FOLIO_NOLOCK);
+
+		if (folio)
+			folio_put(folio);
+		cond_resched();
+	}
+}
+
+static void ksm_reset_unstable_trees(void)
+{
+	for (int nid = 0; nid < ksm_nr_node_ids; nid++)
+		root_unstable_tree[nid] = RB_ROOT;
+}
+
 static struct ksm_rmap_item *scan_get_next_rmap_item(struct page **page)
 {
 	struct mm_struct *mm;
@@ -2617,7 +2646,6 @@ static struct ksm_rmap_item *scan_get_next_rmap_item(struct page **page)
 	struct vm_area_struct *vma;
 	struct ksm_rmap_item *rmap_item;
 	struct vma_iterator vmi;
-	int nid;
 
 	if (list_empty(&ksm_mm_head.slot.mm_node))
 		return NULL;
@@ -2639,28 +2667,8 @@ static struct ksm_rmap_item *scan_get_next_rmap_item(struct page **page)
 		 */
 		lru_add_drain_all();
 
-		/*
-		 * Whereas stale stable_nodes on the stable_tree itself
-		 * get pruned in the regular course of stable_tree_search(),
-		 * those moved out to the migrate_nodes list can accumulate:
-		 * so prune them once before each full scan.
-		 */
-		if (!ksm_merge_across_nodes) {
-			struct ksm_stable_node *stable_node, *next;
-			struct folio *folio;
-
-			list_for_each_entry_safe(stable_node, next,
-						 &migrate_nodes, list) {
-				folio = ksm_get_folio(stable_node,
-						      KSM_GET_FOLIO_NOLOCK);
-				if (folio)
-					folio_put(folio);
-				cond_resched();
-			}
-		}
-
-		for (nid = 0; nid < ksm_nr_node_ids; nid++)
-			root_unstable_tree[nid] = RB_ROOT;
+		ksm_prune_migrate_nodes();
+		ksm_reset_unstable_trees();
 
 		spin_lock(&ksm_mmlist_lock);
 		slot = list_entry(mm_slot->slot.mm_node.next,
-- 
2.34.1

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

[Xu Xin (vivo)]

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