[PATCH v4 2/4] memcontrol: only transfer the memcg data for migration

[Nhat Pham <nphamcs@gmail.com>]

For most migration use cases, only transfer the memcg data from the old
folio to the new folio, and clear the old folio's memcg data. No
charging and uncharging will be done.

This shaves off some work on the migration path, and avoids the
temporary double charging of a folio during its migration.

The only exception is replace_page_cache_folio(), which will use the old
mem_cgroup_migrate() (now renamed to mem_cgroup_replace_folio). In that
context, the isolation of the old page isn't quite as thorough as with
migration, so we cannot use our new implementation directly.

This patch is the result of the following discussion on the new hugetlb
memcg accounting behavior:

https://lore.kernel.org/lkml/20231003171329.GB314430@monkey/

Suggested-by: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Nhat Pham <nphamcs@gmail.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
---
 include/linux/memcontrol.h |  7 +++++++
 mm/filemap.c               |  2 +-
 mm/memcontrol.c            | 40 +++++++++++++++++++++++++++++++++++---
 3 files changed, 45 insertions(+), 4 deletions(-)

diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index 42bf7e9b1a2f..5daf14da3759 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -708,6 +708,8 @@ static inline void mem_cgroup_uncharge_list(struct list_head *page_list)
 
 void mem_cgroup_cancel_charge(struct mem_cgroup *memcg, unsigned int nr_pages);
 
+void mem_cgroup_replace_folio(struct folio *old, struct folio *new);
+
 void mem_cgroup_migrate(struct folio *old, struct folio *new);
 
 /**
@@ -1285,6 +1287,11 @@ static inline void mem_cgroup_cancel_charge(struct mem_cgroup *memcg,
 {
 }
 
+static inline void mem_cgroup_replace_folio(struct folio *old,
+		struct folio *new)
+{
+}
+
 static inline void mem_cgroup_migrate(struct folio *old, struct folio *new)
 {
 }
diff --git a/mm/filemap.c b/mm/filemap.c
index 9481ffaf24e6..673745219c82 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -819,7 +819,7 @@ void replace_page_cache_folio(struct folio *old, struct folio *new)
 	new->mapping = mapping;
 	new->index = offset;
 
-	mem_cgroup_migrate(old, new);
+	mem_cgroup_replace_folio(old, new);
 
 	xas_lock_irq(&xas);
 	xas_store(&xas, new);
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 0219befeae38..b9c479d768e2 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -7281,16 +7281,17 @@ void __mem_cgroup_uncharge_list(struct list_head *page_list)
 }
 
 /**
- * mem_cgroup_migrate - Charge a folio's replacement.
+ * mem_cgroup_replace_folio - Charge a folio's replacement.
  * @old: Currently circulating folio.
  * @new: Replacement folio.
  *
  * Charge @new as a replacement folio for @old. @old will
- * be uncharged upon free.
+ * be uncharged upon free. This is only used by the page cache
+ * (in replace_page_cache_folio()).
  *
  * Both folios must be locked, @new->mapping must be set up.
  */
-void mem_cgroup_migrate(struct folio *old, struct folio *new)
+void mem_cgroup_replace_folio(struct folio *old, struct folio *new)
 {
 	struct mem_cgroup *memcg;
 	long nr_pages = folio_nr_pages(new);
@@ -7329,6 +7330,39 @@ void mem_cgroup_migrate(struct folio *old, struct folio *new)
 	local_irq_restore(flags);
 }
 
+/**
+ * mem_cgroup_migrate - Transfer the memcg data from the old to the new folio.
+ * @old: Currently circulating folio.
+ * @new: Replacement folio.
+ *
+ * Transfer the memcg data from the old folio to the new folio for migration.
+ * The old folio's data info will be cleared. Note that the memory counters
+ * will remain unchanged throughout the process.
+ *
+ * Both folios must be locked, @new->mapping must be set up.
+ */
+void mem_cgroup_migrate(struct folio *old, struct folio *new)
+{
+	struct mem_cgroup *memcg;
+
+	VM_BUG_ON_FOLIO(!folio_test_locked(old), old);
+	VM_BUG_ON_FOLIO(!folio_test_locked(new), new);
+	VM_BUG_ON_FOLIO(folio_test_anon(old) != folio_test_anon(new), new);
+	VM_BUG_ON_FOLIO(folio_nr_pages(old) != folio_nr_pages(new), new);
+
+	if (mem_cgroup_disabled())
+		return;
+
+	memcg = folio_memcg(old);
+	VM_WARN_ON_ONCE_FOLIO(!memcg, old);
+	if (!memcg)
+		return;
+
+	/* Transfer the charge and the css ref */
+	commit_charge(new, memcg);
+	old->memcg_data = 0;
+}
+
 DEFINE_STATIC_KEY_FALSE(memcg_sockets_enabled_key);
 EXPORT_SYMBOL(memcg_sockets_enabled_key);
 
-- 
2.34.1