[PATCH v2 1/2] memcg: move mem_cgroup_swappiness and vm_swappiness to mm/swap.h

[Ridong <ridong.chen@linux.dev>]

From: Ridong Chen <chenridong@xiaomi.com>

The per-memcg swappiness knob is v1-only; v2 always uses global
vm_swappiness and ignores the per-cgroup field.

Both mem_cgroup_swappiness() and vm_swappiness are only used within
mm/ (memcontrol.c, memcontrol-v1.c, vmscan.c), so move them out of the
public include/linux/swap.h into the mm-private mm/swap.h. This keeps
unrelated declarations out of include/linux/swap.h.

Guard memcg->swappiness with CONFIG_MEMCG_V1 as well, so v2-only
kernels drop the unused field.

No functional change for v1; v2-only kernels drop the unused field.

Signed-off-by: Ridong Chen <chenridong@xiaomi.com>
---
 include/linux/memcontrol.h |  4 ++--
 include/linux/swap.h       | 19 -------------------
 mm/memcontrol.c            |  4 ++--
 mm/swap.h                  | 20 ++++++++++++++++++++
 4 files changed, 24 insertions(+), 23 deletions(-)

diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index 957260677678..e7a794dfb40e 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -239,8 +239,6 @@ struct mem_cgroup {
 	 */
 	bool oom_group;
 
-	int swappiness;
-
 	/* memory.events and memory.events.local */
 	struct cgroup_file events_file;
 	struct cgroup_file events_local_file;
@@ -318,6 +316,8 @@ struct mem_cgroup {
 	/* List of events which userspace want to receive */
 	struct list_head event_list;
 	spinlock_t event_list_lock;
+
+	int swappiness;
 #endif /* CONFIG_MEMCG_V1 */
 
 	struct mem_cgroup_per_node *nodeinfo[];
diff --git a/include/linux/swap.h b/include/linux/swap.h
index 696ed01709c2..330a420fd6de 100644
--- a/include/linux/swap.h
+++ b/include/linux/swap.h
@@ -309,7 +309,6 @@ static inline bool lru_cache_disabled(void)
 }
 
 extern unsigned long shrink_all_memory(unsigned long nr_pages);
-extern int vm_swappiness;
 long remove_mapping(struct address_space *mapping, struct folio *folio);
 
 #if defined(CONFIG_SYSFS) && defined(CONFIG_NUMA)
@@ -468,25 +467,7 @@ static inline int add_swap_extent(struct swap_info_struct *sis,
 }
 #endif /* CONFIG_SWAP */
 #ifdef CONFIG_MEMCG
-static inline int mem_cgroup_swappiness(struct mem_cgroup *memcg)
-{
-	/* Cgroup2 doesn't have per-cgroup swappiness */
-	if (cgroup_subsys_on_dfl(memory_cgrp_subsys))
-		return READ_ONCE(vm_swappiness);
-
-	/* root ? */
-	if (mem_cgroup_disabled() || mem_cgroup_is_root(memcg))
-		return READ_ONCE(vm_swappiness);
-
-	return READ_ONCE(memcg->swappiness);
-}
-
 void lru_reparent_memcg(struct mem_cgroup *memcg, struct mem_cgroup *parent, int nid);
-#else
-static inline int mem_cgroup_swappiness(struct mem_cgroup *memcg)
-{
-	return READ_ONCE(vm_swappiness);
-}
 #endif
 
 #if defined(CONFIG_SWAP) && defined(CONFIG_MEMCG) && defined(CONFIG_BLK_CGROUP)
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 22f55aeb94f3..d6ba276740ea 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -63,6 +63,7 @@
 #include <linux/sched/isolation.h>
 #include <linux/kmemleak.h>
 #include "internal.h"
+#include "swap.h"
 #include "swap_table.h"
 #include <net/sock.h>
 #include <net/ip.h>
@@ -4174,11 +4175,10 @@ mem_cgroup_css_alloc(struct cgroup_subsys_state *parent_css)
 #endif
 	page_counter_set_high(&memcg->swap, PAGE_COUNTER_MAX);
 	if (parent) {
-		WRITE_ONCE(memcg->swappiness, mem_cgroup_swappiness(parent));
-
 		page_counter_init(&memcg->memory, &parent->memory, memcg_on_dfl);
 		page_counter_init(&memcg->swap, &parent->swap, false);
 #ifdef CONFIG_MEMCG_V1
+		WRITE_ONCE(memcg->swappiness, mem_cgroup_swappiness(parent));
 		memcg->memory.track_failcnt = !memcg_on_dfl;
 		WRITE_ONCE(memcg->oom_kill_disable, READ_ONCE(parent->oom_kill_disable));
 		page_counter_init(&memcg->kmem, &parent->kmem, false);
diff --git a/mm/swap.h b/mm/swap.h
index b51ad3071a73..a0e04b97a466 100644
--- a/mm/swap.h
+++ b/mm/swap.h
@@ -4,6 +4,7 @@
 
 #include <linux/atomic.h> /* for atomic_long_t */
 #include <linux/mm.h> /* for PAGE_SHIFT */
+#include <linux/memcontrol.h> /* for mem_cgroup_swappiness() */
 struct mempolicy;
 struct swap_iocb;
 struct swap_memcg_table;
@@ -76,6 +77,25 @@ enum swap_cluster_flags {
 	CLUSTER_FLAG_MAX,
 };
 
+extern int vm_swappiness;
+
+static inline int mem_cgroup_swappiness(struct mem_cgroup *memcg)
+{
+#ifdef CONFIG_MEMCG_V1
+	/* Cgroup2 doesn't have per-cgroup swappiness */
+	if (cgroup_subsys_on_dfl(memory_cgrp_subsys))
+		return READ_ONCE(vm_swappiness);
+
+	/* root ? */
+	if (mem_cgroup_disabled() || mem_cgroup_is_root(memcg))
+		return READ_ONCE(vm_swappiness);
+
+	return READ_ONCE(memcg->swappiness);
+#else
+	return READ_ONCE(vm_swappiness);
+#endif
+}
+
 #ifdef CONFIG_SWAP
 #include <linux/swapops.h> /* for swp_offset */
 #include <linux/blk_types.h> /* for bio_end_io_t */
-- 
2.34.1