mm: memcontrol: move memsw charge callbacks to v1

The interweaving of two entirely different swap accounting strategies has
been one of the more confusing parts of the memcg code.  Split out the v1
code to clarify the implementation and a handful of callsites, and to
avoid building the v1 bits when !CONFIG_MEMCG_V1.

   text	  data	   bss	   dec	   hex	filename
  39253	  6446	  4160	 49859	  c2c3	mm/memcontrol.o.old
  38877	  6382	  4160	 49419	  c10b	mm/memcontrol.o

Link: https://lkml.kernel.org/r/20250124054132.45643-1-hannes@cmpxchg.org
Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: Roman Gushchin <roman.gushchin@linux.dev>
Acked-by: Michal Hocko <mhocko@suse.com>
Acked-by: Balbir Singh <balbirs@nvidia.com>
Acked-by: Shakeel Butt <shakeel.butt@linux.dev>
Cc: Muchun Song <muchun.song@linux.dev>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

include/linux/memcontrol.h

@@ -649,8 +649,6 @@ int mem_cgroup_charge_hugetlb(struct folio* folio, gfp_t gfp);
 int mem_cgroup_swapin_charge_folio(struct folio *folio, struct mm_struct *mm,
 				  gfp_t gfp, swp_entry_t entry);
 
-void mem_cgroup_swapin_uncharge_swap(swp_entry_t entry, unsigned int nr_pages);
-
 void __mem_cgroup_uncharge(struct folio *folio);
 
 /**
@@ -1165,10 +1163,6 @@ static inline int mem_cgroup_swapin_charge_folio(struct folio *folio,
 	return 0;
 }
 
-static inline void mem_cgroup_swapin_uncharge_swap(swp_entry_t entry, unsigned int nr)
-{
-}
-
 static inline void mem_cgroup_uncharge(struct folio *folio)
 {
 }
@@ -1848,6 +1842,9 @@ static inline void mem_cgroup_exit_user_fault(void)
 	current->in_user_fault = 0;
 }
 
+void memcg1_swapout(struct folio *folio, swp_entry_t entry);
+void memcg1_swapin(swp_entry_t entry, unsigned int nr_pages);
+
 #else /* CONFIG_MEMCG_V1 */
 static inline
 unsigned long memcg1_soft_limit_reclaim(pg_data_t *pgdat, int order,
@@ -1875,6 +1872,14 @@ static inline void mem_cgroup_exit_user_fault(void)
 {
 }
 
+static inline void memcg1_swapout(struct folio *folio, swp_entry_t entry)
+{
+}
+
+static inline void memcg1_swapin(swp_entry_t entry, unsigned int nr_pages)
+{
+}
+
 #endif /* CONFIG_MEMCG_V1 */
 
 #endif /* _LINUX_MEMCONTROL_H */

include/linux/swap.h

@@ -659,7 +659,6 @@ static inline void folio_throttle_swaprate(struct folio *folio, gfp_t gfp)
 #endif
 
 #if defined(CONFIG_MEMCG) && defined(CONFIG_SWAP)
-void mem_cgroup_swapout(struct folio *folio, swp_entry_t entry);
 int __mem_cgroup_try_charge_swap(struct folio *folio, swp_entry_t entry);
 static inline int mem_cgroup_try_charge_swap(struct folio *folio,
 		swp_entry_t entry)
@@ -680,10 +679,6 @@ static inline void mem_cgroup_uncharge_swap(swp_entry_t entry, unsigned int nr_p
 extern long mem_cgroup_get_nr_swap_pages(struct mem_cgroup *memcg);
 extern bool mem_cgroup_swap_full(struct folio *folio);
 #else
-static inline void mem_cgroup_swapout(struct folio *folio, swp_entry_t entry)
-{
-}
-
 static inline int mem_cgroup_try_charge_swap(struct folio *folio,
 					     swp_entry_t entry)
 {

mm/huge_memory.c

@@ -3740,7 +3740,7 @@ void deferred_split_folio(struct folio *folio, bool partially_mapped)
 
 	/*
 	 * Exclude swapcache: originally to avoid a corrupt deferred split
-	 * queue. Nowadays that is fully prevented by mem_cgroup_swapout();
+	 * queue. Nowadays that is fully prevented by memcg1_swapout();
 	 * but if page reclaim is already handling the same folio, it is
 	 * unnecessary to handle it again in the shrinker, so excluding
 	 * swapcache here may still be a useful optimization.

mm/memcontrol-v1.c

@@ -581,8 +581,59 @@ void memcg1_commit_charge(struct folio *folio, struct mem_cgroup *memcg)
 	local_irq_restore(flags);
 }
 
-void memcg1_swapout(struct folio *folio, struct mem_cgroup *memcg)
+/**
+ * memcg1_swapout - transfer a memsw charge to swap
+ * @folio: folio whose memsw charge to transfer
+ * @entry: swap entry to move the charge to
+ *
+ * Transfer the memsw charge of @folio to @entry.
+ */
+void memcg1_swapout(struct folio *folio, swp_entry_t entry)
 {
+	struct mem_cgroup *memcg, *swap_memcg;
+	unsigned int nr_entries;
+
+	VM_BUG_ON_FOLIO(folio_test_lru(folio), folio);
+	VM_BUG_ON_FOLIO(folio_ref_count(folio), folio);
+
+	if (mem_cgroup_disabled())
+		return;
+
+	if (!do_memsw_account())
+		return;
+
+	memcg = folio_memcg(folio);
+
+	VM_WARN_ON_ONCE_FOLIO(!memcg, folio);
+	if (!memcg)
+		return;
+
+	/*
+	 * In case the memcg owning these pages has been offlined and doesn't
+	 * have an ID allocated to it anymore, charge the closest online
+	 * ancestor for the swap instead and transfer the memory+swap charge.
+	 */
+	swap_memcg = mem_cgroup_id_get_online(memcg);
+	nr_entries = folio_nr_pages(folio);
+	/* Get references for the tail pages, too */
+	if (nr_entries > 1)
+		mem_cgroup_id_get_many(swap_memcg, nr_entries - 1);
+	mod_memcg_state(swap_memcg, MEMCG_SWAP, nr_entries);
+
+	swap_cgroup_record(folio, mem_cgroup_id(memcg), entry);
+
+	folio_unqueue_deferred_split(folio);
+	folio->memcg_data = 0;
+
+	if (!mem_cgroup_is_root(memcg))
+		page_counter_uncharge(&memcg->memory, nr_entries);
+
+	if (memcg != swap_memcg) {
+		if (!mem_cgroup_is_root(swap_memcg))
+			page_counter_charge(&swap_memcg->memsw, nr_entries);
+		page_counter_uncharge(&memcg->memsw, nr_entries);
+	}
+
 	/*
 	 * Interrupts should be disabled here because the caller holds the
 	 * i_pages lock which is taken with interrupts-off. It is
@@ -594,6 +645,42 @@ void memcg1_swapout(struct folio *folio, struct mem_cgroup *memcg)
 	memcg1_charge_statistics(memcg, -folio_nr_pages(folio));
 	preempt_enable_nested();
 	memcg1_check_events(memcg, folio_nid(folio));
+
+	css_put(&memcg->css);
+}
+
+/*
+ * memcg1_swapin - uncharge swap slot
+ * @entry: the first swap entry for which the pages are charged
+ * @nr_pages: number of pages which will be uncharged
+ *
+ * Call this function after successfully adding the charged page to swapcache.
+ *
+ * Note: This function assumes the page for which swap slot is being uncharged
+ * is order 0 page.
+ */
+void memcg1_swapin(swp_entry_t entry, unsigned int nr_pages)
+{
+	/*
+	 * Cgroup1's unified memory+swap counter has been charged with the
+	 * new swapcache page, finish the transfer by uncharging the swap
+	 * slot. The swap slot would also get uncharged when it dies, but
+	 * it can stick around indefinitely and we'd count the page twice
+	 * the entire time.
+	 *
+	 * Cgroup2 has separate resource counters for memory and swap,
+	 * so this is a non-issue here. Memory and swap charge lifetimes
+	 * correspond 1:1 to page and swap slot lifetimes: we charge the
+	 * page to memory here, and uncharge swap when the slot is freed.
+	 */
+	if (do_memsw_account()) {
+		/*
+		 * The swap entry might not get freed for a long time,
+		 * let's not wait for it.  The page already received a
+		 * memory+swap charge, drop the swap entry duplicate.
+		 */
+		mem_cgroup_uncharge_swap(entry, nr_pages);
+	}
 }
 
 void memcg1_uncharge_batch(struct mem_cgroup *memcg, unsigned long pgpgout,

mm/memcontrol-v1.h

@@ -36,6 +36,9 @@ unsigned long memcg_events(struct mem_cgroup *memcg, int event);
 unsigned long memcg_page_state_output(struct mem_cgroup *memcg, int item);
 int memory_stat_show(struct seq_file *m, void *v);
 
+void mem_cgroup_id_get_many(struct mem_cgroup *memcg, unsigned int n);
+struct mem_cgroup *mem_cgroup_id_get_online(struct mem_cgroup *memcg);
+
 /* Cgroup v1-specific declarations */
 #ifdef CONFIG_MEMCG_V1
 
@@ -69,7 +72,6 @@ void memcg1_oom_finish(struct mem_cgroup *memcg, bool locked);
 void memcg1_oom_recover(struct mem_cgroup *memcg);
 
 void memcg1_commit_charge(struct folio *folio, struct mem_cgroup *memcg);
-void memcg1_swapout(struct folio *folio, struct mem_cgroup *memcg);
 void memcg1_uncharge_batch(struct mem_cgroup *memcg, unsigned long pgpgout,
 			   unsigned long nr_memory, int nid);
 
@@ -107,8 +109,6 @@ static inline void memcg1_oom_recover(struct mem_cgroup *memcg) {}
 static inline void memcg1_commit_charge(struct folio *folio,
 					struct mem_cgroup *memcg) {}
 
-static inline void memcg1_swapout(struct folio *folio, struct mem_cgroup *memcg) {}
-
 static inline void memcg1_uncharge_batch(struct mem_cgroup *memcg,
 					 unsigned long pgpgout,
 					 unsigned long nr_memory, int nid) {}

mm/memcontrol.c

@@ -3398,7 +3398,7 @@ static void mem_cgroup_id_remove(struct mem_cgroup *memcg)
 	}
 }
 
-static void __maybe_unused mem_cgroup_id_get_many(struct mem_cgroup *memcg,
+void __maybe_unused mem_cgroup_id_get_many(struct mem_cgroup *memcg,
 					   unsigned int n)
 {
 	refcount_add(n, &memcg->id.ref);
@@ -3419,6 +3419,24 @@ static inline void mem_cgroup_id_put(struct mem_cgroup *memcg)
 	mem_cgroup_id_put_many(memcg, 1);
 }
 
+struct mem_cgroup *mem_cgroup_id_get_online(struct mem_cgroup *memcg)
+{
+	while (!refcount_inc_not_zero(&memcg->id.ref)) {
+		/*
+		 * The root cgroup cannot be destroyed, so it's refcount must
+		 * always be >= 1.
+		 */
+		if (WARN_ON_ONCE(mem_cgroup_is_root(memcg))) {
+			VM_BUG_ON(1);
+			break;
+		}
+		memcg = parent_mem_cgroup(memcg);
+		if (!memcg)
+			memcg = root_mem_cgroup;
+	}
+	return memcg;
+}
+
 /**
  * mem_cgroup_from_id - look up a memcg from a memcg id
  * @id: the memcg id to look up
@@ -4604,40 +4622,6 @@ int mem_cgroup_swapin_charge_folio(struct folio *folio, struct mm_struct *mm,
 	return ret;
 }
 
-/*
- * mem_cgroup_swapin_uncharge_swap - uncharge swap slot
- * @entry: the first swap entry for which the pages are charged
- * @nr_pages: number of pages which will be uncharged
- *
- * Call this function after successfully adding the charged page to swapcache.
- *
- * Note: This function assumes the page for which swap slot is being uncharged
- * is order 0 page.
- */
-void mem_cgroup_swapin_uncharge_swap(swp_entry_t entry, unsigned int nr_pages)
-{
-	/*
-	 * Cgroup1's unified memory+swap counter has been charged with the
-	 * new swapcache page, finish the transfer by uncharging the swap
-	 * slot. The swap slot would also get uncharged when it dies, but
-	 * it can stick around indefinitely and we'd count the page twice
-	 * the entire time.
-	 *
-	 * Cgroup2 has separate resource counters for memory and swap,
-	 * so this is a non-issue here. Memory and swap charge lifetimes
-	 * correspond 1:1 to page and swap slot lifetimes: we charge the
-	 * page to memory here, and uncharge swap when the slot is freed.
-	 */
-	if (do_memsw_account()) {
-		/*
-		 * The swap entry might not get freed for a long time,
-		 * let's not wait for it.  The page already received a
-		 * memory+swap charge, drop the swap entry duplicate.
-		 */
-		mem_cgroup_uncharge_swap(entry, nr_pages);
-	}
-}
-
 struct uncharge_gather {
 	struct mem_cgroup *memcg;
 	unsigned long nr_memory;
@@ -4963,81 +4947,6 @@ static int __init mem_cgroup_init(void)
 subsys_initcall(mem_cgroup_init);
 
 #ifdef CONFIG_SWAP
-static struct mem_cgroup *mem_cgroup_id_get_online(struct mem_cgroup *memcg)
-{
-	while (!refcount_inc_not_zero(&memcg->id.ref)) {
-		/*
-		 * The root cgroup cannot be destroyed, so it's refcount must
-		 * always be >= 1.
-		 */
-		if (WARN_ON_ONCE(mem_cgroup_is_root(memcg))) {
-			VM_BUG_ON(1);
-			break;
-		}
-		memcg = parent_mem_cgroup(memcg);
-		if (!memcg)
-			memcg = root_mem_cgroup;
-	}
-	return memcg;
-}
-
-/**
- * mem_cgroup_swapout - transfer a memsw charge to swap
- * @folio: folio whose memsw charge to transfer
- * @entry: swap entry to move the charge to
- *
- * Transfer the memsw charge of @folio to @entry.
- */
-void mem_cgroup_swapout(struct folio *folio, swp_entry_t entry)
-{
-	struct mem_cgroup *memcg, *swap_memcg;
-	unsigned int nr_entries;
-
-	VM_BUG_ON_FOLIO(folio_test_lru(folio), folio);
-	VM_BUG_ON_FOLIO(folio_ref_count(folio), folio);
-
-	if (mem_cgroup_disabled())
-		return;
-
-	if (!do_memsw_account())
-		return;
-
-	memcg = folio_memcg(folio);
-
-	VM_WARN_ON_ONCE_FOLIO(!memcg, folio);
-	if (!memcg)
-		return;
-
-	/*
-	 * In case the memcg owning these pages has been offlined and doesn't
-	 * have an ID allocated to it anymore, charge the closest online
-	 * ancestor for the swap instead and transfer the memory+swap charge.
-	 */
-	swap_memcg = mem_cgroup_id_get_online(memcg);
-	nr_entries = folio_nr_pages(folio);
-	/* Get references for the tail pages, too */
-	if (nr_entries > 1)
-		mem_cgroup_id_get_many(swap_memcg, nr_entries - 1);
-	mod_memcg_state(swap_memcg, MEMCG_SWAP, nr_entries);
-
-	swap_cgroup_record(folio, mem_cgroup_id(swap_memcg), entry);
-
-	folio_unqueue_deferred_split(folio);
-	folio->memcg_data = 0;
-
-	if (!mem_cgroup_is_root(memcg))
-		page_counter_uncharge(&memcg->memory, nr_entries);
-
-	if (memcg != swap_memcg) {
-		if (!mem_cgroup_is_root(swap_memcg))
-			page_counter_charge(&swap_memcg->memsw, nr_entries);
-		page_counter_uncharge(&memcg->memsw, nr_entries);
-	}
-
-	memcg1_swapout(folio, memcg);
-	css_put(&memcg->css);
-}
-
 /**
  * __mem_cgroup_try_charge_swap - try charging swap space for a folio
  * @folio: folio being added to swap

mm/memory.c

@@ -4407,7 +4407,7 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
 				}
 				need_clear_cache = true;
 
-				mem_cgroup_swapin_uncharge_swap(entry, nr_pages);
+				memcg1_swapin(entry, nr_pages);
 
 				shadow = get_shadow_from_swap_cache(entry);
 				if (shadow)

mm/shmem.c

@@ -2017,7 +2017,7 @@ static struct folio *shmem_swap_alloc_folio(struct inode *inode,
 	__folio_set_swapbacked(new);
 	new->swap = entry;
 
-	mem_cgroup_swapin_uncharge_swap(entry, nr_pages);
+	memcg1_swapin(entry, nr_pages);
 	shadow = get_shadow_from_swap_cache(entry);
 	if (shadow)
 		workingset_refault(new, shadow);

mm/swap_state.c

@@ -521,7 +521,7 @@ struct folio *__read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask,
 	if (add_to_swap_cache(new_folio, entry, gfp_mask & GFP_RECLAIM_MASK, &shadow))
 		goto fail_unlock;
 
-	mem_cgroup_swapin_uncharge_swap(entry, 1);
+	memcg1_swapin(entry, 1);
 
 	if (shadow)
 		workingset_refault(new_folio, shadow);

mm/vmscan.c

@@ -769,7 +769,7 @@ static int __remove_mapping(struct address_space *mapping, struct folio *folio,
 		if (reclaimed && !mapping_exiting(mapping))
 			shadow = workingset_eviction(folio, target_memcg);
 		__delete_from_swap_cache(folio, swap, shadow);
-		mem_cgroup_swapout(folio, swap);
+		memcg1_swapout(folio, swap);
 		xa_unlock_irq(&mapping->i_pages);
 		put_swap_folio(folio, swap);
 	} else {