[PATCH 00/11] mm: memcontrol: constify the read side of the memcg API

[PATCH 00/11] mm: memcontrol: constify the read side of the memcg API

[Tal Zussman]

The memcg accessors, lruvec helpers, and stat readers only read from
the memcg, folio, or lruvec they are given, but take non-const
pointers. Constify them, along with a couple other helper functions in
mm/page_counter.c along the way. 

This started as a follow-up to [1] that quickly grew in proportions :)

[1]: https://lore.kernel.org/linux-mm/anYqSDnC2BbJXv1h@casper.infradead.org/

---
Tal Zussman (11):
      mm: memcontrol: take a const folio in folio_memcg() and friends
      mm: memcontrol: constify obj_cgroup_memcg() and friends
      mm: memcontrol: constify the lruvec helpers
      mm/page_io: take a const folio in bio_associate_blkg_from_folio()
      mm: memcontrol: constify the mem_cgroup accessors
      mm: page_counter: constify page_counter_read() and page_counter_margin()
      mm: memcontrol: constify the reclaim protection helpers
      mm: memcontrol: constify the memcg and lruvec stat readers
      mm: memcontrol: constify the swap accounting helpers
      mm: memcontrol: constify mem_cgroup_swappiness() and mem_cgroup_get_max()
      mm: memcontrol: constify the zswap and socket pressure helpers

 include/linux/memcontrol.h   | 196 ++++++++++++++++++++++---------------------
 include/linux/mmzone.h       |   2 +-
 include/linux/page_counter.h |   4 +-
 include/linux/swap.h         |  12 +--
 mm/memcontrol.c              |  37 ++++----
 mm/page_counter.c            |   2 +-
 mm/page_io.c                 |  14 ++--
 mm/swap.h                    |   2 +-
 8 files changed, 138 insertions(+), 131 deletions(-)
---
base-commit: 178b3d97bf1f15f598ea7cc615a40c115e528e1c
change-id: 20260807-folio_memcg-const-4a67e40d485b

Best regards,
--  
Tal Zussman <tz2294@columbia.edu>

[PATCH 01/11] mm: memcontrol: take a const folio in folio_memcg() and friends

[Tal Zussman]

The folio_memcg() family only reads from the folio, and everything it
calls already takes a const folio. Constify it, along with the page
wrappers built on top of it.

Signed-off-by: Tal Zussman <tz2294@columbia.edu>
---
 include/linux/memcontrol.h | 46 +++++++++++++++++++++++-----------------------
 mm/memcontrol.c            |  6 +++---
 2 files changed, 26 insertions(+), 26 deletions(-)

diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index 945d7dbf1bb2..91ca94667d44 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -377,7 +377,7 @@ static inline struct mem_cgroup *obj_cgroup_memcg(struct obj_cgroup *objcg)
  * or NULL. This function assumes that the folio is known to have a
  * proper object cgroup pointer.
  */
-static inline struct obj_cgroup *folio_objcg(struct folio *folio)
+static inline struct obj_cgroup *folio_objcg(const struct folio *folio)
 {
 	unsigned long memcg_data = folio->memcg_data;
 
@@ -415,7 +415,7 @@ static inline struct obj_cgroup *folio_objcg(struct folio *folio)
  * Note: The caller should hold an rcu read lock or cgroup_mutex to protect
  * memcg associated with a folio from being released.
  */
-static inline struct mem_cgroup *folio_memcg(struct folio *folio)
+static inline struct mem_cgroup *folio_memcg(const struct folio *folio)
 {
 	struct obj_cgroup *objcg = folio_objcg(folio);
 
@@ -428,7 +428,7 @@ static inline struct mem_cgroup *folio_memcg(struct folio *folio)
  *
  * Returns true if folio is charged to a memory cgroup, otherwise returns false.
  */
-static inline bool folio_memcg_charged(struct folio *folio)
+static inline bool folio_memcg_charged(const struct folio *folio)
 {
 	return folio->memcg_data != 0;
 }
@@ -448,7 +448,7 @@ static inline bool folio_memcg_charged(struct folio *folio)
  * A caller should hold an rcu read lock to protect memcg associated with a
  * page from being released.
  */
-static inline struct mem_cgroup *folio_memcg_check(struct folio *folio)
+static inline struct mem_cgroup *folio_memcg_check(const struct folio *folio)
 {
 	/*
 	 * Because folio->memcg_data might be changed asynchronously
@@ -465,11 +465,11 @@ static inline struct mem_cgroup *folio_memcg_check(struct folio *folio)
 	return obj_cgroup_memcg(objcg);
 }
 
-static inline struct mem_cgroup *page_memcg_check(struct page *page)
+static inline struct mem_cgroup *page_memcg_check(const struct page *page)
 {
 	if (PageTail(page))
 		return NULL;
-	return folio_memcg_check((struct folio *)page);
+	return folio_memcg_check((const struct folio *)page);
 }
 
 static inline struct mem_cgroup *get_mem_cgroup_from_objcg(struct obj_cgroup *objcg)
@@ -494,14 +494,14 @@ static inline struct mem_cgroup *get_mem_cgroup_from_objcg(struct obj_cgroup *ob
  * that the folio has an associated memory cgroup. It's not safe to call
  * this function against some types of folios, e.g. slab folios.
  */
-static inline bool folio_memcg_kmem(struct folio *folio)
+static inline bool folio_memcg_kmem(const struct folio *folio)
 {
 	VM_BUG_ON_PGFLAGS(PageTail(&folio->page), &folio->page);
 	VM_BUG_ON_FOLIO(folio->memcg_data & MEMCG_DATA_OBJEXTS, folio);
 	return folio->memcg_data & MEMCG_DATA_KMEM;
 }
 
-static inline bool PageMemcgKmem(struct page *page)
+static inline bool PageMemcgKmem(const struct page *page)
 {
 	return folio_memcg_kmem(page_folio(page));
 }
@@ -743,7 +743,7 @@ struct mem_cgroup *get_mem_cgroup_from_mm(struct mm_struct *mm);
 
 struct mem_cgroup *get_mem_cgroup_from_current(void);
 
-struct mem_cgroup *get_mem_cgroup_from_folio(struct folio *folio);
+struct mem_cgroup *get_mem_cgroup_from_folio(const struct folio *folio);
 
 struct lruvec *folio_lruvec_lock(struct folio *folio);
 struct lruvec *folio_lruvec_lock_irq(struct folio *folio);
@@ -871,8 +871,8 @@ static inline bool mm_match_cgroup(struct mm_struct *mm,
 	return match;
 }
 
-struct cgroup_subsys_state *get_mem_cgroup_css_from_folio(struct folio *folio);
-ino_t page_cgroup_ino(struct page *page);
+struct cgroup_subsys_state *get_mem_cgroup_css_from_folio(const struct folio *folio);
+ino_t page_cgroup_ino(const struct page *page);
 
 static inline bool mem_cgroup_online(struct mem_cgroup *memcg)
 {
@@ -917,7 +917,7 @@ void mem_cgroup_print_oom_group(struct mem_cgroup *memcg);
 void mod_memcg_state(struct mem_cgroup *memcg,
 		     enum memcg_stat_item idx, int val);
 
-static inline void mod_memcg_page_state(struct page *page,
+static inline void mod_memcg_page_state(const struct page *page,
 					enum memcg_stat_item idx, int val)
 {
 	struct mem_cgroup *memcg;
@@ -951,7 +951,7 @@ void mod_lruvec_kmem_state(void *p, enum node_stat_item idx, int val);
 void count_memcg_events(struct mem_cgroup *memcg, enum vm_event_item idx,
 			unsigned long count);
 
-static inline void count_memcg_folio_events(struct folio *folio,
+static inline void count_memcg_folio_events(const struct folio *folio,
 		enum vm_event_item idx, unsigned long nr)
 {
 	struct mem_cgroup *memcg;
@@ -1045,22 +1045,22 @@ static inline struct mem_cgroup *obj_cgroup_memcg(struct obj_cgroup *objcg)
 
 #define root_mem_cgroup		(NULL)
 
-static inline struct mem_cgroup *folio_memcg(struct folio *folio)
+static inline struct mem_cgroup *folio_memcg(const struct folio *folio)
 {
 	return NULL;
 }
 
-static inline bool folio_memcg_charged(struct folio *folio)
+static inline bool folio_memcg_charged(const struct folio *folio)
 {
 	return false;
 }
 
-static inline struct mem_cgroup *folio_memcg_check(struct folio *folio)
+static inline struct mem_cgroup *folio_memcg_check(const struct folio *folio)
 {
 	return NULL;
 }
 
-static inline struct mem_cgroup *page_memcg_check(struct page *page)
+static inline struct mem_cgroup *page_memcg_check(const struct page *page)
 {
 	return NULL;
 }
@@ -1070,12 +1070,12 @@ static inline struct mem_cgroup *get_mem_cgroup_from_objcg(struct obj_cgroup *ob
 	return NULL;
 }
 
-static inline bool folio_memcg_kmem(struct folio *folio)
+static inline bool folio_memcg_kmem(const struct folio *folio)
 {
 	return false;
 }
 
-static inline bool PageMemcgKmem(struct page *page)
+static inline bool PageMemcgKmem(const struct page *page)
 {
 	return false;
 }
@@ -1208,7 +1208,7 @@ static inline struct mem_cgroup *get_mem_cgroup_from_current(void)
 	return NULL;
 }
 
-static inline struct mem_cgroup *get_mem_cgroup_from_folio(struct folio *folio)
+static inline struct mem_cgroup *get_mem_cgroup_from_folio(const struct folio *folio)
 {
 	return NULL;
 }
@@ -1366,7 +1366,7 @@ static inline void mod_memcg_state(struct mem_cgroup *memcg,
 {
 }
 
-static inline void mod_memcg_page_state(struct page *page,
+static inline void mod_memcg_page_state(const struct page *page,
 					enum memcg_stat_item idx, int val)
 {
 }
@@ -1431,7 +1431,7 @@ static inline void count_memcg_events(struct mem_cgroup *memcg,
 {
 }
 
-static inline void count_memcg_folio_events(struct folio *folio,
+static inline void count_memcg_folio_events(const struct folio *folio,
 		enum vm_event_item idx, unsigned long nr)
 {
 }
@@ -1861,7 +1861,7 @@ static inline void count_objcg_events(struct obj_cgroup *objcg,
 {
 }
 
-static inline ino_t page_cgroup_ino(struct page *page)
+static inline ino_t page_cgroup_ino(const struct page *page)
 {
 	return 0;
 }
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 5b9e0ebd42ae..b9bf655cdd2d 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -331,7 +331,7 @@ EXPORT_SYMBOL(memcg_bpf_enabled_key);
  * If memcg is bound to a traditional hierarchy, the css of root_mem_cgroup
  * is returned.
  */
-struct cgroup_subsys_state *get_mem_cgroup_css_from_folio(struct folio *folio)
+struct cgroup_subsys_state *get_mem_cgroup_css_from_folio(const struct folio *folio)
 {
 	struct mem_cgroup *memcg;
 
@@ -356,7 +356,7 @@ struct cgroup_subsys_state *get_mem_cgroup_css_from_folio(struct folio *folio)
  * after page_cgroup_ino() returns, so it only should be used by callers that
  * do not care (such as procfs interfaces).
  */
-ino_t page_cgroup_ino(struct page *page)
+ino_t page_cgroup_ino(const struct page *page)
 {
 	struct mem_cgroup *memcg;
 	unsigned long ino = 0;
@@ -1220,7 +1220,7 @@ struct mem_cgroup *get_mem_cgroup_from_current(void)
  *
  * See folio_memcg() for folio->objcg/memcg binding rules.
  */
-struct mem_cgroup *get_mem_cgroup_from_folio(struct folio *folio)
+struct mem_cgroup *get_mem_cgroup_from_folio(const struct folio *folio)
 {
 	struct mem_cgroup *memcg;
 

-- 
2.39.5

[PATCH 02/11] mm: memcontrol: constify obj_cgroup_memcg() and friends

[Tal Zussman]

obj_cgroup_memcg() and get_mem_cgroup_from_objcg() only read from the
objcg. Constify them.

Signed-off-by: Tal Zussman <tz2294@columbia.edu>
---
 include/linux/memcontrol.h | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index 91ca94667d44..2b3b62e84ab2 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -363,7 +363,7 @@ enum objext_flags {
  *
  * The caller must ensure that the returned memcg won't be released.
  */
-static inline struct mem_cgroup *obj_cgroup_memcg(struct obj_cgroup *objcg)
+static inline struct mem_cgroup *obj_cgroup_memcg(const struct obj_cgroup *objcg)
 {
 	lockdep_assert_once(rcu_read_lock_held() || lockdep_is_held(&cgroup_mutex));
 	return objcg ? READ_ONCE(objcg->memcg) : NULL;
@@ -472,7 +472,7 @@ static inline struct mem_cgroup *page_memcg_check(const struct page *page)
 	return folio_memcg_check((const struct folio *)page);
 }
 
-static inline struct mem_cgroup *get_mem_cgroup_from_objcg(struct obj_cgroup *objcg)
+static inline struct mem_cgroup *get_mem_cgroup_from_objcg(const struct obj_cgroup *objcg)
 {
 	struct mem_cgroup *memcg;
 
@@ -1036,7 +1036,7 @@ void mem_cgroup_flush_workqueue(void);
 extern int mem_cgroup_init(void);
 #else /* CONFIG_MEMCG */
 
-static inline struct mem_cgroup *obj_cgroup_memcg(struct obj_cgroup *objcg)
+static inline struct mem_cgroup *obj_cgroup_memcg(const struct obj_cgroup *objcg)
 {
 	return NULL;
 }
@@ -1065,7 +1065,7 @@ static inline struct mem_cgroup *page_memcg_check(const struct page *page)
 	return NULL;
 }
 
-static inline struct mem_cgroup *get_mem_cgroup_from_objcg(struct obj_cgroup *objcg)
+static inline struct mem_cgroup *get_mem_cgroup_from_objcg(const struct obj_cgroup *objcg)
 {
 	return NULL;
 }

-- 
2.39.5

[PATCH 03/11] mm: memcontrol: constify the lruvec helpers

[Tal Zussman]

The lruvec lookup helpers only read from the memcg, folio, or lruvec
they are given. Constify them, along with lruvec_pgdat().

Signed-off-by: Tal Zussman <tz2294@columbia.edu>
---
 include/linux/memcontrol.h | 34 +++++++++++++++++-----------------
 include/linux/mmzone.h     |  2 +-
 mm/memcontrol.c            |  6 +++---
 3 files changed, 21 insertions(+), 21 deletions(-)

diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index 2b3b62e84ab2..4b7cc97e45b2 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -688,7 +688,7 @@ void mem_cgroup_migrate(struct folio *old, struct folio *new);
  * @pgdat combination. This can be the node lruvec, if the memory
  * controller is disabled.
  */
-static inline struct lruvec *mem_cgroup_lruvec(struct mem_cgroup *memcg,
+static inline struct lruvec *mem_cgroup_lruvec(const struct mem_cgroup *memcg,
 					       struct pglist_data *pgdat)
 {
 	struct mem_cgroup_per_node *mz;
@@ -729,7 +729,7 @@ static inline struct lruvec *mem_cgroup_lruvec(struct mem_cgroup *memcg,
  * their binding is stable if the returned lruvec matches the one the caller has
  * locked. Useful for lock batching.
  */
-static inline struct lruvec *folio_lruvec(struct folio *folio)
+static inline struct lruvec *folio_lruvec(const struct folio *folio)
 {
 	struct mem_cgroup *memcg = folio_memcg(folio);
 
@@ -745,9 +745,9 @@ struct mem_cgroup *get_mem_cgroup_from_current(void);
 
 struct mem_cgroup *get_mem_cgroup_from_folio(const struct folio *folio);
 
-struct lruvec *folio_lruvec_lock(struct folio *folio);
-struct lruvec *folio_lruvec_lock_irq(struct folio *folio);
-struct lruvec *folio_lruvec_lock_irqsave(struct folio *folio,
+struct lruvec *folio_lruvec_lock(const struct folio *folio);
+struct lruvec *folio_lruvec_lock_irq(const struct folio *folio);
+struct lruvec *folio_lruvec_lock_irqsave(const struct folio *folio,
 						unsigned long *flags);
 
 static inline
@@ -827,7 +827,7 @@ static inline struct mem_cgroup *mem_cgroup_from_seq(struct seq_file *m)
 	return mem_cgroup_from_css(seq_css(m));
 }
 
-static inline struct mem_cgroup *lruvec_memcg(struct lruvec *lruvec)
+static inline struct mem_cgroup *lruvec_memcg(const struct lruvec *lruvec)
 {
 	struct mem_cgroup_per_node *mz;
 
@@ -885,7 +885,7 @@ void mem_cgroup_update_lru_size(struct lruvec *lruvec, enum lru_list lru,
 		int zid, long nr_pages);
 
 static inline
-unsigned long mem_cgroup_get_zone_lru_size(struct lruvec *lruvec,
+unsigned long mem_cgroup_get_zone_lru_size(const struct lruvec *lruvec,
 		enum lru_list lru, int zone_idx)
 {
 	struct mem_cgroup_per_node *mz;
@@ -1175,13 +1175,13 @@ static inline void mem_cgroup_migrate(struct folio *old, struct folio *new)
 {
 }
 
-static inline struct lruvec *mem_cgroup_lruvec(struct mem_cgroup *memcg,
+static inline struct lruvec *mem_cgroup_lruvec(const struct mem_cgroup *memcg,
 					       struct pglist_data *pgdat)
 {
 	return &pgdat->__lruvec;
 }
 
-static inline struct lruvec *folio_lruvec(struct folio *folio)
+static inline struct lruvec *folio_lruvec(const struct folio *folio)
 {
 	struct pglist_data *pgdat = folio_pgdat(folio);
 	return &pgdat->__lruvec;
@@ -1241,7 +1241,7 @@ static inline void mem_cgroup_put(struct mem_cgroup *memcg)
 {
 }
 
-static inline struct lruvec *folio_lruvec_lock(struct folio *folio)
+static inline struct lruvec *folio_lruvec_lock(const struct folio *folio)
 {
 	struct pglist_data *pgdat = folio_pgdat(folio);
 
@@ -1250,7 +1250,7 @@ static inline struct lruvec *folio_lruvec_lock(struct folio *folio)
 	return &pgdat->__lruvec;
 }
 
-static inline struct lruvec *folio_lruvec_lock_irq(struct folio *folio)
+static inline struct lruvec *folio_lruvec_lock_irq(const struct folio *folio)
 {
 	struct pglist_data *pgdat = folio_pgdat(folio);
 
@@ -1259,7 +1259,7 @@ static inline struct lruvec *folio_lruvec_lock_irq(struct folio *folio)
 	return &pgdat->__lruvec;
 }
 
-static inline struct lruvec *folio_lruvec_lock_irqsave(struct folio *folio,
+static inline struct lruvec *folio_lruvec_lock_irqsave(const struct folio *folio,
 		unsigned long *flagsp)
 {
 	struct pglist_data *pgdat = folio_pgdat(folio);
@@ -1314,7 +1314,7 @@ static inline struct mem_cgroup *mem_cgroup_from_seq(struct seq_file *m)
 	return NULL;
 }
 
-static inline struct mem_cgroup *lruvec_memcg(struct lruvec *lruvec)
+static inline struct mem_cgroup *lruvec_memcg(const struct lruvec *lruvec)
 {
 	return NULL;
 }
@@ -1325,7 +1325,7 @@ static inline bool mem_cgroup_online(struct mem_cgroup *memcg)
 }
 
 static inline
-unsigned long mem_cgroup_get_zone_lru_size(struct lruvec *lruvec,
+unsigned long mem_cgroup_get_zone_lru_size(const struct lruvec *lruvec,
 		enum lru_list lru, int zone_idx)
 {
 	return 0;
@@ -1465,7 +1465,7 @@ static inline void mem_cgroup_flush_workqueue(void) { }
 static inline int mem_cgroup_init(void) { return 0; }
 #endif /* CONFIG_MEMCG */
 
-static inline struct lruvec *parent_lruvec(struct lruvec *lruvec)
+static inline struct lruvec *parent_lruvec(const struct lruvec *lruvec)
 {
 	struct mem_cgroup *memcg;
 
@@ -1528,8 +1528,8 @@ static inline void lruvec_unlock_irqrestore(struct lruvec *lruvec, unsigned long
 }
 
 /* Test requires a stable folio->memcg binding, see folio_memcg() */
-static inline bool folio_matches_lruvec(struct folio *folio,
-		struct lruvec *lruvec)
+static inline bool folio_matches_lruvec(const struct folio *folio,
+		const struct lruvec *lruvec)
 {
 	return lruvec_pgdat(lruvec) == folio_pgdat(folio) &&
 	       lruvec_memcg(lruvec) == folio_memcg(folio);
diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index 84e237f2c17d..8e1e3a50757a 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -1657,7 +1657,7 @@ extern void init_currently_empty_zone(struct zone *zone, unsigned long start_pfn
 
 extern void lruvec_init(struct lruvec *lruvec);
 
-static inline struct pglist_data *lruvec_pgdat(struct lruvec *lruvec)
+static inline struct pglist_data *lruvec_pgdat(const struct lruvec *lruvec)
 {
 #ifdef CONFIG_MEMCG
 	return lruvec->pgdat;
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index b9bf655cdd2d..e2845f761e37 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -1441,7 +1441,7 @@ void mem_cgroup_scan_tasks(struct mem_cgroup *memcg,
  *
  * Return: The lruvec this folio is on with its lock held and rcu read lock held.
  */
-struct lruvec *folio_lruvec_lock(struct folio *folio)
+struct lruvec *folio_lruvec_lock(const struct folio *folio)
 {
 	struct lruvec *lruvec;
 
@@ -1469,7 +1469,7 @@ struct lruvec *folio_lruvec_lock(struct folio *folio)
  * Return: The lruvec this folio is on with its lock held and interrupts
  * disabled and rcu read lock held.
  */
-struct lruvec *folio_lruvec_lock_irq(struct folio *folio)
+struct lruvec *folio_lruvec_lock_irq(const struct folio *folio)
 {
 	struct lruvec *lruvec;
 
@@ -1498,7 +1498,7 @@ struct lruvec *folio_lruvec_lock_irq(struct folio *folio)
  * Return: The lruvec this folio is on with its lock held and interrupts
  * disabled and rcu read lock held.
  */
-struct lruvec *folio_lruvec_lock_irqsave(struct folio *folio,
+struct lruvec *folio_lruvec_lock_irqsave(const struct folio *folio,
 		unsigned long *flags)
 {
 	struct lruvec *lruvec;

-- 
2.39.5

[PATCH 04/11] mm/page_io: take a const folio in bio_associate_blkg_from_folio()

[Tal Zussman]

bio_associate_blkg_from_folio() and its helpers only read from the
folio. Constify their folio arguments.

Signed-off-by: Tal Zussman <tz2294@columbia.edu>
---
 mm/page_io.c | 14 +++++++++-----
 1 file changed, 9 insertions(+), 5 deletions(-)

diff --git a/mm/page_io.c b/mm/page_io.c
index 1da4ff484f09..5f7756e370f7 100644
--- a/mm/page_io.c
+++ b/mm/page_io.c
@@ -256,12 +256,13 @@ int swap_writeout(struct swap_io_ctx *ctx, struct folio *folio)
 }
 
 #if defined(CONFIG_MEMCG) && defined(CONFIG_BLK_CGROUP)
-static struct cgroup_subsys_state *folio_memcg_blkg_css(struct folio *folio)
+static struct cgroup_subsys_state *folio_memcg_blkg_css(const struct folio *folio)
 {
 	return cgroup_e_css(folio_memcg(folio)->css.cgroup, &io_cgrp_subsys);
 }
 
-static bool folio_blkg_can_merge(struct folio *folio, struct folio *prev_folio)
+static bool folio_blkg_can_merge(const struct folio *folio,
+				 const struct folio *prev_folio)
 {
 	bool can_merge = true;
 
@@ -277,7 +278,8 @@ static bool folio_blkg_can_merge(struct folio *folio, struct folio *prev_folio)
 	return can_merge;
 }
 
-static void bio_associate_blkg_from_folio(struct bio *bio, struct folio *folio)
+static void bio_associate_blkg_from_folio(struct bio *bio,
+					  const struct folio *folio)
 {
 	struct cgroup_subsys_state *css;
 
@@ -294,11 +296,13 @@ static void bio_associate_blkg_from_folio(struct bio *bio, struct folio *folio)
 		css_put(css);
 }
 #else
-static bool folio_blkg_can_merge(struct folio *folio, struct folio *prev_folio)
+static bool folio_blkg_can_merge(const struct folio *folio,
+				 const struct folio *prev_folio)
 {
 	return true;
 }
-static void bio_associate_blkg_from_folio(struct bio *bio, struct folio *folio)
+static void bio_associate_blkg_from_folio(struct bio *bio,
+					  const struct folio *folio)
 {
 }
 #endif /* CONFIG_MEMCG && CONFIG_BLK_CGROUP */

-- 
2.39.5

[PATCH 05/11] mm: memcontrol: constify the mem_cgroup accessors

[Tal Zussman]

mem_cgroup_id(), parent_mem_cgroup(), mem_cgroup_is_root(),
mem_cgroup_is_descendant(), memcg_kmem_id(), and the other memcg
accessors only read from the memcg. Constify them, along with
mem_cgroup_shrink_is_root()'s shrink_control.

mem_cgroup_print_oom_context()'s task stays non-const, as
task_cgroup() takes a non-const task.

Signed-off-by: Tal Zussman <tz2294@columbia.edu>
---
 include/linux/memcontrol.h | 41 +++++++++++++++++++++--------------------
 mm/memcontrol.c            |  3 ++-
 2 files changed, 23 insertions(+), 21 deletions(-)

diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index 4b7cc97e45b2..bda969b3e1d9 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -506,7 +506,7 @@ static inline bool PageMemcgKmem(const struct page *page)
 	return folio_memcg_kmem(page_folio(page));
 }
 
-static inline bool mem_cgroup_is_root(struct mem_cgroup *memcg)
+static inline bool mem_cgroup_is_root(const struct mem_cgroup *memcg)
 {
 	return (memcg == root_mem_cgroup);
 }
@@ -522,7 +522,7 @@ static inline bool mem_cgroup_is_root(struct mem_cgroup *memcg)
  * and do not honour sc->memcg can use this to early-return 0 in per-memcg
  * contexts.
  */
-static inline bool mem_cgroup_shrink_is_root(struct shrink_control *sc)
+static inline bool mem_cgroup_shrink_is_root(const struct shrink_control *sc)
 {
 	return !sc->memcg || mem_cgroup_is_root(sc->memcg);
 }
@@ -806,7 +806,7 @@ void mem_cgroup_iter_break(struct mem_cgroup *, struct mem_cgroup *);
 void mem_cgroup_scan_tasks(struct mem_cgroup *memcg,
 			   int (*)(struct task_struct *, void *), void *arg);
 
-static inline unsigned short mem_cgroup_private_id(struct mem_cgroup *memcg)
+static inline unsigned short mem_cgroup_private_id(const struct mem_cgroup *memcg)
 {
 	if (mem_cgroup_disabled())
 		return 0;
@@ -815,7 +815,7 @@ static inline unsigned short mem_cgroup_private_id(struct mem_cgroup *memcg)
 }
 struct mem_cgroup *mem_cgroup_from_private_id(unsigned short id);
 
-static inline u64 mem_cgroup_id(struct mem_cgroup *memcg)
+static inline u64 mem_cgroup_id(const struct mem_cgroup *memcg)
 {
 	return memcg ? cgroup_id(memcg->css.cgroup) : 0;
 }
@@ -844,13 +844,13 @@ static inline struct mem_cgroup *lruvec_memcg(const struct lruvec *lruvec)
  *
  * Returns the parent memcg, or NULL if this is the root.
  */
-static inline struct mem_cgroup *parent_mem_cgroup(struct mem_cgroup *memcg)
+static inline struct mem_cgroup *parent_mem_cgroup(const struct mem_cgroup *memcg)
 {
 	return mem_cgroup_from_css(memcg->css.parent);
 }
 
-static inline bool mem_cgroup_is_descendant(struct mem_cgroup *memcg,
-			      struct mem_cgroup *root)
+static inline bool mem_cgroup_is_descendant(const struct mem_cgroup *memcg,
+			      const struct mem_cgroup *root)
 {
 	if (root == memcg)
 		return true;
@@ -858,7 +858,7 @@ static inline bool mem_cgroup_is_descendant(struct mem_cgroup *memcg,
 }
 
 static inline bool mm_match_cgroup(struct mm_struct *mm,
-				   struct mem_cgroup *memcg)
+				   const struct mem_cgroup *memcg)
 {
 	struct mem_cgroup *task_memcg;
 	bool match = false;
@@ -904,7 +904,7 @@ static inline void mem_cgroup_handle_over_high(gfp_t gfp_mask)
 
 unsigned long mem_cgroup_get_max(struct mem_cgroup *memcg);
 
-void mem_cgroup_print_oom_context(struct mem_cgroup *memcg,
+void mem_cgroup_print_oom_context(const struct mem_cgroup *memcg,
 				struct task_struct *p);
 
 void mem_cgroup_print_oom_meminfo(struct mem_cgroup *memcg);
@@ -1080,12 +1080,12 @@ static inline bool PageMemcgKmem(const struct page *page)
 	return false;
 }
 
-static inline bool mem_cgroup_is_root(struct mem_cgroup *memcg)
+static inline bool mem_cgroup_is_root(const struct mem_cgroup *memcg)
 {
 	return true;
 }
 
-static inline bool mem_cgroup_shrink_is_root(struct shrink_control *sc)
+static inline bool mem_cgroup_shrink_is_root(const struct shrink_control *sc)
 {
 	return true;
 }
@@ -1187,13 +1187,13 @@ static inline struct lruvec *folio_lruvec(const struct folio *folio)
 	return &pgdat->__lruvec;
 }
 
-static inline struct mem_cgroup *parent_mem_cgroup(struct mem_cgroup *memcg)
+static inline struct mem_cgroup *parent_mem_cgroup(const struct mem_cgroup *memcg)
 {
 	return NULL;
 }
 
 static inline bool mm_match_cgroup(struct mm_struct *mm,
-		struct mem_cgroup *memcg)
+		const struct mem_cgroup *memcg)
 {
 	return true;
 }
@@ -1287,7 +1287,7 @@ static inline void mem_cgroup_scan_tasks(struct mem_cgroup *memcg,
 {
 }
 
-static inline unsigned short mem_cgroup_private_id(struct mem_cgroup *memcg)
+static inline unsigned short mem_cgroup_private_id(const struct mem_cgroup *memcg)
 {
 	return 0;
 }
@@ -1299,7 +1299,7 @@ static inline struct mem_cgroup *mem_cgroup_from_private_id(unsigned short id)
 	return NULL;
 }
 
-static inline u64 mem_cgroup_id(struct mem_cgroup *memcg)
+static inline u64 mem_cgroup_id(const struct mem_cgroup *memcg)
 {
 	return 0;
 }
@@ -1337,7 +1337,8 @@ static inline unsigned long mem_cgroup_get_max(struct mem_cgroup *memcg)
 }
 
 static inline void
-mem_cgroup_print_oom_context(struct mem_cgroup *memcg, struct task_struct *p)
+mem_cgroup_print_oom_context(const struct mem_cgroup *memcg,
+			     struct task_struct *p)
 {
 }
 
@@ -1773,7 +1774,7 @@ static inline void memcg_kmem_uncharge_page(struct page *page, int order)
  * A helper for accessing memcg's kmem_id, used for getting
  * corresponding LRU lists.
  */
-static inline int memcg_kmem_id(struct mem_cgroup *memcg)
+static inline int memcg_kmem_id(const struct mem_cgroup *memcg)
 {
 	return memcg ? memcg->kmemcg_id : -1;
 }
@@ -1845,7 +1846,7 @@ static inline bool memcg_kmem_online(void)
 	return false;
 }
 
-static inline int memcg_kmem_id(struct mem_cgroup *memcg)
+static inline int memcg_kmem_id(const struct mem_cgroup *memcg)
 {
 	return -1;
 }
@@ -1912,7 +1913,7 @@ static inline bool mem_cgroup_zswap_writeback_enabled(struct mem_cgroup *memcg)
 #ifdef CONFIG_MEMCG_V1
 bool mem_cgroup_oom_synchronize(bool wait);
 
-static inline bool task_in_memcg_oom(struct task_struct *p)
+static inline bool task_in_memcg_oom(const struct task_struct *p)
 {
 	return p->memcg_in_oom;
 }
@@ -1930,7 +1931,7 @@ static inline void mem_cgroup_exit_user_fault(void)
 }
 
 #else /* CONFIG_MEMCG_V1 */
-static inline bool task_in_memcg_oom(struct task_struct *p)
+static inline bool task_in_memcg_oom(const struct task_struct *p)
 {
 	return false;
 }
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index e2845f761e37..35f924c9f2a3 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -1812,7 +1812,8 @@ static void memory_stat_format(struct mem_cgroup *memcg, struct seq_buf *s)
  * NOTE: @memcg and @p's mem_cgroup can be different when hierarchy is
  * enabled
  */
-void mem_cgroup_print_oom_context(struct mem_cgroup *memcg, struct task_struct *p)
+void mem_cgroup_print_oom_context(const struct mem_cgroup *memcg,
+				  struct task_struct *p)
 {
 	rcu_read_lock();
 

-- 
2.39.5

[PATCH 06/11] mm: page_counter: constify page_counter_read() and page_counter_margin()

[Tal Zussman]

Both only read the counter. Constify them so that users can read
counters from const memcgs.

Signed-off-by: Tal Zussman <tz2294@columbia.edu>
---
 include/linux/page_counter.h | 4 ++--
 mm/page_counter.c            | 2 +-
 2 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/include/linux/page_counter.h b/include/linux/page_counter.h
index 07b7cb12249c..2baf7a2b29b2 100644
--- a/include/linux/page_counter.h
+++ b/include/linux/page_counter.h
@@ -63,12 +63,12 @@ static inline void page_counter_init(struct page_counter *counter,
 	counter->track_failcnt = false;
 }
 
-static inline unsigned long page_counter_read(struct page_counter *counter)
+static inline unsigned long page_counter_read(const struct page_counter *counter)
 {
 	return atomic_long_read(&counter->usage);
 }
 
-long page_counter_margin(struct page_counter *counter);
+long page_counter_margin(const struct page_counter *counter);
 void page_counter_cancel(struct page_counter *counter, unsigned long nr_pages);
 void page_counter_charge(struct page_counter *counter, unsigned long nr_pages);
 bool page_counter_try_charge(struct page_counter *counter,
diff --git a/mm/page_counter.c b/mm/page_counter.c
index 450543f4b318..8c6f3b45136b 100644
--- a/mm/page_counter.c
+++ b/mm/page_counter.c
@@ -53,7 +53,7 @@ static void propagate_protected_usage(struct page_counter *c,
  * Return: The minimum value of max minus usage across @counter and all of
  * its ancestors. The value may be negative during a concurrent charge.
  */
-long page_counter_margin(struct page_counter *counter)
+long page_counter_margin(const struct page_counter *counter)
 {
 	long margin = PAGE_COUNTER_MAX;
 

-- 
2.39.5

[PATCH 07/11] mm: memcontrol: constify the reclaim protection helpers

[Tal Zussman]

mem_cgroup_protection(), mem_cgroup_unprotected(),
mem_cgroup_below_low(), and mem_cgroup_below_min() only read the
protection state. Constify them.

Signed-off-by: Tal Zussman <tz2294@columbia.edu>
---
 include/linux/memcontrol.h | 32 ++++++++++++++++----------------
 1 file changed, 16 insertions(+), 16 deletions(-)

diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index bda969b3e1d9..bd82628d0d72 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -537,8 +537,8 @@ static inline bool mem_cgroup_disabled(void)
 	return !cgroup_subsys_enabled(memory_cgrp_subsys);
 }
 
-static inline void mem_cgroup_protection(struct mem_cgroup *root,
-					 struct mem_cgroup *memcg,
+static inline void mem_cgroup_protection(const struct mem_cgroup *root,
+					 const struct mem_cgroup *memcg,
 					 unsigned long *min,
 					 unsigned long *low,
 					 unsigned long *usage)
@@ -592,8 +592,8 @@ static inline void mem_cgroup_protection(struct mem_cgroup *root,
 void mem_cgroup_calculate_protection(struct mem_cgroup *root,
 				     struct mem_cgroup *memcg);
 
-static inline bool mem_cgroup_unprotected(struct mem_cgroup *target,
-					  struct mem_cgroup *memcg)
+static inline bool mem_cgroup_unprotected(const struct mem_cgroup *target,
+					  const struct mem_cgroup *memcg)
 {
 	/*
 	 * The root memcg doesn't account charges, and doesn't support
@@ -604,8 +604,8 @@ static inline bool mem_cgroup_unprotected(struct mem_cgroup *target,
 		memcg == target;
 }
 
-static inline bool mem_cgroup_below_low(struct mem_cgroup *target,
-					struct mem_cgroup *memcg)
+static inline bool mem_cgroup_below_low(const struct mem_cgroup *target,
+					const struct mem_cgroup *memcg)
 {
 	if (mem_cgroup_unprotected(target, memcg))
 		return false;
@@ -614,8 +614,8 @@ static inline bool mem_cgroup_below_low(struct mem_cgroup *target,
 		page_counter_read(&memcg->memory);
 }
 
-static inline bool mem_cgroup_below_min(struct mem_cgroup *target,
-					struct mem_cgroup *memcg)
+static inline bool mem_cgroup_below_min(const struct mem_cgroup *target,
+					const struct mem_cgroup *memcg)
 {
 	if (mem_cgroup_unprotected(target, memcg))
 		return false;
@@ -1110,8 +1110,8 @@ static inline void memcg_memory_event_mm(struct mm_struct *mm,
 {
 }
 
-static inline void mem_cgroup_protection(struct mem_cgroup *root,
-					 struct mem_cgroup *memcg,
+static inline void mem_cgroup_protection(const struct mem_cgroup *root,
+					 const struct mem_cgroup *memcg,
 					 unsigned long *min,
 					 unsigned long *low,
 					 unsigned long *usage)
@@ -1124,19 +1124,19 @@ static inline void mem_cgroup_calculate_protection(struct mem_cgroup *root,
 {
 }
 
-static inline bool mem_cgroup_unprotected(struct mem_cgroup *target,
-					  struct mem_cgroup *memcg)
+static inline bool mem_cgroup_unprotected(const struct mem_cgroup *target,
+					  const struct mem_cgroup *memcg)
 {
 	return true;
 }
-static inline bool mem_cgroup_below_low(struct mem_cgroup *target,
-					struct mem_cgroup *memcg)
+static inline bool mem_cgroup_below_low(const struct mem_cgroup *target,
+					const struct mem_cgroup *memcg)
 {
 	return false;
 }
 
-static inline bool mem_cgroup_below_min(struct mem_cgroup *target,
-					struct mem_cgroup *memcg)
+static inline bool mem_cgroup_below_min(const struct mem_cgroup *target,
+					const struct mem_cgroup *memcg)
 {
 	return false;
 }

-- 
2.39.5

[PATCH 08/11] mm: memcontrol: constify the memcg and lruvec stat readers

[Tal Zussman]

memcg_page_state(), memcg_events(), and the lruvec_page_state() family
only read counters. Constify them.

Signed-off-by: Tal Zussman <tz2294@columbia.edu>
---
 include/linux/memcontrol.h | 23 ++++++++++++-----------
 mm/memcontrol.c            | 12 ++++++------
 2 files changed, 18 insertions(+), 17 deletions(-)

diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index bd82628d0d72..10a197886f31 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -932,15 +932,16 @@ static inline void mod_memcg_page_state(const struct page *page,
 	rcu_read_unlock();
 }
 
-unsigned long memcg_events(struct mem_cgroup *memcg, int event);
-unsigned long memcg_page_state(struct mem_cgroup *memcg, int idx);
-unsigned long memcg_page_state_output(struct mem_cgroup *memcg, int item);
+unsigned long memcg_events(const struct mem_cgroup *memcg, int event);
+unsigned long memcg_page_state(const struct mem_cgroup *memcg, int idx);
+unsigned long memcg_page_state_output(const struct mem_cgroup *memcg, int item);
 bool memcg_stat_item_valid(int idx);
 bool memcg_vm_event_item_valid(enum vm_event_item idx);
-unsigned long lruvec_page_state(struct lruvec *lruvec, enum node_stat_item idx);
-unsigned long lruvec_page_state_monotonic(struct lruvec *lruvec,
+unsigned long lruvec_page_state(const struct lruvec *lruvec,
+				enum node_stat_item idx);
+unsigned long lruvec_page_state_monotonic(const struct lruvec *lruvec,
 					  enum node_stat_item idx);
-unsigned long lruvec_page_state_local(struct lruvec *lruvec,
+unsigned long lruvec_page_state_local(const struct lruvec *lruvec,
 				      enum node_stat_item idx);
 
 void mem_cgroup_flush_stats(struct mem_cgroup *memcg);
@@ -1372,12 +1373,12 @@ static inline void mod_memcg_page_state(const struct page *page,
 {
 }
 
-static inline unsigned long memcg_page_state(struct mem_cgroup *memcg, int idx)
+static inline unsigned long memcg_page_state(const struct mem_cgroup *memcg, int idx)
 {
 	return 0;
 }
 
-static inline unsigned long memcg_page_state_output(struct mem_cgroup *memcg, int item)
+static inline unsigned long memcg_page_state_output(const struct mem_cgroup *memcg, int item)
 {
 	return 0;
 }
@@ -1392,19 +1393,19 @@ static inline bool memcg_vm_event_item_valid(enum vm_event_item idx)
 	return false;
 }
 
-static inline unsigned long lruvec_page_state(struct lruvec *lruvec,
+static inline unsigned long lruvec_page_state(const struct lruvec *lruvec,
 					      enum node_stat_item idx)
 {
 	return node_page_state(lruvec_pgdat(lruvec), idx);
 }
 
-static inline unsigned long lruvec_page_state_monotonic(struct lruvec *lruvec,
+static inline unsigned long lruvec_page_state_monotonic(const struct lruvec *lruvec,
 							enum node_stat_item idx)
 {
 	return node_page_state_monotonic(lruvec_pgdat(lruvec), idx);
 }
 
-static inline unsigned long lruvec_page_state_local(struct lruvec *lruvec,
+static inline unsigned long lruvec_page_state_local(const struct lruvec *lruvec,
 						    enum node_stat_item idx)
 {
 	return node_page_state(lruvec_pgdat(lruvec), idx);
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 35f924c9f2a3..3d3eadddc130 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -489,7 +489,7 @@ struct lruvec_stats {
 	long state_pending[NR_MEMCG_NODE_STAT_ITEMS];
 };
 
-unsigned long lruvec_page_state(struct lruvec *lruvec, enum node_stat_item idx)
+unsigned long lruvec_page_state(const struct lruvec *lruvec, enum node_stat_item idx)
 {
 	struct mem_cgroup_per_node *pn;
 	long x;
@@ -530,7 +530,7 @@ unsigned long lruvec_page_state(struct lruvec *lruvec, enum node_stat_item idx)
  * monotonically-incremented event counters are stored in
  * enum node_stat_item.
  */
-unsigned long lruvec_page_state_monotonic(struct lruvec *lruvec,
+unsigned long lruvec_page_state_monotonic(const struct lruvec *lruvec,
 					  enum node_stat_item idx)
 {
 	struct mem_cgroup_per_node *pn;
@@ -547,7 +547,7 @@ unsigned long lruvec_page_state_monotonic(struct lruvec *lruvec,
 	return (unsigned long)READ_ONCE(pn->lruvec_stats->state[i]);
 }
 
-unsigned long lruvec_page_state_local(struct lruvec *lruvec,
+unsigned long lruvec_page_state_local(const struct lruvec *lruvec,
 				      enum node_stat_item idx)
 {
 	struct mem_cgroup_per_node *pn;
@@ -799,7 +799,7 @@ static void flush_memcg_stats_dwork(struct work_struct *w)
 	queue_delayed_work(system_dfl_wq, &stats_flush_dwork, FLUSH_TIME);
 }
 
-unsigned long memcg_page_state(struct mem_cgroup *memcg, int idx)
+unsigned long memcg_page_state(const struct mem_cgroup *memcg, int idx)
 {
 	long x;
 	int i = memcg_stats_index(idx);
@@ -1091,7 +1091,7 @@ void count_memcg_events(struct mem_cgroup *memcg, enum vm_event_item idx,
 	put_cpu();
 }
 
-unsigned long memcg_events(struct mem_cgroup *memcg, int event)
+unsigned long memcg_events(const struct mem_cgroup *memcg, int event)
 {
 	int i = memcg_events_index(event);
 
@@ -1709,7 +1709,7 @@ static int memcg_page_state_output_unit(int item)
 	}
 }
 
-unsigned long memcg_page_state_output(struct mem_cgroup *memcg, int item)
+unsigned long memcg_page_state_output(const struct mem_cgroup *memcg, int item)
 {
 	return memcg_page_state(memcg, item) *
 		memcg_page_state_output_unit(item);

-- 
2.39.5

[PATCH 09/11] mm: memcontrol: constify the swap accounting helpers

[Tal Zussman]

mem_cgroup_get_nr_swap_pages(), mem_cgroup_get_folio_swap_margin(), and
mem_cgroup_swap_full() only read swap counters and limits. Constify
them.

Remove externs from function declarations while at it.

Signed-off-by: Tal Zussman <tz2294@columbia.edu>
---
 include/linux/swap.h | 12 ++++++------
 mm/memcontrol.c      |  6 +++---
 2 files changed, 9 insertions(+), 9 deletions(-)

diff --git a/include/linux/swap.h b/include/linux/swap.h
index a72ecf12c00d..a64b5d37bf36 100644
--- a/include/linux/swap.h
+++ b/include/linux/swap.h
@@ -510,9 +510,9 @@ static inline void mem_cgroup_uncharge_swap(unsigned short id, unsigned int nr_p
 	__mem_cgroup_uncharge_swap(id, nr_pages);
 }
 
-long mem_cgroup_get_folio_swap_margin(struct folio *folio);
-extern long mem_cgroup_get_nr_swap_pages(struct mem_cgroup *memcg);
-extern bool mem_cgroup_swap_full(struct folio *folio);
+long mem_cgroup_get_folio_swap_margin(const struct folio *folio);
+long mem_cgroup_get_nr_swap_pages(const struct mem_cgroup *memcg);
+bool mem_cgroup_swap_full(const struct folio *folio);
 #else
 static inline int mem_cgroup_try_charge_swap(struct folio *folio)
 {
@@ -524,17 +524,17 @@ static inline void mem_cgroup_uncharge_swap(unsigned short id,
 {
 }
 
-static inline long mem_cgroup_get_folio_swap_margin(struct folio *folio)
+static inline long mem_cgroup_get_folio_swap_margin(const struct folio *folio)
 {
 	return PAGE_COUNTER_MAX;
 }
 
-static inline long mem_cgroup_get_nr_swap_pages(struct mem_cgroup *memcg)
+static inline long mem_cgroup_get_nr_swap_pages(const struct mem_cgroup *memcg)
 {
 	return get_nr_swap_pages();
 }
 
-static inline bool mem_cgroup_swap_full(struct folio *folio)
+static inline bool mem_cgroup_swap_full(const struct folio *folio)
 {
 	return vm_swap_full();
 }
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 3d3eadddc130..2b8705a88f6a 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -5839,7 +5839,7 @@ void __mem_cgroup_uncharge_swap(unsigned short id, unsigned int nr_pages)
 	rcu_read_unlock();
 }
 
-long mem_cgroup_get_nr_swap_pages(struct mem_cgroup *memcg)
+long mem_cgroup_get_nr_swap_pages(const struct mem_cgroup *memcg)
 {
 	long nr_swap_pages = get_nr_swap_pages();
 
@@ -5855,7 +5855,7 @@ long mem_cgroup_get_nr_swap_pages(struct mem_cgroup *memcg)
  *
  * Return: Remaining chargeable pages in the folio's memcg hierarchy.
  */
-long mem_cgroup_get_folio_swap_margin(struct folio *folio)
+long mem_cgroup_get_folio_swap_margin(const struct folio *folio)
 {
 	struct mem_cgroup *memcg;
 	long margin;
@@ -5872,7 +5872,7 @@ long mem_cgroup_get_folio_swap_margin(struct folio *folio)
 	return margin;
 }
 
-bool mem_cgroup_swap_full(struct folio *folio)
+bool mem_cgroup_swap_full(const struct folio *folio)
 {
 	struct mem_cgroup *memcg;
 	bool ret = false;

-- 
2.39.5

[PATCH 10/11] mm: memcontrol: constify mem_cgroup_swappiness() and mem_cgroup_get_max()

[Tal Zussman]

Both only read swappiness and the memory limits. Constify them.

Signed-off-by: Tal Zussman <tz2294@columbia.edu>
---
 include/linux/memcontrol.h | 4 ++--
 mm/memcontrol.c            | 2 +-
 mm/swap.h                  | 2 +-
 3 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index 10a197886f31..2d5e43dbec4d 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -902,7 +902,7 @@ static inline void mem_cgroup_handle_over_high(gfp_t gfp_mask)
 		__mem_cgroup_handle_over_high(gfp_mask);
 }
 
-unsigned long mem_cgroup_get_max(struct mem_cgroup *memcg);
+unsigned long mem_cgroup_get_max(const struct mem_cgroup *memcg);
 
 void mem_cgroup_print_oom_context(const struct mem_cgroup *memcg,
 				struct task_struct *p);
@@ -1332,7 +1332,7 @@ unsigned long mem_cgroup_get_zone_lru_size(const struct lruvec *lruvec,
 	return 0;
 }
 
-static inline unsigned long mem_cgroup_get_max(struct mem_cgroup *memcg)
+static inline unsigned long mem_cgroup_get_max(const struct mem_cgroup *memcg)
 {
 	return 0;
 }
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 2b8705a88f6a..e28b2a4f278d 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -1878,7 +1878,7 @@ void mem_cgroup_print_oom_meminfo(struct mem_cgroup *memcg)
 /*
  * Return the memory (and swap, if configured) limit for a memcg.
  */
-unsigned long mem_cgroup_get_max(struct mem_cgroup *memcg)
+unsigned long mem_cgroup_get_max(const struct mem_cgroup *memcg)
 {
 	unsigned long max = READ_ONCE(memcg->memory.max);
 
diff --git a/mm/swap.h b/mm/swap.h
index 0b5d507739bc..94714d9dd003 100644
--- a/mm/swap.h
+++ b/mm/swap.h
@@ -82,7 +82,7 @@ enum swap_cluster_flags {
 
 extern int vm_swappiness;
 
-static inline int mem_cgroup_swappiness(struct mem_cgroup *memcg)
+static inline int mem_cgroup_swappiness(const struct mem_cgroup *memcg)
 {
 #ifdef CONFIG_MEMCG_V1
 	if (!cgroup_subsys_on_dfl(memory_cgrp_subsys) &&

-- 
2.39.5

[PATCH 11/11] mm: memcontrol: constify the zswap and socket pressure helpers

[Tal Zussman]

mem_cgroup_zswap_writeback_enabled() only reads the zswap_writeback
flags, and mem_cgroup_get_socket_pressure() only reads the socket
pressure timestamp. Constify them.

Signed-off-by: Tal Zussman <tz2294@columbia.edu>
---
 include/linux/memcontrol.h | 8 ++++----
 mm/memcontrol.c            | 2 +-
 2 files changed, 5 insertions(+), 5 deletions(-)

diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index 2d5e43dbec4d..e18b44965d05 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -1644,7 +1644,7 @@ static inline void mem_cgroup_set_socket_pressure(struct mem_cgroup *memcg)
 	write_sequnlock_irqrestore(&memcg->socket_pressure_seqlock, flags);
 }
 
-static inline u64 mem_cgroup_get_socket_pressure(struct mem_cgroup *memcg)
+static inline u64 mem_cgroup_get_socket_pressure(const struct mem_cgroup *memcg)
 {
 	unsigned int seq;
 	u64 val;
@@ -1662,7 +1662,7 @@ static inline void mem_cgroup_set_socket_pressure(struct mem_cgroup *memcg)
 	WRITE_ONCE(memcg->socket_pressure, jiffies + HZ);
 }
 
-static inline u64 mem_cgroup_get_socket_pressure(struct mem_cgroup *memcg)
+static inline u64 mem_cgroup_get_socket_pressure(const struct mem_cgroup *memcg)
 {
 	return READ_ONCE(memcg->socket_pressure);
 }
@@ -1887,7 +1887,7 @@ static inline bool memcg_is_dying(struct mem_cgroup *memcg)
 bool obj_cgroup_may_zswap(struct obj_cgroup *objcg);
 void obj_cgroup_charge_zswap(struct obj_cgroup *objcg, size_t size);
 void obj_cgroup_uncharge_zswap(struct obj_cgroup *objcg, size_t size);
-bool mem_cgroup_zswap_writeback_enabled(struct mem_cgroup *memcg);
+bool mem_cgroup_zswap_writeback_enabled(const struct mem_cgroup *memcg);
 #else
 static inline bool obj_cgroup_may_zswap(struct obj_cgroup *objcg)
 {
@@ -1901,7 +1901,7 @@ static inline void obj_cgroup_uncharge_zswap(struct obj_cgroup *objcg,
 					     size_t size)
 {
 }
-static inline bool mem_cgroup_zswap_writeback_enabled(struct mem_cgroup *memcg)
+static inline bool mem_cgroup_zswap_writeback_enabled(const struct mem_cgroup *memcg)
 {
 	/* if zswap is disabled, do not block pages going to the swapping device */
 	return true;
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index e28b2a4f278d..f56e958948ac 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -6140,7 +6140,7 @@ void obj_cgroup_uncharge_zswap(struct obj_cgroup *objcg, size_t size)
 	rcu_read_unlock();
 }
 
-bool mem_cgroup_zswap_writeback_enabled(struct mem_cgroup *memcg)
+bool mem_cgroup_zswap_writeback_enabled(const struct mem_cgroup *memcg)
 {
 	/* if zswap is disabled, do not block pages going to the swapping device */
 	if (!zswap_is_enabled())

-- 
2.39.5

Re: [PATCH 03/11] mm: memcontrol: constify the lruvec helpers

[Tal Zussman]

On 9/2/26 10:27 PM, Tal Zussman wrote:
> The lruvec lookup helpers only read from the memcg, folio, or lruvec
> they are given. Constify them, along with lruvec_pgdat().
> 
> Signed-off-by: Tal Zussman <tz2294@columbia.edu>
> ---
>  include/linux/memcontrol.h | 34 +++++++++++++++++-----------------
>  include/linux/mmzone.h     |  2 +-
>  mm/memcontrol.c            |  6 +++---
>  3 files changed, 21 insertions(+), 21 deletions(-)
> 
> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> index 2b3b62e84ab2..4b7cc97e45b2 100644
> --- a/include/linux/memcontrol.h
> +++ b/include/linux/memcontrol.h
> @@ -688,7 +688,7 @@ void mem_cgroup_migrate(struct folio *old, struct folio *new);
>   * @pgdat combination. This can be the node lruvec, if the memory
>   * controller is disabled.
>   */
> -static inline struct lruvec *mem_cgroup_lruvec(struct mem_cgroup *memcg,
> +static inline struct lruvec *mem_cgroup_lruvec(const struct mem_cgroup *memcg,
>  					       struct pglist_data *pgdat)
>  {
>  	struct mem_cgroup_per_node *mz;

From Sashiko:

[Severity: Medium]
Does this function actually just read from the memcg?

Looking at the rest of mem_cgroup_lruvec() not shown in this diff, it
contains:

     if (unlikely(lruvec->pgdat != pgdat))
         lruvec->pgdat = pgdat;

This mutates the lruvec which is logically part of the memcg's per-node
state.

Since the commit message states these helpers "only read from the memcg",
is it safe to constify this when it can modify the structure on newly
onlined nodes?

Answer:

The write goes to lruvec->pgdat, which is stored in the per-node
structure reached through memcg->nodeinfo[], not in the memcg itself,
so const is fine. I'll clarify the commit message.

> @@ -1528,8 +1528,8 @@ static inline void lruvec_unlock_irqrestore(struct lruvec *lruvec, unsigned long
>  }
>  
>  /* Test requires a stable folio->memcg binding, see folio_memcg() */
> -static inline bool folio_matches_lruvec(struct folio *folio,
> -		struct lruvec *lruvec)
> +static inline bool folio_matches_lruvec(const struct folio *folio,
> +		const struct lruvec *lruvec)
>  {
>  	return lruvec_pgdat(lruvec) == folio_pgdat(folio) &&
>  	       lruvec_memcg(lruvec) == folio_memcg(folio);

From Sashiko:

[Severity: Low]
Were folio_lruvec_relock_irq() and folio_lruvec_relock_irqsave()
intentionally omitted from the constification?

Just below this hunk in include/linux/memcontrol.h, both functions
still take a non-const struct folio pointer:

static inline struct lruvec *folio_lruvec_relock_irq(struct folio *folio,
         struct lruvec *locked_lruvec)

static inline void folio_lruvec_relock_irqsave(struct folio *folio,
         struct lruvec **lruvecp, unsigned long *flags)

Since they only pass the folio along to other now-constified helpers,
should they be updated to maintain API consistency?

Answer:

Will constify their folio as well.

Re: [PATCH 08/11] mm: memcontrol: constify the memcg and lruvec stat readers

[Tal Zussman]

Responses to Sashiko below.

On 9/2/26 10:27 PM, Tal Zussman wrote:
> memcg_page_state(), memcg_events(), and the lruvec_page_state() family
> only read counters. Constify them.
> 
> Signed-off-by: Tal Zussman <tz2294@columbia.edu>
> ---
>  include/linux/memcontrol.h | 23 ++++++++++++-----------
>  mm/memcontrol.c            | 12 ++++++------
>  2 files changed, 18 insertions(+), 17 deletions(-)
> 
> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> index bd82628d0d72e..10a197886f316 100644
> --- a/include/linux/memcontrol.h
> +++ b/include/linux/memcontrol.h
[ ... ]
> @@ -1392,19 +1393,19 @@ static inline bool memcg_vm_event_item_valid(enum vm_event_item idx)
>  	return false;
>  }
>  
> -static inline unsigned long lruvec_page_state(struct lruvec *lruvec,
> +static inline unsigned long lruvec_page_state(const struct lruvec *lruvec,
>  					      enum node_stat_item idx)
>  {
>  	return node_page_state(lruvec_pgdat(lruvec), idx);
>  }

[Severity: Low]
Does passing the newly constified lruvec pointer to lruvec_pgdat() discard
the const qualifier?

Looking at lruvec_pgdat() in include/linux/mmzone.h, it does this:

	return container_of(lruvec, struct pglist_data, __lruvec);

This returns a mutable container from a const pointer, circumventing the
internal const-correctness the patch intends to provide. Should
container_of_const() be used here instead?

Answer:

lruvec_pgdat() purposefully returns a non-const pgdat, as some callers
still need one at the moment (node_page_state()).

> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 35f924c9f2a31..3d3eadddc1307 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -489,7 +489,7 @@ struct lruvec_stats {
>  	long state_pending[NR_MEMCG_NODE_STAT_ITEMS];
>  };
>  
> -unsigned long lruvec_page_state(struct lruvec *lruvec, enum node_stat_item idx)
> +unsigned long lruvec_page_state(const struct lruvec *lruvec, enum node_stat_item idx)
>  {
>  	struct mem_cgroup_per_node *pn;
>  	long x;

[Severity: Low]
Does the use of container_of() later in this function silently cast away the
const qualifier added to the lruvec parameter?

In lruvec_page_state():

	pn = container_of(lruvec, struct mem_cgroup_per_node, lruvec);

This macro expansion discards the const qualifier, undermining the compiler's
ability to enforce const-correctness. Could container_of_const() be used here
instead, with pn declared as a const struct mem_cgroup_per_node *?

Answer:

Yes, this can use container_of_const(). Will change.

[ ... ]
> @@ -530,7 +530,7 @@ unsigned long lruvec_page_state(struct lruvec *lruvec, enum node_stat_item idx)
>   * monotonically-incremented event counters are stored in
>   * enum node_stat_item.
>   */
> -unsigned long lruvec_page_state_monotonic(struct lruvec *lruvec,
> +unsigned long lruvec_page_state_monotonic(const struct lruvec *lruvec,
>  					  enum node_stat_item idx)
>  {
>  	struct mem_cgroup_per_node *pn;

[Severity: Low]
Does this function also silently lose the const qualifier through
container_of(), similar to lruvec_page_state()?

In lruvec_page_state_monotonic():

	pn = container_of(lruvec, struct mem_cgroup_per_node, lruvec);

Would it be better to use container_of_const() and a const pointer for pn
to preserve the safety guarantees of this patch?

Answer:

Yes, this can use container_of_const(). Will change.

> @@ -547,7 +547,7 @@ unsigned long lruvec_page_state_monotonic(struct lruvec *lruvec,
>  	return (unsigned long)READ_ONCE(pn->lruvec_stats->state[i]);
>  }
>  
> -unsigned long lruvec_page_state_local(struct lruvec *lruvec,
> +unsigned long lruvec_page_state_local(const struct lruvec *lruvec,
>  				      enum node_stat_item idx)
>  {
>  	struct mem_cgroup_per_node *pn;

[Severity: Low]
Like the other functions above, does container_of() discard the const
qualifier here?

In lruvec_page_state_local():

	pn = container_of(lruvec, struct mem_cgroup_per_node, lruvec);

Could this be updated to use container_of_const() as well to maintain strict
const-correctness?

Answer:

Yes, this can use container_of_const(). Will change.