[PATCH v2 5/7] mm: list_lru: introduce caller locking for additions and deletions

Johannes Weiner
2026-03-12T20:53:44Z

Locking is currently internal to the list_lru API. However, a caller
might want to keep auxiliary state synchronized with the LRU state.

For example, the THP shrinker uses the lock of its custom LRU to keep
PG_partially_mapped and vmstats consistent.

To allow the THP shrinker to switch to list_lru, provide normal and
irqsafe locking primitives as well as caller-locked variants of the
addition and deletion functions.

Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
---
include/linux/list_lru.h | 34 +++++++++++++
mm/list_lru.c | 104 +++++++++++++++++++++++++++------------
2 files changed, 107 insertions(+), 31 deletions(-)

diff --git a/include/linux/list_lru.h b/include/linux/list_lru.h
index fe739d35a864..4afc02deb44d 100644
--- a/include/linux/list_lru.h
+++ b/include/linux/list_lru.h
@@ -83,6 +83,40 @@ int memcg_list_lru_alloc(struct mem_cgroup *memcg, struct list_lru *lru,
gfp_t gfp);
void memcg_reparent_list_lrus(struct mem_cgroup *memcg, struct mem_cgroup *parent);

+/**
+ * list_lru_lock: lock the sublist for the given node and memcg
+ * @lru: the lru pointer
+ * @nid: the node id of the sublist to lock.
+ * @memcg: the cgroup of the sublist to lock.
+ *
+ * Returns the locked list_lru_one sublist. The caller must call
+ * list_lru_unlock() when done.
+ *
+ * You must ensure that the memcg is not freed during this call (e.g., with
+ * rcu or by taking a css refcnt).
+ *
+ * Return: the locked list_lru_one, or NULL on failure
+ */
+struct list_lru_one *list_lru_lock(struct list_lru *lru, int nid,
+ struct mem_cgroup *memcg);
+
+/**
+ * list_lru_unlock: unlock a sublist locked by list_lru_lock()
+ * @l: the list_lru_one to unlock
+ */
+void list_lru_unlock(struct list_lru_one *l);
+
+struct list_lru_one *list_lru_lock_irqsave(struct list_lru *lru, int nid,
+ struct mem_cgroup *memcg, unsigned long *irq_flags);
+void list_lru_unlock_irqrestore(struct list_lru_one *l,
+ unsigned long *irq_flags);
+
+/* Caller-locked variants, see list_lru_add() etc for documentation */
+bool __list_lru_add(struct list_lru *lru, struct list_lru_one *l,
+ struct list_head *item, int nid, struct mem_cgroup *memcg);
+bool __list_lru_del(struct list_lru *lru, struct list_lru_one *l,
+ struct list_head *item, int nid);
+
/**
* list_lru_add: add an element to the lru list's tail
* @lru: the lru pointer
diff --git a/mm/list_lru.c b/mm/list_lru.c
index 4d74c2e9c2a5..779cb26cec84 100644
--- a/mm/list_lru.c
+++ b/mm/list_lru.c
@@ -15,17 +15,23 @@
#include "slab.h"
#include "internal.h"

-static inline void lock_list_lru(struct list_lru_one *l, bool irq)
+static inline void lock_list_lru(struct list_lru_one *l, bool irq,
+ unsigned long *irq_flags)
{
- if (irq)
+ if (irq_flags)
+ spin_lock_irqsave(&l->lock, *irq_flags);
+ else if (irq)
spin_lock_irq(&l->lock);
else
spin_lock(&l->lock);
}

-static inline void unlock_list_lru(struct list_lru_one *l, bool irq_off)
+static inline void unlock_list_lru(struct list_lru_one *l, bool irq_off,
+ unsigned long *irq_flags)
{
- if (irq_off)
+ if (irq_flags)
+ spin_unlock_irqrestore(&l->lock, *irq_flags);
+ else if (irq_off)
spin_unlock_irq(&l->lock);
else
spin_unlock(&l->lock);
@@ -78,7 +84,7 @@ list_lru_from_memcg_idx(struct list_lru *lru, int nid, int idx)

static inline struct list_lru_one *
lock_list_lru_of_memcg(struct list_lru *lru, int nid, struct mem_cgroup *memcg,
- bool irq, bool skip_empty)
+ bool irq, unsigned long *irq_flags, bool skip_empty)
{
struct list_lru_one *l;

@@ -86,12 +92,12 @@ lock_list_lru_of_memcg(struct list_lru *lru, int nid, struct mem_cgroup *memcg,
again:
l = list_lru_from_memcg_idx(lru, nid, memcg_kmem_id(memcg));
if (likely(l)) {
- lock_list_lru(l, irq);
+ lock_list_lru(l, irq, irq_flags);
if (likely(READ_ONCE(l->nr_items) != LONG_MIN)) {
rcu_read_unlock();
return l;
}
- unlock_list_lru(l, irq);
+ unlock_list_lru(l, irq, irq_flags);
}
/*
* Caller may simply bail out if raced with reparenting or
@@ -132,37 +138,79 @@ list_lru_from_memcg_idx(struct list_lru *lru, int nid, int idx)

static inline struct list_lru_one *
lock_list_lru_of_memcg(struct list_lru *lru, int nid, struct mem_cgroup *memcg,
- bool irq, bool skip_empty)
+ bool irq, unsigned long *irq_flags, bool skip_empty)
{
struct list_lru_one *l = &lru->node[nid].lru;

- lock_list_lru(l, irq);
+ lock_list_lru(l, irq, irq_flags);

return l;
}
#endif /* CONFIG_MEMCG */

-/* The caller must ensure the memcg lifetime. */
-bool list_lru_add(struct list_lru *lru, struct list_head *item, int nid,
- struct mem_cgroup *memcg)
+struct list_lru_one *list_lru_lock(struct list_lru *lru, int nid,
+ struct mem_cgroup *memcg)
{
- struct list_lru_node *nlru = &lru->node[nid];
- struct list_lru_one *l;
+ return lock_list_lru_of_memcg(lru, nid, memcg, false, NULL, false);
+}
+
+void list_lru_unlock(struct list_lru_one *l)
+{
+ unlock_list_lru(l, false, NULL);
+}
+
+struct list_lru_one *list_lru_lock_irqsave(struct list_lru *lru, int nid,
+ struct mem_cgroup *memcg,
+ unsigned long *flags)
+{
+ return lock_list_lru_of_memcg(lru, nid, memcg, true, flags, false);
+}
+
+void list_lru_unlock_irqrestore(struct list_lru_one *l, unsigned long *flags)
+{
+ unlock_list_lru(l, true, flags);
+}

- l = lock_list_lru_of_memcg(lru, nid, memcg, false, false);
+bool __list_lru_add(struct list_lru *lru, struct list_lru_one *l,
+ struct list_head *item, int nid,
+ struct mem_cgroup *memcg)
+{
if (list_empty(item)) {
list_add_tail(item, &l->list);
/* Set shrinker bit if the first element was added */
if (!l->nr_items++)
set_shrinker_bit(memcg, nid, lru_shrinker_id(lru));
- unlock_list_lru(l, false);
- atomic_long_inc(&nlru->nr_items);
+ atomic_long_inc(&lru->node[nid].nr_items);
+ return true;
+ }
+ return false;
+}
+
+bool __list_lru_del(struct list_lru *lru, struct list_lru_one *l,
+ struct list_head *item, int nid)
+{
+ if (!list_empty(item)) {
+ list_del_init(item);
+ l->nr_items--;
+ atomic_long_dec(&lru->node[nid].nr_items);
return true;
}
- unlock_list_lru(l, false);
return false;
}

+/* The caller must ensure the memcg lifetime. */
+bool list_lru_add(struct list_lru *lru, struct list_head *item, int nid,
+ struct mem_cgroup *memcg)
+{
+ struct list_lru_one *l;
+ bool ret;
+
+ l = list_lru_lock(lru, nid, memcg);
+ ret = __list_lru_add(lru, l, item, nid, memcg);
+ list_lru_unlock(l);
+ return ret;
+}
+
bool list_lru_add_obj(struct list_lru *lru, struct list_head *item)
{
bool ret;
@@ -184,19 +232,13 @@ EXPORT_SYMBOL_GPL(list_lru_add_obj);
bool list_lru_del(struct list_lru *lru, struct list_head *item, int nid,
struct mem_cgroup *memcg)
{
- struct list_lru_node *nlru = &lru->node[nid];
struct list_lru_one *l;
+ bool ret;

- l = lock_list_lru_of_memcg(lru, nid, memcg, false, false);
- if (!list_empty(item)) {
- list_del_init(item);
- l->nr_items--;
- unlock_list_lru(l, false);
- atomic_long_dec(&nlru->nr_items);
- return true;
- }
- unlock_list_lru(l, false);
- return false;
+ l = list_lru_lock(lru, nid, memcg);
+ ret = __list_lru_del(lru, l, item, nid);
+ list_lru_unlock(l);
+ return ret;
}

bool list_lru_del_obj(struct list_lru *lru, struct list_head *item)
@@ -269,7 +311,7 @@ __list_lru_walk_one(struct list_lru *lru, int nid, struct mem_cgroup *memcg,
unsigned long isolated = 0;

restart:
- l = lock_list_lru_of_memcg(lru, nid, memcg, irq_off, true);
+ l = lock_list_lru_of_memcg(lru, nid, memcg, irq_off, NULL, true);
if (!l)
return isolated;
list_for_each_safe(item, n, &l->list) {
@@ -310,7 +352,7 @@ __list_lru_walk_one(struct list_lru *lru, int nid, struct mem_cgroup *memcg,
BUG();
}
}
- unlock_list_lru(l, irq_off);
+ unlock_list_lru(l, irq_off, NULL);
out:
return isolated;
}
--
2.53.0