[PATCH 1/3] slab: decouple pointer to barn from kmem_cache_node

Vlastimil Babka (SUSE)
2026-03-11T08:26:29Z

The pointer to barn currently exists in struct kmem_cache_node. That
struct is instantiated for every NUMA node with memory, but we want to
have a barn for every online node (including memoryless).

Thus decouple the two structures. In struct kmem_cache we have an array
for kmem_cache_node pointers that appears to be sized MAX_NUMNODES but
the actual size calculation in kmem_cache_init() uses nr_node_ids.
Therefore we can't just add another array of barn pointers. Instead
change the array to newly introduced struct kmem_cache_per_node_ptrs
holding both kmem_cache_node and barn pointer.

Adjust barn accessor and allocation/initialization code accordingly. For
now no functional change intended, barns are created 1:1 together with
kmem_cache_nodes.

Signed-off-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>
---
mm/slab.h | 7 +++-
mm/slub.c | 128 +++++++++++++++++++++++++++++++++++---------------------------
2 files changed, 78 insertions(+), 57 deletions(-)

diff --git a/mm/slab.h b/mm/slab.h
index e9ab292acd22..c735e6b4dddb 100644
--- a/mm/slab.h
+++ b/mm/slab.h
@@ -191,6 +191,11 @@ struct kmem_cache_order_objects {
unsigned int x;
};

+struct kmem_cache_per_node_ptrs {
+ struct node_barn *barn;
+ struct kmem_cache_node *node;
+};
+
/*
* Slab cache management.
*/
@@ -247,7 +252,7 @@ struct kmem_cache {
struct kmem_cache_stats __percpu *cpu_stats;
#endif

- struct kmem_cache_node *node[MAX_NUMNODES];
+ struct kmem_cache_per_node_ptrs per_node[MAX_NUMNODES];
};

/*
diff --git a/mm/slub.c b/mm/slub.c
index 20cb4f3b636d..609a183f8533 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -59,7 +59,7 @@
* 0. cpu_hotplug_lock
* 1. slab_mutex (Global Mutex)
* 2a. kmem_cache->cpu_sheaves->lock (Local trylock)
- * 2b. node->barn->lock (Spinlock)
+ * 2b. barn->lock (Spinlock)
* 2c. node->list_lock (Spinlock)
* 3. slab_lock(slab) (Only on some arches)
* 4. object_map_lock (Only for debugging)
@@ -136,7 +136,7 @@
* or spare sheaf can handle the allocation or free, there is no other
* overhead.
*
- * node->barn->lock (spinlock)
+ * barn->lock (spinlock)
*
* This lock protects the operations on per-NUMA-node barn. It can quickly
* serve an empty or full sheaf if available, and avoid more expensive refill
@@ -436,26 +436,24 @@ struct kmem_cache_node {
atomic_long_t total_objects;
struct list_head full;
#endif
- struct node_barn *barn;
};

static inline struct kmem_cache_node *get_node(struct kmem_cache *s, int node)
{
- return s->node[node];
+ return s->per_node[node].node;
+}
+
+static inline struct node_barn *get_barn_node(struct kmem_cache *s, int node)
+{
+ return s->per_node[node].barn;
}

/*
- * Get the barn of the current cpu's closest memory node. It may not exist on
- * systems with memoryless nodes but without CONFIG_HAVE_MEMORYLESS_NODES
+ * Get the barn of the current cpu's memory node. It may be a memoryless node.
*/
static inline struct node_barn *get_barn(struct kmem_cache *s)
{
- struct kmem_cache_node *n = get_node(s, numa_mem_id());
-
- if (!n)
- return NULL;
-
- return n->barn;
+ return get_barn_node(s, numa_node_id());
}

/*
@@ -5791,7 +5789,6 @@ bool free_to_pcs(struct kmem_cache *s, void *object, bool allow_spin)

static void rcu_free_sheaf(struct rcu_head *head)
{
- struct kmem_cache_node *n;
struct slab_sheaf *sheaf;
struct node_barn *barn = NULL;
struct kmem_cache *s;
@@ -5814,12 +5811,10 @@ static void rcu_free_sheaf(struct rcu_head *head)
if (__rcu_free_sheaf_prepare(s, sheaf))
goto flush;

- n = get_node(s, sheaf->node);
- if (!n)
+ barn = get_barn_node(s, sheaf->node);
+ if (!barn)
goto flush;

- barn = n->barn;
-
/* due to slab_free_hook() */
if (unlikely(sheaf->size == 0))
goto empty;
@@ -7430,7 +7425,7 @@ static inline int calculate_order(unsigned int size)
}

static void
-init_kmem_cache_node(struct kmem_cache_node *n, struct node_barn *barn)
+init_kmem_cache_node(struct kmem_cache_node *n)
{
n->nr_partial = 0;
spin_lock_init(&n->list_lock);
@@ -7440,9 +7435,6 @@ init_kmem_cache_node(struct kmem_cache_node *n, struct node_barn *barn)
atomic_long_set(&n->total_objects, 0);
INIT_LIST_HEAD(&n->full);
#endif
- n->barn = barn;
- if (barn)
- barn_init(barn);
}

#ifdef CONFIG_SLUB_STATS
@@ -7537,8 +7529,8 @@ static void early_kmem_cache_node_alloc(int node)
n = kasan_slab_alloc(kmem_cache_node, n, GFP_KERNEL, false);
slab->freelist = get_freepointer(kmem_cache_node, n);
slab->inuse = 1;
- kmem_cache_node->node[node] = n;
- init_kmem_cache_node(n, NULL);
+ kmem_cache_node->per_node[node].node = n;
+ init_kmem_cache_node(n);
inc_slabs_node(kmem_cache_node, node, slab->objects);

/*
@@ -7553,15 +7545,20 @@ static void free_kmem_cache_nodes(struct kmem_cache *s)
int node;
struct kmem_cache_node *n;

- for_each_kmem_cache_node(s, node, n) {
- if (n->barn) {
- WARN_ON(n->barn->nr_full);
- WARN_ON(n->barn->nr_empty);
- kfree(n->barn);
- n->barn = NULL;
- }
+ for_each_node(node) {
+ struct node_barn *barn = get_barn_node(s, node);

- s->node[node] = NULL;
+ if (!barn)
+ continue;
+
+ WARN_ON(barn->nr_full);
+ WARN_ON(barn->nr_empty);
+ kfree(barn);
+ s->per_node[node].barn = NULL;
+ }
+
+ for_each_kmem_cache_node(s, node, n) {
+ s->per_node[node].node = NULL;
kmem_cache_free(kmem_cache_node, n);
}
}
@@ -7582,31 +7579,36 @@ static int init_kmem_cache_nodes(struct kmem_cache *s)

for_each_node_mask(node, slab_nodes) {
struct kmem_cache_node *n;
- struct node_barn *barn = NULL;

if (slab_state == DOWN) {
early_kmem_cache_node_alloc(node);
continue;
}

- if (cache_has_sheaves(s)) {
- barn = kmalloc_node(sizeof(*barn), GFP_KERNEL, node);
-
- if (!barn)
- return 0;
- }
-
n = kmem_cache_alloc_node(kmem_cache_node,
GFP_KERNEL, node);
- if (!n) {
- kfree(barn);
+ if (!n)
return 0;
- }

- init_kmem_cache_node(n, barn);
+ init_kmem_cache_node(n);
+ s->per_node[node].node = n;
+ }
+
+ if (slab_state == DOWN || !cache_has_sheaves(s))
+ return 1;
+
+ for_each_node_mask(node, slab_nodes) {
+ struct node_barn *barn;
+
+ barn = kmalloc_node(sizeof(*barn), GFP_KERNEL, node);
+
+ if (!barn)
+ return 0;

- s->node[node] = n;
+ barn_init(barn);
+ s->per_node[node].barn = barn;
}
+
return 1;
}

@@ -7895,10 +7897,15 @@ int __kmem_cache_shutdown(struct kmem_cache *s)
if (cache_has_sheaves(s))
rcu_barrier();

+ for_each_node(node) {
+ struct node_barn *barn = get_barn_node(s, node);
+
+ if (barn)
+ barn_shrink(s, barn);
+ }
+
/* Attempt to free all objects */
for_each_kmem_cache_node(s, node, n) {
- if (n->barn)
- barn_shrink(s, n->barn);
free_partial(s, n);
if (n->nr_partial || node_nr_slabs(n))
return 1;
@@ -8108,14 +8115,18 @@ static int __kmem_cache_do_shrink(struct kmem_cache *s)
unsigned long flags;
int ret = 0;

+ for_each_node(node) {
+ struct node_barn *barn = get_barn_node(s, node);
+
+ if (barn)
+ barn_shrink(s, barn);
+ }
+
for_each_kmem_cache_node(s, node, n) {
INIT_LIST_HEAD(&discard);
for (i = 0; i < SHRINK_PROMOTE_MAX; i++)
INIT_LIST_HEAD(promote + i);

- if (n->barn)
- barn_shrink(s, n->barn);
-
spin_lock_irqsave(&n->list_lock, flags);

/*
@@ -8204,7 +8215,8 @@ static int slab_mem_going_online_callback(int nid)
if (get_node(s, nid))
continue;

- if (cache_has_sheaves(s)) {
+ if (cache_has_sheaves(s) && !get_barn_node(s, nid)) {
+
barn = kmalloc_node(sizeof(*barn), GFP_KERNEL, nid);

if (!barn) {
@@ -8225,13 +8237,17 @@ static int slab_mem_going_online_callback(int nid)
goto out;
}

- init_kmem_cache_node(n, barn);
+ init_kmem_cache_node(n);
+ s->per_node[nid].node = n;

- s->node[nid] = n;
+ if (barn) {
+ barn_init(barn);
+ s->per_node[nid].barn = barn;
+ }
}
/*
* Any cache created after this point will also have kmem_cache_node
- * initialized for the new node.
+ * and barn initialized for the new node.
*/
node_set(nid, slab_nodes);
out:
@@ -8323,7 +8339,7 @@ static void __init bootstrap_cache_sheaves(struct kmem_cache *s)
}

barn_init(barn);
- get_node(s, node)->barn = barn;
+ s->per_node[node].barn = barn;
}

for_each_possible_cpu(cpu) {
@@ -8394,8 +8410,8 @@ void __init kmem_cache_init(void)
slab_state = PARTIAL;

create_boot_cache(kmem_cache, "kmem_cache",
- offsetof(struct kmem_cache, node) +
- nr_node_ids * sizeof(struct kmem_cache_node *),
+ offsetof(struct kmem_cache, per_node) +
+ nr_node_ids * sizeof(struct kmem_cache_per_node_ptrs),
SLAB_HWCACHE_ALIGN | SLAB_NO_OBJ_EXT, 0, 0);

kmem_cache = bootstrap(&boot_kmem_cache);

--
2.53.0