[PATCH v3 13/21] slab: remove defer_deactivate_slab()

Vlastimil Babka
2026-01-16T14:41:43Z

There are no more cpu slabs so we don't need their deferred
deactivation. The function is now only used from places where we
allocate a new slab but then can't spin on node list_lock to put it on
the partial list. Instead of the deferred action we can free it directly
via __free_slab(), we just need to tell it to use _nolock() freeing of
the underlying pages and take care of the accounting.

Since free_frozen_pages_nolock() variant does not yet exist for code
outside of the page allocator, create it as a trivial wrapper for
__free_frozen_pages(..., FPI_TRYLOCK).

Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
---
mm/internal.h | 1 +
mm/page_alloc.c | 5 +++++
mm/slab.h | 8 +-------
mm/slub.c | 56 ++++++++++++++++++++------------------------------------
4 files changed, 27 insertions(+), 43 deletions(-)

diff --git a/mm/internal.h b/mm/internal.h
index e430da900430..1f44ccb4badf 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -846,6 +846,7 @@ static inline struct page *alloc_frozen_pages_noprof(gfp_t gfp, unsigned int ord
struct page *alloc_frozen_pages_nolock_noprof(gfp_t gfp_flags, int nid, unsigned int order);
#define alloc_frozen_pages_nolock(...) \
alloc_hooks(alloc_frozen_pages_nolock_noprof(__VA_ARGS__))
+void free_frozen_pages_nolock(struct page *page, unsigned int order);

extern void zone_pcp_reset(struct zone *zone);
extern void zone_pcp_disable(struct zone *zone);
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index c380f063e8b7..0127e9d661ad 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -2981,6 +2981,11 @@ void free_frozen_pages(struct page *page, unsigned int order)
__free_frozen_pages(page, order, FPI_NONE);
}

+void free_frozen_pages_nolock(struct page *page, unsigned int order)
+{
+ __free_frozen_pages(page, order, FPI_TRYLOCK);
+}
+
/*
* Free a batch of folios
*/
diff --git a/mm/slab.h b/mm/slab.h
index e77260720994..4efec41b6445 100644
--- a/mm/slab.h
+++ b/mm/slab.h
@@ -71,13 +71,7 @@ struct slab {
struct kmem_cache *slab_cache;
union {
struct {
- union {
- struct list_head slab_list;
- struct { /* For deferred deactivate_slab() */
- struct llist_node llnode;
- void *flush_freelist;
- };
- };
+ struct list_head slab_list;
/* Double-word boundary */
struct freelist_counters;
};
diff --git a/mm/slub.c b/mm/slub.c
index b08e775dc4cb..33f218c0e8d6 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -3260,7 +3260,7 @@ static struct slab *new_slab(struct kmem_cache *s, gfp_t flags, int node)
flags & (GFP_RECLAIM_MASK | GFP_CONSTRAINT_MASK), node);
}

-static void __free_slab(struct kmem_cache *s, struct slab *slab)
+static void __free_slab(struct kmem_cache *s, struct slab *slab, bool allow_spin)
{
struct page *page = slab_page(slab);
int order = compound_order(page);
@@ -3271,14 +3271,26 @@ static void __free_slab(struct kmem_cache *s, struct slab *slab)
__ClearPageSlab(page);
mm_account_reclaimed_pages(pages);
unaccount_slab(slab, order, s);
- free_frozen_pages(page, order);
+ if (allow_spin)
+ free_frozen_pages(page, order);
+ else
+ free_frozen_pages_nolock(page, order);
+}
+
+static void free_new_slab_nolock(struct kmem_cache *s, struct slab *slab)
+{
+ /*
+ * Since it was just allocated, we can skip the actions in
+ * discard_slab() and free_slab().
+ */
+ __free_slab(s, slab, false);
}

static void rcu_free_slab(struct rcu_head *h)
{
struct slab *slab = container_of(h, struct slab, rcu_head);

- __free_slab(slab->slab_cache, slab);
+ __free_slab(slab->slab_cache, slab, true);
}

static void free_slab(struct kmem_cache *s, struct slab *slab)
@@ -3294,7 +3306,7 @@ static void free_slab(struct kmem_cache *s, struct slab *slab)
if (unlikely(s->flags & SLAB_TYPESAFE_BY_RCU))
call_rcu(&slab->rcu_head, rcu_free_slab);
else
- __free_slab(s, slab);
+ __free_slab(s, slab, true);
}

static void discard_slab(struct kmem_cache *s, struct slab *slab)
@@ -3387,8 +3399,6 @@ static void *alloc_single_from_partial(struct kmem_cache *s,
return object;
}

-static void defer_deactivate_slab(struct slab *slab, void *flush_freelist);
-
/*
* Called only for kmem_cache_debug() caches to allocate from a freshly
* allocated slab. Allocate a single object instead of whole freelist
@@ -3404,8 +3414,8 @@ static void *alloc_single_from_new_slab(struct kmem_cache *s, struct slab *slab,
void *object;

if (!allow_spin && !spin_trylock_irqsave(&n->list_lock, flags)) {
- /* Unlucky, discard newly allocated slab */
- defer_deactivate_slab(slab, NULL);
+ /* Unlucky, discard newly allocated slab. */
+ free_new_slab_nolock(s, slab);
return NULL;
}

@@ -4276,7 +4286,7 @@ static unsigned int alloc_from_new_slab(struct kmem_cache *s, struct slab *slab,

if (!spin_trylock_irqsave(&n->list_lock, flags)) {
/* Unlucky, discard newly allocated slab */
- defer_deactivate_slab(slab, NULL);
+ free_new_slab_nolock(s, slab);
return 0;
}
}
@@ -6033,7 +6043,6 @@ static void free_to_pcs_bulk(struct kmem_cache *s, size_t size, void **p)

struct defer_free {
struct llist_head objects;
- struct llist_head slabs;
struct irq_work work;
};

@@ -6041,7 +6050,6 @@ static void free_deferred_objects(struct irq_work *work);

static DEFINE_PER_CPU(struct defer_free, defer_free_objects) = {
.objects = LLIST_HEAD_INIT(objects),
- .slabs = LLIST_HEAD_INIT(slabs),
.work = IRQ_WORK_INIT(free_deferred_objects),
};

@@ -6054,10 +6062,9 @@ static void free_deferred_objects(struct irq_work *work)
{
struct defer_free *df = container_of(work, struct defer_free, work);
struct llist_head *objs = &df->objects;
- struct llist_head *slabs = &df->slabs;
struct llist_node *llnode, *pos, *t;

- if (llist_empty(objs) && llist_empty(slabs))
+ if (llist_empty(objs))
return;

llnode = llist_del_all(objs);
@@ -6081,16 +6088,6 @@ static void free_deferred_objects(struct irq_work *work)

__slab_free(s, slab, x, x, 1, _THIS_IP_);
}
-
- llnode = llist_del_all(slabs);
- llist_for_each_safe(pos, t, llnode) {
- struct slab *slab = container_of(pos, struct slab, llnode);
-
- if (slab->frozen)
- deactivate_slab(slab->slab_cache, slab, slab->flush_freelist);
- else
- free_slab(slab->slab_cache, slab);
- }
}

static void defer_free(struct kmem_cache *s, void *head)
@@ -6106,19 +6103,6 @@ static void defer_free(struct kmem_cache *s, void *head)
irq_work_queue(&df->work);
}

-static void defer_deactivate_slab(struct slab *slab, void *flush_freelist)
-{
- struct defer_free *df;
-
- slab->flush_freelist = flush_freelist;
-
- guard(preempt)();
-
- df = this_cpu_ptr(&defer_free_objects);
- if (llist_add(&slab->llnode, &df->slabs))
- irq_work_queue(&df->work);
-}
-
void defer_free_barrier(void)
{
int cpu;

--
2.52.0