mm/slab: introduce kfree_rcu_nolock()

Currently, k[v]free_rcu() cannot be called in unknown context since
it could lead to a deadlock when called in the middle of k[v]free_rcu().

Make users' lives easier by introducing kfree_rcu_nolock() variant,
now that kfree_rcu_sheaf() is available on PREEMPT_RT and
__kfree_rcu_sheaf() handles unknown context.

When sheaves path fails, kfree_rcu_nolock() falls back to
defer_kfree_rcu() that uses an irq work to free the object via
kvfree_call_rcu(). In most cases, the sheaves path is expected to
succeed and therefore it's unnecessary to introduce additional
complexity to the existing kvfree_rcu batching by teaching it
how to handle unknown context.

Since defer_kfree_rcu() can be called on caches without sheaves, move
deferred_work_barrier() and rcu_barrier() outside the branch in
kvfree_rcu_barrier_on_cache().

Now that deferred kvfree_rcu objects are submitted to kvfree_call_rcu()
after deferred_work_barrier() and may end up in RCU sheaves,
deferred_work_barrier() must be invoked before
flush_rcu_sheaves_on_cache().

Since the RCU sheaf path has not been used on !KVFREE_RCU_BATCHED
kernels, always fall back when kvfree_rcu() is not batched, for
consistency. kvfree_rcu_barrier{,_on_cache()}() on
!KVFREE_RCU_BATCHED are moved to mm/slab_common.c to invoke
deferred_work_barrier() before rcu_barrier().

Signed-off-by: Harry Yoo (Oracle) <harry@kernel.org>
Link: https://patch.msgid.link/20260729-kfree_rcu_nolock-v5-7-a28cdcda9673@kernel.org
Signed-off-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>

include/linux/rcupdate.h

@@ -1099,6 +1099,7 @@ static inline void rcu_read_unlock_migrate(void)
  * In mm/slab_common.c, no suitable header to include here.
  */
 void kvfree_call_rcu(struct kvfree_rcu_head *head, void *ptr);
+void kfree_call_rcu_nolock(struct kvfree_rcu_head *head, void *ptr);
 
 /*
  * The BUILD_BUG_ON() makes sure the rcu_head offset can be handled. See the
@@ -1124,6 +1125,27 @@ do {								\
 		kvfree_call_rcu(NULL, (void *) (___p));		\
 } while (0)
 
+/**
+ * kfree_rcu_nolock() - a version of kfree_rcu() that can be called in any context.
+ * @ptr: pointer to kfree for double-argument invocations.
+ * @kvrhf: the name of the struct kvfree_rcu_head within the type of @ptr.
+ *
+ * With KVFREE_RCU_BATCHED, kfree_rcu_nolock() tries hard to free objects
+ * without any deferred processing, but may still defer freeing.
+ * Large kmalloc and vmalloc objects are always deferred.
+ *
+ * kfree_rcu_nolock() supports 2-arg variant only.
+ */
+#define kfree_rcu_nolock(ptr, kvrhf)						\
+do {										\
+	typeof (ptr) ___p = (ptr);						\
+										\
+	if (___p) {								\
+		BUILD_BUG_ON(offsetof(typeof(*(ptr)), kvrhf) >= 4096);		\
+		kfree_call_rcu_nolock(&((___p)->kvrhf), (void *) (___p));	\
+	}									\
+} while (0)
+
 /*
  * Place this after a lock-acquisition primitive to guarantee that
  * an UNLOCK+LOCK pair acts as a full barrier.  This guarantee applies

include/linux/slab.h

@@ -1427,25 +1427,15 @@ extern void kvfree_sensitive(const void *addr, size_t len);
 unsigned int kmem_cache_size(struct kmem_cache *s);
 
 #ifndef CONFIG_KVFREE_RCU_BATCHED
-static inline void kvfree_rcu_barrier(void)
-{
-	rcu_barrier();
-}
-
-static inline void kvfree_rcu_barrier_on_cache(struct kmem_cache *s)
-{
-	rcu_barrier();
-}
-
 static inline void kfree_rcu_scheduler_running(void) { }
 #else
+void kfree_rcu_scheduler_running(void);
+#endif
+
 void kvfree_rcu_barrier(void);
 
 void kvfree_rcu_barrier_on_cache(struct kmem_cache *s);
 
-void kfree_rcu_scheduler_running(void);
-#endif
-
 /**
  * kmalloc_size_roundup - Report allocation bucket size for the given size
  *

mm/slab_common.c

@@ -1280,6 +1280,33 @@ EXPORT_TRACEPOINT_SYMBOL(kmem_cache_alloc);
 EXPORT_TRACEPOINT_SYMBOL(kfree);
 EXPORT_TRACEPOINT_SYMBOL(kmem_cache_free);
 
+void kfree_call_rcu_nolock(struct kvfree_rcu_head *head, void *ptr)
+{
+	struct slab *slab;
+
+	if (!IS_ENABLED(CONFIG_KVFREE_RCU_BATCHED))
+		goto fallback;
+
+	if (unlikely(is_vmalloc_addr(ptr)))
+		goto fallback;
+
+	slab = virt_to_slab(ptr);
+	if (unlikely(!slab))
+		goto fallback;
+
+	if (unlikely(IS_ENABLED(CONFIG_NUMA) && slab_nid(slab) != numa_mem_id()))
+		goto fallback;
+
+	if (unlikely(!__kfree_rcu_sheaf(slab->slab_cache, ptr, SLAB_FREE_NOLOCK)))
+		goto fallback;
+
+	return;
+
+fallback:
+	defer_kfree_rcu(head);
+}
+EXPORT_SYMBOL_GPL(kfree_call_rcu_nolock);
+
 #ifndef CONFIG_KVFREE_RCU_BATCHED
 
 void kvfree_call_rcu(struct kvfree_rcu_head *head, void *ptr)
@@ -1297,6 +1324,20 @@ void kvfree_call_rcu(struct kvfree_rcu_head *head, void *ptr)
 }
 EXPORT_SYMBOL_GPL(kvfree_call_rcu);
 
+void kvfree_rcu_barrier(void)
+{
+	deferred_work_barrier();
+	rcu_barrier();
+}
+EXPORT_SYMBOL_GPL(kvfree_rcu_barrier);
+
+void kvfree_rcu_barrier_on_cache(struct kmem_cache *s)
+{
+	deferred_work_barrier();
+	rcu_barrier();
+}
+EXPORT_SYMBOL_GPL(kvfree_rcu_barrier_on_cache);
+
 void __init kvfree_rcu_init(void)
 {
 }
@@ -2133,14 +2174,16 @@ EXPORT_SYMBOL_GPL(kvfree_rcu_barrier);
  */
 void kvfree_rcu_barrier_on_cache(struct kmem_cache *s)
 {
+	/* kfree_rcu_nolock() might have deferred frees even without sheaves */
+	deferred_work_barrier();
+
 	if (cache_has_sheaves(s)) {
 		cpus_read_lock();
 		flush_rcu_sheaves_on_cache(s);
 		cpus_read_unlock();
-		deferred_work_barrier();
-		rcu_barrier();
 	}
 
+	rcu_barrier();
 	__kvfree_rcu_barrier();
 }
 EXPORT_SYMBOL_GPL(kvfree_rcu_barrier_on_cache);

mm/slub.c

@@ -4050,6 +4050,7 @@ static void flush_all(struct kmem_cache *s)
 
 struct deferred_percpu_work {
 	struct llist_head objects;
+	struct llist_head objects_by_rcu;
 	struct llist_head rcu_sheaves;
 	struct irq_work work;
 };
@@ -4058,6 +4059,7 @@ static void deferred_percpu_work_fn(struct irq_work *work);
 
 static DEFINE_PER_CPU(struct deferred_percpu_work, deferred_percpu_work) = {
 	.objects = LLIST_HEAD_INIT(objects),
+	.objects_by_rcu = LLIST_HEAD_INIT(objects_by_rcu),
 	.rcu_sheaves = LLIST_HEAD_INIT(rcu_sheaves),
 	.work = IRQ_WORK_INIT(deferred_percpu_work_fn),
 };
@@ -4121,6 +4123,8 @@ void flush_all_rcu_sheaves(void)
 {
 	struct kmem_cache *s;
 
+	deferred_work_barrier();
+
 	cpus_read_lock();
 	mutex_lock(&slab_mutex);
 
@@ -4133,7 +4137,6 @@ void flush_all_rcu_sheaves(void)
 	mutex_unlock(&slab_mutex);
 	cpus_read_unlock();
 
-	deferred_work_barrier();
 	rcu_barrier();
 }
 
@@ -6368,13 +6371,14 @@ static void free_to_pcs_bulk(struct kmem_cache *s, size_t size, void **p)
 static void deferred_percpu_work_fn(struct irq_work *work)
 {
 	struct deferred_percpu_work *dpw;
-	struct llist_head *objs, *rcu_sheaves;
+	struct llist_head *objs, *objs_by_rcu, *rcu_sheaves;
 	struct llist_node *llnode, *pos, *t;
 	struct slab_sheaf *sheaf, *next;
 
 	dpw = container_of(work, struct deferred_percpu_work, work);
 	rcu_sheaves = &dpw->rcu_sheaves;
 	objs = &dpw->objects;
+	objs_by_rcu = &dpw->objects_by_rcu;
 
 	llnode = llist_del_all(objs);
 	llist_for_each_safe(pos, t, llnode) {
@@ -6399,6 +6403,14 @@ static void deferred_percpu_work_fn(struct irq_work *work)
 		stat(s, FREE_SLOWPATH);
 	}
 
+	llnode = llist_del_all(objs_by_rcu);
+	llist_for_each_safe(pos, t, llnode) {
+		void *head = pos;
+		void *objp = kvmalloc_obj_start_addr(head);
+
+		kvfree_call_rcu(head, objp);
+	}
+
 	llnode = llist_del_all(rcu_sheaves);
 	llist_for_each_entry_safe(sheaf, next, llnode, llnode)
 		call_rcu(&sheaf->rcu_head, rcu_free_sheaf);
@@ -6417,6 +6429,18 @@ static void defer_free(struct kmem_cache *s, void *head)
 		irq_work_queue(&dpw->work);
 }
 
+void defer_kfree_rcu(struct kvfree_rcu_head *head)
+{
+	struct deferred_percpu_work *dpw;
+
+	guard(preempt)();
+
+	dpw = this_cpu_ptr(&deferred_percpu_work);
+	if (llist_add((struct llist_node *)head, &dpw->objects_by_rcu))
+		irq_work_queue(&dpw->work);
+}
+
+/* Must be called before flush_rcu_sheaves_on_cache() */
 void deferred_work_barrier(void)
 {
 	int cpu;