[PATCH bpf] bpf: do not use kmalloc_nolock when !HAVE_CMPXCHG_DOUBLE

[Levi Zim via B4 Relay]

From: Levi Zim <rsworktech@xxxxxxxxxxx>

kmalloc_nolock always fails for architectures that lack cmpxchg16b.
For example, this causes bpf_task_storage_get with flag
BPF_LOCAL_STORAGE_GET_F_CREATE to fails on riscv64 6.19 kernel.

Fix it by enabling use_kmalloc_nolock only when HAVE_CMPXCHG_DOUBLE

Fixes: f484f4a3e058 ("bpf: Replace bpf memory allocator with kmalloc_nolock() in local storage")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Levi Zim <rsworktech@xxxxxxxxxxx>
---
I find that bpf_task_storage_get with flag BPF_LOCAL_STORAGE_GET_F_CREATE
always fails for me on 6.19 kernel on riscv64 and bisected it.

In f484f4a3e058 ("bpf: Replace bpf memory allocator with kmalloc_nolock()
in local storage"), bpf memory allocator is replaced with kmalloc_nolock.
This approach is problematic for architectures that lack CMPXCHG_DOUBLE
because kmalloc_nolock always fails in this case:

In function kmalloc_nolock (kmalloc_nolock_noprof): 

	if (!(s->flags & __CMPXCHG_DOUBLE) && !kmem_cache_debug(s))
		/*
		 * kmalloc_nolock() is not supported on architectures that
		 * don't implement cmpxchg16b, but debug caches don't use
		 * per-cpu slab and per-cpu partial slabs. They rely on
		 * kmem_cache_node->list_lock, so kmalloc_nolock() can
		 * attempt to allocate from debug caches by
		 * spin_trylock_irqsave(&n->list_lock, ...)
		 */
		return NULL;

Fix it by enabling use_kmalloc_nolock only when HAVE_CMPXCHG_DOUBLE.

Note for stable: this only needs to be picked into v6.19 if the patch
makes it into 7.0.
---
 include/linux/bpf_local_storage.h | 2 ++
 kernel/bpf/bpf_cgrp_storage.c     | 2 +-
 kernel/bpf/bpf_local_storage.c    | 3 ++-
 kernel/bpf/bpf_task_storage.c     | 2 +-
 4 files changed, 6 insertions(+), 3 deletions(-)

diff --git a/include/linux/bpf_local_storage.h b/include/linux/bpf_local_storage.h
index 8157e8da61d40..a7ae5dde15bcb 100644
--- a/include/linux/bpf_local_storage.h
+++ b/include/linux/bpf_local_storage.h
@@ -19,6 +19,8 @@
 
 #define BPF_LOCAL_STORAGE_CACHE_SIZE	16
 
+static const bool KMALLOC_NOLOCK_SUPPORTED = IS_ENABLED(CONFIG_HAVE_CMPXCHG_DOUBLE);
+
 struct bpf_local_storage_map_bucket {
 	struct hlist_head list;
 	rqspinlock_t lock;
diff --git a/kernel/bpf/bpf_cgrp_storage.c b/kernel/bpf/bpf_cgrp_storage.c
index c2a2ead1f466d..09c557e426968 100644
--- a/kernel/bpf/bpf_cgrp_storage.c
+++ b/kernel/bpf/bpf_cgrp_storage.c
@@ -114,7 +114,7 @@ static int notsupp_get_next_key(struct bpf_map *map, void *key, void *next_key)
 
 static struct bpf_map *cgroup_storage_map_alloc(union bpf_attr *attr)
 {
-	return bpf_local_storage_map_alloc(attr, &cgroup_cache, true);
+	return bpf_local_storage_map_alloc(attr, &cgroup_cache, KMALLOC_NOLOCK_SUPPORTED);
 }
 
 static void cgroup_storage_map_free(struct bpf_map *map)
diff --git a/kernel/bpf/bpf_local_storage.c b/kernel/bpf/bpf_local_storage.c
index 9c96a4477f81a..8e4b0fe6d12af 100644
--- a/kernel/bpf/bpf_local_storage.c
+++ b/kernel/bpf/bpf_local_storage.c
@@ -894,7 +894,8 @@ bpf_local_storage_map_alloc(union bpf_attr *attr,
 	 * preemptible context. Thus, enforce all storages to use
 	 * kmalloc_nolock() when CONFIG_PREEMPT_RT is enabled.
 	 */
-	smap->use_kmalloc_nolock = IS_ENABLED(CONFIG_PREEMPT_RT) ? true : use_kmalloc_nolock;
+	smap->use_kmalloc_nolock = IS_ENABLED(CONFIG_PREEMPT_RT) &&
+		KMALLOC_NOLOCK_SUPPORTED ? true : use_kmalloc_nolock;
 
 	smap->cache_idx = bpf_local_storage_cache_idx_get(cache);
 	return &smap->map;
diff --git a/kernel/bpf/bpf_task_storage.c b/kernel/bpf/bpf_task_storage.c
index 605506792b5b4..9f74fd1ef7f46 100644
--- a/kernel/bpf/bpf_task_storage.c
+++ b/kernel/bpf/bpf_task_storage.c
@@ -212,7 +212,7 @@ static int notsupp_get_next_key(struct bpf_map *map, void *key, void *next_key)
 
 static struct bpf_map *task_storage_map_alloc(union bpf_attr *attr)
 {
-	return bpf_local_storage_map_alloc(attr, &task_cache, true);
+	return bpf_local_storage_map_alloc(attr, &task_cache, KMALLOC_NOLOCK_SUPPORTED);
 }
 
 static void task_storage_map_free(struct bpf_map *map)

---
base-commit: e06e6b8001233241eb5b2e2791162f0585f50f4b
change-id: 20260314-bpf-kmalloc-nolock-60da80e613de

Best regards,
-- 
Levi Zim <rsworktech@xxxxxxxxxxx>