Re: [PATCH bpf v3] bpf: do not use kmalloc_nolock when !HAVE_CMPXCHG_DOUBLE

[Paul Chaignon <paul.chaignon@gmail.com>]

On Sun, Mar 15, 2026 at 12:02:48AM +0800, Levi Zim via B4 Relay wrote:
> From: Levi Zim <rsworktech@outlook.com>
> 
> 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.
> But leave the PREEMPT_RT case as is because it requires kmalloc_nolock
> for correctness. Add a comment about this limitation that architecture's
> lack of CMPXCHG_DOUBLE combined with PREEMPT_RT could make
> bpf_local_storage_alloc always fail.
> 
> Fixes: f484f4a3e058 ("bpf: Replace bpf memory allocator with kmalloc_nolock() in local storage")
> Cc: stable@vger.kernel.org
> Signed-off-by: Levi Zim <rsworktech@outlook.com>
> ---

Note there may be something broken with your setup as lore is reporting
that you sent this v3 email three times. Not sure if it could be an
issue.

[...]

> diff --git a/include/linux/bpf_local_storage.h b/include/linux/bpf_local_storage.h
> index 8157e8da61d40..d8f2c5d63a80e 100644
> --- a/include/linux/bpf_local_storage.h
> +++ b/include/linux/bpf_local_storage.h
> @@ -18,6 +18,7 @@
>  #include <asm/rqspinlock.h>
>  
>  #define BPF_LOCAL_STORAGE_CACHE_SIZE	16
> +#define KMALLOC_NOLOCK_SUPPORTED IS_ENABLED(CONFIG_HAVE_CMPXCHG_DOUBLE)
>  
>  struct bpf_local_storage_map_bucket {
>  	struct hlist_head list;
> diff --git a/kernel/bpf/bpf_cgrp_storage.c b/kernel/bpf/bpf_cgrp_storage.c
> index c2a2ead1f466d..cd18193c44058 100644
> --- a/kernel/bpf/bpf_cgrp_storage.c
> +++ b/kernel/bpf/bpf_cgrp_storage.c
> @@ -114,7 +114,8 @@ 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..a6c240da87668 100644
> --- a/kernel/bpf/bpf_local_storage.c
> +++ b/kernel/bpf/bpf_local_storage.c
> @@ -893,6 +893,10 @@ bpf_local_storage_map_alloc(union bpf_attr *attr,
>  	/* In PREEMPT_RT, kmalloc(GFP_ATOMIC) is still not safe in non
>  	 * preemptible context. Thus, enforce all storages to use
>  	 * kmalloc_nolock() when CONFIG_PREEMPT_RT is enabled.
> +	 *
> +	 * However, kmalloc_nolock would fail on architectures that do not
> +	 * have CMPXCHG_DOUBLE. On such architectures with PREEMPT_RT,
> +	 * bpf_local_storage_alloc would always fail.
>  	 */
>  	smap->use_kmalloc_nolock = IS_ENABLED(CONFIG_PREEMPT_RT) ? true : use_kmalloc_nolock;
>  
> diff --git a/kernel/bpf/bpf_task_storage.c b/kernel/bpf/bpf_task_storage.c
> index 605506792b5b4..6e8597edea314 100644
> --- a/kernel/bpf/bpf_task_storage.c
> +++ b/kernel/bpf/bpf_task_storage.c
> @@ -212,7 +212,8 @@ 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);

I can confirm that this does fix one selftest using
BPF_LOCAL_STORAGE_GET_F_CREATE on riscv64: test_ls_map_kptr_ref1 in
map_kptr. Other tests using BPF_LOCAL_STORAGE_GET_F_CREATE are still
failing so I guess they have other issues.

Tested-by: Paul Chaignon <paul.chaignon@gmail.com>

>  }
>  
>  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@outlook.com>
> 
> 
>