Re: [PATCH v2 bpf-next 13/13] bpf: Convert bpf_cpumask to bpf_mem_cache_free_rcu.

[David Vernet <void@manifault.com>]

On Fri, Jun 23, 2023 at 08:13:33PM -0700, Alexei Starovoitov wrote:
> From: Alexei Starovoitov <ast@kernel.org>
> 
> Convert bpf_cpumask to bpf_mem_cache_free_rcu.
> 
> Signed-off-by: Alexei Starovoitov <ast@kernel.org>

Acked-by: David Vernet <void@manifault.com>

LGTM, thanks for cleaning this up. I left one drive-by comment /
observation below, but it's not a blocker for this patch / series.

> ---
>  kernel/bpf/cpumask.c | 20 ++++++--------------
>  1 file changed, 6 insertions(+), 14 deletions(-)
> 
> diff --git a/kernel/bpf/cpumask.c b/kernel/bpf/cpumask.c
> index 938a60ff4295..6983af8e093c 100644
> --- a/kernel/bpf/cpumask.c
> +++ b/kernel/bpf/cpumask.c
> @@ -9,7 +9,6 @@
>  /**
>   * struct bpf_cpumask - refcounted BPF cpumask wrapper structure
>   * @cpumask:	The actual cpumask embedded in the struct.
> - * @rcu:	The RCU head used to free the cpumask with RCU safety.
>   * @usage:	Object reference counter. When the refcount goes to 0, the
>   *		memory is released back to the BPF allocator, which provides
>   *		RCU safety.
> @@ -25,7 +24,6 @@
>   */
>  struct bpf_cpumask {
>  	cpumask_t cpumask;
> -	struct rcu_head rcu;
>  	refcount_t usage;
>  };
>  
> @@ -82,16 +80,6 @@ __bpf_kfunc struct bpf_cpumask *bpf_cpumask_acquire(struct bpf_cpumask *cpumask)
>  	return cpumask;
>  }
>  
> -static void cpumask_free_cb(struct rcu_head *head)
> -{
> -	struct bpf_cpumask *cpumask;
> -
> -	cpumask = container_of(head, struct bpf_cpumask, rcu);
> -	migrate_disable();
> -	bpf_mem_cache_free(&bpf_cpumask_ma, cpumask);
> -	migrate_enable();
> -}
> -
>  /**
>   * bpf_cpumask_release() - Release a previously acquired BPF cpumask.
>   * @cpumask: The cpumask being released.
> @@ -102,8 +90,12 @@ static void cpumask_free_cb(struct rcu_head *head)
>   */
>  __bpf_kfunc void bpf_cpumask_release(struct bpf_cpumask *cpumask)
>  {
> -	if (refcount_dec_and_test(&cpumask->usage))
> -		call_rcu(&cpumask->rcu, cpumask_free_cb);
> +	if (!refcount_dec_and_test(&cpumask->usage))
> +		return;
> +
> +	migrate_disable();
> +	bpf_mem_cache_free_rcu(&bpf_cpumask_ma, cpumask);
> +	migrate_enable();

The fact that callers have to disable migration like this in order to
safely free the memory feels a bit leaky. Is there any reason we can't
move this into bpf_mem_{cache_}free_rcu()?