Re: [PATCH RFC bpf-next v2 05/18] bpf: Implement get_next_key and free_internal_structs for resizable hashtab

["Emil Tsalapatis" <emil@etsalapatis.com>]

On Wed Apr 8, 2026 at 11:10 AM EDT, Mykyta Yatsenko wrote:
> From: Mykyta Yatsenko <yatsenko@meta.com>
>
> Implement rhtab_map_get_next_key() and rhtab_map_free_internal_structs()
> of the BPF resizable hashtable. Both are only called from syscall, so
> it's safe to use rhashtable walk API that uses spinlocks internally.
>
> Signed-off-by: Mykyta Yatsenko <yatsenko@meta.com>
> ---
>  kernel/bpf/hashtab.c | 78 +++++++++++++++++++++++++++++++++++++++++++++++++++-
>  1 file changed, 77 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/bpf/hashtab.c b/kernel/bpf/hashtab.c
> index ea7314cc3703..7eee450a321e 100644
> --- a/kernel/bpf/hashtab.c
> +++ b/kernel/bpf/hashtab.c
> @@ -2921,13 +2921,89 @@ static int rhtab_map_gen_lookup(struct bpf_map *map, struct bpf_insn *insn_buf)
>  	return insn - insn_buf;
>  }
>  
> +/* Helper to get next element, handling -EAGAIN during resize */
> +static struct rhtab_elem *rhtab_iter_next(struct rhashtable_iter *iter)
> +{
> +	struct rhtab_elem *elem;
> +
> +	while ((elem = rhashtable_walk_next(iter))) {
> +		if (IS_ERR(elem)) {
> +			if (PTR_ERR(elem) == -EAGAIN)
> +				continue;
> +			return NULL;
> +		}
> +		return elem;
> +	}
> +
> +	return NULL;
> +}
> +
>  static void rhtab_map_free_internal_structs(struct bpf_map *map)
>  {
> +	struct bpf_rhtab *rhtab = container_of(map, struct bpf_rhtab, map);
> +	struct rhashtable_iter iter;
> +	struct rhtab_elem *elem;
> +
> +	if (!bpf_map_has_internal_structs(map))
> +		return;
> +
> +	/*
> +	 * An element can be processed twice if rhashtable resized concurrently.
> +	 * Special structs freeing handles duplicate cancel_and_free.
> +	 */
> +	rhashtable_walk_enter(&rhtab->ht, &iter);
> +	rhashtable_walk_start(&iter);
> +
> +	for (elem = rhtab_iter_next(&iter); elem; elem = rhtab_iter_next(&iter))
> +		bpf_map_free_internal_structs(map, rhtab_elem_value(elem, map->key_size));
> +
> +	rhashtable_walk_stop(&iter);
> +	rhashtable_walk_exit(&iter);
>  }
>  
>  static int rhtab_map_get_next_key(struct bpf_map *map, void *key, void *next_key)
>  {
> -	return -EOPNOTSUPP;
> +	struct bpf_rhtab *rhtab = container_of(map, struct bpf_rhtab, map);
> +	struct rhashtable_iter iter;
> +	struct rhtab_elem *elem;
> +	bool key_found;
> +	int ret = -ENOENT;
> +
> +	/*
> +	 * Hold RCU across enter_from + walk_start to prevent the
> +	 * element cached by enter_from from being freed before
> +	 * walk_start re-acquires RCU.
> +	 */
> +	guard(rcu)();
> +	rhashtable_walk_enter_from(&rhtab->ht, &iter, key, rhtab->params);
> +	key_found = key && iter.p;
> +	rhashtable_walk_start(&iter);
> +
> +	elem = rhtab_iter_next(&iter);
> +	if (elem) {
> +		memcpy(next_key, elem->data, map->key_size);
> +		ret = 0;
> +	}
> +
> +	rhashtable_walk_stop(&iter);
> +	rhashtable_walk_exit(&iter);
> +
> +	if (ret == 0 || key_found)
> +		return ret;
> +
> +	/* Key was not found restart from the beginning */
> +	rhashtable_walk_enter(&rhtab->ht, &iter);
> +	rhashtable_walk_start(&iter);
> +
> +	elem = rhtab_iter_next(&iter);
> +	if (elem) {
> +		memcpy(next_key, elem->data, map->key_size);
> +		ret = 0;
> +	}
> +
> +	rhashtable_walk_stop(&iter);
> +	rhashtable_walk_exit(&iter);
Nit: The two attempts are partly identical, can you try to
roll them together and use a goto to retry? Or do you think
this will be too messy?

Otherwise:

Reviewed-by: Emil Tsalapatis <emil@etsalapatis.com>

> +	return ret;
>  }
>  
>  static void rhtab_map_seq_show_elem(struct bpf_map *map, void *key, struct seq_file *m)