Re: [PATCH RFC bpf-next v2 07/18] bpf: Implement batch ops 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>
>
> Add batch operations for BPF_MAP_TYPE_RHASH.
>
> Batch operations:
>  * rhtab_map_lookup_batch: Bulk lookup of elements by bucket
>  * rhtab_map_lookup_and_delete_batch: Atomic bulk lookup and delete
>
> The batch implementation uses rhashtable_walk_enter_from() to resume
> iteration from the last collected key. When the buffer fills, the last
> key becomes the cursor for the next batch call.
>
> Also implements rhtab_map_mem_usage() to report memory consumption.
>
> Signed-off-by: Mykyta Yatsenko <yatsenko@meta.com>
> ---
>  kernel/bpf/hashtab.c | 137 +++++++++++++++++++++++++++++++++++++++++++++++++--
>  1 file changed, 134 insertions(+), 3 deletions(-)
>
> diff --git a/kernel/bpf/hashtab.c b/kernel/bpf/hashtab.c
> index e79c194e2779..a79d434dc626 100644
> --- a/kernel/bpf/hashtab.c
> +++ b/kernel/bpf/hashtab.c
> @@ -3051,19 +3051,150 @@ static long bpf_each_rhash_elem(struct bpf_map *map, bpf_callback_t callback_fn,
>  
>  static u64 rhtab_map_mem_usage(const struct bpf_map *map)
>  {
> -	return 0;
> +	struct bpf_rhtab *rhtab = container_of(map, struct bpf_rhtab, map);
> +	u64 num_entries;
> +
> +	num_entries = atomic_read(&rhtab->ht.nelems);
> +	return sizeof(struct bpf_rhtab) + rhtab->elem_size * num_entries;
> +}
> +
> +static int __rhtab_map_lookup_and_delete_batch(struct bpf_map *map,
> +					       const union bpf_attr *attr,
> +					       union bpf_attr __user *uattr,
> +					       bool do_delete)
> +{
> +	struct bpf_rhtab *rhtab = container_of(map, struct bpf_rhtab, map);
> +	void __user *uvalues = u64_to_user_ptr(attr->batch.values);
> +	void __user *ukeys = u64_to_user_ptr(attr->batch.keys);
> +	void __user *ubatch = u64_to_user_ptr(attr->batch.in_batch);
> +	void *buf = NULL, *keys = NULL, *values = NULL, *dst_key, *dst_val;
> +	struct rhtab_elem **del_elems = NULL;
> +	u32 max_count, total, key_size, value_size, i;
> +	struct rhashtable_iter iter;
> +	struct rhtab_elem *elem;
> +	u64 elem_map_flags, map_flags;
> +	int ret = 0;
> +
> +	elem_map_flags = attr->batch.elem_flags;
> +	if ((elem_map_flags & ~BPF_F_LOCK) ||
> +	    ((elem_map_flags & BPF_F_LOCK) &&
> +	     !btf_record_has_field(map->record, BPF_SPIN_LOCK)))
> +		return -EINVAL;
> +
> +	map_flags = attr->batch.flags;
> +	if (map_flags)
> +		return -EINVAL;
> +
> +	max_count = attr->batch.count;
> +	if (!max_count)
> +		return 0;
> +
> +	if (put_user(0, &uattr->batch.count))
> +		return -EFAULT;
> +
> +	key_size = map->key_size;
> +	value_size = map->value_size;
> +
> +	keys = kvmalloc_array(max_count, key_size, GFP_USER | __GFP_NOWARN);
> +	values = kvmalloc_array(max_count, value_size, GFP_USER | __GFP_NOWARN);
> +	if (do_delete)
> +		del_elems = kvmalloc_array(max_count, sizeof(void *),
> +					   GFP_USER | __GFP_NOWARN);
> +
> +	if (!keys || !values || (do_delete && !del_elems)) {
> +		ret = -ENOMEM;
> +		goto free;
> +	}
> +
> +	/*
> +	 * Use the last key from the previous batch as cursor.
> +	 * enter_from positions at that key's bucket, walk_next
> +	 * returns the successor in O(1).
> +	 * First call (ubatch == NULL): starts from bucket 0.
> +	 */
> +	if (ubatch) {
> +		buf = kmalloc(key_size, GFP_USER | __GFP_NOWARN);
> +		if (!buf) {
> +			ret = -ENOMEM;
> +			goto free;
> +		}
> +		if (copy_from_user(buf, ubatch, key_size)) {
> +			ret = -EFAULT;
> +			goto free;
> +		}
> +	}
> +
> +	scoped_guard(rcu) {

AFAICT this guard makes sure the RCU critical section extends from the
beginning of rhashtable_walk_enter_from all the way to walk_stop(), is
that correct?

> +		rhashtable_walk_enter_from(&rhtab->ht, &iter, buf, rhtab->params);
> +		rhashtable_walk_start(&iter);
> +	}
> +
> +	dst_key = keys;
> +	dst_val = values;
> +	total = 0;
> +
> +	while (total < max_count) {
> +		elem = rhtab_iter_next(&iter);
> +		if (!elem)
> +			break;
> +
> +		memcpy(dst_key, elem->data, key_size);
> +		rhtab_read_elem_value(map, dst_val, elem, elem_map_flags);
> +		check_and_init_map_value(map, dst_val);
> +
> +		if (do_delete)
> +			del_elems[total] = elem;
> +
> +		dst_key += key_size;
> +		dst_val += value_size;
> +		total++;
> +	}
> +
> +	if (do_delete) {
> +		for (i = 0; i < total; i++)
> +			rhtab_delete_elem(rhtab, del_elems[i]);
> +	}
> +
> +	rhashtable_walk_stop(&iter);
> +	rhashtable_walk_exit(&iter);
> +
> +	if (total == 0) {
> +		ret = -ENOENT;
> +		goto free;
> +	}
> +
> +	/* Signal end of table when we collected fewer than requested */
> +	if (total < max_count)
> +		ret = -ENOENT;
> +
> +	/* Write last key as cursor for the next batch call */
> +	if (copy_to_user(ukeys, keys, total * key_size) ||
> +	    copy_to_user(uvalues, values, total * value_size) ||
> +	    put_user(total, &uattr->batch.count) ||
> +	    copy_to_user(u64_to_user_ptr(attr->batch.out_batch),
> +			 dst_key - key_size, key_size)) {
> +		ret = -EFAULT;
> +		goto free;
> +	}
> +
> +free:
> +	kfree(buf);
> +	kvfree(keys);
> +	kvfree(values);
> +	kvfree(del_elems);
> +	return ret;
>  }
>  
>  static int rhtab_map_lookup_batch(struct bpf_map *map, const union bpf_attr *attr,
>  				  union bpf_attr __user *uattr)
>  {
> -	return 0;
> +	return __rhtab_map_lookup_and_delete_batch(map, attr, uattr, false);
>  }
>  
>  static int rhtab_map_lookup_and_delete_batch(struct bpf_map *map, const union bpf_attr *attr,
>  					     union bpf_attr __user *uattr)
>  {
> -	return 0;
Shouldn't these have been -EINVAL or -ENOSUPP in previous patches?

> +	return __rhtab_map_lookup_and_delete_batch(map, attr, uattr, true);
>  }
>  
>  struct bpf_iter_seq_rhash_map_info {