Subject: Re: [PATCH bpf-next 1/2] bpf: implement MAP_GET_NEXT_KEY command for LPM_TRIE map

From: Eric Dumazet <[email-masked]>
Date: Mon, 22 Jan 2018 11:28:32 -0800

On Thu, 2018-01-18 at 15:08 -0800, Yonghong Song wrote:
> Current LPM_TRIE map type does not implement MAP_GET_NEXT_KEY
> command. This command is handy when users want to enumerate
> keys. Otherwise, a different map which supports key
> enumeration may be required to store the keys. If the
> map data is sparse and all map data are to be deleted without
> closing file descriptor, using MAP_GET_NEXT_KEY to find
> all keys is much faster than enumerating all key space.
> 
> This patch implements MAP_GET_NEXT_KEY command for LPM_TRIE map.
> If user provided key pointer is NULL or the key does not have
> an exact match in the trie, the first key will be returned.
> Otherwise, the next key will be returned.
> 
> In this implemenation, key enumeration follows a postorder
> traversal of internal trie. More specific keys
> will be returned first than less specific ones, given
> a sequence of MAP_GET_NEXT_KEY syscalls.
> 
> Signed-off-by: Yonghong Song <[email-masked]>
> ---
>  kernel/bpf/lpm_trie.c | 95 +++++++++++++++++++++++++++++++++++++++++++++++++--
>  1 file changed, 93 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/bpf/lpm_trie.c b/kernel/bpf/lpm_trie.c
> index 584e022..d7ea962 100644
> --- a/kernel/bpf/lpm_trie.c
> +++ b/kernel/bpf/lpm_trie.c
> @@ -591,9 +591,100 @@ static void trie_free(struct bpf_map *map)
>  	raw_spin_unlock(&trie->lock);
>  }
>  
> -static int trie_get_next_key(struct bpf_map *map, void *key, void *next_key)
> +static int trie_get_next_key(struct bpf_map *map, void *_key, void *_next_key)
>  {
> -	return -ENOTSUPP;
> +	struct lpm_trie *trie = container_of(map, struct lpm_trie, map);
> +	struct bpf_lpm_trie_key *key = _key, *next_key = _next_key;
> +	struct lpm_trie_node *node, *next_node = NULL, *parent;
> +	struct lpm_trie_node **node_stack = NULL;
> +	struct lpm_trie_node __rcu **root;
> +	int err = 0, stack_ptr = -1;
> +	unsigned int next_bit;
> +	size_t matchlen;
> +
> +	/* The get_next_key follows postorder. For the 4 node example in
> +	 * the top of this file, the trie_get_next_key() returns the following
> +	 * one after another:
> +	 *   192.168.0.0/24
> +	 *   192.168.1.0/24
> +	 *   192.168.128.0/24
> +	 *   192.168.0.0/16
> +	 *
> +	 * The idea is to return more specific keys before less specific ones.
> +	 */
> +
> +	/* Empty trie */
> +	if (!rcu_dereference(trie->root))
> +		return -ENOENT;
> +
> +	/* For invalid key, find the leftmost node in the trie */
> +	if (!key || key->prefixlen > trie->max_prefixlen) {
> +		root = &trie->root;
> +		goto find_leftmost;
> +	}
> +
> +	node_stack = kmalloc(trie->max_prefixlen * sizeof(struct lpm_trie_node *),
> +			     GFP_USER | __GFP_NOWARN);

It is illegal to use a blocking kmalloc() while holding RCU.

CONFIG_DEBUG_ATOMIC_SLEEP=y is your friend

> +	if (!node_stack)
> +		return -ENOMEM;
> +
> +	/* Try to find the exact node for the given key */
> +	for (node = rcu_dereference(trie->root); node;) {
> +		node_stack[++stack_ptr] = node;
> +		matchlen = longest_prefix_match(trie, node, key);
> +		if (node->prefixlen != matchlen ||
> +		    node->prefixlen == key->prefixlen)
> +			break;
> +
> +		next_bit = extract_bit(key->data, node->prefixlen);
> +		node = rcu_dereference(node->child[next_bit]);
> +	}
> +	if (!node || node->prefixlen != key->prefixlen ||
> +	    (node->flags & LPM_TREE_NODE_FLAG_IM)) {
> +		root = &trie->root;
> +		goto find_leftmost;
> +	}
> +
> +	/* The node with the exactly-matching key has been found,
> +	 * find the first node in postorder after the matched node.
> +	 */
> +	node = node_stack[stack_ptr];
> +	while (stack_ptr > 0) {
> +		parent = node_stack[stack_ptr - 1];
> +		if (rcu_dereference(parent->child[0]) == node &&
> +		    rcu_dereference(parent->child[1])) {
> +			root = &parent->child[1];
> +			goto find_leftmost;
> +		}
> +		if (!(parent->flags & LPM_TREE_NODE_FLAG_IM)) {
> +			next_node = parent;
> +			goto do_copy;
> +		}
> +
> +		node = parent;
> +		stack_ptr--;
> +	}
> +
> +	/* did not find anything */
> +	err = -ENOENT;
> +	goto free_stack;
> +
> +find_leftmost:
> +	/* Find the leftmost non-intermediate node, all intermediate nodes
> +	 * have exact two children, so this function will never return NULL.
> +	 */
> +	for (node = rcu_dereference(*root); node;) {
> +		if (!(node->flags & LPM_TREE_NODE_FLAG_IM))
> +			next_node = node;
> +		node = rcu_dereference(node->child[0]);
> +	}
> +do_copy:
> +	next_key->prefixlen = next_node->prefixlen;
> +	memcpy((void *)next_key + offsetof(struct bpf_lpm_trie_key, data),
> +	       next_node->data, trie->data_size);
> +free_stack:
> +	kfree(node_stack);
> +	return err;
>  }
>  
>  const struct bpf_map_ops trie_map_ops = {