Hi
On Sun, Nov 5, 2023, at 9:58 AM, Florian Lehner wrote:
> When looking up an element in LPM trie, the condition 'matchlen ==
> trie->max_prefixlen' will never return true, if key->prefixlen is larger
> than trie->max_prefixlen. Consequently all elements in the LPM trie will
> be visited and no element is returned in the end.
>
> To resolve this, check key->prefixlen first before walking the LPM trie.
Am I understanding you right that this is an optimization to avoid walking the entire trie? Because the way I read your commit-message I assume the output has always been NULL? Or am I missing something.
Do you have a specific use-case where such lookups are common? Can you explain why it is important to optimize this case? Because you now add a condition for every lookup just to optimize for the lookup-miss of a special case. I don't think I understand your reasoning here, but I might be missing some context.
Thanks!
David
> Fixes: b95a5c4db09b ("bpf: add a longest prefix match trie map implementation")
> Signed-off-by: Florian Lehner <dev@xxxxxxxxxxx>
> ---
> kernel/bpf/lpm_trie.c | 3 +++
> 1 file changed, 3 insertions(+)
>
> diff --git a/kernel/bpf/lpm_trie.c b/kernel/bpf/lpm_trie.c
> index 17c7e7782a1f..b32be680da6c 100644
> --- a/kernel/bpf/lpm_trie.c
> +++ b/kernel/bpf/lpm_trie.c
> @@ -231,6 +231,9 @@ static void *trie_lookup_elem(struct bpf_map *map,
> void *_key)
> struct lpm_trie_node *node, *found = NULL;
> struct bpf_lpm_trie_key *key = _key;
>
> + if (key->prefixlen > trie->max_prefixlen)
> + return NULL;
> +
> /* Start walking the trie from the root node ... */
>
> for (node = rcu_dereference_check(trie->root, rcu_read_lock_bh_held());
> --
> 2.39.2
