BPF_MAP_TYPE_HASH, with PERCPU and LRU Variants

Note

  • BPF_MAP_TYPE_HASH was introduced in kernel version 3.19

  • BPF_MAP_TYPE_PERCPU_HASH was introduced in version 4.6

  • Both BPF_MAP_TYPE_LRU_HASH and BPF_MAP_TYPE_LRU_PERCPU_HASH were introduced in version 4.10

BPF_MAP_TYPE_HASH and BPF_MAP_TYPE_PERCPU_HASH provide general purpose hash map storage. Both the key and the value can be structs, allowing for composite keys and values.

The kernel is responsible for allocating and freeing key/value pairs, up to the max_entries limit that you specify. Hash maps use pre-allocation of hash table elements by default. The BPF_F_NO_PREALLOC flag can be used to disable pre-allocation when it is too memory expensive.

BPF_MAP_TYPE_PERCPU_HASH provides a separate value slot per CPU. The per-cpu values are stored internally in an array.

The BPF_MAP_TYPE_LRU_HASH and BPF_MAP_TYPE_LRU_PERCPU_HASH variants add LRU semantics to their respective hash tables. An LRU hash will automatically evict the least recently used entries when the hash table reaches capacity. An LRU hash maintains an internal LRU list that is used to select elements for eviction. This internal LRU list is shared across CPUs but it is possible to request a per CPU LRU list with the BPF_F_NO_COMMON_LRU flag when calling bpf_map_create.

Usage

long bpf_map_update_elem(struct bpf_map *map, const void *key, const void *value, u64 flags)

Hash entries can be added or updated using the bpf_map_update_elem() helper. This helper replaces existing elements atomically. The flags parameter can be used to control the update behaviour:

  • BPF_ANY will create a new element or update an existing element

  • BPF_NOEXIST will create a new element only if one did not already exist

  • BPF_EXIST will update an existing element

bpf_map_update_elem() returns 0 on success, or negative error in case of failure.

void *bpf_map_lookup_elem(struct bpf_map *map, const void *key)

Hash entries can be retrieved using the bpf_map_lookup_elem() helper. This helper returns a pointer to the value associated with key, or NULL if no entry was found.

long bpf_map_delete_elem(struct bpf_map *map, const void *key)

Hash entries can be deleted using the bpf_map_delete_elem() helper. This helper will return 0 on success, or negative error in case of failure.

Per CPU Hashes

For BPF_MAP_TYPE_PERCPU_HASH and BPF_MAP_TYPE_LRU_PERCPU_HASH the bpf_map_update_elem() and bpf_map_lookup_elem() helpers automatically access the hash slot for the current CPU.

void *bpf_map_lookup_percpu_elem(struct bpf_map *map, const void *key, u32 cpu)

The bpf_map_lookup_percpu_elem() helper can be used to lookup the value in the hash slot for a specific CPU. Returns value associated with key on cpu , or NULL if no entry was found or cpu is invalid.

Concurrency

Values stored in BPF_MAP_TYPE_HASH can be accessed concurrently by programs running on different CPUs. Since Kernel version 5.1, the BPF infrastructure provides struct bpf_spin_lock to synchronise access. See tools/testing/selftests/bpf/progs/test_spin_lock.c.

Userspace

int bpf_map_get_next_key(int fd, const void *cur_key, void *next_key)

In userspace, it is possible to iterate through the keys of a hash using libbpf’s bpf_map_get_next_key() function. The first key can be fetched by calling bpf_map_get_next_key() with cur_key set to NULL. Subsequent calls will fetch the next key that follows the current key. bpf_map_get_next_key() returns 0 on success, -ENOENT if cur_key is the last key in the hash, or negative error in case of failure.

Note that if cur_key gets deleted then bpf_map_get_next_key() will instead return the first key in the hash table which is undesirable. It is recommended to use batched lookup if there is going to be key deletion intermixed with bpf_map_get_next_key().

Examples

Please see the tools/testing/selftests/bpf directory for functional examples. The code snippets below demonstrates API usage.

This example shows how to declare an LRU Hash with a struct key and a struct value.

#include <linux/bpf.h>
#include <bpf/bpf_helpers.h>

struct key {
    __u32 srcip;
};

struct value {
    __u64 packets;
    __u64 bytes;
};

struct {
        __uint(type, BPF_MAP_TYPE_LRU_HASH);
        __uint(max_entries, 32);
        __type(key, struct key);
        __type(value, struct value);
} packet_stats SEC(".maps");

This example shows how to create or update hash values using atomic instructions:

static void update_stats(__u32 srcip, int bytes)
{
        struct key key = {
                .srcip = srcip,
        };
        struct value *value = bpf_map_lookup_elem(&packet_stats, &key);

        if (value) {
                __sync_fetch_and_add(&value->packets, 1);
                __sync_fetch_and_add(&value->bytes, bytes);
        } else {
                struct value newval = { 1, bytes };

                bpf_map_update_elem(&packet_stats, &key, &newval, BPF_NOEXIST);
        }
}

Userspace walking the map elements from the map declared above:

#include <bpf/libbpf.h>
#include <bpf/bpf.h>

static void walk_hash_elements(int map_fd)
{
        struct key *cur_key = NULL;
        struct key next_key;
        struct value value;
        int err;

        for (;;) {
                err = bpf_map_get_next_key(map_fd, cur_key, &next_key);
                if (err)
                        break;

                bpf_map_lookup_elem(map_fd, &next_key, &value);

                // Use key and value here

                cur_key = &next_key;
        }
}