bpf: devmap memory usage

A new helper is introduced to calculate the memory usage of devmap and
devmap_hash. The number of dynamically allocated elements are recored
for devmap_hash already, but not for devmap. To track the memory size of
dynamically allocated elements, this patch also count the numbers for
devmap.

The result as follows,
- before
40: devmap  name count_map  flags 0x80
        key 4B  value 4B  max_entries 65536  memlock 524288B
41: devmap_hash  name count_map  flags 0x80
        key 4B  value 4B  max_entries 65536  memlock 524288B

- after
40: devmap  name count_map  flags 0x80  <<<< no elements
        key 4B  value 4B  max_entries 65536  memlock 524608B
41: devmap_hash  name count_map  flags 0x80 <<<< no elements
        key 4B  value 4B  max_entries 65536  memlock 524608B

Note that the number of buckets is same with max_entries for devmap_hash
in this case.
Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
Link: https://lore.kernel.org/r/20230305124615.12358-11-laoar.shao@gmail.comSigned-off-by: Alexei Starovoitov <ast@kernel.org>

kernel/bpf/devmap.c

@@ -819,8 +819,10 @@ static int dev_map_delete_elem(struct bpf_map *map, void *key)
 		return -EINVAL;
 
 	old_dev = unrcu_pointer(xchg(&dtab->netdev_map[k], NULL));
-	if (old_dev)
+	if (old_dev) {
 		call_rcu(&old_dev->rcu, __dev_map_entry_free);
+		atomic_dec((atomic_t *)&dtab->items);
+	}
 	return 0;
 }
 
@@ -931,6 +933,8 @@ static int __dev_map_update_elem(struct net *net, struct bpf_map *map,
 	old_dev = unrcu_pointer(xchg(&dtab->netdev_map[i], RCU_INITIALIZER(dev)));
 	if (old_dev)
 		call_rcu(&old_dev->rcu, __dev_map_entry_free);
+	else
+		atomic_inc((atomic_t *)&dtab->items);
 
 	return 0;
 }
@@ -1016,6 +1020,20 @@ static int dev_hash_map_redirect(struct bpf_map *map, u64 ifindex, u64 flags)
 				      __dev_map_hash_lookup_elem);
 }
 
+static u64 dev_map_mem_usage(const struct bpf_map *map)
+{
+	struct bpf_dtab *dtab = container_of(map, struct bpf_dtab, map);
+	u64 usage = sizeof(struct bpf_dtab);
+
+	if (map->map_type == BPF_MAP_TYPE_DEVMAP_HASH)
+		usage += (u64)dtab->n_buckets * sizeof(struct hlist_head);
+	else
+		usage += (u64)map->max_entries * sizeof(struct bpf_dtab_netdev *);
+	usage += atomic_read((atomic_t *)&dtab->items) *
+			 (u64)sizeof(struct bpf_dtab_netdev);
+	return usage;
+}
+
 BTF_ID_LIST_SINGLE(dev_map_btf_ids, struct, bpf_dtab)
 const struct bpf_map_ops dev_map_ops = {
 	.map_meta_equal = bpf_map_meta_equal,
@@ -1026,6 +1044,7 @@ const struct bpf_map_ops dev_map_ops = {
 	.map_update_elem = dev_map_update_elem,
 	.map_delete_elem = dev_map_delete_elem,
 	.map_check_btf = map_check_no_btf,
+	.map_mem_usage = dev_map_mem_usage,
 	.map_btf_id = &dev_map_btf_ids[0],
 	.map_redirect = dev_map_redirect,
 };
@@ -1039,6 +1058,7 @@ const struct bpf_map_ops dev_map_hash_ops = {
 	.map_update_elem = dev_map_hash_update_elem,
 	.map_delete_elem = dev_map_hash_delete_elem,
 	.map_check_btf = map_check_no_btf,
+	.map_mem_usage = dev_map_mem_usage,
 	.map_btf_id = &dev_map_btf_ids[0],
 	.map_redirect = dev_hash_map_redirect,
 };
@@ -1109,9 +1129,11 @@ static int dev_map_notification(struct notifier_block *notifier,
 				if (!dev || netdev != dev->dev)
 					continue;
 				odev = unrcu_pointer(cmpxchg(&dtab->netdev_map[i], RCU_INITIALIZER(dev), NULL));
-				if (dev == odev)
+				if (dev == odev) {
 					call_rcu(&dev->rcu,
 						 __dev_map_entry_free);
+					atomic_dec((atomic_t *)&dtab->items);
+				}
 			}
 		}
 		rcu_read_unlock();