bpf, net: xskmap memory usage

A new helper is introduced to calculate xskmap memory usage.

The xfsmap memory usage can be dynamically changed when we add or remove
a xsk_map_node. Hence we need to track the count of xsk_map_node to get
its memory usage.

The result as follows,
- before
10: xskmap  name count_map  flags 0x0
        key 4B  value 4B  max_entries 65536  memlock 524288B

- after
10: xskmap  name count_map  flags 0x0 <<< no elements case
        key 4B  value 4B  max_entries 65536  memlock 524608B
Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
Link: https://lore.kernel.org/r/20230305124615.12358-17-laoar.shao@gmail.comSigned-off-by: Alexei Starovoitov <ast@kernel.org>

include/net/xdp_sock.h

@@ -38,6 +38,7 @@ struct xdp_umem {
 struct xsk_map {
 	struct bpf_map map;
 	spinlock_t lock; /* Synchronize map updates */
+	atomic_t count;
 	struct xdp_sock __rcu *xsk_map[];
 };
 

net/xdp/xskmap.c

@@ -24,6 +24,7 @@ static struct xsk_map_node *xsk_map_node_alloc(struct xsk_map *map,
 		return ERR_PTR(-ENOMEM);
 
 	bpf_map_inc(&map->map);
+	atomic_inc(&map->count);
 
 	node->map = map;
 	node->map_entry = map_entry;
@@ -32,8 +33,11 @@ static struct xsk_map_node *xsk_map_node_alloc(struct xsk_map *map,
 
 static void xsk_map_node_free(struct xsk_map_node *node)
 {
+	struct xsk_map *map = node->map;
+
 	bpf_map_put(&node->map->map);
 	kfree(node);
+	atomic_dec(&map->count);
 }
 
 static void xsk_map_sock_add(struct xdp_sock *xs, struct xsk_map_node *node)
@@ -85,6 +89,14 @@ static struct bpf_map *xsk_map_alloc(union bpf_attr *attr)
 	return &m->map;
 }
 
+static u64 xsk_map_mem_usage(const struct bpf_map *map)
+{
+	struct xsk_map *m = container_of(map, struct xsk_map, map);
+
+	return struct_size(m, xsk_map, map->max_entries) +
+		   (u64)atomic_read(&m->count) * sizeof(struct xsk_map_node);
+}
+
 static void xsk_map_free(struct bpf_map *map)
 {
 	struct xsk_map *m = container_of(map, struct xsk_map, map);
@@ -267,6 +279,7 @@ const struct bpf_map_ops xsk_map_ops = {
 	.map_update_elem = xsk_map_update_elem,
 	.map_delete_elem = xsk_map_delete_elem,
 	.map_check_btf = map_check_no_btf,
+	.map_mem_usage = xsk_map_mem_usage,
 	.map_btf_id = &xsk_map_btf_ids[0],
 	.map_redirect = xsk_map_redirect,
 };