bpf: cpumap memory usage

A new helper is introduced to calculate cpumap memory usage. The size of
cpu_entries can be dynamically changed when we update or delete a cpumap
element, but this patch doesn't include the memory size of cpu_entry
yet. We can dynamically calculate the memory usage when we alloc or free
a cpu_entry, but it will take extra runtime overhead, so let just put it
aside currently. Note that the size of different cpu_entry may be
different as well.

The result as follows,
- before
48: cpumap  name count_map  flags 0x4
        key 4B  value 4B  max_entries 64  memlock 4096B

- after
48: cpumap  name count_map  flags 0x4
        key 4B  value 4B  max_entries 64  memlock 832B

Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
Link: https://lore.kernel.org/r/20230305124615.12358-10-laoar.shao@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

kernel/bpf/cpumap.c

@@ -673,6 +673,15 @@ static int cpu_map_redirect(struct bpf_map *map, u64 index, u64 flags)
 				      __cpu_map_lookup_elem);
 }
 
+static u64 cpu_map_mem_usage(const struct bpf_map *map)
+{
+	u64 usage = sizeof(struct bpf_cpu_map);
+
+	/* Currently the dynamically allocated elements are not counted */
+	usage += (u64)map->max_entries * sizeof(struct bpf_cpu_map_entry *);
+	return usage;
+}
+
 BTF_ID_LIST_SINGLE(cpu_map_btf_ids, struct, bpf_cpu_map)
 const struct bpf_map_ops cpu_map_ops = {
 	.map_meta_equal		= bpf_map_meta_equal,
@@ -683,6 +692,7 @@ const struct bpf_map_ops cpu_map_ops = {
 	.map_lookup_elem	= cpu_map_lookup_elem,
 	.map_get_next_key	= cpu_map_get_next_key,
 	.map_check_btf		= map_check_no_btf,
+	.map_mem_usage		= cpu_map_mem_usage,
 	.map_btf_id		= &cpu_map_btf_ids[0],
 	.map_redirect		= cpu_map_redirect,
 };