bpf: add percpu stats for bpf_map elements insertions/deletions

Add a generic percpu stats for bpf_map elements insertions/deletions in order
to keep track of both, the current (approximate) number of elements in a map
and per-cpu statistics on update/delete operations.

To expose these stats a particular map implementation should initialize the
counter and adjust it as needed using the 'bpf_map_*_elem_count' helpers
provided by this commit.
Signed-off-by: Anton Protopopov <aspsk@isovalent.com>
Link: https://lore.kernel.org/r/20230706133932.45883-2-aspsk@isovalent.comSigned-off-by: Alexei Starovoitov <ast@kernel.org>

include/linux/bpf.h

@@ -275,6 +275,7 @@ struct bpf_map {
 	} owner;
 	bool bypass_spec_v1;
 	bool frozen; /* write-once; write-protected by freeze_mutex */
+	s64 __percpu *elem_count;
 };
 
 static inline const char *btf_field_type_name(enum btf_field_type type)
@@ -2040,6 +2041,35 @@ bpf_map_alloc_percpu(const struct bpf_map *map, size_t size, size_t align,
 }
 #endif
 
+static inline int
+bpf_map_init_elem_count(struct bpf_map *map)
+{
+	size_t size = sizeof(*map->elem_count), align = size;
+	gfp_t flags = GFP_USER | __GFP_NOWARN;
+
+	map->elem_count = bpf_map_alloc_percpu(map, size, align, flags);
+	if (!map->elem_count)
+		return -ENOMEM;
+
+	return 0;
+}
+
+static inline void
+bpf_map_free_elem_count(struct bpf_map *map)
+{
+	free_percpu(map->elem_count);
+}
+
+static inline void bpf_map_inc_elem_count(struct bpf_map *map)
+{
+	this_cpu_inc(*map->elem_count);
+}
+
+static inline void bpf_map_dec_elem_count(struct bpf_map *map)
+{
+	this_cpu_dec(*map->elem_count);
+}
+
 extern int sysctl_unprivileged_bpf_disabled;
 
 static inline bool bpf_allow_ptr_leaks(void)