bpf: Remove migrate_{disable|enable} in ->map_for_each_callback

BPF program may call bpf_for_each_map_elem(), and it will call
the ->map_for_each_callback callback of related bpf map. Considering the
running context of bpf program has already disabled migration, remove
the unnecessary migrate_{disable|enable} pair in the implementations of
->map_for_each_callback. To ensure the guarantee will not be voilated
later, also add cant_migrate() check in the implementations.

Signed-off-by: Hou Tao <houtao1@huawei.com>
Link: https://lore.kernel.org/r/20250108010728.207536-3-houtao@huaweicloud.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

Author: Hou Tao

kernel/bpf/arraymap.c

@@ -735,13 +735,13 @@ static long bpf_for_each_array_elem(struct bpf_map *map, bpf_callback_t callback
 	u64 ret = 0;
 	void *val;
 
+	cant_migrate();
+
 	if (flags != 0)
 		return -EINVAL;
 
 	is_percpu = map->map_type == BPF_MAP_TYPE_PERCPU_ARRAY;
 	array = container_of(map, struct bpf_array, map);
-	if (is_percpu)
-		migrate_disable();
 	for (i = 0; i < map->max_entries; i++) {
 		if (is_percpu)
 			val = this_cpu_ptr(array->pptrs[i]);
@@ -756,8 +756,6 @@ static long bpf_for_each_array_elem(struct bpf_map *map, bpf_callback_t callback
 			break;
 	}
 
-	if (is_percpu)
-		migrate_enable();
 	return num_elems;
 }
 

kernel/bpf/hashtab.c

@@ -2208,17 +2208,18 @@ static long bpf_for_each_hash_elem(struct bpf_map *map, bpf_callback_t callback_
 	bool is_percpu;
 	u64 ret = 0;
 
+	cant_migrate();
+
 	if (flags != 0)
 		return -EINVAL;
 
 	is_percpu = htab_is_percpu(htab);
 
 	roundup_key_size = round_up(map->key_size, 8);
-	/* disable migration so percpu value prepared here will be the
-	 * same as the one seen by the bpf program with bpf_map_lookup_elem().
+	/* migration has been disabled, so percpu value prepared here will be
+	 * the same as the one seen by the bpf program with
+	 * bpf_map_lookup_elem().
 	 */
-	if (is_percpu)
-		migrate_disable();
 	for (i = 0; i < htab->n_buckets; i++) {
 		b = &htab->buckets[i];
 		rcu_read_lock();
@@ -2244,8 +2245,6 @@ static long bpf_for_each_hash_elem(struct bpf_map *map, bpf_callback_t callback_
 		rcu_read_unlock();
 	}
 out:
-	if (is_percpu)
-		migrate_enable();
 	return num_elems;
 }