Re: [PATCH bpf-next v3 2/6] bpf: Introduce BPF_F_CPU flag for percpu_array maps

["Leon Hwang" <leon.hwang@linux.dev>]

On Sat Aug 23, 2025 at 6:14 AM +08, Andrii Nakryiko wrote:
> On Thu, Aug 21, 2025 at 9:08 AM Leon Hwang <leon.hwang@linux.dev> wrote:
>>
>> Introduce support for the BPF_F_ALL_CPUS flag in percpu_array maps to
>> allow updating values for all CPUs with a single value.
>>
>> Introduce support for the BPF_F_CPU flag in percpu_array maps to allow
>> updating value for specified CPU.
>>
>> This enhancement enables:
>>
>> * Efficient update values across all CPUs with a single value when
>>   BPF_F_ALL_CPUS is set for update_elem and update_batch APIs.
>> * Targeted update or lookup for a specified CPU when BPF_F_CPU is set.
>>
>> The BPF_F_CPU flag is passed via:
>>
>> * map_flags of lookup_elem and update_elem APIs along with embedded cpu
>>   field.
>> * elem_flags of lookup_batch and update_batch APIs along with embedded
>>   cpu field.
>>
>> Signed-off-by: Leon Hwang <leon.hwang@linux.dev>
>> ---
>>  include/linux/bpf.h            |  3 +-
>>  include/uapi/linux/bpf.h       |  2 ++
>>  kernel/bpf/arraymap.c          | 56 ++++++++++++++++++++++++++--------
>>  kernel/bpf/syscall.c           | 27 ++++++++++------
>>  tools/include/uapi/linux/bpf.h |  2 ++
>>  5 files changed, 67 insertions(+), 23 deletions(-)
>>
>
> [...]
>
>> diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
>> index 19f7f5de5e7dc..6251ac9bc7e42 100644
>> --- a/kernel/bpf/syscall.c
>> +++ b/kernel/bpf/syscall.c
>> @@ -131,9 +131,11 @@ bool bpf_map_write_active(const struct bpf_map *map)
>>         return atomic64_read(&map->writecnt) != 0;
>>  }
>>
>> -static u32 bpf_map_value_size(const struct bpf_map *map)
>> +static u32 bpf_map_value_size(const struct bpf_map *map, u64 flags)
>>  {
>> -       if (map->map_type == BPF_MAP_TYPE_PERCPU_HASH ||
>> +       if (map->map_type == BPF_MAP_TYPE_PERCPU_ARRAY && (flags & (BPF_F_CPU | BPF_F_ALL_CPUS)))
>> +               return round_up(map->value_size, 8);
>
> this doesn't depend on the PERCPU_ARRAY map type, right? Any map for
> which we allowed BPF_F_CPU or BPF_F_ALL_CPUS would use this formula?
> (and if map doesn't support those flags, you should have filtered that
> out earlier, no?) So maybe add this is first separate condition before
> all this map type specific logic?
>

Right.

I will remove this map type specific logic in next revision.

>> +       else if (map->map_type == BPF_MAP_TYPE_PERCPU_HASH ||
>>             map->map_type == BPF_MAP_TYPE_LRU_PERCPU_HASH ||
>>             map->map_type == BPF_MAP_TYPE_PERCPU_ARRAY ||
>>             map->map_type == BPF_MAP_TYPE_PERCPU_CGROUP_STORAGE)
>> @@ -314,7 +316,7 @@ static int bpf_map_copy_value(struct bpf_map *map, void *key, void *value,
>>             map->map_type == BPF_MAP_TYPE_LRU_PERCPU_HASH) {
>>                 err = bpf_percpu_hash_copy(map, key, value);
>>         } else if (map->map_type == BPF_MAP_TYPE_PERCPU_ARRAY) {
>> -               err = bpf_percpu_array_copy(map, key, value);
>> +               err = bpf_percpu_array_copy(map, key, value, flags);
>>         } else if (map->map_type == BPF_MAP_TYPE_PERCPU_CGROUP_STORAGE) {
>>                 err = bpf_percpu_cgroup_storage_copy(map, key, value);
>>         } else if (map->map_type == BPF_MAP_TYPE_STACK_TRACE) {
>> @@ -1656,12 +1658,19 @@ static void *___bpf_copy_key(bpfptr_t ukey, u64 key_size)
>>
>>  static int check_map_flags(struct bpf_map *map, u64 flags, bool check_flag)
>
> you are later moving this into bpf.h header, so do it in previous
> patch early on, less unnecessary code churn and easier to review
> actual changes to that function in subsequent patches
>

Ack.

Thanks,
Leon

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