Re: [PATCH bpf-next v7 4/7] bpf: Add BPF_F_CPU and BPF_F_ALL_CPUS flags support for percpu_hash and lru_percpu_hash maps

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

On Sat Sep 20, 2025 at 6:31 AM +08, Andrii Nakryiko wrote:
> On Thu, Sep 18, 2025 at 10:25 PM Leon Hwang <leon.hwang@linux.dev> wrote:
>>
>>
>>
>> >> @@ -1724,7 +1742,7 @@ __htab_map_lookup_and_delete_batch(struct bpf_map *map,
>> >>         value_size = htab->map.value_size;
>> >>         size = round_up(value_size, 8);
>> >>         if (is_percpu)
>> >> -               value_size = size * num_possible_cpus();
>> >> +               value_size = (elem_map_flags & BPF_F_CPU) ? size : size * num_possible_cpus();
>> >
>> > if (is_percpu && !(elem_map_flags & BPF_F_CPU))
>> >     value_size = size * num_possible_cpus();
>> >
>> > ?
>> >
>>
>> After looking at it again, I’d like to keep my approach.
>>
>> When 'elem_map_flags & BPF_F_CPU' is set, 'value_size' has to be
>> assigned to 'size' ('round_up(value_size, 8)') instead of keeping
>> 'htab->map.value_size'.
>>
>
> isn't that what will happen here as well? There is
>
> size = round_up(value_size, 8);
>
> right before that if
>

As for percpu maps, both 'size' and 'value_size' need to be 8-byte
aligned here, because 'map.value_size' itself is not guarenteed to be
aligned.

In 'htab_map_alloc_check()', there is no alignment check for percpu
maps.

So 'map.value_size' can be unaligned.

Let's look at how 'value_size' is used:

values = kvmalloc_array(value_size, bucket_size, GFP_USER | __GFP_NOWARN);
dst_val = values;
hlist_nulls_for_each_entry_safe(l, n, head, hash_node) {
        if (is_percpu) {
                if (elem_map_flags & BPF_F_CPU) {
                        copy_map_value_long(&htab->map, dst_val, per_cpu_ptr(pptr, cpu));
                }
        }
        dst_val += value_size;
}
copy_to_user(uvalues + total * value_size, values,
             value_size * bucket_cnt)

Here, 'value_size' determines how values are laid out and copied.

As a result, when 'is_percpu && (elem_map_flags & BPF_F_CPU)',
'value_size' must be assigned to 'size' in order to make sure it's
8-byte aligned.

Thanks,
Leon