Re: [PATCH bpf-next v4 3/7] bpf: Allow per unit prefill for non-fix-size percpu memory allocator

[Yonghong Song <yonghong.song@linux.dev>]

On 12/19/23 8:37 PM, Alexei Starovoitov wrote:
> On Sun, Dec 17, 2023 at 10:30:47PM -0800, Yonghong Song wrote:
>> @@ -2963,7 +2963,9 @@ static int __init bpf_global_ma_init(void)
>>   
>>   	ret = bpf_mem_alloc_init(&bpf_global_ma, 0, false);
>>   	bpf_global_ma_set = !ret;
>> -	return ret;
>> +	ret = bpf_mem_alloc_percpu_init(&bpf_global_percpu_ma);
>> +	bpf_global_percpu_ma_set = !ret;
>> +	return !bpf_global_ma_set || !bpf_global_percpu_ma_set;
> ...
>> -				if (meta.func_id == special_kfunc_list[KF_bpf_percpu_obj_new_impl]) {
>> -					if (!bpf_global_percpu_ma_set) {
>> -						mutex_lock(&bpf_percpu_ma_lock);
>> -						if (!bpf_global_percpu_ma_set) {
>> -							err = bpf_mem_alloc_init(&bpf_global_percpu_ma, 0, true);
>> -							if (!err)
>> -								bpf_global_percpu_ma_set = true;
>> -						}
>> -						mutex_unlock(&bpf_percpu_ma_lock);
>> -						if (err)
>> -							return err;
>> -					}
>> -				}
>> -
>>   				if (((u64)(u32)meta.arg_constant.value) != meta.arg_constant.value) {
>>   					verbose(env, "local type ID argument must be in range [0, U32_MAX]\n");
>>   					return -EINVAL;
>> @@ -12096,6 +12079,17 @@ static int check_kfunc_call(struct bpf_verifier_env *env, struct bpf_insn *insn,
>>   					return -EINVAL;
>>   				}
>>   
>> +				if (meta.func_id == special_kfunc_list[KF_bpf_percpu_obj_new_impl]) {
>> +					if (!bpf_global_percpu_ma_set)
>> +						return -ENOMEM;
> The patch set looks great except I don't understand this part of the patch
> that goes back to allocating bpf_global_percpu_ma by default.
> Why allocate even small amount if no bpf prog will use it?
> It seems delaying allocation until the verifier sees the need is better.
> The rest of the series makes sense.

Thanks for suggestion. Will move early bpf_global_percpu_ma initialization
from __init stage to verifier then. This way, we have zero memory consumption
if bpf_global_percpu_ma is not used.