Re: [PATCH bpf-next 4/5] selftests/bpf: Avoid flaky resize value test for percpu data

[Leon Hwang]

On 15/8/26 02:24, bot+bpf-ci@xxxxxxxxxx wrote:
>> diff --git a/tools/testing/selftests/bpf/prog_tests/global_data_init.c b/tools/testing/selftests/bpf/prog_tests/global_data_init.c
>> index 7d6bda909295..2f0dbeb88934 100644
>> --- a/tools/testing/selftests/bpf/prog_tests/global_data_init.c
>> +++ b/tools/testing/selftests/bpf/prog_tests/global_data_init.c
> 
> [ ... ]
> 
>> @@ -126,11 +123,11 @@ static void test_percpu_data_on_cpus(struct bpf_map *map, int map_fd, int prog_f
>>  static void test_global_percpu_data_init(void)
>>  {
>>  	struct test_global_percpu_data__percpu init_value = {};
>> +	const __u32 desired_sz = sysconf(_SC_PAGE_SIZE) * 2;
>>  	struct test_global_percpu_data__percpu *init_data;
>> -	const __u32 desired_sz = sysconf(_SC_PAGE_SIZE);
> 
> Does multiplying by 2 here break the test on 64K page systems?
> 
> Per-CPU array value sizes are capped by the kernel:
> 
>   /* percpu map value size is bound by PCPU_MIN_UNIT_SIZE */
>   if (percpu && round_up(attr->value_size, 8) > PCPU_MIN_UNIT_SIZE)
>           return -E2BIG;
> 
> in kernel/bpf/arraymap.c:array_map_alloc_check(), where PCPU_MIN_UNIT_SIZE
> is defined as:
> 
>   #define PCPU_MIN_UNIT_SIZE  PFN_ALIGN(32 << 10)
> 
> which equals max(32768, PAGE_SIZE).
> 
> The old code was safe by construction: desired_sz == PAGE_SIZE <=
> PFN_ALIGN(32768) == PCPU_MIN_UNIT_SIZE. Doubling it breaks that:
> 
>   PAGE_SIZE  4096 -> limit 32768, request   8192  OK
>   PAGE_SIZE 16384 -> limit 32768, request  32768  OK (exactly at limit)
>   PAGE_SIZE 65536 -> limit 65536, request 131072  -E2BIG
> 
> On kernels built with 64K pages (arm64 CONFIG_ARM64_64K_PAGES, ppc64)
> bpf_map__set_value_size() still succeeds (it is purely userspace
> bookkeeping), all four assertions that follow it pass, and then map
> creation fails inside test_global_percpu_data__load():
> 
>>  	err = test_global_percpu_data__load(skel);
>>  	if (!ASSERT_OK(err, "test_global_percpu_data__load"))
>>  		goto out;
> 
> The "global_percpu_data/init" subtest fails, and test_percpu_data_on_cpus()
> is never reached. That is a new failure introduced by this patch (with
> desired_sz == PAGE_SIZE the same configuration created a 65536-byte per-CPU
> value, which is exactly at the limit and accepted).
> 
> Note that the neighbouring resize test in global_map_resize.c deliberately
> does not scale a per-CPU map by page size for this reason. At line 46 it
> uses:
> 
>   new_sz = sizeof(skel->data_percpu_arr->percpu_arr[0]) *
>            libbpf_num_possible_cpus();
> 
> A size that is independent of PAGE_SIZE would still force the mmap region
> to move, which is what the new skel->percpu_arr reassignment and the
> arr[0] == -1 check are there to exercise, without hitting the per-CPU
> allocator bound.
> 

To avoid breaking the test on 64K page systems, run the resize subtest
only on 4K page systems.

>From maintenance view, I think it's better to add a dedicated subtest to
test bpf_map__set_value_size() for percpu data. Then, it will be able to
skip the subtest when sysconf(_SC_PAGE_SIZE) != 4K.

Thanks,
Leon