Re: [PATCH bpf-next v4 4/9] selftests/bpf: libarena: Raise arena spinlock CPU limits

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential
issue to consider:
- [Medium] selftests/bpf: enlarged qnodes exceeds arena max_entries in
  arena_spin_lock.c
--

commit b35a00e6a80fa689cd0b2cdb4e1bcd8899fded17
Author: Emil Tsalapatis <emil@etsalapatis.com>

selftests/bpf: libarena: Raise arena spinlock CPU limits

This commit increases the maximum number of CPUs supported by the BPF
arena spinlocks from 1024 to 8192.

> diff --git a/tools/testing/selftests/bpf/libarena/include/bpf_arena_spin_lock.h b/tools/testing/selftests/bpf/libarena/include/bpf_arena_spin_lock.h
> index 560ee18fd287f..230bd112e8d59 100644
> --- a/tools/testing/selftests/bpf/libarena/include/bpf_arena_spin_lock.h
> +++ b/tools/testing/selftests/bpf/libarena/include/bpf_arena_spin_lock.h
> @@ -77,7 +77,7 @@ struct arena_qnode {
>   * 16-17: tail index
>   * 18-31: tail cpu (+1)
>   */
> -#define _Q_MAX_CPUS		1024
> +#define _Q_MAX_CPUS		8192

[Severity: Medium]
Does raising _Q_MAX_CPUS to 8192 exceed the 100-page arena map limit in
tools/testing/selftests/bpf/progs/arena_spin_lock.c?

tools/testing/selftests/bpf/progs/arena_spin_lock.c {
	struct {
		__uint(type, BPF_MAP_TYPE_ARENA);
		__uint(map_flags, BPF_F_MMAPABLE);
		__uint(max_entries, 100); /* number of pages */
	...
	} arena SEC(".maps");
	...
	struct arena_qnode __arena __hidden qnodes[_Q_MAX_CPUS][_Q_MAX_NODES];
}

With 16-byte struct arena_qnode and _Q_MAX_NODES of 4, qnodes grows from
64 KiB to 512 KiB (128 4-KiB pages), causing bpf_object__relocate() to
reject loading the arena_spin_lock selftest with -E2BIG on 4-KiB page
architectures.

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20261109182055.7581-1-emil@etsalapatis.com?part=4