Subject: Re: [PATCH bpf] cpumap: Avoid warning when CONFIG_DEBUG_PER_CPU_MAPS is enabled

From: Jesper Dangaard Brouer <[email-masked]>
Date: Thu, 16 Apr 2020 08:55:37 +0200

On Wed, 15 Apr 2020 16:01:51 +0200
Toke Høiland-Jørgensen <[email-masked]> wrote:

> When the kernel is built with CONFIG_DEBUG_PER_CPU_MAPS, the cpumap code
> can trigger a spurious warning if CONFIG_CPUMASK_OFFSTACK is also set. This
> happens because in this configuration, NR_CPUS can be larger than
> nr_cpumask_bits, so the initial check in cpu_map_alloc() is not sufficient
> to guard against hitting the warning in cpumask_check().
> 
> Fix this by using the nr_cpumask_bits variable in the map creation code
> instead of the NR_CPUS constant.

Shouldn't you use 'nr_cpu_ids' instead of 'nr_cpumask_bits' ?

Else this will still fail on systems with CONFIG_CPUMASK_OFFSTACK=n.

> 
> Fixes: 6710e1126934 ("bpf: introduce new bpf cpu map type BPF_MAP_TYPE_CPUMAP")
> Cc: Jesper Dangaard Brouer <[email-masked]>
> Reported-by: Xiumei Mu <[email-masked]>
> Signed-off-by: Toke Høiland-Jørgensen <[email-masked]>
> ---
>  kernel/bpf/cpumap.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/bpf/cpumap.c b/kernel/bpf/cpumap.c
> index 70f71b154fa5..23902afb3bba 100644
> --- a/kernel/bpf/cpumap.c
> +++ b/kernel/bpf/cpumap.c
> @@ -99,8 +99,8 @@ static struct bpf_map *cpu_map_alloc(union bpf_attr *attr)
>  
>  	bpf_map_init_from_attr(&cmap->map, attr);
>  
> -	/* Pre-limit array size based on NR_CPUS, not final CPU check */
> -	if (cmap->map.max_entries > NR_CPUS) {
> +	/* Pre-limit array size based on nr_cpumask_bits, not final CPU check */
> +	if (cmap->map.max_entries > nr_cpumask_bits) {

Shouldn't you use 'nr_cpu_ids' instead of 'nr_cpumask_bits' ?

>  		err = -E2BIG;
>  		goto free_cmap;
>  	}

-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer