Re: [PATCH 6/9] bpf: arraymap: introduce BPF_MAP_TYPE_ARRAY_PERCPU

[Alexei Starovoitov]

On Mon, Jan 11, 2016 at 11:56:58PM +0800, Ming Lei wrote:
> This patch introduces percpu array map so that expensive
> atomic operations can be avoided in eBPF prog in case of
> ARRAY map.
> 
> PERCPU MAP uses the percpu version of update/lookup element
> helpers and callbacks to access element in the map, and
> the previous update/lookup element helpers and callbacks
> don't work at the same time.
> 
> Signed-off-by: Ming Lei <tom.leiming@xxxxxxxxx>

useful stuff!

> +	if (percpu) {
> +		if (alloc_percpu_array(array, attr->max_entries,
> +				       attr->value_size)) {
> +			kvfree(array);
> +			return ERR_PTR(-ENOMEM);
> +		}
> +		array->map.pages = round_up(attr->max_entries *
> +				attr->value_size * num_possible_cpus(),
> +				PAGE_SIZE) >> PAGE_SHIFT;

I think it would be more accurate to add it to array_size instead of
doing page rounding here.

> -	array->map.pages = round_up(array_size, PAGE_SIZE) >> PAGE_SHIFT;
> +	array->map.pages += round_up(array_size, PAGE_SIZE) >> PAGE_SHIFT;

and this line wouldn't need to change...