Re: [PATCH] perf_counter: allow and require one-page mmap on counting counters

[Ingo Molnar <mingo@elte.hu>]

* Paul Mackerras <paulus@samba.org> wrote:

> +++ b/kernel/perf_counter.c
> @@ -1362,8 +1362,13 @@ static int perf_mmap(struct file *file, struct vm_area_struct *vma)
>  	vma_size = vma->vm_end - vma->vm_start;
>  	nr_pages = (vma_size / PAGE_SIZE) - 1;
>  
> -	if (nr_pages == 0 || !is_power_of_2(nr_pages))
> -		return -EINVAL;
> +	if (counter->hw_event.record_type == PERF_RECORD_SIMPLE) {
> +		if (nr_pages)
> +			return -EINVAL;
> +	} else {
> +		if (nr_pages == 0 || !is_power_of_2(nr_pages))
> +			return -EINVAL;
> +	}

Hm, is_power_of_2() is buggy then as 1 page is a power of two as 
well: 1 == 2^0.

Hm, it seems fine:

 static inline __attribute__((const))
 bool is_power_of_2(unsigned long n)
 {
         return (n != 0 && ((n & (n - 1)) == 0));
 }

that should return true for an input of 1.

What am i missing?

	Ingo