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

[Peter Zijlstra <a.p.zijlstra@chello.nl>]

On Wed, 2009-03-25 at 09:28 +0100, Ingo Molnar wrote:
> * 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?

nr_pages is the number of data pages.