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

Ingo Molnar
2009-03-25T08:29:30Z

* 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