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

Paul Mackerras
2009-03-25T08:57:11Z

Ingo Molnar writes:

> * 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;

To answer your question below... this: ^^^

> >
> > - 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

We have one page as a header that contains the info for reading the
counter value in userspace plus the head pointer, followed by (for a
sampling counter) 2^N pages of ring buffer.

Paul.