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

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Date:	Wed, 25 Mar 2009 19:56:47 +1100
From:	Paul Mackerras <paulus@...ba.org>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] perf_counter: allow and require one-page mmap on
	counting counters

Ingo Molnar writes:

> * Paul Mackerras <paulus@...ba.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.
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