Date: Wed, 25 Mar 2009 09:28:44 +0100
From: Ingo Molnar <mingo@elte.hu>
To: Paul Mackerras <paulus@samba.org>,
       Andrew Morton <akpm@linux-foundation.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] perf_counter: allow and require one-page mmap on
	counting counters
Message-ID: <20090325082844.GA11217@elte.hu>
References: <18889.59409.260586.87939@cargo.ozlabs.ibm.com>
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* 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
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