linux-kernel - RE: [tip:perf/core] Revert "perf: Remove the extra validity check on nr

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Date:	Tue, 3 Mar 2015 20:04:42 +0000
From:	"Liang, Kan" <kan.liang@...el.com>
To:	Ingo Molnar <mingo@...nel.org>, "hpa@...or.com" <hpa@...or.com>,
	"ak@...ux.intel.com" <ak@...ux.intel.com>,
	"acme@...hat.com" <acme@...hat.com>,
	"a.p.zijlstra@...llo.nl" <a.p.zijlstra@...llo.nl>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"xiakaixu@...wei.com" <xiakaixu@...wei.com>,
	"tglx@...utronix.de" <tglx@...utronix.de>
CC:	"linux-tip-commits@...r.kernel.org" 
	<linux-tip-commits@...r.kernel.org>
Subject: RE: [tip:perf/core] Revert "perf: Remove the extra validity check
 on nr_pages"



> 
> 
> * tip-bot for Kan Liang <tipbot@...or.com> wrote:
> 
> > --- a/kernel/events/core.c
> > +++ b/kernel/events/core.c
> > @@ -4446,7 +4446,7 @@ static int perf_mmap(struct file *file, struct
> vm_area_struct *vma)
> >  	 * If we have rb pages ensure they're a power-of-two number, so
> we
> >  	 * can do bitmasks instead of modulo.
> >  	 */
> > -	if (!is_power_of_2(nr_pages))
> > +	if (nr_pages != 0 && !is_power_of_2(nr_pages))
> >  		return -EINVAL;
> 
> Hm, what does is_power_of_2(0) return? It should return 0, because 0 is
> not a power of 2!
> 
> and if it's fixed to return 0, then the check should properly be something
> like:
> 
> 	if (!nr_pages || !is_power_of_2(nr_pages))
> 
> or so?

nr_pages == 0 is valid, we cannot return -EINVAL. Because 
	vma_size = vma->vm_end - vma->vm_start;
	nr_pages = (vma_size / PAGE_SIZE) - 1;

So here we only want to check is_power_of_2
when the pages > 1 (or nr_pages > 0).

Thanks,
Kan

> 
> Thanks,
> 
> 	Ingo
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