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

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Date:	Wed, 4 Mar 2015 21:28:28 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	"Liang, Kan" <kan.liang@...el.com>
Cc:	"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>,
	"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"


* Liang, Kan <kan.liang@...el.com> wrote:

> > * Liang, Kan <kan.liang@...el.com> wrote:
> > 
> > > > * 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))
> > > >  		return -EINVAL;
> > > >
> > > > 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).
> > 
> > I don't think you answered my first question, on which my later
> > comments rely:
> > 
> > > > Hm, what does is_power_of_2(0) return? It should return 0, because 0
> > is
> > > > not a power of 2!
> 
> Yes,  is_power_of_2(0) returns 0.

Oh, indeed, and I clearly cannot read code: your original argument and 
the revert is correct, sorry about the confusion!

Thanks,

	Ingo
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