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

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Message-ID: <37D7C6CF3E00A74B8858931C1DB2F077017119BD@SHSMSX103.ccr.corp.intel.com>
Date:	Wed, 4 Mar 2015 14:06:01 +0000
From:	"Liang, Kan" <kan.liang@...el.com>
To:	Ingo Molnar <mingo@...nel.org>
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:
> 
> > > * 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).
> 
> 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.

Thanks,
Kan

> 
> Thanks,
> 
> 	Ingo
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