Re: [tip:perf/core] Revert "perf: Remove the extra validity check on nr_pages"

[Ingo Molnar <mingo@kernel.org>]

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

> > * tip-bot for Kan Liang <tipbot@zytor.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!

Thanks,

	Ingo