Re: [RFC PATCH 03/14] mm/migrate: Add copy_pages_mthread function

[Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>]

On Thu, Feb 23, 2017 at 01:20:16PM +0530, Anshuman Khandual wrote:
...
> > 
> >> +
> >> +	cthreads = nr_copythreads;
> >> +	cthreads = min_t(unsigned int, cthreads, cpumask_weight(cpumask));
> > 
> > nitpick, but looks a little wordy, can it be simply like below?
> > 
> >   cthreads = min_t(unsigned int, nr_copythreads, cpumask_weight(cpumask));
> > 
> >> +	cthreads = (cthreads / 2) * 2;
> > 
> > I'm not sure the intention here. # of threads should be even number?
> 
> Yes.
> 
> > If cpumask_weight() is 1, cthreads is 0, that could cause zero division.
> > So you had better making sure to prevent it.
> 
> If cpumask_weight() is 1, then min_t(unsigned int, 8, 1) should be
> greater that equal to 1. Then cthreads can end up in 0. That is
> possible. But how there is a chance of zero division ? 

Hi Anshuman,

I just thought like above when reading the line your patch introduces:

       chunk_size = PAGE_SIZE * nr_pages / cthreads
                                           ~~~~~~~~
                                           (this can be 0?)

- Naoya

> May be its
> possible if we are trying move into a CPU less memory only node
> where cpumask_weight() is 0 ?
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