linux-kernel - Re: [patch v2] mm, hugetlb_cgroup: round limit_in

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Date:	Fri, 15 Apr 2016 15:24:51 +0200
From:	Michal Hocko <mhocko@...nel.org>
To:	David Rientjes <rientjes@...gle.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Nikolay Borisov <kernel@...p.com>,
	Johannes Weiner <hannes@...xchg.org>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [patch v2] mm, hugetlb_cgroup: round limit_in_bytes down to
 hugepage size

On Thu 14-04-16 13:22:30, David Rientjes wrote:
> On Thu, 7 Apr 2016, Michal Hocko wrote:
> 
> > > +static void hugetlb_cgroup_init(struct hugetlb_cgroup *h_cgroup,
> > > +				struct hugetlb_cgroup *parent_h_cgroup)
> > > +{
> > > +	int idx;
> > > +
> > > +	for (idx = 0; idx < HUGE_MAX_HSTATE; idx++) {
> > > +		struct page_counter *counter = &h_cgroup->hugepage[idx];
> > > +		struct page_counter *parent = NULL;
> > > +		unsigned long limit;
> > > +		int ret;
> > > +
> > > +		if (parent_h_cgroup)
> > > +			parent = &parent_h_cgroup->hugepage[idx];
> > > +		page_counter_init(counter, parent);
> > > +
> > > +		limit = round_down(PAGE_COUNTER_MAX,
> > > +				   1 << huge_page_order(&hstates[idx]));
> > > +		ret = page_counter_limit(counter, limit);
> > > +		VM_BUG_ON(ret);
> > > +	}
> > > +}
> > 
> > I fail to see the point for this. Why would want to round down
> > PAGE_COUNTER_MAX? It will never make a real difference. Or am I missing
> > something?
> 
> Did you try the patch?
> 
> If we're rounding down the user value, it makes sense to be consistent 
> with the upper bound default to specify intent.

The point I've tried to raise is why do we care and add a code if we can
never reach that value? Does actually anybody checks for the alignment.
-- 
Michal Hocko
SUSE Labs