Date: Fri, 15 Apr 2016 15:24:51 +0200
From: Michal Hocko <mhocko@kernel.org>
To: David Rientjes <rientjes@google.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
        Nikolay Borisov <kernel@kyup.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org
Subject: Re: [patch v2] mm, hugetlb_cgroup: round limit_in_bytes down to
 hugepage size
Message-ID: <20160415132451.GL32377@dhcp22.suse.cz>
References: <alpine.DEB.2.10.1604051824320.32718@chino.kir.corp.google.com>
 <5704BA37.2080508@kyup.com>
 <5704BBBF.8040302@kyup.com>
 <alpine.DEB.2.10.1604061510040.10401@chino.kir.corp.google.com>
 <20160407125145.GD32755@dhcp22.suse.cz>
 <alpine.DEB.2.10.1604141321350.6593@chino.kir.corp.google.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <alpine.DEB.2.10.1604141321350.6593@chino.kir.corp.google.com>
User-Agent: Mutt/1.5.24 (2015-08-30)
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 1290
Lines: 39

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