On Mon, 18 Apr 2016 14:23:58 -0700 (PDT) David Rientjes <rientjes@xxxxxxxxxx> wrote:
> On Fri, 15 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.
>
> If the user modifies the value successfully, it can never be restored to
> the default since the write handler rounds down. It's a matter of
> consistency for a long-term maintainable kernel and prevents bug reports.
Can we please get the above reasoning into the changelog?
Also, the runtime effects of the patch are unclear - "not possible to
charge partial hugepages" sounds serious, but there's no cc:stable.
Some clarification there also please.
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