Re: [v3 PATCH 4/4] mm: thp: make deferred split shrinker memcg aware

[Yang Shi]

On 6/25/19 3:00 PM, Andrew Morton wrote:
> On Thu, 13 Jun 2019 05:56:49 +0800 Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx> wrote:
>
> > Currently THP deferred split shrinker is not memcg aware, this may cause
> > premature OOM with some configuration. For example the below test would
> > run into premature OOM easily:
> >
> > $ cgcreate -g memory:thp
> > $ echo 4G > /sys/fs/cgroup/memory/thp/memory/limit_in_bytes
> > $ cgexec -g memory:thp transhuge-stress 4000
> >
> > transhuge-stress comes from kernel selftest.
> >
> > It is easy to hit OOM, but there are still a lot THP on the deferred
> > split queue, memcg direct reclaim can't touch them since the deferred
> > split shrinker is not memcg aware.
> >
> > Convert deferred split shrinker memcg aware by introducing per memcg
> > deferred split queue.  The THP should be on either per node or per memcg
> > deferred split queue if it belongs to a memcg.  When the page is
> > immigrated to the other memcg, it will be immigrated to the target
> > memcg's deferred split queue too.
> >
> > Reuse the second tail page's deferred_list for per memcg list since the
> > same THP can't be on multiple deferred split queues.
> >
> > ...
> >
> > --- a/mm/memcontrol.c
> > +++ b/mm/memcontrol.c
> > @@ -4579,6 +4579,11 @@ static struct mem_cgroup *mem_cgroup_alloc(void)
> >   #ifdef CONFIG_CGROUP_WRITEBACK
> >   	INIT_LIST_HEAD(&memcg->cgwb_list);
> >   #endif
> > +#ifdef CONFIG_TRANSPARENT_HUGEPAGE
> > +	spin_lock_init(&memcg->deferred_split_queue.split_queue_lock);
> > +	INIT_LIST_HEAD(&memcg->deferred_split_queue.split_queue);
> > +	memcg->deferred_split_queue.split_queue_len = 0;
> > +#endif
> >   	idr_replace(&mem_cgroup_idr, memcg, memcg->id.id);
> >   	return memcg;
> >   fail:
> > @@ -4949,6 +4954,14 @@ static int mem_cgroup_move_account(struct page *page,
> >   		__mod_memcg_state(to, NR_WRITEBACK, nr_pages);
> >   	}
> >   
> > +#ifdef CONFIG_TRANSPARENT_HUGEPAGE
> > +	if (compound && !list_empty(page_deferred_list(page))) {
> > +		spin_lock(&from->deferred_split_queue.split_queue_lock);
> > +		list_del(page_deferred_list(page));
> It's worrisome that this page still appears to be on the deferred_list
> and that the above if() would still succeed.  Should this be
> list_del_init()?

list_del_init() sounds safe although I'm not quite sure this is 
possible. Will update this with fixing build issue together.

> > +		from->deferred_split_queue.split_queue_len--;
> > +		spin_unlock(&from->deferred_split_queue.split_queue_lock);
> > +	}
> > +#endif