Re: [PATCH] mm, memcg: reset low limit during memcg offlining

Vladimir Davydov
2017-07-25T12:05:42Z

On Tue, Jul 25, 2017 at 12:40:47PM +0100, Roman Gushchin wrote:
> A removed memory cgroup with a defined low limit and some belonging
> pagecache has very low chances to be freed.
>
> If a cgroup has been removed, there is likely no memory pressure inside
> the cgroup, and the pagecache is protected from the external pressure
> by the defined low limit. The cgroup will be freed only after
> the reclaim of all belonging pages. And it will not happen until
> there are any reclaimable memory in the system. That means,
> there is a good chance, that a cold pagecache will reside
> in the memory for an undefined amount of time, wasting
> system resources.
>
> Fix this issue by zeroing memcg->low during memcg offlining.
>
> Signed-off-by: Roman Gushchin <guro@fb.com>
> Cc: Tejun Heo <tj@kernel.org>
> Cc: Johannes Weiner <hannes@cmpxchg.org>
> Cc: Michal Hocko <mhocko@kernel.org>
> Cc: Vladimir Davydov <vdavydov.dev@gmail.com>
> Cc: kernel-team@fb.com
> Cc: cgroups@vger.kernel.org
> Cc: linux-mm@kvack.org
> Cc: linux-kernel@vger.kernel.org
> ---
> mm/memcontrol.c | 2 ++
> 1 file changed, 2 insertions(+)
>
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index aed11b2d0251..2aa204b8f9fd 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -4300,6 +4300,8 @@ static void mem_cgroup_css_offline(struct cgroup_subsys_state *css)
> }
> spin_unlock(&memcg->event_list_lock);
>
> + memcg->low = 0;
> +
> memcg_offline_kmem(memcg);
> wb_memcg_offline(memcg);
>

We already have that - see mem_cgroup_css_reset().