Re: [PATCH 2/3] mm: memcontrol: do not try to do swap when force empty

From: Yang Shi
Date: Thu Jan 03 2019 - 13:20:33 EST

On 1/3/19 9:03 AM, Shakeel Butt wrote:
On Thu, Jan 3, 2019 at 8:57 AM Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx> wrote:

On 1/2/19 1:45 PM, Shakeel Butt wrote:
On Wed, Jan 2, 2019 at 12:06 PM Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx> wrote:
The typical usecase of force empty is to try to reclaim as much as
possible memory before offlining a memcg. Since there should be no
attached tasks to offlining memcg, the tasks anonymous pages would have
already been freed or uncharged.
Anon pages can come from tmpfs files as well.
Yes, but they are charged to swap space as regular anon pages.

The point was the lifetime of tmpfs anon pages are not tied to any
task. Even though there aren't any task attached to a memcg, the tmpfs
anon pages will remain charged. Other than that, the old anon pages of
a task which have migrated away might still be charged to the old
memcg (if move_charge_at_immigrate is not set).

Yes, my understanding is even though they are swapped out but they are still charged to memsw for cgroupv1. force_empty is supposed to reclaim as much as possible memory, here I'm supposed "reclaim" also means "uncharge".

Even though the anon pages are still charged to the old memcg, it will be moved the new memcg when the old one is deleted, or the pages will be just released if the task is killed.

So, IMHO, I don't see the point why swapping anon pages when doing force_empty.


Even though anonymous pages get
swapped out, but they still get charged to swap space. So, it sounds
pointless to do swap for force empty.

I understand that force_empty is typically used before rmdir'ing a
memcg but it might be used differently by some users. We use this
interface to test memory reclaim behavior (anon and file).
Thanks for sharing your usecase. So, you uses this for test only?