On Mon, Mar 20, 2023 at 03:06:47AM +0000, Cai Xinchen <caixinchen1@huawei.com> wrote:
> There are two problems left:
> 
>      root
>      /  \
>     A    B
>    / \    \
>   C   E    D
> 
> 1. In some case of reparent, some page cache may be used by other memcg
> D but it charges to the parent memcg A of dying memcg E. D is getting
> away with using the page for free while A is taxed.

Note that A is (effectively) taxed even before E is removed due to
hierarchical nature of charging. Then what you describe transforms into
"well-known" problem of shared charging (with not well-known solution
:-/).

HTH,
Michal