Re: [PATCH 1/4] mm: Trial do_wp_page() simplification

From: Michal Hocko
Date: Mon Sep 21 2020 - 09:42:05 EST

[Cc Tejun and Christian - this is a part of a larger discussion which is
not directly related to this particular question so let me trim the
original email to the bare minimum.]

On Fri 18-09-20 12:40:32, Peter Xu wrote:
> One issue is when we charge for cgroup we probably can't do that onto the new
> mm/task, since copy_namespaces() is called after copy_mm(). I don't know
> enough about cgroup, I thought the child will inherit the parent's, but I'm not
> sure. Or, can we change that order of copy_namespaces() && copy_mm()? I don't
> see a problem so far but I'd like to ask first..

I suspect you are referring to CLONE_INTO_CGROUP, right? I have only now
learned about this feature so I am not deeply familiar with all the
details and I might be easily wrong. Normally all the cgroup aware
resources are accounted to the parent's cgroup. For memcg that includes
all the page tables, early CoW and other allocations with __GFP_ACCOUNT.
IIUC CLONE_INTO_CGROUP properly then this hasn't changed as the child is
associated to its new cgroup (and memcg) only in cgroup_post_fork. If
that is correct then we might have quite a lot of resources bound to
child's lifetime but accounted to the parent's memcg which can lead to
all sorts of interesting problems (e.g. unreclaimable memory - even by
the oom killer).

Christian, Tejun is this the expected semantic or I am just misreading
the code?
Michal Hocko