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

Christian Brauner
2020-09-21T14:42:01Z

On Mon, Sep 21, 2020 at 03:42:00PM +0200, Michal Hocko wrote:
> [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?

Hey Michal,

Thanks for the Cc!

If I understand your question correctly, then you are correct. The logic
is split in three simple parts:
1. Child gets created and doesn't live in any cset
- This should mean that resources are still charged against the
parent's memcg which is what you're asking afiu.
1. cgroup_can_fork()
- create new or find existing matching cset for the child
3. cgroup_post_fork()
- move/attach child to the new or found cset

_Purely from a CLONE_INTO_CGROUP perspective_ you should be ok to
reverse the order of copy_mm() and copy_namespaces().

Christian