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

[Michal Hocko]

On Mon 21-09-20 16:41:34, Christian Brauner wrote:
> 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().

Switching the order wouldn't make much of a difference right. At least
not for memcg where all the accounted allocations will still go to
current's memcg.
-- 
Michal Hocko
SUSE Labs