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

[Michal Hocko]

On Mon 21-09-20 16:43:55, Christian Brauner wrote:
> On Mon, Sep 21, 2020 at 10:38:47AM -0400, Tejun Heo wrote:
> > Hello,
> > 
> > On Mon, Sep 21, 2020 at 04:28:34PM +0200, Michal Hocko wrote:
> > > Fundamentaly CLONE_INTO_CGROUP is similar to regular fork + move to the
> > > target cgroup after the child gets executed. So in principle there
> > > shouldn't be any big difference. Except that the move has to be explicit
> > > and the the child has to have enough privileges to move itself. I am not
> > 
> > Yeap, they're supposed to be the same operations. We've never clearly
> > defined how the accounting gets split across moves because 1. it's
> > inherently blurry and difficult 2. doesn't make any practical difference for
> > the recommended and vast majority usage pattern which uses migration to seed
> > the new cgroup. CLONE_INTO_CGROUP doesn't change any of that.
> > 
> > > completely sure about CLONE_INTO_CGROUP model though. According to man
> > > clone(2) it seems that O_RDONLY for the target cgroup directory is
> > > sufficient. That seems much more relaxed IIUC and it would allow to fork
> > > into a different cgroup while keeping a lot of resources in the parent's
> > > proper.
> > 
> > If the man page is documenting that, it's wrong. cgroup_css_set_fork() has
> > an explicit cgroup_may_write() test on the destination cgroup.
> > CLONE_INTO_CGROUP should follow exactly the same rules as regular
> > migrations.
> 
> Indeed!
> The O_RDONLY mention on the manpage doesn't make sense but it is
> explained that the semantics are exactly the same for moving via the
> filesystem:

OK, if the semantic is the same as for the task migration then I do not
see any (new) problems. Care to point me where the actual check is
enforced? For the migration you need a write access to cgroup.procs but
if the API expects directory fd then I am not sure how that would expose
the same behavior.

-- 
Michal Hocko
SUSE Labs