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

[Christian Brauner]

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:

"In order to place the child process in a different cgroup, the caller
specifies CLONE_INTO_CGROUP in cl_args.flags and passes a file
descriptor  that  refers  to  a  version  2  cgroup  in  the
cl_args.cgroup  field.  (This file descriptor can be obtained by opening
a cgroup v2 directory using either the O_RDONLY or the O_PATH flag.)
Note that all of the usual restrictions (described in cgroups(7)) on
placing a process into a version 2 cgroup apply."

Christian