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
