"Serge E. Hallyn" <se...@hallyn.com> writes: > Quoting Eric W. Biederman (ebied...@xmission.com): >> "Serge E. Hallyn" <se...@hallyn.com> writes: >> >> > This is so that userspace can distinguish a mount made in a cgroup >> > namespace from a bind mount from a cgroup subdirectory. >> >> To do that do you need to print the path, or is an extra option that >> reveals nothing except that it was a cgroup mount sufficient? >> >> Is there any practical difference between a mount in a namespace and a >> bind mount? >> >> Given the way the conversation has been going I think it would be good >> to see the answers to these questions. Perhaps I missed it but I >> haven't seen the answers to those questions. > > Yup, I tried to answer those in my last email, let me try again. > > Let's say I start a container using cgroup namespaces, /lxc/x1. It mounts > freezer at /sys/fs/cgroup so it has field three of mountinfo as /lxc/x1, > and /sys/fs/cgroup/ is the path to the container's cgroup (/lxc/x1). In > that container, I start another container x1, not using cgroup namespaces. > It also wants a cgroup mount, and a common way to handle that (to prevent > container rewriting its limits) is to mount a tmpfs at /sys/fs/cgroup, > create /sysfs/cgroup/lxc/x1, and bind mount /sys/fs/cgroup/lxc/x1 from > the parent container onto /sys/fs/cgroup/lxc/x1 in the child container. > Now for that bind mount, the mountinfo field 3 will show /lxc/x1/lxc/x1, > with mount target /sys/fs/cgroup/lxc/x1, while /proc/self/cgroup for a task > in that container will show '/lxc/x1'. Unless it has been moved into > /lxc/x1/lxc/x1 in the container (/lxc/x1/lxc/x1/lxc/x1 on the host)... > Every time I've thought "maybe we can just..." I've found a case where it > wouldn't work. > > At first in lxc we simply said if /proc/self/ns/cgroup exists assume that > the cgroupfs mounts are not bind mounts. However, old userspace (and > container drivers) on new kernels is certainly possible, especially an > older distro in a container on a newer distro on the host. That completely > breaks with this approach. > > I also personally think there *is* value in letting a task know its > place on the system, so hiding the full cgroup path is imo not only not > a valid goal, it's counter-productive. Part of making for better > virtualization is to give userspace all the info it needs about its > current limits. Consider that with the unified hierarchy, you cannot > have tasks in a cgroup that also has child cgroups - except for the > root. Cgroup namespaces do not make an exception for this, so knowing > that you are not in the absolute cgroup root actually can prevent you > from trying something that cannot work. Or, I suppose, at least > understanding why you're unable to do what you're trying to do (namely > your container manager messed up). I point this out because finding > a way to only show the namespaced root in field 3 of mountinfo would > fix the base problem, but at the cost of hiding useful information > from a container.
It is just the superblock show_path method. And regardless of the rest of the usefullness of your mount option implementing show_path appears to be fundamentally the right thing in this context. As that field appears to have the same issue as /proc/self/cgroup. Eric
