Re: [PATCH] cgroup: fix to allow mounting a hierarchy by name

[Tejun Heo <tj@kernel.org>]

Hello,

On Fri, Dec 30, 2011 at 01:58:52PM +0800, Li Zefan wrote:
> Normal filesystems can have multi mount points, and an fs instance
> is identified by device name, but cgroupfs ignores device name like
> other pseudo filesystems. Instead a set of subsystems is used, so
> to mount the same cgroupfs instance in different mount points, we
> can do this:
> 
> 	# mount -t cgroup -o cpuset xxx /cgroup1
> 	# mount -t cgroup -o cpuset xxx /cgroup2
>
> Now we have the "none" option, so a cgroupfs can have no subsystems
> bound to it, and we allow multi instances of such cgroupfs, so we
> have to assign names to each instance:
> 
> 	# mount -t cgroup -o none,name=hier1 xxx /cgroup1
> 	# mount -t cgroup -o none,name=hier2 xxx /cgroup2
> 
> Then we want to also mount "hier1" in another mount point, we can't
> do this:
> 
> 	# mount -t cgroup -o none xxx /mnt
> 
> because we have two different instances with "none" subsystem. So
> we specify its name:
> 
> 	# mount -t cgroup -o none,name=hier1 xxx /mnt
> 
> Hope I have made things clear to you?

mount --bind?  It's not exactly the same thing but I don't think the
differences would matter for cgroup.  Also, what's the use case for
mounting the same cgroup directory multiple times?  Why is that
necessary?  Is it useful for some namespace-savvy setup?

> What I try to fix here is the behavior of "mount -t cgroup -o name=xxx ..."
> (no other options are specified), so what behavior do we want?
> 
> 1. find if any existing cgroupfs instance matches the name, which is
> the orginal behavior.
> 
> 2. the same as "mount -t cgroup -o all,name=xxx ...", which is the
> current behavior due to the commit that broke (1).
> 
> 3. make it invalid and fail to mount.
> 
> 4. any other idea?

I guess I'll apply the patches but it still seems like a silly
redundant feature.  If not, please enlighten me.

Thanks.

-- 
tejun