Re: [PATCH] cgroup: support checking of subsystem dependencies

["Paul Menage" <menage@google.com>]

On Wed, Jun 18, 2008 at 1:01 AM, Li Zefan <lizf@cn.fujitsu.com> wrote:
> This allows one subsystem to require that it only be mounted when some
> other subsystems are also present in the proposed hierarchy.
>
> For example if subsystem foo depends on bar, the following will fail:
>  # mount -t cgroup -ofoo xxx /dev/cgroup
>
> You should mount with both subsystems:
>  # mount -t cgroup -ofoo,bar xxx /dev/cgroup
>
> Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
> ---
>  Documentation/cgroups.txt |    6 ++++++
>  include/linux/cgroup.h    |    2 ++
>  kernel/cgroup.c           |   30 ++++++++++++++++++++++++++++++
>  3 files changed, 38 insertions(+), 0 deletions(-)
>
> diff --git a/Documentation/cgroups.txt b/Documentation/cgroups.txt
> index 824fc02..8252f5b 100644
> --- a/Documentation/cgroups.txt
> +++ b/Documentation/cgroups.txt
> @@ -530,6 +530,12 @@ and root cgroup. Currently this will only involve movement between
>  the default hierarchy (which never has sub-cgroups) and a hierarchy
>  that is being created/destroyed (and hence has no sub-cgroups).
>
> +int subsys_depend(struct cgroup_subsys *ss, unsigned long subsys_bits)
> +
> +Called when a cgroup subsystem wants to check if some other subsystems
> +are also present in the proposed hierarchy. If this method returns error,
> +the mount of the cgroup filesystem will fail.
> +
>  4. Questions
>  ============
>
> diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
> index e155aa7..fc99ba4 100644
> --- a/include/linux/cgroup.h
> +++ b/include/linux/cgroup.h
> @@ -305,6 +305,8 @@ struct cgroup_subsys {
>                        struct cgroup *cgrp);
>        void (*post_clone)(struct cgroup_subsys *ss, struct cgroup *cgrp);
>        void (*bind)(struct cgroup_subsys *ss, struct cgroup *root);
> +       int (*subsys_depend)(struct cgroup_subsys *ss,
> +                             unsigned long subsys_bits);
>        /*
>         * This routine is called with the task_lock of mm->owner held
>         */
> diff --git a/kernel/cgroup.c b/kernel/cgroup.c
> index 15ac0e1..a4c8671 100644
> --- a/kernel/cgroup.c
> +++ b/kernel/cgroup.c
> @@ -837,6 +837,25 @@ static int parse_cgroupfs_options(char *data,
>        return 0;
>  }
>
> +static int check_subsys_dependency(unsigned long subsys_bits)
> +{
> +       int i;
> +       int ret;
> +       struct cgroup_subsys *ss;
> +
> +       for (i = 0; i < CGROUP_SUBSYS_COUNT; i++) {
> +               ss = subsys[i];
> +
> +               if (test_bit(i, &subsys_bits) && ss->subsys_depend) {
> +                       ret = ss->subsys_depend(ss, subsys_bits);
> +                       if (ret)
> +                               return ret;
> +               }
> +       }
> +
> +       return 0;
> +}
> +
>  static int cgroup_remount(struct super_block *sb, int *flags, char *data)
>  {
>        int ret = 0;
> @@ -852,6 +871,10 @@ static int cgroup_remount(struct super_block *sb, int *flags, char *data)
>        if (ret)
>                goto out_unlock;
>
> +       ret = check_subsys_dependency(opts.subsys_bits);
> +       if (ret)
> +               goto out_unlock;
> +

Can we just call this from one place, in parse_cgroupfs_options() ?

Paul