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


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