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    SubjectRe: [RFC] Control Groups Roadmap ideas
    Paul Menage wrote:
    > 3) Subsystem dependencies
    > -----
    >
    > This would be a fairly simple change, essentially allowing one
    > subsystem to require that it only be mounted on a hierarchy when some
    > other subsystem was also present. The implementation would probably be
    > a callback that allows a subsystem to confirm whether it's prepared to
    > be included in a proposed hierarchy containing a specified subsystem
    > bitmask; it would be able to prevent the hierarchy from being created
    > by giving an error return. An example of a use for this would be a
    > swap subsystem that is mostly independent of the memory controller,
    > but uses the page-ownership tracking of the memory controller to
    > determine which cgroup to charge swap pages to. Hence it would require
    > that it only be mounted on a hierarchy that also included a memory
    > controller. The memory controller would make no such requirement by
    > itself, so could be used on its own without the swap controller.
    >

    Sounds good, and I wrote a prototype in a quick:

    diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
    index a6a6035..091bc21 100644
    --- a/include/linux/cgroup.h
    +++ b/include/linux/cgroup.h
    @@ -254,6 +254,7 @@ 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 (*can_mount)(struct cgroup_subsys *ss, unsigned long subsys_bits);
    int subsys_id;
    int active;
    int disabled;
    diff --git a/kernel/cgroup.c b/kernel/cgroup.c
    index 62f1a52..3d43ff2 100644
    --- a/kernel/cgroup.c
    +++ b/kernel/cgroup.c
    @@ -824,6 +824,25 @@ static int parse_cgroupfs_options(char *data,
    return 0;
    }

    +static int check_mount(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->can_mount) {
    + ret = ss->can_mount(ss, subsys_bits);
    + if (ret)
    + return ret;
    + }
    + }
    +
    + return 0;
    +}
    +
    static int cgroup_remount(struct super_block *sb, int *flags, char *data)
    {
    int ret = 0;
    @@ -839,6 +858,10 @@ static int cgroup_remount(struct super_block *sb, int *flags, char *data)
    if (ret)
    goto out_unlock;

    + ret = check_mount(opts.subsys_bits);
    + if (ret)
    + goto out_unlock;
    +
    /* Don't allow flags to change at remount */
    if (opts.flags != root->flags) {
    ret = -EINVAL;
    @@ -959,6 +982,13 @@ static int cgroup_get_sb(struct file_system_type *fs_type,
    return ret;
    }

    + ret = check_mount(opts.subsys_bits);
    + if (ret) {
    + if (opts.release_agent)
    + kfree(opts.release_agent);
    + return ret;
    + }
    +
    root = kzalloc(sizeof(*root), GFP_KERNEL);
    if (!root) {
    if (opts.release_agent)
    -------
    for the example about swap controller and memory controller:

    static int swap_cgroup_can_mount(struct cgroup_subsys *ss,
    unsigned long subsys_bits)
    {
    if (!test_bit(mem_cgroup_subsys_id, &subsys_bits))
    return -EINVAL;
    return 0;
    }
    'mem_cgroup_subsys_id' is a member of enum cgroup_subsys_id defined in cgroup.h


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