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SubjectRe: [PATCH v10 4/4] cgroups: implement the PIDs subsystem
> @@ -0,0 +1,368 @@
> +/*
> + * Process number limiting controller for cgroups.
> + *
> + * Used to allow a cgroup hierarchy to stop any new processes
> + * from fork()ing after a certain limit is reached.
> + *
> + * Since it is trivial to hit the task limit without hitting
> + * any kmemcg limits in place, PIDs are a fundamental resource.
> + * As such, PID exhaustion must be preventable in the scope of
> + * a cgroup hierarchy by allowing resource limiting of the
> + * number of tasks in a cgroup.
> + *
> + * In order to use the `pids` controller, set the maximum number
> + * of tasks in pids.max (this is not available in the root cgroup
> + * for obvious reasons). The number of processes currently
> + * in the cgroup is given by pids.current. Organisational operations
> + * are not blocked by cgroup policies, so it is possible to have
> + * pids.current > pids.max. However, fork()s will still not work.
> + *
> + * To set a cgroup to have no limit, set pids.max to "max". fork()
> + * will return -EBUSY if forking would cause a cgroup policy to be
> + * violated.
> + *
> + * pids.current tracks all child cgroup hierarchies, so
> + * parent/pids.current is a superset of parent/child/pids.current.
> + *
> + * Copyright (C) 2015 Aleksa Sarai <cyphar@cyphar.com>

The above text looks wrapped too narrow.

> +struct pids_cgroup {
> + struct cgroup_subsys_state css;
> +
> + /*
> + * Use 64-bit types so that we can safely represent "max" as
> + * (PID_MAX_LIMIT + 1).
^^^^^^^^^^^^^^^^^
...
> +static struct cgroup_subsys_state *
> +pids_css_alloc(struct cgroup_subsys_state *parent)
> +{
> + struct pids_cgroup *pids;
> +
> + pids = kzalloc(sizeof(struct pids_cgroup), GFP_KERNEL);
> + if (!pids)
> + return ERR_PTR(-ENOMEM);
> +
> + pids->limit = PIDS_MAX;
^^^^^^^^^

> + atomic64_set(&pids->counter, 0);
> + return &pids->css;
> +}
...
> +static void pids_detach(struct cgroup_subsys_state *old_css,
> + struct task_struct *task)
> +{
> + struct pids_cgroup *old_pids = css_pids(old_css);
> +
> + pids_uncharge(old_pids, 1);
> +}

You can do the above as a part of can/cancel.

> +static int pids_can_fork(struct task_struct *task, void **private)

Maybe @priv_p or something which signifies it's of different type from
others?

> +{
...
> + rcu_read_lock();
> + css = task_css(current, pids_cgrp_id);
> + if (!css_tryget_online(css)) {
> + retval = -EBUSY;
> + goto err_rcu_unlock;
> + }
> + rcu_read_unlock();

Hmmm... so, the above is guaranteed to succeed in finite amount of
time (the race window is actually very narrow) and it'd be silly to
fail fork because a task was being moved across cgroups.

I think it'd be a good idea to implement task_get_css() which loops
and returns the current css for the requested subsystem with reference
count bumped and it can use css_tryget() too. Holding a ref doesn't
prevent css from dying anyway, so it doesn't make any difference.

> +static void pids_fork(struct task_struct *task, void *private)
> +{
...
> + rcu_read_lock();
> + css = task_css(task, pids_cgrp_id);
> + css_get(css);

Why is this safe? What guarantees that css's ref isn't already zero
at this point?

> + rcu_read_unlock();
> +
> + pids = css_pids(css);
> +
> + /*
> + * The association has changed, we have to revert and reapply the
> + * charge/uncharge on the wrong hierarchy to the current one. Since
> + * the association can only change due to an organisation event, its
> + * okay for us to ignore the limit in this case.
> + */
> + if (pids != old_pids) {
> + pids_uncharge(old_pids, 1);
> + pids_charge(pids, 1);
> + }
> +
> + css_put(css);
> + css_put(old_css);
> +}
...
> +static ssize_t pids_max_write(struct kernfs_open_file *of, char *buf,
> + size_t nbytes, loff_t off)
> +{
> + struct cgroup_subsys_state *css = of_css(of);
> + struct pids_cgroup *pids = css_pids(css);
> + int64_t limit;
> + int err;
> +
> + buf = strstrip(buf);
> + if (!strcmp(buf, PIDS_MAX_STR)) {
> + limit = PIDS_MAX;
> + goto set_limit;
> + }
> +
> + err = kstrtoll(buf, 0, &limit);
> + if (err)
> + return err;
> +
> + /* We use INT_MAX as the maximum value of pid_t. */
> + if (limit < 0 || limit > INT_MAX)

This is kinda weird if we're using PIDS_MAX for max as it may end up
showing "max" after some larger number is written to the file.

Thanks.

--
tejun


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