Task Control Groups: add tasks file interface

Add the per-directory "tasks" file for cgroupfs mounts; this allows the
user to determine which tasks are members of a cgroup by reading a
cgroup's "tasks", and to move a task into a cgroup by writing its pid to
its "tasks".

Signed-off-by: Paul Menage <menage@google.com>
Cc: Serge E. Hallyn <serue@us.ibm.com>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Dave Hansen <haveblue@us.ibm.com>
Cc: Balbir Singh <balbir@in.ibm.com>
Cc: Paul Jackson <pj@sgi.com>
Cc: Kirill Korotaev <dev@openvz.org>
Cc: Herbert Poetzl <herbert@13thfloor.at>
Cc: Srivatsa Vaddagiri <vatsa@in.ibm.com>
Cc: Cedric Le Goater <clg@fr.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

2 files changed, 368 insertions, 1 deletions

diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index 60735dcf427..e2dd44f68f9 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -144,6 +144,16 @@ int cgroup_is_removed(const struct cgroup *cont);
 
 int cgroup_path(const struct cgroup *cont, char *buf, int buflen);
 
+int __cgroup_task_count(const struct cgroup *cont);
+static inline int cgroup_task_count(const struct cgroup *cont)
+{
+        int task_count;
+        rcu_read_lock();
+        task_count = __cgroup_task_count(cont);
+        rcu_read_unlock();
+        return task_count;
+}
+
 /* Return true if the cgroup is a descendant of the current cgroup */
 int cgroup_is_descendant(const struct cgroup *cont);
 
diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 6ba857bec71..356c40d5d20 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -40,7 +40,7 @@
 #include <linux/magic.h>
 #include <linux/spinlock.h>
 #include <linux/string.h>
-
+#include <linux/sort.h>
 #include <asm/atomic.h>
 
 /* Generate an array of cgroup subsystem pointers */
@@ -700,6 +700,127 @@ int cgroup_path(const struct cgroup *cont, char *buf, int buflen)
         return 0;
 }
 
+/*
+ * Return the first subsystem attached to a cgroup's hierarchy, and
+ * its subsystem id.
+ */
+
+static void get_first_subsys(const struct cgroup *cont,
+                        struct cgroup_subsys_state **css, int *subsys_id)
+{
+        const struct cgroupfs_root *root = cont->root;
+        const struct cgroup_subsys *test_ss;
+        BUG_ON(list_empty(&root->subsys_list));
+        test_ss = list_entry(root->subsys_list.next,
+                             struct cgroup_subsys, sibling);
+        if (css) {
+                *css = cont->subsys[test_ss->subsys_id];
+                BUG_ON(!*css);
+        }
+        if (subsys_id)
+                *subsys_id = test_ss->subsys_id;
+}
+
+/*
+ * Attach task 'tsk' to cgroup 'cont'
+ *
+ * Call holding cgroup_mutex.  May take task_lock of
+ * the task 'pid' during call.
+ */
+static int attach_task(struct cgroup *cont, struct task_struct *tsk)
+{
+        int retval = 0;
+        struct cgroup_subsys *ss;
+        struct cgroup *oldcont;
+        struct css_set *cg = &tsk->cgroups;
+        struct cgroupfs_root *root = cont->root;
+        int i;
+        int subsys_id;
+
+        get_first_subsys(cont, NULL, &subsys_id);
+
+        /* Nothing to do if the task is already in that cgroup */
+        oldcont = task_cgroup(tsk, subsys_id);
+        if (cont == oldcont)
+                return 0;
+
+        for_each_subsys(root, ss) {
+                if (ss->can_attach) {
+                        retval = ss->can_attach(ss, cont, tsk);
+                        if (retval) {
+                                return retval;
+                        }
+                }
+        }
+
+        task_lock(tsk);
+        if (tsk->flags & PF_EXITING) {
+                task_unlock(tsk);
+                return -ESRCH;
+        }
+        /* Update the css_set pointers for the subsystems in this
+         * hierarchy */
+        for (i = 0; i < CGROUP_SUBSYS_COUNT; i++) {
+                if (root->subsys_bits & (1ull << i)) {
+                        /* Subsystem is in this hierarchy. So we want
+                         * the subsystem state from the new
+                         * cgroup. Transfer the refcount from the
+                         * old to the new */
+                        atomic_inc(&cont->count);
+                        atomic_dec(&cg->subsys[i]->cgroup->count);
+                        rcu_assign_pointer(cg->subsys[i], cont->subsys[i]);
+                }
+        }
+        task_unlock(tsk);
+
+        for_each_subsys(root, ss) {
+                if (ss->attach) {
+                        ss->attach(ss, cont, oldcont, tsk);
+                }
+        }
+
+        synchronize_rcu();
+        return 0;
+}
+
+/*
+ * Attach task with pid 'pid' to cgroup 'cont'. Call with
+ * cgroup_mutex, may take task_lock of task
+ */
+static int attach_task_by_pid(struct cgroup *cont, char *pidbuf)
+{
+        pid_t pid;
+        struct task_struct *tsk;
+        int ret;
+
+        if (sscanf(pidbuf, "%d", &pid) != 1)
+                return -EIO;
+
+        if (pid) {
+                rcu_read_lock();
+                tsk = find_task_by_pid(pid);
+                if (!tsk || tsk->flags & PF_EXITING) {
+                        rcu_read_unlock();
+                        return -ESRCH;
+                }
+                get_task_struct(tsk);
+                rcu_read_unlock();
+
+                if ((current->euid) && (current->euid != tsk->uid)
+                    && (current->euid != tsk->suid)) {
+                        put_task_struct(tsk);
+                        return -EACCES;
+                }
+        } else {
+                tsk = current;
+                get_task_struct(tsk);
+        }
+
+        ret = attach_task(cont, tsk);
+        put_task_struct(tsk);
+        return ret;
+}
+
 /* The various types of files and directories in a cgroup file system */
 
 enum cgroup_filetype {
@@ -708,6 +829,55 @@ enum cgroup_filetype {
         FILE_TASKLIST,
 };
 
+static ssize_t cgroup_common_file_write(struct cgroup *cont,
+                                           struct cftype *cft,
+                                           struct file *file,
+                                           const char __user *userbuf,
+                                           size_t nbytes, loff_t *unused_ppos)
+{
+        enum cgroup_filetype type = cft->private;
+        char *buffer;
+        int retval = 0;
+
+        if (nbytes >= PATH_MAX)
+                return -E2BIG;
+
+        /* +1 for nul-terminator */
+        buffer = kmalloc(nbytes + 1, GFP_KERNEL);
+        if (buffer == NULL)
+                return -ENOMEM;
+
+        if (copy_from_user(buffer, userbuf, nbytes)) {
+                retval = -EFAULT;
+                goto out1;
+        }
+        buffer[nbytes] = 0;     /* nul-terminate */
+
+        mutex_lock(&cgroup_mutex);
+
+        if (cgroup_is_removed(cont)) {
+                retval = -ENODEV;
+                goto out2;
+        }
+
+        switch (type) {
+        case FILE_TASKLIST:
+                retval = attach_task_by_pid(cont, buffer);
+                break;
+        default:
+                retval = -EINVAL;
+                goto out2;
+        }
+
+        if (retval == 0)
+                retval = nbytes;
+out2:
+        mutex_unlock(&cgroup_mutex);
+out1:
+        kfree(buffer);
+        return retval;
+}
+
 static ssize_t cgroup_file_write(struct file *file, const char __user *buf,
                                                 size_t nbytes, loff_t *ppos)
 {
@@ -914,6 +1084,189 @@ int cgroup_add_files(struct cgroup *cont,
         return 0;
 }
 
+/* Count the number of tasks in a cgroup. Could be made more
+ * time-efficient but less space-efficient with more linked lists
+ * running through each cgroup and the css_set structures that
+ * referenced it. Must be called with tasklist_lock held for read or
+ * write or in an rcu critical section.
+ */
+int __cgroup_task_count(const struct cgroup *cont)
+{
+        int count = 0;
+        struct task_struct *g, *p;
+        struct cgroup_subsys_state *css;
+        int subsys_id;
+
+        get_first_subsys(cont, &css, &subsys_id);
+        do_each_thread(g, p) {
+                if (task_subsys_state(p, subsys_id) == css)
+                        count ++;
+        } while_each_thread(g, p);
+        return count;
+}
+
+/*
+ * Stuff for reading the 'tasks' file.
+ *
+ * Reading this file can return large amounts of data if a cgroup has
+ * *lots* of attached tasks. So it may need several calls to read(),
+ * but we cannot guarantee that the information we produce is correct
+ * unless we produce it entirely atomically.
+ *
+ * Upon tasks file open(), a struct ctr_struct is allocated, that
+ * will have a pointer to an array (also allocated here).  The struct
+ * ctr_struct * is stored in file->private_data.  Its resources will
+ * be freed by release() when the file is closed.  The array is used
+ * to sprintf the PIDs and then used by read().
+ */
+struct ctr_struct {
+        char *buf;
+        int bufsz;
+};
+
+/*
+ * Load into 'pidarray' up to 'npids' of the tasks using cgroup
+ * 'cont'.  Return actual number of pids loaded.  No need to
+ * task_lock(p) when reading out p->cgroup, since we're in an RCU
+ * read section, so the css_set can't go away, and is
+ * immutable after creation.
+ */
+static int pid_array_load(pid_t *pidarray, int npids, struct cgroup *cont)
+{
+        int n = 0;
+        struct task_struct *g, *p;
+        struct cgroup_subsys_state *css;
+        int subsys_id;
+
+        get_first_subsys(cont, &css, &subsys_id);
+        rcu_read_lock();
+        do_each_thread(g, p) {
+                if (task_subsys_state(p, subsys_id) == css) {
+                        pidarray[n++] = pid_nr(task_pid(p));
+                        if (unlikely(n == npids))
+                                goto array_full;
+                }
+        } while_each_thread(g, p);
+
+array_full:
+        rcu_read_unlock();
+        return n;
+}
+
+static int cmppid(const void *a, const void *b)
+{
+        return *(pid_t *)a - *(pid_t *)b;
+}
+
+/*
+ * Convert array 'a' of 'npids' pid_t's to a string of newline separated
+ * decimal pids in 'buf'.  Don't write more than 'sz' chars, but return
+ * count 'cnt' of how many chars would be written if buf were large enough.
+ */
+static int pid_array_to_buf(char *buf, int sz, pid_t *a, int npids)
+{
+        int cnt = 0;
+        int i;
+
+        for (i = 0; i < npids; i++)
+                cnt += snprintf(buf + cnt, max(sz - cnt, 0), "%d\n", a[i]);
+        return cnt;
+}
+
+/*
+ * Handle an open on 'tasks' file.  Prepare a buffer listing the
+ * process id's of tasks currently attached to the cgroup being opened.
+ *
+ * Does not require any specific cgroup mutexes, and does not take any.
+ */
+static int cgroup_tasks_open(struct inode *unused, struct file *file)
+{
+        struct cgroup *cont = __d_cont(file->f_dentry->d_parent);
+        struct ctr_struct *ctr;
+        pid_t *pidarray;
+        int npids;
+        char c;
+
+        if (!(file->f_mode & FMODE_READ))
+                return 0;
+
+        ctr = kmalloc(sizeof(*ctr), GFP_KERNEL);
+        if (!ctr)
+                goto err0;
+
+        /*
+         * If cgroup gets more users after we read count, we won't have
+         * enough space - tough.  This race is indistinguishable to the
+         * caller from the case that the additional cgroup users didn't
+         * show up until sometime later on.
+         */
+        npids = cgroup_task_count(cont);
+        if (npids) {
+                pidarray = kmalloc(npids * sizeof(pid_t), GFP_KERNEL);
+                if (!pidarray)
+                        goto err1;
+
+                npids = pid_array_load(pidarray, npids, cont);
+                sort(pidarray, npids, sizeof(pid_t), cmppid, NULL);
+
+                /* Call pid_array_to_buf() twice, first just to get bufsz */
+                ctr->bufsz = pid_array_to_buf(&c, sizeof(c), pidarray, npids) + 1;
+                ctr->buf = kmalloc(ctr->bufsz, GFP_KERNEL);
+                if (!ctr->buf)
+                        goto err2;
+                ctr->bufsz = pid_array_to_buf(ctr->buf, ctr->bufsz, pidarray, npids);
+
+                kfree(pidarray);
+        } else {
+                ctr->buf = 0;
+                ctr->bufsz = 0;
+        }
+        file->private_data = ctr;
+        return 0;
+
+err2:
+        kfree(pidarray);
+err1:
+        kfree(ctr);
+err0:
+        return -ENOMEM;
+}
+
+static ssize_t cgroup_tasks_read(struct cgroup *cont,
+                                    struct cftype *cft,
+                                    struct file *file, char __user *buf,
+                                    size_t nbytes, loff_t *ppos)
+{
+        struct ctr_struct *ctr = file->private_data;
+
+        return simple_read_from_buffer(buf, nbytes, ppos, ctr->buf, ctr->bufsz);
+}
+
+static int cgroup_tasks_release(struct inode *unused_inode,
+                                        struct file *file)
+{
+        struct ctr_struct *ctr;
+
+        if (file->f_mode & FMODE_READ) {
+                ctr = file->private_data;
+                kfree(ctr->buf);
+                kfree(ctr);
+        }
+        return 0;
+}
+
+/*
+ * for the common functions, 'private' gives the type of file
+ */
+static struct cftype cft_tasks = {
+        .name = "tasks",
+        .open = cgroup_tasks_open,
+        .read = cgroup_tasks_read,
+        .write = cgroup_common_file_write,
+        .release = cgroup_tasks_release,
+        .private = FILE_TASKLIST,
+};
+
 static int cgroup_populate_dir(struct cgroup *cont)
 {
         int err;
@@ -922,6 +1275,10 @@ static int cgroup_populate_dir(struct cgroup *cont)
         /* First clear out any existing files */
         cgroup_clear_directory(cont->dentry);
 
+        err = cgroup_add_file(cont, NULL, &cft_tasks);
+        if (err < 0)
+                return err;
+
         for_each_subsys(cont->root, ss) {
                 if (ss->populate && (err = ss->populate(ss, cont)) < 0)
                         return err;