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    Subject[PATCH] fs, proc: Introduce the /proc/<pid>/children entry v2
    There is no easy way to make a reverse parent->children chain
    from the task status, in turn children->parent provided with "PPid"
    field.

    So instead of walking over all pids in system to figure out what
    children the task have -- we add explicit /proc/<pid>/children entry,
    since kernel already knows this kind of information but it was not
    yet exported.

    v2:
    - Kame suggested to use a separated /proc/<pid>/children entry
    instead of poking /proc/<pid>/status
    - Andew suggested to use rcu facility instead of locking
    tasklist_lock
    - Tejun pointed that non-seekable seq file might not be
    enough for tasks with large number of children

    Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
    Cc: Andrew Morton <akpm@linux-foundation.org>
    Cc: Pavel Emelyanov <xemul@parallels.com>
    Cc: Serge Hallyn <serge.hallyn@canonical.com>
    Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
    Cc: Pavel Emelyanov <xemul@parallels.com>
    Cc: Tejun Heo <tj@kernel.org>
    ---

    Please review. I hope I didn't miss anything here.

    fs/proc/array.c | 66 +++++++++++++++++++++++++++++++++++++++++++++++++++++
    fs/proc/base.c | 1
    fs/proc/internal.h | 1
    3 files changed, 68 insertions(+)

    Index: linux-2.6.git/fs/proc/array.c
    ===================================================================
    --- linux-2.6.git.orig/fs/proc/array.c
    +++ linux-2.6.git/fs/proc/array.c
    @@ -547,3 +547,69 @@ int proc_pid_statm(struct seq_file *m, s

    return 0;
    }
    +
    +static int children_seq_show(struct seq_file *seq, void *v)
    +{
    + struct task_struct *task = container_of(v, struct task_struct, sibling);
    + return seq_printf(seq, " %d", pid_vnr(task_pid(task)));
    +}
    +
    +static void *children_seq_start(struct seq_file *seq, loff_t *pos)
    +{
    + struct task_struct *task;
    +
    + rcu_read_lock();
    + task = seq->private;
    + if (task)
    + return seq_list_start(&task->children, *pos);
    +
    + return NULL;
    +}
    +
    +static void *children_seq_next(struct seq_file *seq, void *v, loff_t *pos)
    +{
    + struct task_struct *task = seq->private;
    + if (task)
    + return seq_list_next(v, &task->children, pos);
    + return NULL;
    +}
    +
    +static void children_seq_stop(struct seq_file *seq, void *v)
    +{
    + rcu_read_unlock();
    + seq_printf(seq, "\n");
    +}
    +
    +static const struct seq_operations children_seq_ops = {
    + .start = children_seq_start,
    + .next = children_seq_next,
    + .stop = children_seq_stop,
    + .show = children_seq_show,
    +};
    +
    +static int children_seq_open(struct inode *inode, struct file *file)
    +{
    + int ret = seq_open(file, &children_seq_ops);
    + if (!ret) {
    + struct seq_file *m = file->private_data;
    + m->private = (void *)get_proc_task(inode);
    + }
    +
    + return ret;
    +}
    +int children_seq_release(struct inode *inode, struct file *file)
    +{
    + struct seq_file *m = file->private_data;
    + if (m->private)
    + put_task_struct(m->private);
    +
    + seq_release(inode, file);
    + return 0;
    +}
    +
    +const struct file_operations proc_pid_children_operations = {
    + .open = children_seq_open,
    + .read = seq_read,
    + .llseek = seq_lseek,
    + .release = children_seq_release,
    +};
    Index: linux-2.6.git/fs/proc/base.c
    ===================================================================
    --- linux-2.6.git.orig/fs/proc/base.c
    +++ linux-2.6.git/fs/proc/base.c
    @@ -3204,6 +3204,7 @@ static const struct pid_entry tgid_base_
    INF("cmdline", S_IRUGO, proc_pid_cmdline),
    ONE("stat", S_IRUGO, proc_tgid_stat),
    ONE("statm", S_IRUGO, proc_pid_statm),
    + REG("children", S_IRUGO, proc_pid_children_operations),
    REG("maps", S_IRUGO, proc_maps_operations),
    #ifdef CONFIG_NUMA
    REG("numa_maps", S_IRUGO, proc_numa_maps_operations),
    Index: linux-2.6.git/fs/proc/internal.h
    ===================================================================
    --- linux-2.6.git.orig/fs/proc/internal.h
    +++ linux-2.6.git/fs/proc/internal.h
    @@ -53,6 +53,7 @@ extern int proc_pid_statm(struct seq_fil
    struct pid *pid, struct task_struct *task);
    extern loff_t mem_lseek(struct file *file, loff_t offset, int orig);

    +extern const struct file_operations proc_pid_children_operations;
    extern const struct file_operations proc_maps_operations;
    extern const struct file_operations proc_numa_maps_operations;
    extern const struct file_operations proc_smaps_operations;

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