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    Subject[patch 1/4] fs, proc: Introduce /proc/<pid>/task/<tid>/children entry v8
    When we do checkpoint of a task we need to know the list of children
    the task, has but there is no easy and fast way to generate reverse
    parent->children chain from arbitrary <pid> (while a parent pid is
    provided in "PPid" field of /proc/<pid>/status).

    So instead of walking over all pids in the system (creating one big process
    tree in memory, just to figure out which children a task has) -- we add
    explicit /proc/<pid>/task/<tid>/children entry, because the kernel already has
    this kind of information but it is not yet exported.

    This is a first level children, not the whole process tree.

    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

    v3:
    - To be on a safe side use %lu format for pid_t printing

    v4:
    - New line get printed when sequence ends not at seq->stop,
    a nit pointed by Tejun
    - Documentation update
    - tasklist_lock is back, Oleg pointed that ->children list
    is actually not rcu-safe

    v5:
    - Oleg suggested to make /proc/<pid>/task/<tid>/children
    instead of global /proc/<pid>/children, which eliminates
    hardness related to threads and children migration, and
    allows patch to be a way simplier.

    v6:
    - Drop ptrace_may_access tests, pids are can be found anyway
    so nothing to protect here.
    - Update comments and docs, pointed by Oleg.

    v7:
    - Use get_pid over proc-pid directly, to simplify
    code, pointed by Oleg.

    v8:
    - Obtain a starting pid from the proc's inode directly.

    Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
    Reviewed-by: Oleg Nesterov <oleg@redhat.com>
    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>
    ---
    Documentation/filesystems/proc.txt | 18 +++++
    fs/proc/array.c | 123 +++++++++++++++++++++++++++++++++++++
    fs/proc/base.c | 1
    fs/proc/internal.h | 1
    4 files changed, 143 insertions(+)

    Index: linux-2.6.git/Documentation/filesystems/proc.txt
    ===================================================================
    --- linux-2.6.git.orig/Documentation/filesystems/proc.txt
    +++ linux-2.6.git/Documentation/filesystems/proc.txt
    @@ -40,6 +40,7 @@ Table of Contents
    3.4 /proc/<pid>/coredump_filter - Core dump filtering settings
    3.5 /proc/<pid>/mountinfo - Information about mounts
    3.6 /proc/<pid>/comm & /proc/<pid>/task/<tid>/comm
    + 3.7 /proc/<pid>/task/<tid>/children - Information about task children

    4 Configuring procfs
    4.1 Mount options
    @@ -1549,6 +1550,23 @@ then the kernel's TASK_COMM_LEN (current
    comm value.


    +3.7 /proc/<pid>/task/<tid>/children - Information about task children
    +-------------------------------------------------------------------------
    +This file provides a fast way to retrieve first level children pids
    +of a task pointed by <pid>/<tid> pair. The format is a space separated
    +stream of pids.
    +
    +Note the "first level" here -- if a child has own children they will
    +not be listed here, one needs to read /proc/<children-pid>/task/<tid>/children
    +to obtain the descendants.
    +
    +Since this interface is intended to be fast and cheap it doesn't
    +guarantee to provide precise results and some children might be
    +skipped, especially if they've exited right after we printed their
    +pids, so one need to either stop or freeze processes being inspected
    +if precise results are needed.
    +
    +
    ------------------------------------------------------------------------------
    Configuring procfs
    ------------------------------------------------------------------------------
    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 +547,126 @@ int proc_pid_statm(struct seq_file *m, s

    return 0;
    }
    +
    +static struct pid *
    +get_children_pid(struct inode *inode, struct pid *pid_prev, loff_t pos)
    +{
    + struct task_struct *start, *task;
    + struct pid *pid = NULL;
    +
    + read_lock(&tasklist_lock);
    +
    + start = pid_task(proc_pid(inode), PIDTYPE_PID);
    + if (!start)
    + goto out;
    +
    + /*
    + * Lets try to continue searching first, this gives
    + * us significant speedup on children-rich processes.
    + */
    + if (pid_prev) {
    + task = pid_task(pid_prev, PIDTYPE_PID);
    + if (task && task->real_parent == start &&
    + !(list_empty(&task->sibling))) {
    + if (list_is_last(&task->sibling, &start->children))
    + goto out;
    + task = list_first_entry(&task->sibling,
    + struct task_struct, sibling);
    + pid = get_pid(task_pid(task));
    + goto out;
    + }
    + }
    +
    + /*
    + * Slow search case.
    + *
    + * We might miss some children here if children
    + * are exited while we were not holding the lock,
    + * but it was never promised to be accurate that
    + * much.
    + *
    + * "Just suppose that the parent sleeps, but N children
    + * exit after we printed their tids. Now the slow paths
    + * skips N extra children, we miss N tasks." (c)
    + *
    + * So one need to stop or freeze the leader and all
    + * its children to get a precise result.
    + */
    + list_for_each_entry(task, &start->children, sibling) {
    + if (pos-- == 0) {
    + pid = get_pid(task_pid(task));
    + break;
    + }
    + }
    +
    +out:
    + read_unlock(&tasklist_lock);
    + return pid;
    +}
    +
    +static int children_seq_show(struct seq_file *seq, void *v)
    +{
    + struct inode *inode = seq->private;
    + pid_t pid;
    +
    + pid = pid_nr_ns(v, inode->i_sb->s_fs_info);
    + return seq_printf(seq, " %d", pid);
    +}
    +
    +static void *children_seq_start(struct seq_file *seq, loff_t *pos)
    +{
    + return get_children_pid(seq->private, NULL, *pos);
    +}
    +
    +static void *children_seq_next(struct seq_file *seq, void *v, loff_t *pos)
    +{
    + struct pid *pid = NULL;
    +
    + pid = get_children_pid(seq->private, v, *pos + 1);
    + if (!pid)
    + seq_printf(seq, "\n");
    + put_pid(v);
    +
    + ++*pos;
    + return pid;
    +}
    +
    +static void children_seq_stop(struct seq_file *seq, void *v)
    +{
    + put_pid(v);
    +}
    +
    +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)
    +{
    + struct seq_file *m;
    + int ret;
    +
    + ret = seq_open(file, &children_seq_ops);
    + if (ret)
    + return ret;
    +
    + m = file->private_data;
    + m->private = inode;
    +
    + return ret;
    +}
    +
    +int children_seq_release(struct inode *inode, struct file *file)
    +{
    + seq_release(inode, file);
    + return 0;
    +}
    +
    +const struct file_operations proc_tid_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
    @@ -3384,6 +3384,7 @@ static const struct pid_entry tid_base_s
    ONE("stat", S_IRUGO, proc_tid_stat),
    ONE("statm", S_IRUGO, proc_pid_statm),
    REG("maps", S_IRUGO, proc_maps_operations),
    + REG("children", S_IRUGO, proc_tid_children_operations),
    #ifdef CONFIG_NUMA
    REG("numa_maps", S_IRUGO, proc_numa_maps_operations),
    #endif
    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_tid_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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