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Subject[RFC] fs, proc: Introduce /proc/<pid>/task/<tid>/children entry v5
When we do checkpoint of a task we need to know the list of children
the task has but there is no easy way to make a reverse parent->children
chain from an 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 the big process
tree just to figure out which children a task has -- we add the 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.

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: Oleg Nesterov <oleg@redhat.com>
---
Documentation/filesystems/proc.txt | 20 ++++
fs/proc/array.c | 162 +++++++++++++++++++++++++++++++++++++
fs/proc/base.c | 1
fs/proc/internal.h | 1
4 files changed, 183 insertions(+), 1 deletion(-)
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
@@ -40,7 +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

------------------------------------------------------------------------------
Preface
@@ -1545,3 +1545,21 @@ a task to set its own or one of its thre
is limited in size compared to the cmdline value, so writing anything longer
then the kernel's TASK_COMM_LEN (currently 16 chars) will result in a truncated
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 shown here, one needs to read /proc/<children-pid>/task/<tid>/children
+to obtain descendants.
+
+Because this interface is intended to be fast and cheap it doesn't
+guarantee to provide precise results, which means if a child is exiting
+it might or might not be counted. The same applies to freshly created
+children -- they might or might not be counted.
+
+If one needs precise pids -- the task and children should be stopped
+or frozen.
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 +547,165 @@ int proc_pid_statm(struct seq_file *m, s

return 0;
}
+
+struct proc_pid_children_iter {
+ struct pid_namespace *pid_ns;
+ struct pid *pid_start;
+};
+
+static struct pid *
+get_children_pid(struct proc_pid_children_iter *iter, struct pid *pid_prev, loff_t pos)
+{
+ struct task_struct *start, *task;
+ struct pid *pid = NULL;
+
+ read_lock(&tasklist_lock);
+
+ start = pid_task(iter->pid_start, PIDTYPE_PID);
+ if (!start)
+ goto out;
+
+ /*
+ * Lets try to continue searching this would speed
+ * search significantly.
+ */
+ if (pid_prev) {
+ task = pid_task(pid_prev, PIDTYPE_PID);
+ if (task && task->real_parent == start &&
+ !(list_empty(&task->sibling))) {
+ /*
+ * OK, ltes try the fastpath, we might
+ * miss some freshly created children
+ * here, but it was never promised to be
+ * accurate.
+ *
+ * Also note if we have not enough rights
+ * to access the next children pid we simply
+ * fall into slow-search version.
+ */
+ if (!list_is_last(&task->sibling, &start->children)) {
+ task = list_first_entry(&task->sibling,
+ struct task_struct, sibling);
+ if (ptrace_may_access(task, PTRACE_MODE_READ)) {
+ pid = get_pid(task_pid(task));
+ goto out;
+ }
+ } else
+ goto out;
+ }
+ }
+
+ list_for_each_entry(task, &start->children, sibling) {
+ if (pos-- == 0) {
+ if (ptrace_may_access(task, PTRACE_MODE_READ)) {
+ pid = get_pid(task_pid(task));
+ goto out;
+ } else {
+ /* Maybe we success with the next children */
+ pos++;
+ }
+ }
+ }
+out:
+ read_unlock(&tasklist_lock);
+ return pid;
+}
+
+static int children_seq_show(struct seq_file *seq, void *v)
+{
+ struct proc_pid_children_iter *iter = seq->private;
+ unsigned long pid = (unsigned long)pid_nr_ns(v, iter->pid_ns);
+
+ return seq_printf(seq, " %lu", 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 proc_pid_children_iter *iter = seq->private;
+ struct pid *pid = NULL;
+
+ pid = get_children_pid(iter, 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 proc_pid_children_iter *iter = NULL;
+ struct task_struct *task = NULL;
+ int ret = 0;
+
+ task = get_proc_task(inode);
+ if (!task) {
+ ret = -ENOENT;
+ goto err;
+ }
+
+ if (!ptrace_may_access(task, PTRACE_MODE_READ)) {
+ ret = -EACCES;
+ goto err;
+ }
+
+ iter = kmalloc(sizeof(*iter), GFP_KERNEL);
+ if (!iter) {
+ ret = -ENOMEM;
+ goto err;
+ }
+
+ ret = seq_open(file, &children_seq_ops);
+ if (!ret) {
+ struct seq_file *m = file->private_data;
+ m->private = iter;
+
+ iter->pid_start = get_pid(task_pid(task));
+ iter->pid_ns = inode->i_sb->s_fs_info;
+ }
+
+err:
+ if (task)
+ put_task_struct(task);
+ if (ret)
+ kfree(iter);
+
+ return ret;
+}
+
+int children_seq_release(struct inode *inode, struct file *file)
+{
+ struct seq_file *m = file->private_data;
+ struct proc_pid_children_iter *iter = m->private;
+
+ put_pid(iter->pid_start);
+ kfree(iter);
+
+ 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
@@ -3497,6 +3497,7 @@ 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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