Re: cgroups(7): documenting /sys/kernel/cgroup files

Hello Michael,

On Wed, Jan 10, 2018 at 05:23:32AM +0100, Michael Kerrisk (man-pages) wrote:

Hello Roman,

On 8 January 2018 at 22:11, Michael Kerrisk (man-pages)
[off-list ref] wrote:

quoted

Hello Roman,

I wrote cgroups(7) text below to document the files added by you in
Linux 4.15. Could you let me know if the following text is okay please:

   /sys/kernel/cgroup files
       /sys/kernel/cgroup/delegate (since Linux 4.15)
              This file exports a list of the cgroups v2 files  (one  per
              line) that are delegatable (i.e., whose ownership should be
              changed to the user ID of the delegatee).  In  the  future,
              the  set  of delegatable files may change or grow, and this
              file provides a way for the  kernel  to  inform  user-space
              applications of which files must be delegated.  As at Linux
              4.15, one sees the following when inspecting this file:

                  $ cat /sys/kernel/cgroup/delegate
                  cgroup.procs
                  cgroup.subtree_control

I have a question about /sys/kernel/cgroup/delegate: why does this
file not list cgroup.threads, since that is also a file that
(potentially) should be delegated?

cc Tejun

cgroup.threads is not listed because it's not delegatable (doesn't have
CFTYPE_NS_DELEGATABLE flag set). I don't know if it was intended or just
an accident (I believe the latter).

Thanks!

PS In the latter case the following patch should fix it:

From fdf19edb6e9594e48e89d4510767b9eb6ca2d9dd Mon Sep 17 00:00:00 2001
From: Roman Gushchin <redacted>
Date: Wed, 10 Jan 2018 04:35:12 -0800
Subject: [PATCH] cgroup: make cgroup.threads delegatable

Make cgroup.threads file delegatable.
The behavior of cgroup.threads should follow the behavior of cgroup.procs.

Signed-off-by: Roman Gushchin <redacted>
Discovered-by: Michael Kerrisk [off-list ref]
Cc: Tejun Heo <tj@kernel.org>
---
 kernel/cgroup/cgroup.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
index 0b1ffe147f24..b74d9ee1425c 100644
--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c

@@ -4449,6 +4449,7 @@ static struct cftype cgroup_base_files[] = {
 	},
 	{
 		.name = "cgroup.threads",
+		.flags = CFTYPE_NS_DELEGATABLE,
 		.release = cgroup_procs_release,
 		.seq_start = cgroup_threads_start,
 		.seq_next = cgroup_procs_next,

-- 
2.14.3