cgroups.7: Document /sys/kernel/cgroup/delegate

Reviewed-by: Roman Gushchin <guro@fb.com>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man7/cgroups.7

@@ -906,6 +906,10 @@ in order to distribute resources into the subtree,
 and the delegatee should not have permission to change
 the resources that are distributed into the delegated subtree.
 .PP
+See also the discussion of the
+.IR /sys/kernel/cgroup/delegate
+file in NOTES.
+.PP
 After the aforementioned steps have been performed,
 the delegatee can create child cgroups within the delegated subtree
 and move processes between cgroups in the subtree.
@@ -1506,6 +1510,26 @@ This field contains the pathname of the control group in the hierarchy
 to which the process belongs.
 This pathname is relative to the mount point of the hierarchy.
 .RE
+.\"
+.SS /sys/kernel/cgroup files
+.TP
+.IR /sys/kernel/cgroup/delegate " (since Linux 4.15)"
+.\" commit 01ee6cfb1483fe57c9cbd8e73817dfbf9bacffd3
+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:
+.IP
+.EX
+.in +4n
+$ \fBcat /sys/kernel/cgroup/delegate\fP
+cgroup.procs
+cgroup.subtree_control
+.in
+.EE
 .SH ERRORS
 The following errors can occur for
 .BR mount (2):