linux-kernel - [PATCH 1/1] user namespaces: document CFS behavior

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Message-ID: <20081208215221.GA3954@us.ibm.com>
Date:	Mon, 8 Dec 2008 15:52:21 -0600
From:	"Serge E. Hallyn" <serue@...ibm.com>
To:	James Morris <jmorris@...ei.org>
Cc:	lkml <linux-kernel@...r.kernel.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Michael Kerrisk <mtk.manpages@...il.com>,
	Dhaval Giani <dhaval@...ux.vnet.ibm.com>
Subject: [PATCH 1/1] user namespaces: document CFS behavior

Documented the currently bogus state of support for CFS user groups with
user namespaces.  In particular, all users in a user namespace should be
children of the user which created the user namespace.  This is yet to
be implemented.

Signed-off-by: Serge E. Hallyn <serue@...ibm.com>
Acked-by: Dhaval Giani <dhaval@...ux.vnet.ibm.com>

Signed-off-by: Serge E. Hallyn <serue@...ibm.com>
---
 Documentation/scheduler/sched-design-CFS.txt |   21 +++++++++++++++++++++
 kernel/user.c                                |    8 +++++++-
 2 files changed, 28 insertions(+), 1 deletions(-)

diff --git a/Documentation/scheduler/sched-design-CFS.txt b/Documentation/scheduler/sched-design-CFS.txt
index eb471c7..8398ca4 100644
--- a/Documentation/scheduler/sched-design-CFS.txt
+++ b/Documentation/scheduler/sched-design-CFS.txt
@@ -273,3 +273,24 @@ task groups and modify their CPU share using the "cgroups" pseudo filesystem.
 
 	# #Launch gmplayer (or your favourite movie player)
 	# echo <movie_player_pid> > multimedia/tasks
+
+8. Implementation note: user namespaces
+
+User namespaces are intended to be hierarchical.  But they are currently
+only partially implemented.  Each of those has ramifications for CFS.
+
+First, since user namespaces are hierarchical, the /sys/kernel/uids
+presentation is inadequate.  Eventually we will likely want to use sysfs
+tagging to provide private views of /sys/kernel/uids within each user
+namespace.
+
+Second, the hierarchical nature is intended to support completely
+unprivileged use of user namespaces.  So if using user groups, then
+we want the users in a user namespace to be children of the user
+who created it.
+
+That is currently unimplemented.  So instead, every user in a new
+user namespace will receive 1024 shares just like any user in the
+initial user namespace.  Note that at the moment creation of a new
+user namespace requires each of CAP_SYS_ADMIN, CAP_SETUID, and
+CAP_SETGID.
diff --git a/kernel/user.c b/kernel/user.c
index 6c924bc..6608a3d 100644
--- a/kernel/user.c
+++ b/kernel/user.c
@@ -239,7 +239,13 @@ static struct kobj_type uids_ktype = {
 	.release = uids_release,
 };
 
-/* create /sys/kernel/uids/<uid>/cpu_share file for this user */
+/*
+ * Create /sys/kernel/uids/<uid>/cpu_share file for this user
+ * We do not create this file for users in a user namespace (until
+ * sysfs tagging is implemented).
+ *
+ * See Documentation/scheduler/sched-design-CFS.txt for ramifications.
+ */
 static int uids_user_create(struct user_struct *up)
 {
 	struct kobject *kobj = &up->kobj;
-- 
1.5.4.3

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