[PATCH 2/2] cgroupfs: Support user xattrs

[Daniel Xu]

This patch turns on xattr support for cgroupfs. This is useful for
letting non-root owners of delegated subtrees attach metadata to
cgroups.

One use case is for subtree owners to tell a userspace out of memory
killer to bias away from killing specific subtrees.

Tests:

    [/sys/fs/cgroup]# for i in $(seq 0 130); \
        do setfattr workload.slice -n user.name$i -v wow; done
    setfattr: workload.slice: No space left on device
    setfattr: workload.slice: No space left on device
    setfattr: workload.slice: No space left on device

    [/sys/fs/cgroup]# for i in $(seq 0 130); \
        do setfattr workload.slice --remove user.name$i; done
    setfattr: workload.slice: No such attribute
    setfattr: workload.slice: No such attribute
    setfattr: workload.slice: No such attribute

    [/sys/fs/cgroup]# for i in $(seq 0 130); \
        do setfattr workload.slice -n user.name$i -v wow; done
    setfattr: workload.slice: No space left on device
    setfattr: workload.slice: No space left on device
    setfattr: workload.slice: No space left on device

`seq 0 130` is inclusive, and 131 - 128 = 3, which is the number of
errors we expect to see.

Signed-off-by: Daniel Xu <dxu@xxxxxxxxx>
---
 kernel/cgroup/cgroup.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
index 75f687301bbf..21621b43a8ab 100644
--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -1954,7 +1954,8 @@ int cgroup_setup_root(struct cgroup_root *root, u16 ss_mask)
 
 	root->kf_root = kernfs_create_root(kf_sops,
 					   KERNFS_ROOT_CREATE_DEACTIVATED |
-					   KERNFS_ROOT_SUPPORT_EXPORTOP,
+					   KERNFS_ROOT_SUPPORT_EXPORTOP |
+					   KERNFS_ROOT_SUPPORT_USER_XATTR,
 					   root_cgrp);
 	if (IS_ERR(root->kf_root)) {
 		ret = PTR_ERR(root->kf_root);
-- 
2.21.1