cgroup: implement CFTYPE_NO_PREFIX

When cgroup files are created, cgroup core automatically prepends the
name of the subsystem as prefix.  This patch adds CFTYPE_NO_ which
disables the automatic prefix.  This is to work around historical
baggages and shouldn't be used for new files.

This will be used to move "cgroup.event_control" from cgroup core to
memcg.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Glauber Costa <glommer@gmail.com>

2 files changed, 3 insertions, 1 deletions

diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index 21ba29869eb8..3561d305b1e0 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -411,6 +411,7 @@ enum {
         CFTYPE_ONLY_ON_ROOT     = (1 << 0),     /* only create on root cgrp */
         CFTYPE_NOT_ON_ROOT      = (1 << 1),     /* don't create on root cgrp */
         CFTYPE_INSANE           = (1 << 2),     /* don't create if sane_behavior */
+        CFTYPE_NO_PREFIX        = (1 << 3),     /* (DON'T USE FOR NEW FILES) no subsys prefix */
 };
 
 #define MAX_CFTYPE_NAME         64
diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 7516668d8325..a41dc87cd07e 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -2756,7 +2756,8 @@ static int cgroup_add_file(struct cgroup *cgrp, struct cftype *cft)
         umode_t mode;
         char name[MAX_CGROUP_TYPE_NAMELEN + MAX_CFTYPE_NAME + 2] = { 0 };
 
-        if (cft->ss && !(cgrp->root->flags & CGRP_ROOT_NOPREFIX)) {
+        if (cft->ss && !(cft->flags & CFTYPE_NO_PREFIX) &&
+            !(cgrp->root->flags & CGRP_ROOT_NOPREFIX)) {
                 strcpy(name, cft->ss->name);
                 strcat(name, ".");
         }