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Subject[PATCH 9/9] [RFC] Example multi-bindable subsystem: a max-depth controller
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[RFC] Example multi-bindable subsystem: a max-depth controller

This subsystem introduces a new file, "maxdepth.children", in each
cgroup in its hierarchy. This value defaults to -1, meaning no
limit. If this maxdepth.children is >= 0, then no child cgroup may be
created below this cgroup at a depth of more than maxdepth.children.
The limit is checked at cgroup creation time for all ancestor cgroups.

Signed-off-by: Paul Menage <menage@google.com>

---
include/linux/cgroup_subsys.h | 6 +++
init/Kconfig | 8 ++++
kernel/Makefile | 1 +
kernel/maxdepth_cgroup.c | 80 +++++++++++++++++++++++++++++++++++++++++
4 files changed, 95 insertions(+), 0 deletions(-)
create mode 100644 kernel/maxdepth_cgroup.c
diff --git a/include/linux/cgroup_subsys.h b/include/linux/cgroup_subsys.h
index 5dfea38..021bfc1 100644
--- a/include/linux/cgroup_subsys.h
+++ b/include/linux/cgroup_subsys.h
@@ -66,3 +66,9 @@ MULTI_SUBSYS(info)
#endif

/* */
+
+#ifdef CONFIG_CGROUP_MAXDEPTH
+MULTI_SUBSYS(maxdepth)
+#endif
+
+/* */
diff --git a/init/Kconfig b/init/Kconfig
index 3bd4685..69907cc 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -613,6 +613,14 @@ config CGROUP_INFO
application-specific configuration data about a cgroup. Can
be mounted on multiple hierarchies at once.

+config CGROUP_MAXDEPTH
+ bool "CGroups controller to limit the max depth of a hierarchy"
+ depends on CGROUPS
+ help
+ Provides a simple cgroups subsystem with an
+ "maxdepth.children" field that limits the maximum number of
+ child generations permitted to a cgroup.
+
endif # CGROUPS

config MM_OWNER
diff --git a/kernel/Makefile b/kernel/Makefile
index e713a67..5712cd5 100644
--- a/kernel/Makefile
+++ b/kernel/Makefile
@@ -62,6 +62,7 @@ obj-$(CONFIG_CGROUP_FREEZER) += cgroup_freezer.o
obj-$(CONFIG_CPUSETS) += cpuset.o
obj-$(CONFIG_CGROUP_NS) += ns_cgroup.o
obj-$(CONFIG_CGROUP_INFO) += info_cgroup.o
+obj-$(CONFIG_CGROUP_MAXDEPTH) += maxdepth_cgroup.o
obj-$(CONFIG_UTS_NS) += utsname.o
obj-$(CONFIG_USER_NS) += user_namespace.o
obj-$(CONFIG_PID_NS) += pid_namespace.o
diff --git a/kernel/maxdepth_cgroup.c b/kernel/maxdepth_cgroup.c
new file mode 100644
index 0000000..2ca92d8
--- /dev/null
+++ b/kernel/maxdepth_cgroup.c
@@ -0,0 +1,80 @@
+/*
+ * maxdepth_cgroup.c - simple cgroup providing a child generation
+ * limit field
+ */
+
+#include "linux/cgroup.h"
+#include "linux/err.h"
+#include "linux/seq_file.h"
+
+struct maxdepth_cgroup {
+ struct cgroup_subsys_state css;
+ int maxdepth;
+};
+
+static inline struct maxdepth_cgroup *cg_md(struct cgroup *cg)
+{
+ return container_of(cgroup_subsys_state(cg, maxdepth_subsys_id),
+ struct maxdepth_cgroup, css);
+}
+
+static struct cgroup_subsys_state *md_create(struct cgroup_subsys *ss,
+ struct cgroup *cg)
+{
+ struct maxdepth_cgroup *md;
+ int depth = 1, maxdepth;
+ while (1) {
+ cg = cg->parent;
+ if (!cg)
+ break;
+ maxdepth = cg_md(cg)->maxdepth;
+ if ((maxdepth >= 0) && (depth > maxdepth))
+ return ERR_PTR(-EINVAL);
+ depth++;
+ }
+
+ md = kzalloc(sizeof(*md), GFP_KERNEL);
+ if (!md)
+ return ERR_PTR(-ENOMEM);
+ md->maxdepth = -1;
+ return &md->css;
+}
+
+static void md_destroy(struct cgroup_subsys *ss, struct cgroup *cont)
+{
+ kfree(cg_md(cont));
+}
+
+static s64 md_read(struct cgroup *cont, struct cftype *cft)
+{
+ return cg_md(cont)->maxdepth;
+}
+
+static int md_write(struct cgroup *cont, struct cftype *cft, s64 val)
+{
+ if ((val < -1) || (val >= INT_MAX))
+ return -EINVAL;
+ cg_md(cont)->maxdepth = val;
+ return 0;
+}
+
+static struct cftype md_files[] = {
+ {
+ .name = "children",
+ .read_s64 = md_read,
+ .write_s64 = md_write,
+ }
+};
+
+static int md_populate(struct cgroup_subsys *ss, struct cgroup *cont)
+{
+ return cgroup_add_files(cont, ss, md_files, ARRAY_SIZE(md_files));
+}
+
+struct cgroup_subsys maxdepth_subsys = {
+ .name = "maxdepth",
+ .create = md_create,
+ .destroy = md_destroy,
+ .populate = md_populate,
+ .subsys_id = maxdepth_subsys_id,
+};


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