mm: hugetlb controller for cgroups v2

In the effort of supporting cgroups v2 into Kubernetes, I stumped on
the lack of the hugetlb controller.

When the controller is enabled, it exposes four new files for each
hugetlb size on non-root cgroups:

- hugetlb.<hugepagesize>.current
- hugetlb.<hugepagesize>.max
- hugetlb.<hugepagesize>.events
- hugetlb.<hugepagesize>.events.local

The differences with the legacy hierarchy are in the file names and
using the value "max" instead of "-1" to disable a limit.

The file .limit_in_bytes is renamed to .max.

The file .usage_in_bytes is renamed to .current.

.failcnt is not provided as a single file anymore, but its value can
be read through the new flat-keyed files .events and .events.local,
through the "max" key.

Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

Documentation/admin-guide/cgroup-v2.rst

@@ -61,6 +61,8 @@ v1 is available under Documentation/admin-guide/cgroup-v1/.
      5-6. Device
      5-7. RDMA
        5-7-1. RDMA Interface Files
+     5-8. HugeTLB
+       5.8-1. HugeTLB Interface Files
      5-8. Misc
        5-8-1. perf_event
      5-N. Non-normative information
@@ -2056,6 +2058,33 @@ RDMA Interface Files
 	  mlx4_0 hca_handle=1 hca_object=20
 	  ocrdma1 hca_handle=1 hca_object=23
 
+HugeTLB
+-------
+
+The HugeTLB controller allows to limit the HugeTLB usage per control group and
+enforces the controller limit during page fault.
+
+HugeTLB Interface Files
+~~~~~~~~~~~~~~~~~~~~~~~
+
+  hugetlb.<hugepagesize>.current
+	Show current usage for "hugepagesize" hugetlb.  It exists for all
+	the cgroup except root.
+
+  hugetlb.<hugepagesize>.max
+	Set/show the hard limit of "hugepagesize" hugetlb usage.
+	The default value is "max".  It exists for all the cgroup except root.
+
+  hugetlb.<hugepagesize>.events
+	A read-only flat-keyed file which exists on non-root cgroups.
+
+	  max
+		The number of allocation failure due to HugeTLB limit
+
+  hugetlb.<hugepagesize>.events.local
+	Similar to hugetlb.<hugepagesize>.events but the fields in the file
+	are local to the cgroup i.e. not hierarchical. The file modified event
+	generated on this file reflects only the local events.
 
 Misc
 ----