[PATCH 3/3] docs: cgroup-v1: note the v1 memory controller implementation boundary

[Tao Cui <cui.tao@linux.dev>]

From: Tao Cui <cuitao@kylinos.cn>

Document that the legacy memory controller is now isolated behind
CONFIG_MEMCG_V1, with its state in struct mem_cgroup_v1, and that new
functionality belongs to the v2 memory controller.

Signed-off-by: Tao Cui <cuitao@kylinos.cn>
---
 Documentation/admin-guide/cgroup-v1/memory.rst | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/Documentation/admin-guide/cgroup-v1/memory.rst b/Documentation/admin-guide/cgroup-v1/memory.rst
index 7d2a44af52c9..2eff1dc6e13b 100644
--- a/Documentation/admin-guide/cgroup-v1/memory.rst
+++ b/Documentation/admin-guide/cgroup-v1/memory.rst
@@ -8,6 +8,13 @@ Memory Resource Controller
       here but make sure to check the current code if you need a deeper
       understanding.
 
+.. note::
+      The legacy (v1) memory controller implementation is isolated behind
+      ``CONFIG_MEMCG_V1``: its interface lives in ``mm/memcontrol-v1.c`` and
+      its per-cgroup state in ``struct mem_cgroup_v1`` (see
+      ``include/linux/memcontrol.h``).  New functionality belongs to the
+      cgroup v2 memory controller.
+
 .. note::
       The Memory Resource Controller has generically been referred to as the
       memory controller in this document. Do not confuse memory controller
-- 
2.43.0